# Ampeg Classic Svt issue.



## Church-Audio (Sep 27, 2014)

I have an Svt in my shop a classic series. I have 125 volts ac on the plates!! With no signal of course  Changed all diodes in amp I am thinking filter cap or a mis wired power transformer or defective transformer amp biases up great plate voltage looks good accept for the 125 volts av on the plates  any ideas? Btw I did check the caps for leakage but again at higher current and voltage one of the might be breaking down...


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## WCGill (Mar 27, 2009)

I would think that a shorted cap would take out a fuse. Check the voltage out of the power transformer before the diodes, possibly lifting them, to see if voltage output is normal.


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## Church-Audio (Sep 27, 2014)

WCGill said:


> I would think that a shorted cap would take out a fuse. Check the voltage out of the power transformer before the diodes, possibly lifting them, to see if voltage output is normal.


I will check that out. But if the plate voltage is ok... And the amp biases ok so we're talking with the supply tap unloaded I would think it would be 800v to 1000v 


question.... What about a reversed diode in the full wave rectifier for the plate supply?? Thinking out loud here...pardon the pun lol


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## WCGill (Mar 27, 2009)

Oops, sorry, 125*AC* on the plates, misread your post. No, wouldn't be in the PT or B+ supply.


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## Church-Audio (Sep 27, 2014)

So what do you think? It's a complex design I have talked to a few friends of mine that also run Ampeg service centres and they have never seen something like this before. Oh yeah it does it with or with out power tubes installed. So it's coming from the output transformer going in on the primary.


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## dcole (Oct 8, 2008)

Man, I hate those amps. So much stuff shoved into a little space. I have a few questions:

1. So, you are showing correct DC plate voltage and correct DC bias, but their is a 125 VAC signal on the DC plate voltage? 
2. Is this 125 VAC peak to peak or RMS? 
3. Have you looked at the waveform on a oscilloscope? 
4. Is it 60 Hz?


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## Church-Audio (Sep 27, 2014)

I will measure it again. Today.


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## jb welder (Sep 14, 2010)

So was the complaint that there was loud hum at the output? I assume you had it connected to a speaker and it does have loud hum?
Can you scope the B+ and see if the hum is 60 or 120hz?
Otherwise, if you are not hearing it, and it biases up good, maybe there is a ground fault or measurement issue?
Disconnect J13 to the OT primary. Now the OT is out of the equation. Is the 120VAC still there (on J13)?
If so, and the scope shows it to be 120hz, then the main filter caps or their connections are a problem.
How about the other side of R54, do you still measure 120VAC there?


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## greco (Jul 15, 2007)

Cool thread! 

Troubleshooting is so "exciting" for (some of) us spectators. It is also educational (again, for some) but most of this is beyond my basic grasp of electronics theory. Too bad it is so frustrating for the tech involved, as you just want to complete the repair and move on.

Cheers

Dave


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## jb welder (Sep 14, 2010)

Amprepair said:


> What about a reversed diode in the full wave rectifier for the plate supply??


D6 thru D13 should all be oriented the same way on the board:


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## nonreverb (Sep 19, 2006)

125VAC on the plates??? Open or broken solder joint on main B+ cap would be my first guess...


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## Church-Audio (Sep 27, 2014)

I have it back on the bench today I will take another look. And oh yeah I hear the hum and it's there with out tubes installed on the op. I am pretty sure it's 125 volts ac rms. I replaced all diodes in the power supply even though I was sure that would do nothing...in4007 are cheap anyway... Pulled most of the filters and checked at high voltage for leakage 500volts dc on my Sencore cap analyser. But the supply voltage is higher than that so I was thinking some high voltage breakdown of a filter cap but the amp is not drawing accessive current at idle on the primary input of the pt. I will check the items mentoned here and report back. I have sunk at least two days of my time on this already needed to take a break and customer is very understanding. I have never seen this before.


