Ampeg SVT 3 pro tube preamp section troubleshooting

[Paul Szegedin]

Hello all -- I picked up an Ampeg svt 3 that was barely producing sound and followed @ejschultz fantastic instructions for biasing the amp in this thread; Mine was reading less than 1mv and I successfully biased it up to spec around 25mv.

Amp sounds great but there is an issue with the tube preamp section. When the tube gain is all the way down it makes a buzzing, then a whump as you turn it up and the amp sounds good with tube gain at 10, but I think because it is out of the circuit and not functioning at 10. Using the preamp output, it sounds good, but the tube gain does nothing.

I tried swapping out all the tubes for known goodies, and tapped them with a chopstick looking for bad sockets.

Could anyone advise on possible weak links here -- the potentiometer for tube gain, or something else? That pot is hard to get contact cleaner into.

I'm not sure that the amp is producing all the power it should -- I have the master at 10 and use the gain to adjust volume -- but maybe it is. I'm not used to cranking the master, but it clips with the solid state gain at about 6.

Any input would be welcome.

edit: while I'm here, is there a known, quieter replacement fan?

 

[agedhorse]

Do you have a scope and do you understand how the circuits are supposed to work?

Do you know what the DC voltages and operating points should be?

 

[rickdog]

The behavior of the "Tube Gain" control you describe is perfectly normal. I have an SVT-3Pro, it does the same thing, and it still has a little sticker on it that says this. I suggest downloading the user manual from the Ampeg web site. It describes this, as well as providing a helpful block diagram of the whole amp.

Tube Gain is part of the power amp, which is why it doesn't affect the preamp output. What it does is adjust the power supply voltage for the last 12AX7 voltage gain stage and the 12AU7 current gain stage that drives the MOSFETs. Turning the knob to the left lowers the voltage, causing these tubes to saturate at lower signal levels.

The Gain control (next to the input jack and Peak LED) adjusts the signal level through the preamp. If you like a warm, thick tube preamp sound, crank that knob up, and light up the Peak LED. That LED should be labeled "Ampeg Goodness." Alternatively, if you want maximum clean headroom, turn the Tube Gain and Master up all the way, and control your volume with Gain.

Congratulations on a careful (and successful) adjustment of the bias. That's essential to get the most out of the SVT-3Pro.

 

[Paul Szegedin]

The behavior of the "Tube Gain" control you describe is perfectly normal. I have an SVT-3Pro, it does the same thing, and it still has a little sticker on it that says this. I suggest downloading the user manual from the Ampeg web site. It describes this, as well as providing a helpful block diagram of the whole amp.

Tube Gain is part of the power amp, which is why it doesn't affect the preamp output. What it does is adjust the power supply voltage for the last 12AX7 voltage gain stage and the 12AU7 current gain stage that drives the MOSFETs. Turning the knob to the left lowers the voltage, causing these tubes to saturate at lower signal levels.

The Gain control (next to the input jack and Peak LED) adjusts the signal level through the preamp. If you like a warm, thick tube preamp sound, crank that knob up, and light up the Peak LED. That LED should be labeled "Ampeg Goodness." Alternatively, if you want maximum clean headroom, turn the Tube Gain and Master up all the way, and control your volume with Gain.

Congratulations on a careful (and successful) adjustment of the bias. That's essential to get the most out of the SVT-3Pro.

Thanks, you're 100% right. I copped the amp last night and hadn't read up on it -- now I saw this thread that describes what you just said. I was just impatient, and now I find that it makes that crackling noise when the gain was moved for a good 15 seconds or so before settling down.

But man, what a sweet sounding amp for $0! A kind soul on FB gave it away because he couldn't fix it. I really got to hand it to @ejschultz for taking the time to post that thread. I owe the guy a beer. Adjusting the bias was nerve-wracking because you move it a hair, and it keeps climbing after you stop, so you really have to take it light. I blew a Gallien-Krueger once trying to adjust bias. But I followed his suggestion to a T and used a vice grip to get a long lever on that tiny screwdriver. Good as gold. I was afraid to dollop silicone or something on there because the slightest touch would move it, so I will just go back in there again and check at some point.

