# Troubleshooting a 5E3



## BMW-KTM (Apr 7, 2015)

First off, let me say that I am in no way whatsoever an amp tech. Electronics do not come to me naturally and most of the time I feel like I am in over my head. That said, I am reasonably intelligent, good with my hands, mechanically inclined, and not easily frightened by a challenge.

I have built myself a 5E3 from parts. I acquired a partial MOJO kit some years ago in a private deal and had to buy the rest of the missing components individually over the years as well as a few extra tools to complete the build.

I completed the build earlier this week and have been enjoying my new toy up until I tried placing a jumper between the bright and normal jacks. I had been using only the normal jack. It was when I did this and was experimenting with the individual volume knobs that I started noticing something was amiss. The first thing I noticed is that there was no sound when plugged into the bright jacks. The MOJO drawing I downloaded and used to build the amp showed 3 out of the 4 input jacks using the chassis as ground and I wondered about that while building it. I went ahead and wired up a ground to the other 3 jacks and then there was sound from the bright jacks but only at half volume compared to the normal jacks. The second thing I noticed is that the bright volume control does very little when plugged into the normal jack, almost nothing when plugged into the bright jack and shuts off all volume if dialled past 11 on either jack. Using a jumper between normal and bright does not seem to have the same affect as it normally should. In fact, it almost seems as though it's not even there.

This is the drawing I used as my reference to build the amp.
I've been around forums for a long time so I know people will want more information.
I will try to provide whatever I can.
I appreciate any help I can get.


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## BMW-KTM (Apr 7, 2015)

I did not follow the MOJO drawing to the letter. There were two deviations.

One, I wanted a mute/standby switch and saw no need for a ground lift. I facilitated that by eliminating the cap, connecting the red/yellow from the PT to one side of the ground switch and the other side to ground. The third contact at the other end of the switch is unused.

Two, I did not get the MOJO 768 OT shown on the drawing and instead used a Mercury Magnetics FTDO-59 so the wire colors are not exactly as shown.


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## RBlakeney (Mar 12, 2017)

Do you have a picture of your input jacks?


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## BMW-KTM (Apr 7, 2015)

This was taken before I added a ground.


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## BMW-KTM (Apr 7, 2015)

After.


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## RBlakeney (Mar 12, 2017)

BMW-KTM said:


> After.


I don’t know much of anything, so someone here who is smart will be helpful. 
only thing I notice that may look different is the extra ground in the bottom right input Jack.🤷‍♂️


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## BMW-KTM (Apr 7, 2015)

That jack works.
It's the two on the left that are the bright jacks.


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## Lincoln (Jun 2, 2008)

I'm no amp tech either, but I find the way Mojo drew the grounding on the schematic you posted a bit unusual. Not just the input jacks - all the way down the line. I'll be waiting to hear from one of the techs on this. Maybe there is some learning here for me. (and for us all) 

Wondering why they separated the pre-amp grounds from the power amp grounds. What the benefit is and should I be doing it.


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## BMW-KTM (Apr 7, 2015)

Excellent question. I was having exactly that discussion with a guy about grounding about a week ago. He was telling me some amp experts believe the preamp section should be grounded in a location separate from the power section. Everything I have been taught tells me that is a recipe for ground looping and that various components all need to be grounded to one location.


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## Lincoln (Jun 2, 2008)

BMW-KTM said:


> Excellent question. I was having exactly that discussion with a guy about grounding about a week ago. He was telling me some amp experts believe the preamp section should be grounded in a location separate from the power section. Everything I have been taught tells me that is a recipe for ground looping and that various components all need to be grounded to one location.


That's what I've been "taught" as well. In that diagram, mojo has even gone so far as to ground the filter cap for the 12AX7's power supply with the pre-amp stuff and separate from the rest of the filter caps. Interesting.


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## BMW-KTM (Apr 7, 2015)

Well, I didn't do that. 
I have one area with two strips connected to each other as a central ground location just to the right of the PT.


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## jb welder (Sep 14, 2010)

Grounding can almost be an art as much as a science. The theory and practice do not always gel. Classic Fender wiring that is often hum free is in some ways technically 'wrong'.
So don't get hung up on it unless you have noise/hum issues. Different (or 'non-traditional') grounding techniques will not make things work/not work.

