Ampeg SVT-4 PRO. Mono mode excessive current.

[Indyryder02]

ok, so….
Took amp in with intermittent static concern. (Generally try to avoid these but there’s little alternative for these guys locally). It was the V1 preamp tube.
I swapped back and forth a couple times to be sure.. because surely I’m not getting this lucky on one of these…. No more static. (Was only checking the individual amp outputs for the noise to be gone, in stereo mode) Sent amp home.

Customer states now in bridged mode, which is how he’s always used it.. there’s very low output. Individual amps work fine. (I of course didn’t do anything but change a preamp tube.. so already miffed at what could have happened). The mono switch hasn’t ever been used so I was the first to switch it in a long time.

ok.. have the amp back. Have also noticed the main filter caps are leaking fluid up and out of the top of their sleeves… first I’ve ever seen on one of these amps.
With mono mode and original mono cable that came with amp.. not only is the output low, but it’s pulling what seems to be excessive current.. even throwing internal relays if too much signal is given. So something is surely wrong. Looking at the individual amp outputs. One is putting out more than the other.. so thats a flag..
I can see where someone previously replaced half of the output transistors and all of the associated emitter resistors.

I guess I don’t even know where to start, as far as what would allow both amps to work ok on their own ( be it at a bit different levels, maybe that’s it?). But when in mono it seems it wants to implode. I definitely don’t want to make it any worse..

since it needs those large caps for sure already.
and there’s a couple of dark marks at a couple of the output transistors that were previously replaced. (They must have got pretty hot). And who knows what else it’s had.. A replacement board is tempting. They’re a bit pricey though it looks. About $500 from what I’ve found.
I just don’t want to lose a ton of time on this amp, for what was a simple preamp tube replacement.

so I’d appreciate any guidance and advise from you much more experienced techs. Ugh. I know better than taking these in! And I knew I couldn’t be lucky enough for it to be a simple tube swap and done! ♂️

[Jon Snell]

"With mono mode and original mono cable that came with amp.. not only is the output low, but it’s pulling what seems to be excessive current.. "
Very odd, unless the customer is pairing the output stage by connecting both outputs together ... that'll certainly cause issues.
Mono mode connects the inputs of the amplifiers together, NEVER connect the outputs together or the amplifiers will become damaged after a while and because they do not produce absolutely identicle signals, they will be fighting each other.

I take it you mean C4, 5, 6 & 7 for replacement as they have been under a lot of strain from shorted output stages.

[Indyryder02]

"With mono mode and original mono cable that came with amp.. not only is the output low, but it’s pulling what seems to be excessive current.. "
Very odd, unless the customer is pairing the output stage by connecting both outputs together ... that'll certainly cause issues.
Mono mode connects the inputs of the amplifiers together, NEVER connect the outputs together or the amplifiers will become damaged after a while and because they do not produce absolutely identicle signals, they will be fighting each other.



I take it you mean C4, 5, 6 & 7 for replacement as they have been under a lot of strain from shorted output stages.

*only using the original supplied mono cable that’s special for the mono output. Switched in mono mode.

hooked up this way, if the switch wasn’t set to mono. Could that have caused an issue?

and yes. Those are the ones leaking. They still read ok capacitance wise.. but they’re definitely physically leaking fluid. First I’ve seen on these amps.

[g1]

You may have 2 issues with the amp.
1) For sure the different levels in stereo mode is a problem.
2) Then, if the switch is not working right, it may not be going back to mono. That would leave the outputs in phase, and with the special bridging cable, you would normally get zero output, but now you get a bit of output because the channels have different levels (problem 1).

[Indyryder02]

You may have 2 issues with the amp.
1) For sure the different levels in stereo mode is a problem.
2) Then, if the switch is not working right, it may not be going back to mono. That would leave the outputs in phase, and with the special bridging cable, you would normally get zero output, but now you get a bit of output because the channels have different levels (problem 1).

and that leads me back to where one sides outputs and resistors were all replaced.. maybe it’s not a well matched set etc.

the switch does seem to operate. Although I’m not sure it’s fully working.

but apparently this all just happened after bringing it to me for the preamp tube swap. Cracked solder joint from the ride home?

a new board sounds better all the time if I can sell it.
unfortunate mostly for it have this issue all of a sudden after I put the tube in it. Which I just can’t see how I could have caused any damage or changes even doing that.