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## Church-Audio (Sep 27, 2014)

I confirmed they are all in there correctly. I used a high tech semiconductor analyser to check all the diodes I have installed, they are all in correctly as per the stripe on the parts location silkscreen unless that's wrong... But I can't see that happening. And they are all orientated the same way. It just ocured to me if I am missing a cap ground that could be an issue..... That might cause vac to appear where it's not supposed to be ! 
This is what I use to analyse semi conductors it works great for matching op transistors and checking diodes although almost anything can check a diode. I use the dca plus not the exact one in this video. 

http://m.youtube.com/watch?v=zo_BejqYSF8&feature=youtu.be


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## nonreverb (Sep 19, 2006)

You won't draw excessive current if the cap is open....just the unfiltered B+ which on those is around 630V +/- when working properly.
What's the present DC component?


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## Church-Audio (Sep 27, 2014)

nonreverb said:


> You won't draw excessive current if the cap is open....just the unfiltered B+ which on those which is around 630V +/- when working properly.
> What's the present DC component?


So yeah think I found the issue. Funny thing is I swear I TESTED all the filter caps!!! but c16 shows 4NF..... i have never seen a 100uf measure as 4 nf before maybe its open I think thats safe to assume. So I would say thats the issue with the amp. I will order new caps for it install them and report back. I always have to remind my self if it walks like a duck talks like a duck its a duck always trust your first thoughts when it comes to an issue, I just don't like "throwing parts at a repair unless I am sure. And funny enough I must have missed this one cap! when I tested them all. Because of my concern for not wanting to see this amp come back and because this guy is a real touring musician I am going to replace all the filter caps on the amp they are cheap enough and if you have one bad one made in the same batch how far can the others be behind?? Thanks so much for your help guys!! I really do appreciate it and you all got me back on the proper track. Sometimes you get a repair that just drives you nuts and it always turns out to be the most basic stuff that you overlook. 
I hope this thread saves someone else from having to spend the time I had to on this amp. I really did think it was filters but I convinced my self I checked them all. I guess not!!!!


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## WCGill (Mar 27, 2009)

Wow, I've never seen an open cap, lots of shorts however. Quick and dirty, measure them with VOM.


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## nonreverb (Sep 19, 2006)

I've seen plenty. Not the most common failure but I do see a few examples every year. A far more common problem is broken solder on big caps mounted to PCB's.



WCGill said:


> Wow, I've never seen an open cap, lots of shorts however. Quick and dirty, measure them with VOM.


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## Church-Audio (Sep 27, 2014)

It's tricky to measure in circuit. There are esr meters that will do it. But the best way is always to pull it out. The issue here was I reflowed all the cap joints first... Then powered it up again. Same issue so I pulled the filter caps one by one... Only problem was I thought I had pulled them all and I did everyone except the one that was bad!!! Doh!! It happens. Sometimes you overthink a problem. Simple first ac in plate supply = possible bad diode or bad cap or bad connection to a cap. Especially if the power tubes are pulled and you still have a 125v on the output. I don't feel bad two authorized Ampeg techs my self and Ampegs factory service centre support could not find the issue.


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## jb welder (Sep 14, 2010)

Amprepair said:


> two authorized Ampeg techs my self and Ampegs factory service centre support could not find the issue.


Wow. As you're not the factory service center you get a pass :smile-new:, but this doesn't say much for the others. This is very basic troubleshooting, lack of filtering (as evidenced by hum on output and AC on B+), caused by bad filter cap.
The filters or their connections were about the only thing it _could_ have been.
I've had my share of overlooking the obvious, so I know it can happen, especially as you have probably not worked on SVT's before. But for the authorized guys to get beat on this is disheartening.

Glad to hear you got it up and running, maybe the other guys will send all their Ampeg's to you now :congratulatory:.


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## Church-Audio (Sep 27, 2014)

I think the real problem was the fact that I said I "checked" them all  I missed the one bad one!!!  oops now the amp puts out 10mv of ac with no input perfectly acceptable replaced all the caps for $30 and the output is 340 watts clean. More than happy with the end result. Customer will be very happy to get it back. New tungsol output tubes the old eh were all over the place current wise.