Your jacks all look correct as far as the wiring goes.
Does everything work correctly without the jumper cable? Lower jacks give different loudness than upper jacks? Bright side is bright, normal jacks normal?

If all that is correct, can you run through this page and say where there are differences? He goes through what happens with the jumper installed: Input Jacks


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## BMW-KTM (Apr 7, 2015)

On the normal jacks one is hotter than the other.
I do no perceive the bright jacks to be brighter but that might be because the output is so much lower.
It is significantly lower. Much more than just being noticable. It is enough that those jacks are basically unusable.


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## dtsaudio (Apr 15, 2009)

Check the wiring to pin 7 of the first tube. The bright and normal go to differnt halves of the first tube. Maybe that first stage isn't working properly.


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## BMW-KTM (Apr 7, 2015)

dtsaudio said:


> Check the wiring to pin 7 of the first tube. The bright and normal go to differnt halves of the first tube. Maybe that first stage isn't working properly.


I have no way of knowing what you mean by "the first tube".
If you mean the 12AY7 socket, the bright is connected to pin 7, the normal is connected to pin 2.
Except for the two variations I mentioned, both of which should have nothing to do with this issue, it is wired up as shown on the diagram in my original post.


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## BMW-KTM (Apr 7, 2015)

Is it possible the 12AY7 is faulty?
Is there any way that a faulty preamp tube could explain the volume knob issues I explained in my OP?

I do not have an alternate tube to try.


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## BMW-KTM (Apr 7, 2015)




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## dtsaudio (Apr 15, 2009)

Yes, the 12ay7. You could pop in a 12ax7 to try. Gain and noise will be different, but it will at least tell you if it is the tube.


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## BMW-KTM (Apr 7, 2015)

I tried a 12AX7 known to be good. (a J/J EEC83)
I like the gain better this way.
It seemed too clean for me before.
It was not at all why I wanted a 5E3 in the first place.
Are there any issues with running it this way all the time?

I think I may have discovered one of my problems. 
The one associated with the jacks.
After replacing the preamp tube and powering it back up I thought I had made it worse because only one of the normal jacks was working properly and the other one was now like the brights jacks at about 1/3 volume.
Simply by way of accident, while I was pulling the cord out and trying the different jacks, I noticed that for a split second it went back up to full volume.
I decided to try wiggling the cord while plugged into the jack and there was a huge difference.
If I push on the jack in such a way that the tip moves further away from the shorting "thingie" it comes in loud and clear.

Should I attempt to slightly bend the shorting part away from the tip contact on all the jacks and see if that cures it?

I have no way of knowing if it will affect my volume knob issue though.


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## Hammerhands (Dec 19, 2016)

Do you have a continuity tester?


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## BMW-KTM (Apr 7, 2015)

Yes I have a multimeter and I know how to test for continuity.


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## Lincoln (Jun 2, 2008)

Sounds like you found the problem alright. Good going!


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## BMW-KTM (Apr 7, 2015)

No.
I did not find the problem.
Bending the contact solved the problem only on that one normal jack but it did not solve anything regarding the bright jacks.
With the top cover off so I can see inside the chassis and the amp sitting on the table at eye level I have visually confirmed that plugging the cord into the jack does in fact separate the contacts on the shorting jacks, all of them.
The problem persists on the bright jacks, even with a known good 12AX7 in the 12AT7 socket.
The volume control problem persists as well.


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## Lincoln (Jun 2, 2008)

Have you tested the resistance of the volume pot on the channel you're having trouble with? have you compared the voltage readings on the first half of V1 tube (terminals 1,2,3) with the voltages on the second half (terminals 6,7,8)?


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## BMW-KTM (Apr 7, 2015)

I will need some assistance here.
As I said earlier I may be handy but I am not a tech.
Using this crop from the MOJO drawing and if we number the pot contacts from 1 to 3 going left to right, where do I place my probes?
I assume the black will go to ground (1) but I don't know if the red should go to middle (2) or the rightmost one (3).
I also assume you mean I should check the pot throughout the range of motion.
It is the middle pot that is misbehaving.
It makes sense to my layman's mind that if this pot were toast it could explain a lot.


