# Vibrochamp Hi B+



## Gizmo (Aug 7, 2008)

Hi guys.

I picked up a mid 70's non-functioning Vibro Champ at a decent price.

The guy told me that because it didn't sound too good, he'd had it it to a tech who put in new tubes and "rebiased".

After which it sounded great and he used it for recordings....until one day it just stopped.

Anyway, first thing I did was to remove the chassis and took a look....no bad smells or signs of burning, fuse ok, so I powered it up and the tubes lit up but dead silence.

I measured the PT output voltage....375v on each leg as opposed to the 315v on the schematic...some increase due to line voltage was expected but not this much.

Then I measured rectifier o/p ...520V...ouch...I guess the tubes are probably shot and no current to drop the voltage. I didn't have another 5Y3, but I pulled all the tubes and tried a 5AR4 with exactly the same results.

Measured the dropping resistors in the B+ and they're all ok. Filter caps seemed ok (at least they held their charge with no tubes in).

Then I noticed something strange about the PT

It had a thermal circuit breaker in the input leg. Notice that it was attached with Robertson screws.












Then the numbers on the PT looked strange...especially the "CSA" 












On the top of the chassis was this label (not a stick-on)












The transformer is a good fit and looks like it belongs, but something just doesn't seem right about this to me, especially with such a high o/p voltage.

Any input appreciated.

Hmmm just noticed death cap still there...

Gizmo


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## nonreverb (Sep 19, 2006)

It's correct Ken. All mid '70's Fenders have thermal breakers. I'm not sure if it's just a Canadian thing but every one from that era I've seen have 'em.
Load it with new tubes and the readings will probably be better. All the amps from this era had higher voltages than the early stuff. They got even higher when the big boys went ultralinear a couple of years later....


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## WCGill (Mar 27, 2009)

It's probably the way it should be, unbelievably high B+, as I've seen some like this also. The output tube will not survive long in this environment and either you can put in a JJ 6V6S or a 6L6GC, no guarantees on either of these, or change the power transformer for one with lower output.


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## StevieMac (Mar 4, 2006)

At the risk of sounding/looking stupid, I'd suggest checking the speaker while you're examing things. You mentioned "dead silence" which, to me at least, raises a red flag around the speaker. IME, whenever an amp fires up but makes no sound _whatsoever_, I wonder about the speaker being fried. I once picked up a DR for next to nothing based on that very suspicion and it turned out to be correct. Popped in a new speaker and....voila! 

Of course this does NOTHING to address your concerns regarding the B+ voltage...

Steve


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## epis (Feb 25, 2012)

Something looks weird on your image, red/black and yellow/black wires are soldered together to the fuse holder, usually these are center taps for secondaries.Fuse should be in primary circuit (usually black wires)
http://vibrochamp.org/wp-content/uploads/2011/06/vibrochamp-layout.jpg

Get some real tech to take a look at it. I could help if you need it. Cheers, Damir


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## Gizmo (Aug 7, 2008)

Hi Richard, I know you've opened a lot of amps, so I hear what you say...although my '71 SR nor my '75 PR has one...and I don't recall seeing one in my '74 DR either. (Maybe they're just a bit to early).


Also.the Robertsons screws made me think this was an add on...along with the "CSA" on the Tx...that just doesn't look right!
Ken





nonreverb said:


> It's correct Ken. All mid '70's Fenders have thermal breakers. I'm not sure if it's just a Canadian thing but every one from that era I've seen have 'em.
> Load it with new tubes and the readings will probably be better. All the amps from this era had higher voltages than the early stuff. They got even higher when the big boys went ultralinear a couple of years later....


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## Gizmo (Aug 7, 2008)

Unbelievably high is right! 

I wonder if I could put a dropping resistor straight after the rectifier and before the first filter cap...that could (should) drop the B+. I have a few 5 watters that I could try.



WCGill said:


> It's probably the way it should be, unbelievably high B+, as I've seen some like this also. The output tube will not survive long in this environment and either you can put in a JJ 6V6S or a 6L6GC, no guarantees on either of these, or change the power transformer for one with lower output.


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## Gizmo (Aug 7, 2008)

Good call Steve, but the speaker is ok.