Now on to a Ampeg v4 that needs a new power transformer after a local hamilton tech installed a 15 amp fuse in it because the customer kept coming back with blown fuses..... Obviously it was a shorted hum balance. Blew up the heater winding. fliptops had a rebuild kit for $300 that replaces all the filters ( amp needed it anyway) dickhead tech cut the leads on the transformer and maretted them back togeather on the hvt heater tap and primary!!!! Guess he was to lazyto unsolder it??? Don't know what that was about. Customer Paid the guy $300+++ for that craptastic repair.


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## nonreverb (Sep 19, 2006)

I've lost count how many times I've been stuck undoing someone else's work to get to the real problem but hey! keeps me in biz! :smile-new:



Amprepair said:


> I think the real problem was the fact that I said I "checked" them all  I missed the one bad one!!!  oops now the amp puts out 10mv of ac with no input perfectly acceptable replaced all the caps for $30 and the output is 340 watts clean. More than happy with the end result. Customer will be very happy to get it back. New tungsol output tubes the old eh were all over the place current wise.
> 
> Now on to a Ampeg v4 that needs a new power transformer after a local hamilton tech installed a 15 amp fuse in it because the customer kept coming back with blown fuses..... Obviously it was a shorted hum balance. Blew up the heater winding. fliptops had a rebuild kit for $300 that replaces all the filters ( amp needed it anyway) dickhead tech cut the leads on the transformer and maretted them back togeather on the hvt heater tap and primary!!!! Guess he was to lazyto unsolder it??? Don't know what that was about. Customer Paid the guy $300+++ for that craptastic repair.


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## jb welder (Sep 14, 2010)

Yikes! The nice thing about the V4's was the "idiot" backup fuse inside the chassis in series with the mains fuse. It's a little larger than the mains so if the external fuse blows and the user installs something big like a 10 or 15, the internal fuse will blow and protect the amp.
But I guess someone like that tech would just remove it or oversize it anyway. :confusion:


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## Church-Audio (Sep 27, 2014)

This is an original v4 from the late 60s early 70s no internal fuse  here is a shot of the new hardware. Waiting for me to install it.


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## jb welder (Sep 14, 2010)

That's just like mine, earlier version with metal knobs. They came stock with a 10A pigtail fuse in series with the fuse holder. If it still has the schematic in the lid it will show a 6A and 10A fuse in series.
Someone removed the one from that amp, might have saved the PT :confusion:.

P.S. You've probably already done this, but the board mounted tube sockets are prone to solder joint failure and usually need to be resoldered.


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## nonreverb (Sep 19, 2006)

There some resistor solder joints that can be bad too if I recall...




jb welder said:


> That's just like mine, earlier version with metal knobs. They came stock with a 10A pigtail fuse in series with the fuse holder. If it still has the schematic in the lid it will show a 6A and 10A fuse in series.
> Someone removed the one from that amp, might have saved the PT :confusion:.
> 
> P.S. You've probably already done this, but the board mounted tube sockets are prone to solder joint failure and usually need to be resoldered.


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## Church-Audio (Sep 27, 2014)

nonreverb said:


> There some resistor solder joints that can be bad too if I recall...


These amps are famous for dry solder joints. I retouch pretty much everything on these boards.
As you can see from the picture I replaced all the screen resistors they were deep brown from heat. Even though the value was still pretty good anything that has seen high current and shows it I replace it. Why because the customer does not end up coming back for a stupid screen resistor  I also replaced the grid stoppers with metal film. I find it helps to use metal film here. I ended up replacing a wire wound resistor r55 it had also taken very high current this was a 7w I replaced it with a 6.5 watt again I think I am safe here in this circuit with new tubes / transformer / and filter caps its going to draw much less current then it once did. Checked all the candy caps for leakage they were fine. I will put the bias range resistor on the trace side of the pcb so the next tech that works on it can change it out if need be for a different value for the bias.


