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Ampeg SVT3 Pro question

audiomitch

audiomitch

Oct 10, 2010
1,734
128
San Franciso Bay Area
#1
It's been a while since I posted here, but have been lurking.

I have a question about my Craigslist Ampeg SVT3 Pro head I just bought (not working) for $50. I was hoping it was an easy fix, but R27 in the power section left a crater in the PCB when somebody made a mistake. I've read that's the flaming resistor when you make a mistake.

Oh well, a $50 dollar gamble.

But the preamp section works perfectly, and I when send it to the "amp in" of my Ampeg PF-500 it sounds glorious. Yay! I can also reamp to it in my DAW either direct or with a speaker IR. Not bad for 50 bucks.

My question is about removing the two tubes in the cratered power section. I don't see a point of having the tubes light up and add all kinds of heat in the chassis. I removed them and all seems well, it even stopped the fan from blowing. But will it cause any problems? Capacitors exploding or anything like that?

Intuitively I would think removing the tubes would cut even more power feeding in to the defunct power section, but "intuition" could lead to a house fire.

Thanks in advance!
 
#2
audiomitch said:
It's been a while since I posted here, but have been lurking.

I have a question about my Craigslist Ampeg SVT3 Pro head I just bought (not working) for $50. I was hoping it was an easy fix, but R27 in the power section left a crater in the PCB when somebody made a mistake. I've read that's the flaming resistor when you make a mistake.

Oh well, a $50 dollar gamble.

But the preamp section works perfectly, and I when send it to the "amp in" of my Ampeg PF-500 it sounds glorious. Yay! I can also reamp to it in my DAW either direct or with a speaker IR. Not bad for 50 bucks.

My question is about removing the two tubes in the cratered power section. I don't see a point of having the tubes light up and add all kinds of heat in the chassis. I removed them and all seems well, it even stopped the fan from blowing. But will it cause any problems? Capacitors exploding or anything like that?

Intuitively I would think removing the tubes would cut even more power feeding in to the defunct power section, but "intuition" could lead to a house fire.

Thanks in advance!
Click to expand...
Pictures? There aren't any tubes in the power section of an SVT 3 pro. I have one. I have done a good amount of work to it ( I also bought mine pretty messed up. But got it up to speed.)
 
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#6
audiomitch said:
Yes, the 12AX7 and 12AU7 driver tubes in the power section.
Click to expand...
So, the back left of your amp has a giant burn in it? But you still have your di and everything else functioning normally? I can't imagine what caused this, I know that's not what your question was, because the trim pot would cause the right side of the amp to smoke (the resistors would give off too much current, and fry the mosfet power section. There are no tubes in the path of any of this. Those two aren't even on the same board). My guess is, you might want to pull out the back left board, flip it over, and find out what's going on underneath it all before even turning the amp on again. Then he'll, if it runs fine without them, go for it. If it stops the fan when you pull them out, leave the top cover off and use a desk fan. Right?
 
#9
JeezyMcNuggles said:
So, the back left of your amp has a giant burn in it? But you still have your di and everything else functioning normally? I can't imagine what caused this, I know that's not what your question was, because the trim pot would cause the right side of the amp to smoke (the resistors would give off too much current, and fry the mosfet power section. There are no tubes in the path of any of this. Those two aren't even on the same board). My guess is, you might want to pull out the back left board, flip it over, and find out what's going on underneath it all before even turning the amp on again. Then he'll, if it runs fine without them, go for it. If it stops the fan when you pull them out, leave the top cover off and use a desk fan. Right?
Click to expand...

The PCB has a burnt crater where R27 used to be. There's a thread somewhere (maybe here) about somebody trying connect the speaker out to the wrong thing, and that same resistor catching on fire. Looks the same issue here, it could even be the same amp for all I know. I have zero interest in going down that rabbit hole. A flaming resistor is not a good sign for the rest of the circuit. I would consider buying an Ampeg part #2041114-00, but it would have to be easy and cheap. A rack mount power amp (probably from Craigslist) would be cheaper, easier, and more powerful. Or continue using my PF-500 and have two different Ampeg preamps to choose from.