StevieMac said:


> At the risk of sounding/looking stupid, I'd suggest checking the speaker while you're examing things. You mentioned "dead silence" which, to me at least, raises a red flag around the speaker. IME, whenever an amp fires up but makes no sound _whatsoever_, I wonder about the speaker being fried. I once picked up a DR for next to nothing based on that very suspicion and it turned out to be correct. Popped in a new speaker and....voila!
> 
> Of course this does NOTHING to address your concerns regarding the B+ voltage...
> 
> Steve


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## Gizmo (Aug 7, 2008)

My thoughts exactly...plus the stretch of one of the ac feeds to the rectifier....just too tight.



epis said:


> Something looks weird on your image, red/black and yellow/black wires are soldered together to the fuse holder, usually these are center taps for secondaries.Fuse should be in primary circuit (usually black wires)
> http://vibrochamp.org/wp-content/uploads/2011/06/vibrochamp-layout.jpg
> 
> Get some real tech to take a look at it. I could help if you need it. Cheers, Damir


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## nonreverb (Sep 19, 2006)

The transformer part number is 010020. It's a dual primary version that can be found on the Vibro Champ and is indicated on the schematic as an option....



epis said:


> Something looks weird on your image, red/black and yellow/black wires are soldered together to the fuse holder, usually these are center taps for secondaries.Fuse should be in primary circuit (usually black wires)
> http://vibrochamp.org/wp-content/uploads/2011/06/vibrochamp-layout.jpg
> 
> Get some real tech to take a look at it. I could help if you need it. Cheers, Damir


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## nonreverb (Sep 19, 2006)

The Thermal breaker was mainly found on the UL amps by '76/'77. I'm pretty sure it was mandated by CSA hence the Robertson screw head. I have seen them with Philips as well though. Don't worry Ken, that amp is STOCK trust me...


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## Gizmo (Aug 7, 2008)

Haha...thanks Richard...I do!

Any chance of that schematic...any I've seen doesn't have the option...and maybe other details.

Ken



nonreverb said:


> The Thermal breaker was mainly found on the UL amps by '76/'77. I'm pretty sure it was mandated by CSA hence the Robertson screw head. I have seen them with Philips as well though. Don't worry Ken, that amp is STOCK trust me...


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## greco (Jul 15, 2007)

Very interesting and educational thread.

Cheers

Dave


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## nonreverb (Sep 19, 2006)

The following is the AB64 circuit. Unfortunately, I haven't found the later version yet with all the CSA revisions on it. It clearly shows 3 options for power transformers. One is the domestic tranny and the other two were just different export configurations. 
By the time your amp was made, Fender was well into the notion that louder was better. It was around this time they came out with the monstrous PS400.
There were two major overhauls in the designs regarding output power. The first was around '68 and the second was around '75/'76. Most if not all amps from the second revision will destroy shitty tubes rather quickly as the plate voltages are considerably high in those amps.

http://www.webphix.com/schematic%20heaven/www.schematicheaven.com/fenderamps/fender_vibrochamp_ab764.pdf




Gizmo said:


> Haha...thanks Richard...I do!
> 
> Any chance of that schematic...any I've seen doesn't have the option...and maybe other details.
> 
> Ken


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## Gizmo (Aug 7, 2008)

Got it, cheers Richard...better than the one I had.

So after all the good advice here I decided to risk another 6V6.

B+ was 416v vs 355v still way too high and I don't want to wreck my tubes.

I don't have any JJ 6V6s so I figured what the heck and put a 470ohm 5W on the rectifier output before the first filter cap.

B+ is now 366v and all other voltages are only 10v higher than schematic.

The resistor is dropping 50v so it will get a bit warm, but it's close to the thermal circuit breaker haha!

Thanks for the input everyone.....

Gizmo

Edit...and the amp sounds just great!

Edit 2... Just had a closer look at the schematic you sent Richard....voltages are close to what I was getting before my mod. eg 420v on yours vs 355v on mine..now I see why these things wreck tubes. Think I'll stick with my mod and see....don't need LOUD! Probably wouldn't have been so concerned if I saw this schematic first.


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## nonreverb (Sep 19, 2006)

Yeah Ken, JJ's or NOS only in that amp! It's a Sovtek eater....



Gizmo said:


> Got it, cheers Richard...better than the one I had.
> 
> So after all the good advice here I decided to risk another 6V6.
> 
> ...


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## jb welder (Sep 14, 2010)

50V across 470 ohm means it is dissipating 5.3 watts. Suggest you upgrade the wattage, maybe an aluminum type you can bolt to the chassis for better heat sinking.


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## Gizmo (Aug 7, 2008)

jb welder said:


> 50V across 470 ohm means it is dissipating 5.3 watts. Suggest you upgrade the wattage, maybe an aluminum type you can bolt to the chassis for better heat sinking.


Thanks, yeah...I could probably get away with two 1k in parallel until I can get a higher rating one.


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## nonreverb (Sep 19, 2006)

I think I have some 10watt ones Ken if you're going that route...


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## Wild Bill (May 3, 2006)

Gizmo said:


> Thanks, yeah...I could probably get away with two 1k in parallel until I can get a higher rating one.


I wouldn't do it that way myself! Using a resistor in series with the main B+ is not just extremely inefficient, it adds a LOT more sag to the tone! You already have a 5Y3 rectifier adding tons of bluesy sag. Your resistor will take it likely way over the top!