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## dtsaudio (Apr 15, 2009)

To check the pot, set meter to ohms (highest setting). First measure across the two outside contacts. That will be the full resistance of the pot.
Then put one probe in the middle. Measure the resistance at either outside contact and rotate. Resistance should smoothly change. Then repeat with the other outside contact. Both pots should measure approximately the same.
Once you know the pots are good. Check the jack's resistance. Plug in a guitar cord into one jack. Measure between the two connections at the other end. Resistance should be high on each jack. If it's low, you have a short or mis-wired jack. Do each jack.
Once you've done that, it's time to measure voltages.


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## BMW-KTM (Apr 7, 2015)

My meter is auto-ranging so I did not have to keep changing the setting. I believe I have at least one faulty pot, if not two. Maybe even three. I did not test the tone pot. I was getting some really confusing readings at first until I realized the position of one pot would affect the reading on the other pot. After I noticed that I did all of my measurements with the other pots turned all the way down to (1). I changed my pointy probes out for alligator clips so I could have my hands and eyes free to work slowly and observe. I moved slowly through the sweep from (1) to (12) to look for abnormalities and I think I found some.



*Normal Pot:*

Outside to outside
Measured 495kΩ all through the range except for a slight dip to 488kΩ around (8) on the dial, then back up by (9).

Middle to right _(right being the one connecting to bright pot, right)_
Measured approximately 495kΩ from (1) through (8) on the dial and then it started dropping.
(9) = 446kΩ, (10) = 228kΩ, (11) = 49kΩ, (12) = 0.2Ω.

Middle to left_ (ground)_
Measured 0.2Ω at (1), (2) = 0.4Ω, jumped up to 5.9kΩ at (3)
(4) = 47kΩ, (5) = 64kΩ, (6) = 81kΩ, (7) = 118kΩ, (8) = 197kΩ, (9) = 339kΩ, (10) = 457kΩ, (11) = 506kΩ, (12) = 495kΩ. That little bump at (11) struck me as odd, particularly since it was not at (8) where the bump was testing the other way.


*Bright Pot:*

Outside to outside
A fairly smooth transition from (1) = 495kΩ through (8) = 452kΩ. Then it began dropping faster. (9) = 403kΩ, (10) = 298kΩ, (11) = 131kΩ, (12) = 0.2Ω.

Middle _(connected to tone)_ to right _(connected to Normal)_
Very similar to outside/outside. Slightly different numbers but the very same curve/shape and starting and ending at the same points.

Middle _(connected to tone)_ to left _(ground)_
Reads as 0.1Ω al the way through from (1) through (12).
It seemed odd so I lifted one clip from the pot and then it showed as open so the 0.1Ω was not a bad connection to the meter.


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## dtsaudio (Apr 15, 2009)

Showing a short circuit. Disconnect the pot and test again.


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## blue_dog (Feb 7, 2013)

Double check your lead dressing it looks a bit tight. Swap out V1 and check the plate voltages to make sure they are within spec.


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## BMW-KTM (Apr 7, 2015)

blue_dog said:


> Double check your lead dressing it looks a bit tight. Swap out V1 and check the plate voltages to make sure they are within spec.


You saw that I am not a tech, right?
I don't know what lead dressing means. I don't know which tube is V1 and I don't know which pins to check or what values to look for.


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## blue_dog (Feb 7, 2013)

It's the shield wrap that is connecting to the 0.1uf capacitor, just check that it's not shorting to the eyelet.



BMW-KTM said:


> You saw that I am not a tech, right?
> I don't know what lead dressing means. I don't know which tube is V1 and I don't know which pins to check or what values to look for.





BMW-KTM said:


> You saw that I am not a tech, right?
> I don't know what lead dressing means. I don't know which tube is V1 and I don't know which pins to check or what values to look for.


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## BMW-KTM (Apr 7, 2015)

Well, that was interesting. I checked continuity at the capacitor connection to ground as blue_dog mentioned and sure enough there was a short. I decoupled it, stripped the shielding back a quarter inch, made sure there were no frays and put it back in the eyelet. Now that pot works as expected only now the whole amp seems choked. All four jacks work but all four are at about 1/4 volume.


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## blue_dog (Feb 7, 2013)

Check the ground on your power capacitors ( blue capacitors), maybe the one in the middle is not well grounded. Try to swap the 12AX7 V2 tube. Measure the plate voltages and post a picture of the power sockets.