This amp with its EQ has a lot more going on in the 30hz area than what I had, so I'm enjoying that ampegy goodness. Just got to quiet the fan now so I can use it at a volume where my longsuffering neighbors won't call the cops.

 

[rickdog]

Some people have had issues with the bias drifting, but I adjusted mine nine years ago, and haven't had a problem since.

My amp's fan starts full speed when I first power it on, but then slows down, and only speeds up if the amp gets hot. Does yours behave the same way, but the fan is just noisy? Or is it running full speed all the time?

 

[Paul Szegedin]

Some people have had issues with the bias drifting, but I adjusted mine nine years ago, and haven't had a problem since.

My amp's fan starts full speed when I first power it on, but then slows down, and only speeds up if the amp gets hot. Does yours behave the same way, but the fan is just noisy? Or is it running full speed all the time?

Mine the same; fan starts full speed then slows down to minimum. But it's too loud to use at low-ish volume -- I'm going to bring the GK back out, which has no fan and is dead quiet at idle. I'm sure the ampeg is fine at concert volume, but it's not even that powerful. For example, it would no good to record, micing the cab right under this head.

I went in there and put foam buffers under, over, and between each mounting point of the fan. That did bupkiss, really. I'm considering putting the fan in the next room and running a cooling duct.

The fan has a little bit of unbalanced vibration but I'm not optimistic a replacement with the same will be better. Lots of power amps have fans but they're not right in front of you in your living room. There has to be a better solution, and I'm considering trying things that will get me scolded by the Ampeg police -- the amp is a 'rescue' after all.

 

[rickdog]

Huh. The fan in mine isn't silent, but it's pretty quiet.

A few long shots: I haven't had the lid off mine for a while (and I don't feel like unracking it and taking it apart right now), so I don't remember much about the fan. Any chance it has accumulated dust that can be cleaned off? Sometimes that will make a fan noisy. Is there any obvious way to lubricate it? And last, try contacting Ampeg support, I haven't had occasion to need them, but I've read comments they're actually quite responsive and helpful, maybe you can just get a new fan. Even if it's "expensive", a nice amp for the price of a new fan sounds like a good deal.

And - you're aware you can operate it with no speaker connected? I've used just the head as a preamp for recording, no speaker involved, works fine and you can set the DI to "post" and get all that tube preamp goodness.

 

[Paul Szegedin]

Yeah I agree, on all that.

Now, this is going to get me flamed, so retired amp engineers and the 'take it to a qualified technician' crowd, please just tune out now. But experimenting in there, I can't help but notice that the fan is a LOT quieter when mounted above the heatsink, pointing straight down with padding at the point of contact. It's closer to the elements that way. So it runs slower, and quieter per a given speed. I figure, airflow is airflow, it's doing its job, and better and quieter -- but the amp wasn't made that way because it would make the chassis larger and less rack-mount usable among other things.

I have half a mind to re-engineer the lid using a piece of aluminum I have at hand, with the fan mounted down. Still, I will miss the zero noise of the GK at low volume. And using my Super Reverb as the amp is probably still quieter and better sounding. Ampeg just gives me that round, soft, big bottom. Embarrassment of riches.

 

[agedhorse]

Except that’s not how fan assisted cooling works. It may be quieter but also way less effective too.

The “elements” (transistors) have terrible (high) thermal resistance at the top of the case and are designed (by some pretty smart guys, called engineers) to transfer 99% of the thermal energy (heat) through the BOTTOM surface of the transistor to the heat sink.

This heat sink to transistor interface is designed specifically for low thermal resistance (high thermal conductivity) by the management of flatness, surface finish and thermal gap filler (for gaps in the 1 or 2 thousandths range) and material profile.

The thermal energy travels through the profile which has hundreds or thousands of times more thermal radiating capacity than the top of the device (transistor) itself.