Things aren't as bad as you think. Lots of Fender amps run a bit high in the B+ department. Back in the Golden Years 115vac in the walls was common. Nowadays, it's more like 123vac. Those few extra volts are multiplied by the same ratio as the HV winding and that's why we see those extra volts after the tube rectifier at the 1st filter cap.

The 5Y3 has a lot of voltage drop. Don't even think about a 5AR4! It has a low voltage drop. No sag and even higher B+.

Lots of Deluxes and such play for years with 410-420 volts on the plates of the 6V6s.

You get a bonus with your Champ! The 6V6 is NOT seeing 410 volts, it is only seeing maybe 390 vdc on the plate!

You see, it is a cathode biased power tube circuit. Tubes are not rated between plate and ground. Who's to say that ground is always 0 volts? That 6V6 is rated at a voltage between the PLATE and the CATHODE! That's how it develops its bias, after all!

So the 6V6 is really seeing B+ minus the cathode voltage. The cathode voltage is only 0 if the cathode is grounded, which it is NOT!

I agree that some brands of 6V6s are really crappy but I think things are not as bad as you think. 

I would just crank it up and play the damn thing!:rockon2:

If you really are worried, check the filament voltage to see if it is running high as well. That will tell you if the PT primary is designed for a lower line voltage. If so, you can easily install a line-bucking transformer to knock it down a bit. You will find out all about this at New Page 1. I've used this trick any number of times with antique gear and it works great!

Wild Bill/Busen Amps


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## Gizmo (Aug 7, 2008)

Thanks Richard, I'll see how I like it when I play it a bit.



nonreverb said:


> I think I have some 10watt ones Ken if you're going that route...


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## Gizmo (Aug 7, 2008)

Great info Bill and thanks. I'm aware there will be more sag but your numbers make a lot of sense.

It does appear though that this thing is eating tubes. The guy told me he'd had a tech install new tubes recently and he'd used it since...when I got it the 6V6 was gone again (although it was an EH, not a NOS or JJ.

Filament voltage is good...just over 6.

Maybe I'll risk another one and pull the resistor...never thought about the cathode voltage, and the schematic that Nonreverb pointed me to did have much higher voltages than the schematic I had, so I'm feeling a lot more comfortable.

Thanks for that great info...this forum is a great place to learn stuff.




Wild Bill said:


> I wouldn't do it that way myself! Using a resistor in series with the main B+ is not just extremely inefficient, it adds a LOT more sag to the tone! You already have a 5Y3 rectifier adding tons of bluesy sag. Your resistor will take it likely way over the top!
> 
> Things aren't as bad as you think. Lots of Fender amps run a bit high in the B+ department. Back in the Golden Years 115vac in the walls was common. Nowadays, it's more like 123vac. Those few extra volts are multiplied by the same ratio as the HV winding and that's why we see those extra volts after the tube rectifier at the 1st filter cap.
> 
> ...


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## nonreverb (Sep 19, 2006)

I have some US made 6v6's around here somewhere Ken if you're interested...


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## Wild Bill (May 3, 2006)

Well, if it is really a tube eater then I would guess there is still an undiscovered problem!

Have you checked the 1k 1w resistor in the power supply string? What's the voltage it leaves for the 6V6 screen? It might not hurt to put a 470 ohm 1w from that tap to the screen, for a bit of extra protection.

Or if the voltage for the 12AX7 plates is too high, you could change that 1k 1w to a 2k2 2w or even a 4k7 2w. That will knock the preamp voltages down to spec and give more screen protection at the same time. Might change the tone, however. Although the schematic calls for 200vdc an extra 20 or 30 vdc shouldn't hurt anything and will add much gain, a la a classic Marshall!

The schematic I'm looking at shows a 330pf from 6V6 grid to cathode. That would be to suppress any oscillations in the output circuit, caused by high plate voltage and maybe poor lead dress, which was a common problem for Fender once they started the SilverFace amps. It is not so large as to seriously kill any audible highs and might be a good idea for your amp. There could be an oscillation too high to hear that is killing output tubes! A little ceramic or mica cap is cheap insurance.

Just my .02.

Wild Bill/Busen Amps


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## nonreverb (Sep 19, 2006)

Good ideas Bill. I'm still hedging my bets on those Sovtek 6V6's. They don't hold up well in the new amps and they sure as sh*t won't in anything with higher plate voltage.
Having said that, it's always prudent to look for failures or potential ones regardless...



Wild Bill said:


> Well, if it is really a tube eater then I would guess there is still an undiscovered problem!
> 
> Have you checked the 1k 1w resistor in the power supply string? What's the voltage it leaves for the 6V6 screen? It might not hurt to put a 470 ohm 1w from that tap to the screen, for a bit of extra protection.
> 
> ...


