Power drop in Super Reverb

[homerayvaughan]

'66 super reverb, new caps - Once I turn it on, it sounds fine, but within 10 to 30 minutes I will hear a loud pop, and then the volume drops considerably. I can put it on standby and turn it back on and it seems ok, but then it will happen again. I changed out the power tubes and it still does it - someone suggested it was the power tubes. If it's a preamp tube, where would I look first? Should I try another set of power tubes?

[Blue Strat]

Have you biased the power tubes? If not, start by doing that.

Are both channels affected?

Try cleaning all tube pins and sockets as per this FAQ (near the bottom).
http://kcanostubes.com/content/newsletter_details.asp?ArticleID=2

[homerayvaughan]

Have you biased the power tubes? If not, start by doing that.

Are both channels affected?

Try cleaning all tube pins and sockets as per this FAQ (near the bottom).
http://kcanostubes.com/content/newsletter_details.asp?ArticleID=2

The amp was completely gone through a few years ago - all the tube sockets were cleaned and tightened, and it does not leave my house - amp I have 4 other amps, so it's not like the tubes have had a lot of use. I bought a matched quad of 7581's - 2 went in the amp and the other 2 were backups. When it started acting up I put in the other set - seeing that they were a matching set I did not rebias the amp. Someone else suggested maybe replacing the phase inverter?

[bob-i]

The first thing is to replace the tubes one at a time in any intermittant tube amp issue.

Next I'd suspect a cold solder joint. Even though it was gone through recently, it's possible. Also, pots are worth checking out.

The thing that concerns me is the POP. Without having the amp on a bench it's hard to determine, but something is overheating and giving up. Possibly a cap, even a new one, might be giving up.

[Blue Strat]

The amp was completely gone through a few years ago - all the tube sockets were cleaned and tightened, and it does not leave my house - amp I have 4 other amps, so it's not like the tubes have had a lot of use. I bought a matched quad of 7581's - 2 went in the amp and the other 2 were backups. When it started acting up I put in the other set - seeing that they were a matching set I did not rebias the amp. Someone else suggested maybe replacing the phase inverter?

Yes, but the amp isn't working, right? So something is wrong and we don't know what it is.

You've got to start with the basics and move on.

ANY of the preamp tubes could have a problem, but the fact that the amp is intermittant leads me to suspect a bad connection (tube pins, sockets) or something else internally.

Good luck.

[homerayvaughan]

Yes, but the amp isn't working, right? So something is wrong and we don't know what it is.

You've got to start with the basics and move on.

ANY of the preamp tubes could have a problem, but the fact that the amp is intermittant leads me to suspect a bad connection (tube pins, sockets) or something else internally.

Good luck.
Actually the amp IS working. It will make a noise ( a crackly pop?) and then the volume drops. I set it to standby for a couple of seconds and the volume is right again. Should I avoid playing it until I can get it checked?

It's hard to check one tube at at time - I'd have to change one tube and play for up to 30 minutes to wait for a noise, that may or may not happen.

[Blue Strat]

Troubleshooting intermittent problems isn't usually easy and is often time consuming. I would start by reading/performing my original recommendations and go on from there.

You may have to bring it to an amp tech.

[donnyjaguar]

This is a problem that could be caused by a number of different things. The easy things to check, or rule out, would be dirty tube sockets and amp losing bias. Wiggle the tubes in their sockets - with the volume turned down otherwise it'll blow your mind, and see if that buys you some more full-volume time. As for the tubes losing bias, they'll be glowing red (very bad!). :) It could also be a bum tube that has a whipped filament or is arcing through. Replacing the tube can help here.

There are still a whole bunch of other conditions that could cause this - none of which are easily remedied without opening it up and doing some basic troubleshooting.

DJ

[slider313]

This sounds like a cold solder joint on a cathode bypass cap. Check by hitting the eyelet with the iron and if the wires lift then use a solder vacuum and do the connection over. Make sure there is enough lead on the cap to go into the eyelet as not to have it pop out after soldering.

[Neil]

I have the exact same symptoms on my 66 showman. It sounds great for ~15 mins and then goes crackle pop and gets quieter and thinner. I have changed all the electrolytics and tubes (which improved the tone but didnt fix the problem). Subjectively it doesnt seem to matter how long I have it on standby it plays ok for the same length of time before going pop.

My (uneducated) guess is that it is a HT problem and not a thermal (eg cold joint, poor valve base contact etc) problem/ Hitting it doesnt help (although it makes me feel better!).

Any one know what might be wrong?

Thanks
Neil

[Fretts]

Hey it's a long shot but...
If your tagboard is wavy, up & down, i.e. Warped, look really close at any uninsulated wires that go from the tag board to ground. On my amp, one was broken but just barely touching. Any change in temperature or a good smack on the top of the amp would make it drop to about 1/4 volume. The two broken ends were just barely touching; it would work for a few minutes, but as soon as it warmed up, the wires would separate and then -- no more volume.
Also any resistors that get hot in operation such as in the power supply or power amp section are suspects for changing behavior after a brief warmup.