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## jb welder (Sep 14, 2010)

While we're talking about V4's in the SVT thread :smile-new:, V4's are wired to handle 6L6 or 7027 pinouts. Modern production 7027's are just 6L6's with extra pins connected that are needed in some amps but not the Ampeg V series. So don't waste your money on modern 7027's for these.
On the adjustable bias issue, they are biased very cold stock. Installing adjustable bias is not a bad thing, as long as you don't get hung up on the 70 to 80% bias thing. Guys that set up these amps like that go through a lot more power tubes, or sometimes can't find any reliable enough to run at that level with the high plate voltages these amps have.
The difference in tone may not even be noticeable, I think stock bias is probably in the 50% range but would need to double check that.


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## Church-Audio (Sep 27, 2014)

jb welder said:


> While we're talking about V4's in the SVT thread :smile-new:, V4's are wired to handle 6L6 or 7027 pinouts. Modern production 7027's are just 6L6's with extra pins connected that are needed in some amps but not the Ampeg V series. So don't waste your money on modern 7027's for these.
> On the adjustable bias issue, they are biased very cold stock. Installing adjustable bias is not a bad thing, as long as you don't get hung up on the 70 to 80% bias thing. Guys that set up these amps like that go through a lot more power tubes, or sometimes can't find any reliable enough to run at that level with the high plate voltages these amps have.
> The difference in tone may not even be noticeable, I think stock bias is probably in the 50% range but would need to double check that.


Been biasing amps for 20 years at 70% or close to it never had an issue with tube wear out. I have had many of the amps I service go out on major tours where they are being abused every single night. BUT the amp must be in perfect working order and you cant use shit tubes.


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## nonreverb (Sep 19, 2006)

Exceedingly high plate voltages are the bane of new production tubes. Particularly where vintage amps are concerned. Most '60's and '70's amps were designed around 110 -115VAC. Today's typical wall voltage is 120 - 125. 10% doesn't seem like much until you consider an original SVT spec'd @ around 685V or worse, a Fender PS400 @ 760V now running 10% higher.
Compound that with basically a small number of choices for tubes and you get the picture. Recently, I've run into some real problems with new tubes and some vintage gear. Early plexi Marshalls and Stramps come to mind as they have very high plate voltage....basically tube eaters. Even biasing colder, they kill 'em. One way to control the problem is increase the screen resistance a bit to rein them in. Either way, special attention must be paid as these amps are HARD on tubes.


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## Church-Audio (Sep 27, 2014)

nonreverb said:


> Exceedingly high plate voltages are the bane of new production tubes. Particularly where vintage amps are concerned. Most '60's and '70's amps were designed around 110 -115VAC. Today's typical wall voltage is 120 - 125. 10% doesn't seem like much until you consider an original SVT spec'd @ around 685V or worse, a Fender PS400 @ 760V now running 10% higher.
> Compound that with basically a small number of choices for tubes and you get the picture. Recently, I've run into some real problems with new tubes and some vintage gear. Early plexi Marshalls and Stramps come to mind as they have very high plate voltage....basically tube eaters. Even biasing colder, they kill 'em. One way to control the problem is increase the screen resistance a bit to rein them in. Either way, special attention must be paid as these amps are HARD on tubes.


 I am lucky to have my shop two blocks from one of North americas largest tube retailers. They pretty much know what tubes work in wich amps. Largest plate supply amp I have restored was a 1968 Marshall major... And funny enough when I started fixing amps when I was 16 working for the guitar clinic in Hamilton was the first amp I ever got a shock from!! Let's see one hand on the chassis one hand on the scope probe and no ground cap insulator on the probe!!! Touched the plate on the kt88 and got the shit shocked out of me. I can count on one hand the shocks I have had since some 28 years later. And I intend on keeping it that way... Last one was a 1961 bandmaster... Had the standby switch on drains the filters so I thought..... Then touched the wrong side of said standby switch and yeah....


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