I'm thinking that it's probably OK to remove those tubes, but the paranoia in me made me post here to get opinions. I think the fan was for the power section, and was just making noise for no reason anymore. Again, an assumption on my part...
 
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#10
audiomitch said:
The PCB has a burnt crater where R27 used to be. There's a thread somewhere (maybe here) about somebody trying connect the speaker out to the wrong thing, and that same resistor catching on fire. Looks the same issue here, it could even be the same amp for all I know. I have zero interest in going down that rabbit hole. A flaming resistor is not a good sign for the rest of the circuit. I would consider buying an Ampeg part #2041114-00, but it would have to be easy and cheap. A rack mount power amp (probably from Craigslist) would be cheaper, easier, and more powerful. Or continue using my PF-500 and have two different Ampeg preamps to choose from.

I'm thinking that it's probably OK to remove those tubes, but the paranoia in me made me post here to get opinions. I think the fan was for the power section, and was just making noise for no reason anymore. Again, an assumption on my part...
Click to expand...
Oh, I gotcha. Yeah, if the pre amp works fine without them, why not right? But, I'd still try to keep it all nice and cool if your going to still use it.
 
#12
audiomitch said:
It's been a while since I posted here, but have been lurking.

I have a question about my Craigslist Ampeg SVT3 Pro head I just bought (not working) for $50. I was hoping it was an easy fix, but R27 in the power section left a crater in the PCB when somebody made a mistake. I've read that's the flaming resistor when you make a mistake.

Oh well, a $50 dollar gamble.

But the preamp section works perfectly, and I when send it to the "amp in" of my Ampeg PF-500 it sounds glorious. Yay! I can also reamp to it in my DAW either direct or with a speaker IR. Not bad for 50 bucks.

My question is about removing the two tubes in the cratered power section. I don't see a point of having the tubes light up and add all kinds of heat in the chassis. I removed them and all seems well, it even stopped the fan from blowing. But will it cause any problems? Capacitors exploding or anything like that?

Intuitively I would think removing the tubes would cut even more power feeding in to the defunct power section, but "intuition" could lead to a house fire.

Thanks in advance!
Click to expand...
Horse is here bro. He'll help you out
 
#13
When R27 fails, this is an indication that the output latched or shorted to the positive rail and took out components in the VI limiter as well as other components.

I have seen this kind of failure when somebody unqualified has tried to adjust the bias based on what they thought they learned in the internet without the proper safety precautions in place. Under these conditions, when something goes wrong, it goes very wrong.

While a qualified tech familiar with this product line could likely make these repairs, it may be less costly to replace the entire PCB (also should be done by a qualified tech because there are safety procedures involved with this as well).

The amp was likely so cheap because the seller had already received an idea amount the likely cost.
 
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#15
agedhorse said:
When R27 fails, this is an indication that the output latched or shorted to the positive rail and took out components in the VI limiter as well as other components.

I have seen this kind of failure when somebody unqualified has tried to adjust the bias based on what they thought they learned in the internet without the proper safety precautions in place. Under these conditions, when something goes wrong, it goes very wrong.

While a qualified tech familiar with this product line could likely make these repairs, it may be less costly to replace the entire PCB (also should be done by a qualified tech because there are safety procedures involved with this as well).

The amp was likely so cheap because the seller had already received an idea amount the likely cost.
Click to expand...
Ha, yeah I got really lucky when I biased mine. I was scared, but I did it anyway :)

I then replaced all the tubes, soldered in a new fan, replaced the DI out, input jack, pre out jack, tuner out jack, new bass control pot, gain pot, tube gain pot. It was a good little adventure for me. Thing's running better than new now. And much quieter thanks to the new fan.
20210310_173856.jpg
20210310_173907.jpg
Nothing cost much at all either. Tube set was about 80 bucks, all the other components were like 3 bucks each. Good amount of time though. Oh, fan cost 40 bucks.
 
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#16
agedhorse said:
A lot of speculative guessing going on here without much thought or even accurate descriptions.
Click to expand...

actually why I posted here. I wanted knowledgeable opinions. Not sure if I could get more accurate description , the power section does not work, the R27 resistor obviously failed in a spectacular fashion, I have zero interest in repairing it. Just asking about the power tubes which has still not been addressed. Thanks!
 

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