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## BMW-KTM (Apr 7, 2015)

I put an alligator clip on the main ground and used a pointy probe on the negative (top) side of the blue power caps. All three measure 0.1Ω

I don't know how to measure plate voltage.


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## BMW-KTM (Apr 7, 2015)




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## blue_dog (Feb 7, 2013)

Double check your shield on the other wire connecting to the second tube. ChecK the value of the 1.5k resistors on the 6v6 tubes pins 5 and 6?

Measuring voltage is a bit hard to explain, check this video


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## Latole (Aug 11, 2020)

Hello BMW-KTM,
Were you a member of the closed Fender.com forum? If so, good to see you back here.


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## BMW-KTM (Apr 7, 2015)

It was very frustrating how that was handled.
Almost no notice and no provisions at all for handing it off to someone who might want to take it over.
Someone (I don't know who) did go to the trouble of preserving the valuable content but it is unsearchable in its present state, making it very difficult and extremely time consuming to find anything.









Fender® Forums: Index page


Fender Guitars: Official Enthusiast Forums




web.archive.org


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## BMW-KTM (Apr 7, 2015)

I had a busy Sunday and did not touch the amp until this afternoon. 
As requested by blue-dog, I checked the shielded wire connected to pin 2 on the 12AX7 socket.

Okay, wait a second.
Maybe first in order to simplify my understanding here I should learn something.
Am I correct in thinking the 12AY7 socket is referred to as V1?
The 12AX7 as V2? 
None of the documentation I downloaded to complete this build ever made any reference to V1, V2, etc.
That said, I do know from previous experience that Mesa-Boogie numbers the sockets beginning with the one closest to the input jack.
Is this kind of a universal guitar amp law? Do all amps number this way?
Would that make my rectifier socket V5?

Anyway, there is no shielding short on pin 2 at the 12AX7 socket.
Touching the shielding right next to the pin I see 0.1Ω to ground.
Touching the pin itself I read 496kΩ to ground.

I will now watch the video and learn how to do final voltage testing.


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## greco (Jul 15, 2007)

BMW-KTM said:


> learn how to do final voltage testing.


PLEASE BE CAREFUL !


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## BMW-KTM (Apr 7, 2015)

So, after viewing the video once I viewed it a second time, this time pausing to take notes and almost immediately noticed that I had missed a step in my build. It was during the part where he ensures all ground points are properly grounded. I needed to put a bridge across here to ground. I can't believe I never noticed I missed that. I was sure I was checking and double checking before I put iron to joint. I will fix that first and then proceed with the voltage tests.


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## Hammerhands (Dec 19, 2016)

This is where you need a discharging tool.


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## BMW-KTM (Apr 7, 2015)

I have a system for discharging the caps.
I put an alligator clip on the black lead of my meter and clamp it to chassis.
The red on my meter is the pointy probe.
I also place one clip of a test lead (alligator clips both ends) on the chassis.
Other end of test lead is clipped to a 560 ohm 5 watt resistor.
I measure the voltage on the cap with one hand and rub the resistor lead on it with the other hand and watch the meter until it goes to zero.
Takes about 10 seconds per cap.


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## dtsaudio (Apr 15, 2009)

So only the last cap was grounded? I'm surprised it didn't hum.


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## BMW-KTM (Apr 7, 2015)

Grounding bridge installed.
Back to the beginning because I've changed things.
With all tubes out, power up and read 795 VAC at pins 4/6, rectifier. 
Everything I've read says it should be between 600 & 800 so I assume that's good and move on.
Power down and install rectifier tube.
Problem here now.
Reading only 7.9 VDC between pin 8 and ground.
Power down immediately.
Attempt to pull rectifier tube.
It is EXTREMELY hot despite having been under power less than 15 seconds.
Very nearly burned myself.
Wait for it to cool down.
Bottle breaks in my hand trying to remove it.
I will discharge the caps again and remove the base of the tube.


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## BMW-KTM (Apr 7, 2015)

Upon pulling the tube base out I confirmed that I did not try to force the tube in the wrong way. I did put it in the correct way. The key tab was aligned with the slot. 

I am confused by this development. At one point the amp was running but with some bad bright jacks. It sounded good. The rectifier previously showed about 500 VDC at pin 8 with no other tubes installed ... before I installed the ground bridge.


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## dtsaudio (Apr 15, 2009)

Check your wiring again coming from the rectifier to those caps. The ultra low voltage points to a short.