This is why real engineers go to school and study subjects like thermodynamics, so we can design products based on solid physics and science rather than incorrect assumptions. We do the calculations and know that what we designed is backed by knowledge.

 

[JeezyMcNuggles]

Thanks, you're 100% right. I copped the amp last night and hadn't read up on it -- now I saw this thread that describes what you just said. I was just impatient, and now I find that it makes that crackling noise when the gain was moved for a good 15 seconds or so before settling down.

But man, what a sweet sounding amp for $0! A kind soul on FB gave it away because he couldn't fix it. I really got to hand it to @ejschultz for taking the time to post that thread. I owe the guy a beer. Adjusting the bias was nerve-wracking because you move it a hair, and it keeps climbing after you stop, so you really have to take it light. I blew a Gallien-Krueger once trying to adjust bias. But I followed his suggestion to a T and used a vice grip to get a long lever on that tiny screwdriver. Good as gold. I was afraid to dollop silicone or something on there because the slightest touch would move it, so I will just go back in there again and check at some point.

This amp with its EQ has a lot more going on in the 30hz area than what I had, so I'm enjoying that ampegy goodness. Just got to quiet the fan now so I can use it at a volume where my longsuffering neighbors won't call the cops.

I replaced my fan. 40 bucks. Way way better. The bias is going to drift back. It's really a pain in the butt. I fixed mine when I got it, and it was back to distorted after a few trips back and forth to practice.the "tube gain" is a dampening knob. Exactly like the mesa wd800. Left for slow attack, right for fast attack. Left = loose, tube like, power section. Right = tight, solid state like, power section.

The kicker is that the more to the right the knob is, actually boosts your signal as well. It sounds best at about 10 o'clock, but then you lose gain. So, I boost it at the graphic to get the volume I want. That amp has a pre gain knob, and a post gain slider. You can get pretty darn good volume from it without having to crank your gain knob to clipping. You just boost your signal after the eq at the graphic instead.

 

[AlexanderB]

Except that’s not how fan assisted cooling works. It may be quieter but also way less effective too.

The “elements” (transistors) have terrible (high) thermal resistance at the top of the case and are designed (by some pretty smart guys, called engineers) to transfer 99% of the thermal energy (heat) through the BOTTOM surface of the transistor to the heat sink.

This heat sink to transistor interface is designed specifically for low thermal resistance (high thermal conductivity) by the management of flatness, surface finish and thermal gap filler (for gaps in the 1 or 2 thousandths range) and material profile.

The thermal energy travels through the profile which has hundreds or thousands of times more thermal radiating capacity than the top of the device (transistor) itself.

This is why real engineers go to school and study subjects like thermodynamics, so we can design products based on solid physics and science rather than incorrect assumptions. We do the calculations and know that what we designed is backed by knowledge.

Hmmm... given the many and widespread issues in Ampeg products since decades, I assume nothing about how well the development and testing was done. The production quality is already below many (most?) competitors, at least until Yamaha took over.

 

[LetItGrowTone]

Probably as a result of reading idealized accounts of the development of e.g. the SR-71, the Bomb, etc, engineering, in my youthful imagination, was about maximizing performance, an heroic challenge (learned from Walter Cronkite that the word "an" is correct there). But I learned in engineering school the degree to which it is (almost always) a game of performance/dollar (i.e. getting it done as cheaply as possible). That's a different kind of challenge, and could also be heroic if "winning the war" depends upon it. Well, come to think of it, but I digress. That was only one of the reasons why I left engineering school. Left myself woefully ignorant I did. But my experience is that it isn't always impossible to improve something, even so, with "a little bit of knowledge", as the proverb goes.

And, oh yeah, Walter Cronkite wasn't as we imagined either, but that would also be a digression. Better than regression I guess.

 

[Paul Szegedin]

Thank God there is an "ignore user" feature on this board.

 

[LetItGrowTone]

I was defending you, actually.

Edit: I hid it under so many layers that you didn't even notice. I'm sorry for that.

 

[Paul Szegedin]

I was defending you, actually.