[g1]

The mono switch hasn’t ever been used so I was the first to switch it in a long time.

That's why I brought attention to the switch.
When using the bridge cable, none of your equipment connected to the output can be grounded. That may be what is creating the excess current.

[Indyryder02]

That's why I brought attention to the switch.
When using the bridge cable, none of your equipment connected to the output can be grounded. That may be what is creating the excess current.

I’ve had it just hooked up to a speaker cabinet with nothing else connected (scope etc). Same.

[Jon Snell]

Where do you plug the mono cable in ... input sude or speaker side?

In mono mode the input to one of the amplifiers is inverted, that is the function of IC201 and the outputs are then in bridge mode. In other words, when the LH amp goes possitive, the RH amp goes negative, effectively doubling the voltage output of the amplifier and thereby increasing the power output BUT take note of the minimum impedances!

[Indyryder02]

Where do you plug the mono cable in ... input sude or speaker side?

In mono mode the input to one of the amplifiers is inverted, that is the function of IC201 and the outputs are then in bridge mode. In other words, when the LH amp goes possitive, the RH amp goes negative, effectively doubling the voltage output of the amplifier and thereby increasing the power output BUT take note of the minimum impedances!

Speaker side. Sorry I didn’t clarify. It’s Ampeg’s specific wired Speakon to 1/4” jack mono speaker cable.

[x-pro]

Those are the ones leaking. They still read ok capacitance wise.. but they’re definitely physically leaking fluid.

https://music-electronics-forum.com/...channels/page4
Post 47

[g1]

I guess you have verified that it is the exact same cable and checked the wiring inside is still good?
It gets a little complicated with the setup depending on when the amp was made (see attachment), but if the cable was correct before, then it should work now.

[Indyryder02]

I guess you have verified that it is the exact same cable and checked the wiring inside is still good?
It gets a little complicated with the setup depending on when the amp was made (see attachment), but if the cable was correct before, then it should work now.

Yes. He supplied his cable he was using that originally came with the amp.

really trying to think of what could up and change from just changing a preamp tube. Cleaning jacks etc and pressing the mono switch in. And a car ride home.. broken solder joint is all I can assume. Unless something coincidentally up and failed rather inconveniently for me.

with the past work that’s been done.. and needing the caps replaced anyway. A new board sounds better all the time if I can sell him on it.

I have the board out now. So I will check for bad joints while it’s out. One on the mono switch already appears suspect

[Tom Phillips]

...really trying to think of what could up and change from just changing a preamp tube...

If you do enough work you will encounter situations like this and there could be absolutely no relation between your work and the timing of the other failure. Unfortunately, the customer doesn't always see it that way. Especially if it is a new, first time, customer.

This also reminds me of the common comments an owner makes when the amp suddenly fails. They just keep repeating "But it was working fine just before this happened."
My reply is "everything is fine until it's not."

[Indyryder02]

If you do enough work you will encounter situations like this and there could be absolutely no relation between your work and the timing of the other failure. Unfortunately, the customer doesn't always see it that way. Especially if it is a new, first time, customer.

This also reminds me of the common comments an owner makes when the amp suddenly fails. They just keep repeating "But it was working fine just before this happened."
My reply is "everything is fine until it's not."

yes. Especially anything electrical. I used to explain to customers when I was in the car biz this same thing..
id tell them.. same as when you go in the bathroom and flick the light switch. Poof. One day it just doesn’t work.

but definitely never looks good when I just had it! And now it doesn’t work! Ugh haha.

Customer seems pretty understanding so far.

[Indyryder02]

I never followed up on this!

So I took out the board. Replaced the caps.
Touched up any suspect solder joints especially the mono switch.
Back together. Adjusted bias.
Everything worked as normal again.
Customer was satisfied. Still no clue what the real culprit was…. I’d guess maybe the solder joints for the mono switch that never got used.
But I sure was happy to see it leave!! Haha

Thanks again for all the help and suggestions