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## blue_dog (Feb 7, 2013)

Sounds weird, double check your grounds and the where the wires coming from that caps are coming from. What rectifier tube are you using and can you swap with another?


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## BMW-KTM (Apr 7, 2015)

It was a J/J 5Y3-S. 
I will be going into the city today (I live in a small rural town) to get another tube. 
I will also get a 1/4" phone plug and a 25 watt 8 ohm resistor while I am in town so I can build a speaker dummy load like the one used in the instructional video.


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## jb welder (Sep 14, 2010)

This is odd. From your post #34 I had assumed you verified those caps were grounded, but perhaps you were using a jumper wire rather than the meter?


BMW-KTM said:


> I put an alligator clip on the main ground and used a pointy probe on the negative (top) side of the blue power caps. All three measure 0.1Ω


In any case, triple check your work. If it's all good, there is a possibility one of those caps is shorted. It would not have caused a problem while one end of it was disconnected.


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## BMW-KTM (Apr 7, 2015)

jb welder said:


> This is odd. From your post #34 I had assumed you verified those caps were grounded, but perhaps you were using a jumper wire rather than the meter?
> 
> 
> In any case, triple check your work. If it's all good, there is a possibility one of those caps is shorted. It would not have caused a problem while one end of it was disconnected.


Really good catch there. When checking those in my post #34 I used the meter on each individually. I agree it is odd. I may have to go to the trouble of removing the board from the chassis. It's possible I connected those three on the back side and don't remember. I should have taken a picture of the back side of the board for future reference.


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## BMW-KTM (Apr 7, 2015)

It seems very strange to me that in a city of 900,000 people I was unable to find a 5Y3 for less than $46 (highway robbery) and was also unable to find a 25 watt, 8 ohm resistor. I have the resistor on order and will get a call in a few days to pick it up. I have (2) 5Y3s ordered from The Tube Store in Ontario for $26 each. $32 each all in. (Still pricey)


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## BMW-KTM (Apr 7, 2015)

I'm learning here so please correct me if I am wrong here but if a cap was shorted would it still be able to hold a charge?
I just checked them and even though I zeroed them out yesterday, they all still showed a very small amount of charge, kind of like how a depleted battery will do that with some rest time. Less than a quarter of a volt on all of the big blue ones. Those are called filter caps, correct?
How about my question about V2, V2, etc?


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## jb welder (Sep 14, 2010)

Sometimes a cap will fail in a way that is not a dead short, and it may only occur after the applied voltage reaches some threshold. So one of those caps could still be bad and show a residual voltage. (yes those are the filter caps)
V1 and so on starting from the input end is sort of traditional, and how we would prefer it to be. Sometimes it is not followed, but in this case it is. So 12AY would be V1, and so on down the line in numerical sequence.
It is usually based on the schematic and what is happening circuit wise. This is not always how things are layed out physically, so there is where you get exceptions.


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## BMW-KTM (Apr 7, 2015)

So how does one determine if a filter cap is faulty?


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## jb welder (Sep 14, 2010)

Ideally you would be on a lamp limiter which would be lighting up (to save the amp from damage and avoid using up fuses). Then you would disconnect one at a time til the bulb limiter quit shining bright.


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## blue_dog (Feb 7, 2013)

BMW-KTM said:


> Really good catch there. When checking those in my post #34 I used the meter on each individually. I agree it is odd. I may have to go to the trouble of removing the board from the chassis. It's possible I connected those three on the back side and don't remember. I should have taken a picture of the back side of the board for future reference.


jb-welder is right, check again. There is something odd, check your ground again at your the terminals. You might be missing a ground wire on the right terminal?


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## BMW-KTM (Apr 7, 2015)

For the sake of overall clarity here I am going to do a little catch up on previous comments



blue_dog said:


> Double check your shield on the other wire connecting to the second tube. ChecK the value of the 1.5k resistors on the 6v6 tubes pins 5 and 6?


V4 = 1.59kΩ
V3 = 1.62kΩ
The other shielded wire does not seem to be shorting.



dtsaudio said:


> Check your wiring again coming from the rectifier to those caps. The ultra low voltage points to a short.


