Power tubes cutting out on Super Reverb

[IggyRoyale]

All,

I'm very close to finishing my first tube amp. It's based on Super Reverb circuit minus tremolo and plus mid control on both clean and crunch channels. Unfortunately my very good friend (who I first met through this forum, by the way) who has been guiding and mentoring me through this project passed away last December. So here I am, back where it all started, hoping someone will be able to help.

The problem appears to be related to the clean channel and is somewhat weird in nature. After playing for two or three minutes on the clean channel, power to the circuit cuts out. Fuse doesn't blow. Volume rapidly drops off and the amplifier goes quiet as though someone has pulled out the power lead. When I flick the standby switch off, I can see the heaters starting to glow again after a few seconds. Flicking it back on, sound comes back just fine for a few mintues and then the same thing happens again.

This does not happen when I use the crunch channel .... would anyone have any idea why this is happening? Happy to provide the schematic if that would help. There are one or two other issues, but I might start another thread for those ...

Cheers

[Tom Phillips]

The symptoms are just as have experienced in amps that had a bad colder connection to a pre-amp tube heater supply. (Slow fade out) You say this only happens when playing the clean channel. Is the heater in the clean channel pre-amp tube still glowing when you lose the sound? Check both halves of the heater. Can you make this happen with the chassis on the bench so you can wiggle wires and measure voltages? This is just one idea based on the slow fad out you described. Could be other things.
HTH,
Tom

[IggyRoyale]

Thanks Tom. Will check and let you know ... would the fault you describe affect the power tubes or just preamp tubes? The problem I was describing seems to affect power tubes, not sure about preamp. And yeah, by the way, the chassis is out of the cabinet and on the bench. I can do measurements and tweaks.

Cheers! Speak soon.

[Tom Phillips]

The problem could be anywhere but I based my answer on your report that it only happens when you use the clean channel. If the power section was the problem, then you would observe the same smptoms with the crunch channel too.
Remember, this is just one idea. There could be many other causes but this is worth a try IMO.
Tom

[IggyRoyale]

Tom,

Just had a chance to have a play with my amp. As devil would have it, much to my frustration, I could not reproduce the problem. I've been playing for a solid hour and been switching between cleand and crunch channles. Dunno ... I'm confused. I checked all the solder joints on the plate heater pins and they all seem solid. Please feel free to throw any other theories at me ... In the mean time I'll keep at it and revisit this thread as soon as it happens again.

I have another probelm to throw at this forum regarding output volume of this amp but I'll start another thread to keep it clean.

Regards

Iggy

[Tom Phillips]

Just had a chance to have a play with my amp. As devil would have it, much to my frustration, I could not reproduce the problem. I've been playing for a solid hour and been switching between cleand and crunch channles. Dunno ... I'm confused. I checked all the solder joints on the plate heater pins and they all seem solid. Please feel free to throw any other theories at me ... In the mean time I'll keep at it and revisit this thread as soon as it happens again.

I have another probelm to throw at this forum regarding output volume of this amp but I'll start another thread to keep it clean.

Regards

Intermittent problems are often difficult or worse. The first task it to make the problem appear and stay long enough to find the cause. Sometimes just messing with the amp temporarily fixes the problem and that can be frustrating. Things you can try are:

• Let the amp heat up on the bench with a signal applied so you can tell when it cuts out.
• Tap and prod around with a stick to check for loose joints.
• Really good visual inspection is always good
• Cover the open chassis with a board to hold the heat in if necessary. Sometimes I use a heat gun to help the process.
• Wiggle the tubes in their sockets
• Let the amp cool down and start over if necessary
• Freeze spray sometimes helps bring out a problem
• See the tube amp debugging page at http://www.geofex.com/ There is a section on intermittent problems.
• Pay close attention and keep an open mind as you go thru the process.

Pay really close attention to the symptoms to help you isolate the problem. For example, based on your initial input I assumed that the problem was not in the power amp or PI because only the clean channel was cutting out. (Comment: The thread title may therefore get people off on the wrong track) The heater circuit idea popped into my head because you mentioned a slow fad and return rather than an abrupt cut out. Could also be a high voltage power supply problem.