Edit: I hid that under so many layers that you didn't even notice. I'm sorry for that.

Yes, thanks I appreciate it - I was referring to the other guy, who responds to every technical post with self-serving, condescending, unhelpful and often wrong answers. Every internet board, whether it's talking about volkswagens, coding, or knitting cat sweaters has those guys in their 60s who are there to make themselves appear honored for their expertise in their own eyes, out of insecurity about the internet sweeping away the value of specialized human knowledge. In fairness, engineers often have serious problems communicating with human beings. Let's all mellow out -- it's a beautiful summer day, and free damn bass amp.

 

[agedhorse]

“In fairness”, some folks don’t understand as much as they think they do either.

Everything I posted was absolutely factual, correct and accurate.

 

[JeezyMcNuggles]

Yes, thanks I appreciate it - I was referring to the other guy, who responds to every technical post with self-serving, condescending, unhelpful and often wrong answers. Every internet board, whether it's talking about volkswagens, coding, or knitting cat sweaters has those guys in their 60s who are there to make themselves appear honored for their expertise in their own eyes, out of insecurity about the internet sweeping away the value of specialized human knowledge. In fairness, engineers often have serious problems communicating with human beings. Let's all mellow out -- it's a beautiful summer day, and free damn bass amp.

You mean horse? He's the developer/engineer from Mesa Boogie. If there's anyone here who knows anything about amps on here, it's him. But yes, he does come off quite pompous at times. Most of the time though, it's actually not intended to sound that way. It's just him. There's almost always the answer hidden between three messages.

 

[Paul Szegedin]

You mean horse? He's the developer/engineer from Mesa Boogie. If there's anyone here who knows anything about amps on here, it's him. But yes, he does come off quite pompous at times. Most of the time though, it's actually not intended to sound that way. It's just him. There's almost always the answer hidden between three messages.

So did you get the exact same fan as stock replacement from Ampeg, or a different fan? I think they have it on digikey or something. Some engineering genius on another board tested a few and posted their noise levels.

As for the horse guy, I don't care if he's the head of NASA -- his posts are not helpful to the people asking -- his posts are self-serving to the point of parody, off-topic, and always wrong. I knew he would chime in here with "do you even OWN an oscilloscope!?" in total ignorance of this amp -- the other guy had the right answer about the tube gain, and another guy humbly posted detailed instructions for biasing the amp. BTW "righty tighty, lefty loosey" is exactly right - good one.

 

[Engle]

So did you get the exact same fan as stock replacement from Ampeg, or a different fan? I think they have it on digikey or something. Some engineering genius on another board tested a few and posted their noise levels.

As for the horse guy, I don't care if he's the head of NASA -- his posts are not helpful to the people asking -- his posts are self-serving to the point of parody, off-topic, and always wrong. I knew he would chime in here with "do you even OWN an oscilloscope!?" in total ignorance of this amp -- the other guy had the right answer about the tube gain, and another guy humbly posted detailed instructions for biasing the amp. BTW "righty tighty, lefty loosey" is exactly right - good one.

I’ve seen agedhorse provide a lot of good information here. His comments have probably saved quite a number of people from getting shocked, stopped people from wrecking their equipment and throwing good money after bad.

It’s good that you got your amp working, but that doesn’t make agedhorse’s statements wrong.

 

[JeezyMcNuggles]

So did you get the exact same fan as stock replacement from Ampeg, or a different fan? I think they have it on digikey or something. Some engineering genius on another board tested a few and posted their noise levels.

As for the horse guy, I don't care if he's the head of NASA -- his posts are not helpful to the people asking -- his posts are self-serving to the point of parody, off-topic, and always wrong. I knew he would chime in here with "do you even OWN an oscilloscope!?" in total ignorance of this amp -- the other guy had the right answer about the tube gain, and another guy humbly posted detailed instructions for biasing the amp. BTW "righty tighty, lefty loosey" is exactly right - good one.

Replacement. I'll look through my purchases to find what it was. It was a sprint. Made in Mexico. But, not sure if the product number.