How would I check the wiring for a short? The only wiring between the rectifier and the filter caps that I can make out is one wire from pin 8 to the + side of the first (left) filter cap. I find continuity between those points. Checking between same filter cap connection and ground I find resistance that is constantly changing. Upon first contact it showed around 200kΩ and then it kept climbing. It would take about 10 seconds or so to reach about 660kΩ. Then the meter would show an open circuit and then begin again, subsequent times beginning around 45kΩ instead of 200.

Question:
Why is that called "B" plus? What is B? Is it because it is DC? Would the AC then be known as A? Trying to learn here.


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## Lincoln (Jun 2, 2008)

"I believe"

B+ is term left over from the beginnings of radio when everything was battery powered. It means "battery positive". The term stuck, and these days it means the unregulated secondary power/voltage available to the amp. The tap coming off the rectifier tube, headed for the first filter cap is your B+.

With the ground lead of your multi meter on the chassis, and touching the positive leads on the filter caps, you should be seeing measurements in the mega-ohm range.


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## BMW-KTM (Apr 7, 2015)

I'm going to be going through these one at a time so please bear with me.


blue_dog said:


> Sounds weird, double check your grounds and the where the wires coming from that caps are coming from.


So, double checking my grounds, all three filter caps are grounded as well as the smaller cap/resistor pair to the left of it. Also the far right cap/resistor pair are grounded as well as the middle pair. I believe these are all supposed to be grounded. Strangely, both of the 0.1uF caps on the right side, the ones that connect to the volume pots also show continuity to ground and that doesn't seem right to my layman's mind. I visually checked and the shielding repair that I did looks sound but the meter shows it still shorting.


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## jb welder (Sep 14, 2010)

Your meter reading on the filter caps (climbing resistance) indicates there is no short at this time. It is possible that the rectifier tube just failed.
I'd suggest building a lamp limiter to start it up with once you get the tube. Other stuff is just academic at this point.
You don't have to build one, but it will be very handy if there is still a fault, or if you plan on doing other builds. See attached.


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## BMW-KTM (Apr 7, 2015)

I removed the shielded wire that goes to the middle pot and it no longer shows continuity to ground at that cap but the other one still does and it was never shielded. I am starting to regret listening to the guy who told me to shield those two connections.


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## Lincoln (Jun 2, 2008)

BMW-KTM said:


> I removed the shielded wire that goes to the middle pot and it no longer shows continuity to ground at that cap but the other one still does and it was never shielded. I am starting to regret listening to the guy who told me to shield those two connections.


It's very easy to heat up shielded wire to the point that the insulation between the inner wire and the braided jacket melts and they short out. I've done it many times. It really takes a gentle and quick hand. Otherwise, shielding is good. it really cuts down hum and also stops ticks and other weird things.


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## jb welder (Sep 14, 2010)

Check your pot settings.  Controls turned all the way down will ground things out.


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## BMW-KTM (Apr 7, 2015)

jb welder said:


> I'd suggest building a lamp limiter to start it up with once you get the tube. Other stuff is just academic at this point.
> You don't have to build one, but it will be very handy if there is still a fault, or if you plan on doing other builds. See attached.


Thanks. I've seen guys use these before and I think I will build one. The drawing you provided seems more complicated than others I have seen. I am unsure why there is a need to pass through a second outlet as well as through a bulb.


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## BMW-KTM (Apr 7, 2015)

jb welder said:


> Check your pot settings.  Controls turned all the way down will ground things out.


That was in fact the case.
I have reinstalled my shielded wire and the ground issue disappeared.
Some of these things are slowly starting to make sense to me.


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## jb welder (Sep 14, 2010)

BMW-KTM said:


> The drawing you provided seems more complicated than others I have seen. I am unsure why there is a need to pass through a second outlet as well as through a bulb.


Yeah, there are simpler methods and drawings, that was just one I had handy. 
Hopefully someone else here could post a simpler one?


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## RBlakeney (Mar 12, 2017)

If you’re anything like me, when you build these from essentially an adult paint by numbers approach, when things don’t work is when you start to learn.


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## greco (Jul 15, 2007)

I'm following every post. I'm learning a lot and totally enjoying the thread.

Just ordered my second bag...


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## BMW-KTM (Apr 7, 2015)

jb welder said:


> Yeah, there are simpler methods and drawings, that was just one I had handy.
> Hopefully someone else here could post a simpler one?