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## Gizmo (Aug 7, 2008)

Sorry for the silence guys...a couple of gigs this weekend kept me busy.

As Bill advised, I pulled the 470 ohm and measured...

The voltages are pretty damn close to the AB764 schematic Richard sent me. I had been working from an AA764.

12AX7 plates are 240v (AB764 schematic says 255-260), so quite a bit higher than the AA764 values of 200, but the preamp tubes don't seem to be blowing.

330pf is there and no oscillations to speak of.

A little bit of 50Hz hum, but nothing to worry about.

So I think I'll just play it as is and when this tube goes I'll either get a JJ or see Richard for one of your tubes.

Thanks a lot for the input.


Ken


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## Gizmo (Aug 7, 2008)

*New Vibrochamp output transformer*

So I'm resurrecting this old Champ thread 'cos it might be of interest to other Champ users....

Anyway, I've played the amp on and off since the last posting here sometime last year. No tubes blown and it sounds ok.

I want to use it with an 8 ohm cab so I bought a replacement o/p Tx from a forum member. 

It's a brand new Magnetic Components "Classic Tone" Tx with 5k and 8k primary taps and 4, 8 and 16 ohm secondaries.

I wired it up using the 5k primary and everything sounded just fine using only the 4ohm output. With the 4ohm AND 8ohm both connected it also sounded ok.

I then tried it with just the 8ohm and I now have a high frequency squeal when I turn the volume down below about 2.

I then re-wired it to use the 8k primary...same deal,...I even moved the NFB resistor to be on the 8ohm tap just in case...no difference.

As noted in an earlier post, there is a 330pf across the 6V6 grid and cathode. I have not had any oscillations previously and I still get none when the 4ohm output is connected to a speaker.

Any ideas gents?? Why would it squeal with only the 8ohm output in use and not the 4ohm?

(haha...I just noticed in my last post I said "50hz" instead 60Hz....even after being out of the UK for 30 years, old habits die hard!)


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## jb welder (Sep 14, 2010)

*Re: New Vibrochamp output transformer*

You may have incorrect phasing which turns the NFB into a positive feedback loop. It may be that it is only causing problems on the 8 ohm tap.
You can test by disconnecting the NFB wire. Signal level should increase with NFB disconnected. If signal level decreases when you disconnect the NFB, then you have a phasing problem. If this is the case, just reverse the wires on the primary side of the OT.


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## Church-Audio (Sep 27, 2014)

*Re: New Vibrochamp output transformer*

I think this transformer is wired for export. Also blow out that thermal fuse. Not needed and could have issues they don't last forever. And I woukd only use JJ 6v6 in this amp 400+ volts are no issue for this tube.


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## Gizmo (Aug 7, 2008)

*Re: New Vibrochamp output transformer*

Thanks JB, it is strange that it happens only on one of the o/p taps but I will try that.

Chris…subject has changed….it's now about oscillation after changing output Tx.

Perhaps I should have started a new thread.




jb welder said:


> You may have incorrect phasing which turns the NFB into a positive feedback loop. It may be that it is only causing problems on the 8 ohm tap.
> You can test by disconnecting the NFB wire. Signal level should increase with NFB disconnected. If signal level decreases when you disconnect the NFB, then you have a phasing problem. If this is the case, just reverse the wires on the primary side of the OT.


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## Church-Audio (Sep 27, 2014)

*Re: New Vibrochamp output transformer*

This is a single ended amp reverse the output leads that are going to the speaker output. I have seen companies make mistakes with this before. I have a heyboer transformer that had this issue.

Chris


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## Gizmo (Aug 7, 2008)

*Re: New Vibrochamp output transformer*

I think you nailed it JB, I took off the NFB completely and no squeal.
Next step will be to flip the primary and see if I can reconnect the NFB line.

Chris, it's a multi tap o/p transformer…not sure I can just reverse the output connections as I need to use two of the outputs…..good call though.



Gizmo said:


> Thanks JB, it is strange that it happens only on one of the o/p taps but I will try that.
> 
> Chris…subject has changed….it's now about oscillation after changing output Tx.
> 
> Perhaps I should have started a new thread.


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## Gizmo (Aug 7, 2008)

*Re: New Vibrochamp output transformer*

Thanks gents.
I reversed the primary and all is well…guess the supplier must do their color codes different to Fender….or something!

I should probably put a note in the chassis saying why the o/p tx is reversed for the next guy who might have this amp and looks inside ;-)


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## Church-Audio (Sep 27, 2014)

*Re: New Vibrochamp output transformer*

I did not know it was multitap primary is always the first choice. But secondary is also fine if its a single ended amp with one tap.


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