'68 Custom Deluxe Reverb Losing Power

aFewGoodTaters

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Looking for assistance troubleshooting an issue I'm having with my '68 Custom Deluxe Reverb amp. The last couple times I've had it plugged in I've lost quite a bit of power after 20 minutes to a half hour of playing. I play around 3.5 to 4 on the volume on the Custom (left) side usually...it sounds great then all the sudden there is a significant drop in volume and it becomes very thin and bad sounding. This has happened once in our rehearsal space and then a week later in the studio right before we were about to record. I also noticed the amp gets pretty hot to the touch. I know tube amps get hot, but the faceplate near the jewel light seemed a bit hotter than normal.

Wondering if there are a few logical places to start troubleshooting before I bring it to a tech. I understand amp safety and discharging filter caps, and while a novice I'd like to begin to learn how to do some of this myself it possible.

Thanks in advance -
 

4pickupguy

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Mine gets abnormally hot as well. Been running a fan and it helps keep it reasonable. Probably biased really hot or something. Hasn’t done the losing power thing though.. 😕
 

Lynxtrap

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It might very well be heat related if it starts after 20-30 minutes of playing. Could be a bad solder joint. If it is caused by heat, the area around the output tubes is a good place to start looking. It can be hard to find, you might have to inspect the circuit board with a magnifying glass.

Before that, perhaps do some more checking and testing when the problem occurs. Like does it affect both channels, does it help turning it off and on again or does it need cooling, twist pots and cables/jacks, etc.
 

ElPositivo

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Judging by your story I would also expect a bad (solder) connection first.

Wiggle the tubes around in their sockets while the amp is on and see if you can hear a clear crackle or pop or a bad connection. In this case it is either a bad tube socket or bad solder connection on the tube socket. Discharge the caps, make sure there is no remaining voltage in the amp and retension the tube socket or reflow the solder on the tube sockets then check if problem is solved.

If that didn’t help I’d take the chassis out, connect the speaker with an extension speaker cable and play the amp until the problem shows up. You can get a wooden chop stick, or something that doesnt conduct electricity, and carefully push on solder joints in the amp and see if you hear a crackle or pop somewhere. If so you have a bad connection. Be very careful as you are now working in a live amp, do not touch anything with your hands (!!). If you are not comfortable doing this or dont know what you are doing I strongly advice not to do this. If you found a bad solder joint reflow the connection with some fresh solder and see if that helped.

If that didn’t help I’d suspect a component that is affected by heat, like filter caps or carbon comp resistors (although I think your amp doesnt have carbon composite resistors).

Hope this helps as a start.
 

dan40

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Have you tried fresh tubes yet? Your amp may be fairly new but tubes can fail at anytime. I agree with what the others said above but I would try ruling out a tube issue before pulling the chassis.
 

aFewGoodTaters

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Thanks for the input and suggestions. I started with the least invasive suggestion first and checked the reverb tank leads. I had upgraded them to higher quality right angle RCA leads a year ago and they appear to be in good shape - hit the plugs with a bit of DeOxit, plugged them back in and twisted around a bit.

I'll be able to play for an extended session tomorrow and see if that was the issue. If I experience the same issue I'll investigate the tube sockets / socket solder joints next.
 

aFewGoodTaters

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Have you tried fresh tubes yet? Your amp may be fairly new but tubes can fail at anytime. I agree with what the others said above but I would try ruling out a tube issue before pulling the chassis.
I replaced V1 through V5 within the past six months but haven't replaced the power tubes or rectifier. They seem OK, glow just fine and don't make any noise when lightly tapping with a pencil eraser...but I don't know how to really test them.
 

Lynxtrap

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Thanks for the input and suggestions. I started with the least invasive suggestion first and checked the reverb tank leads. I had upgraded them to higher quality right angle RCA leads a year ago and they appear to be in good shape - hit the plugs with a bit of DeOxit, plugged them back in and twisted around a bit.

I'll be able to play for an extended session tomorrow and see if that was the issue. If I experience the same issue I'll investigate the tube sockets / socket solder joints next.

It might not be necessary to wait for extended playing sessions. You can start by just leaving it on (not in standby) for half an hour or more, then check if it is acting up.
 

King Fan

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I replaced V1 through V5 within the past six months but haven't replaced the power tubes or rectifier. They seem OK, glow just fine and don't make any noise when lightly tapping with a pencil eraser...but I don't know how to really test them.
If you want noninvasive first, this is a good time to pick up the spare/backup output and rectifier tubes every tube amp needs. JJs are cheap and robust. I admit tubes may not be your *most* likely problem, but these tubes can fail as soon as others — even sooner — so you’ll want them on hand.

After that it’s up to you. Solder joints on PCBs aren’t trivial. And ribbon connectors? Ugh. Chopsticking involves working in a live amp. And so on. No reason not to try, but if I was wondering when to think “tech time” I might be getting close. I’m not trying to discourage you, I’m just saying it’s also OK to consult a pro.
 
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aFewGoodTaters

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So I played the amp for an hour yesterday after cleaning the RCA leads and it didn't act up on me.

I'm wondering if this may have been part of my issue: The RCA cable I bought was from StewMac and it has a shield wire that I'm assuming can be soldered to ground if desired (see link). I didn't solder it, and I'm wondering if it was coming in contact with a tube retainer or somewhere on the reverb pan that was causing an issue? I've trimmed the excess back so hopefully not causing a short going forward.

 




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