No need. I understand the concept. It's just an extension cord really, except you pass the hot through a lamp fixture. I have everything I need already to build one except for an electrical box big enough for a lamp base and a duplex. I'll go to Crappy Tire tomorrow and get one. Or I guess Bartle & Gibson if Crap Tire doesn't have a big enough one.


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## dtsaudio (Apr 15, 2009)

BMW-KTM said:


> How would I check the wiring for a short? The only wiring between the rectifier and the filter caps that I can make out is one wire from pin 8 to the + side of the first (left) filter cap. I find continuity between those points. Checking between same filter cap connection and ground I find resistance that is constantly changing. Upon first contact it showed around 200kΩ and then it kept climbing. It would take about 10 seconds or so to reach about 660kΩ. Then the meter would show an open circuit and then begin again, subsequent times beginning around 45kΩ instead of 200.
> 
> Question:
> Why is that called "B" plus? What is B? Is it because it is DC? Would the AC then be known as A? Trying to learn here.


You've done exactly what you should. Giving it some more thought, you've already had the amp sort of working so I'm thinking, like JB, that your tube just died. Of all the tubes being made today, rectifiers seem to be the worst for reliability.

It's called B+ from the old days when a lot of this stuff was battery powered. "B" meaning battery, and + for the positive. It's since developed into a bit more where you have B+, C+, D+, which are now just points further upstream from the first power point.


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## BMW-KTM (Apr 7, 2015)

I already had a brand new extension cord that had never been opened, already had the lamp fixture, found a piece of scrap wood from an old shelving project and already had the PVC glue and marettes. I bought the box, the bulb and the two strain reliefs and made myself a very simple current limiter.


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## blue_dog (Feb 7, 2013)

This is awesome post. I’ll have some of the fine Gibson’s whiskey.


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## BMW-KTM (Apr 7, 2015)

I love the Venerable. 
It is soooo silky smooth.
Almost no burn or bite.
I've have tried hundreds of expensive whiskies, a few of them costing hundreds per bottle and this one is my favourite.
I hide it when people come over (or used to come over before covid) because someone will always insist they can taste the difference when they mix it 4:1 in a highball and they think that justifies them wasting my good whisky.
I bring out the Royal Reserve for company and hide the Venerable.

For Scotch, I have found pretty good value in Glen Farclas 15.
It's about $90 but it fights in a much higher weight class than its dollar value would suggest.
I don't recommend the Glen Farclas 12 though. Different animal altogether.

Whiskey is as good a thing to talk about as anything else while I wait for my tubes to arrive, right?
LOL


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## jb welder (Sep 14, 2010)

Tell us how you turned that Oatey PVC cement into the Venerable.


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## BMW-KTM (Apr 7, 2015)

Abra Cadabra .....


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## BMW-KTM (Apr 7, 2015)

Yesterday my tubes arrived and the resistor I had ordered came in as well.
First thing I did was to assemble my speaker dummy load last night.


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## BMW-KTM (Apr 7, 2015)

I also made a dedicated tool for bleeding off the caps.
I did a mock run and this is noticeably easier to use that what I was doing before.


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## BMW-KTM (Apr 7, 2015)

So, going back to square one, I powered it up with no tubes installed and found 807 VAC across pins 4/6 at the rectifier, V5. It's a little high according to what I've been reading but I remember it is in an unloaded state so I proceed. I power down and install the rectifier tube and power back up. I read 556 VDC between pin 8 and ground. The first time I went through this process I got to this point and found 560VDC and stopped because I had been reading it should be 400VDC ± 20. At the time I contacted a professional who told me it was OK but now I am wondering if maybe I should not have been so eager to take his word for it. I mean my tube did behave strangely and then failed. I am reluctant to proceed further without some input here.


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## Lincoln (Jun 2, 2008)

well, it's rectified at that point, but still unloaded really. If you are using one of those current limiting, light bulb contraptions, you won't have another "meltdown" no matter what.


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## BMW-KTM (Apr 7, 2015)

With all tubes in place, with the dummy load in the speaker jack and the amp plugged into the current limiting thingie I built a few days ago I went back to the power-supply I found 716VAC at V5/P4-6. Checking both sides against ground I found identical values of 358VAC.

I then checked the pilot light and found 6.43VAC. Then following the twisted pair down the line from there I found it gradually decreasing ending up as 6.36 at V1/P45-8.