Best of luck to you. Can you post pictures?

Tom

[IggyRoyale]

Tom,

Thanks for above suggestions. Will do the "heat, shake and wiggle" test ;-). Good idea bout covering the open chassis. "Geofx" site is fantastic. I already found potential answers to other bugs in this amp. As far as intermittent nature of this problem goes, I think it will be worthwhile resoldering all tube pin connections.

Yep, I can post pictures. Are you interested in pictures of wiring and circuitry?

Regards

Iggy

[Tom Phillips]

Yep, I can post pictures. Are you interested in pictures of wiring and circuitry?

Yep. Love that amp porn.
Sometimes pictures help people give you better assistance too.
Tom

[IggyRoyale]

Tom,

Here are some pictures I took a couple of months ago. Some components in the power supply circuit (smaller square board on the right) have changed a little bit but the photos are still a good representation of what it looks like today. Feel free to comment on anything you see ... just do go easy on me though. I know this is not the best wiring job ... it could have been much cleaner. Just remember that this is the first point to point wiring project I've ever done.

Regards

Iggy

[IggyRoyale]

Tom.

I believe that I have found at least one bad solder connection ... the main one being on the heater supply of the right hand power tube as viewed in the second picture above. Coincidentally it is placed right underneath the IEC mains plug and therefore hard to see. I positioned my desk lamp to get as much light in the tight spots as possible, and used a magnifying glass to see better. It appeared that one of the pins was not completely covered with solder and was showing some light through the bottom wire hole.

What gave it away was that, by fluke, I managed to reproduce the problem as I was showing off my amp to a friend. As soon as I switched it on and started playing it started fading away. So I quickly went for the power tubes and started wiggling while giving a little strum on the guitar as I was doing so. Sure enough the sound came back. This warranted a closer inspection of all the soldering work and ... yay!!!.... there it was.

I intend to keep on monitoring to see if it happens again. Will post an update if the problem occurs again.

Cheers mate! Thanks for your input

Iggy

[Tom Phillips]

Iggy,
That's good news. In your first report you thought that only the clean channel was affected. Did BOTH channels go dead when this last time?
If the problem happens again and the amp is opened up it would be a good opportunity to verify what is and is not working. Also a chance to take voltage readings to help isolate any additional problems.
Cheers,
Tom

[IggyRoyale]

Tom,

Hmmm ... this time around both channels were being used in parallel ("high" and "low" input jacks of my amp's clean channel are wired in such way that when the guitar is plugged into "high", "low" jack acts as a guitar signal "out", allowing me to run another cable to the crunch channel and get some extra overdrive). This appears to disprove my initial observation that the amp cuts out only when the clean channel is used. Nevertheless I will keep on monitoring it for a while and see whether it happens again.

If it does, what do you suggest I check first? Obvious checks are 1) mains AC voltage 2) B+ 3) AC voltage across the heater pins 4) -ve bias voltage ... anything else?

Cheers mate! Speak soon.

Iggy

[Tom Phillips]

...what do you suggest I check first? Obvious checks are 1) mains AC voltage 2) B+ 3) AC voltage across the heater pins 4) -ve bias voltage ... anything else?Iggy

The goal is to isolate the problem so if the amp cuts out...
Play through each channel separatly
Measute the voltages you listed very carefully so you don't disturb any loose joints and possibly make it start working before you isolate the problem area.
You can also use visual clues such as which tube heaters may have turned off.
Listen carefully. For example, if both channels appear to be dead but there is any hum or hiss comming from the speaker then you know that the power amp is still working.
Hopefully, you have already fixed the problem.
Cheers,
Tom

[Enzo]

If the heaters are going out, then it is not a matter of bias or B+, THEY don't associate with the heater circuit. If ALL the heateers fade, then look for the point where the transformer 6v wires connect with the wiring to the tube sockets. I'd bet one joint is bad, regardless of how it looks.

Since the tube heaters are generally wired socket to socket to socket, if only some of them go out, then redo the solder at the last working socket. I don't care how good it looks, pull it apart and redo the connection.