Switching the meter to VDC I again check V5/P8-G and find 361.6VDC, a far cry from the 550+ unloaded. I assume this is close enough to move on. I don't hear anything, I don't see anything out of the ordinary and I don't smell anything. So far, so good.

Using the Mojo diagram as a reference I measure the VDC voltages on the plus side of the board going from left to right and placing the Mojo reference numbers in parentheses after my found values, this is what I find.

21.6(22) . 362(386) . 330(342) . 251(252)
19.7(19) . 53.4(53) . 199(198) . 166(160)
1.20(1.4) . 163(125) . 157(125) . 1.30(2.1)










Some of these numbers are really close to what is expected but some are off.
How concerned do I need to be with this?
Note: I have a 12AX7 in V1 instead of a 12AY7 and I don't know what affect that will have on any of these as-found values.

Nobody ever did answer my previous query about the possibility of running a 12AX7 in V1 full time.


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## jb welder (Sep 14, 2010)

Your numbers are all close enough, the only significantly different are due to the 12AX rather than AY so those are to be expected.
No problem with a 12AX in there full time if you prefer the sound compared to the AY.


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## BMW-KTM (Apr 7, 2015)

Still using the current limiter I plugged the speaker in and tried the amp.
The first thing I noticed was that it was very quiet and had almost no drive to it.
I wondered if the limiter might be the cause so I tried again straight into the wall.
Then it was big and loud and full of drive.
Nuther lesson learned; don't play the amp using the limiter.

Checking all four jacks I find the normal and bright channels are actually normal and bright and the #1 jacks are hotter than the #2 jacks. The volume drop in the bright side is no longer there, presumably corrected with the earlier shielding repair. This was my original reason for starting this thread and I think we have it basically sussed out but I'm not going to call this finished just yet as I want to do more real world testing. I noticed a bit of crackling pulling the cable in and out of the jacks and not the normal kind. I will experiment with different cables and take a closer look at the jacks. After I have that issue sussed out I will try again with the channel jumper to see if that works as it is supposed to.

I have also noticed that the 12AX7 seems a bit much now. I will try it this way for a while but I may end up going back to the 12AY7. Just in case the AY is too low, is there anything in between them?


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## epis (Feb 25, 2012)

Yes, split load on 12ax7.


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## BMW-KTM (Apr 7, 2015)

I tried a known good cable and it still crackled so I took a closer look at the jacks with a light and a magnifying glass and two of them (the two that I did NOT bend earlier in the thread) needed to be bent like I did to the other two. The tip contact was coming back into very slight contact with the shorting thing as it fell into the notch on the phone plug. I bent them both away just slightly from the tip contact and now there is some space when a plug is inserted. The shorting thing does still short with no plug so that's all fixed now. No more crackles. No noise with all jacks empty and volumes at 6 to 8. 

I played it for a few minutes and could not stand the 12AX7 in V1. I could not dial in a tone I liked. I cooled it off and put the 12AY7 back in. The gain structure is now much closer to what I was originally expecting and the reason I took on this project in the first place. Out of curiosity I reinstalled the dummy load and tested the B+ values again with the 12AY7. Much closer.

23.2(22) . 387(386) . 350(342) . 252(252)
19.1(19) . 53.2(53) . 201(198) . 166(160)
1.3(1.4) . 117(125) . 118(125) . 1.9(2.1)

I again tried all four jacks and they all work as they should. When plugged into one channel the volume of the other channel does have a small amount of influence on the channel being used, which is what I expected. Jumpering the channels high/high, low/low, high/low and low/high all have the effect I was expecting. All three pots behave as expected with nothing unusual except a bit more noise entering the signal at very high volume settings than I would like but it is acceptable as I will not likely run the amp full tilt anyway. The noise starts between 10 and 11 and I likely won't go much higher than 9 and more than likely I will live around 7 or 8 and use guitar volume to clean up.

I am confident enough that I am ready to call this finished that I installed the upper back cover and placed the amp back in the studio.

Now it is time to express gratitude.
A big and warm thank-you to all who participated in this project with their wisdom and information.
Your help is greatly appreciated.

I am going to create a bookmark to this thread to make it easier to find just in case any trouble crops up down the road.

Thanks again.
I'm going to play my new toy now.


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