Tonefiend Forum

OK I am sure hoping somebody out there who actually has some bench experience with tube amps can help an old SPICE head. I promise not to post any SPICE simulations for a whole month if I can get some help.

My amp has 3 tubes - 12X4 rectifier, 12AT6 preamp, 12AB5 power amp.

Amp works fine for about 3 minutes after it warms up. Then there is a sudden volume drop accompanied by crackling. It kinda cuts in and out if I gently wiggle the single power tube. I have changed the power tube for another one and the same thing happens with the other tube.

I thought maybe the socket contacts were cruddy so I sprayed some Deoxit on it and also pinched together the contacts from the bottom side so that the pins would have to scrape through any crud on their way in. But it didn't help a bit. However, based on the crackling when I wiggle the tube, I still get the impression it's a bad connection down there.

For the record, here's an article about retensioning tube sockets:

https://billmaudio.com/wp/?page_id=171

Another one with similar symptoms...

https://www.tedsforums.com/forum/index.php?action=printpage;topic=8463.0

I remelted all the solder joints on the power amp tube socket and likewise it did not help.

All the resistors have already been replaced, along with one of the 0.01 uF ceramic caps.

Looked like the "orange glow" was also flickering (not consistently though) during one particular wiggle. Makes me think the heater pin socket contacts for that tube are not swell. But I don't want to replace the socket (which looks to be quite a bit of effort) if that is not going to fix it.

Ideas?

Thanks for the response!

Here's a better copy of the schematic which was nicely pasted to the inside of the cabinet.

Please ignore the numbers on the schematic, they have nothing to do with my list below.

Here are some more details:

1) If I keep the volume at about 50% and play softly, I do not experience any sound cutting out, even after 5 minutes. But if I turn up the volume all the way and then hit a big old first position E major, then the sound starts cutting out.

2) Signal is still getting through in all cases. It is just reduced a lot in volume (and apparently, low end - though that may be an auditory illusion of the Fletcher-Munson variety).

3) The underneath of the tube socket contacts are BLACK on a number of the pins.

4) I got a DeOxit set from Radio Shack. This includes a small spray can of DeOxit and another of DeOxit Gold. I didn't really pay close attention to the difference - I thought maybe the "Gold" gave you a groovier rush when you concentrated and inhaled the contents.

5) The first can of DeOxit was empty although I'd only used it a few times. I asked my sons if they knew anything about it and one of them said "I sprayed it on [his friend] Connor's head because he was being an idiot". I said "you shouldn't do that" and he said "I thought it was compressed air". I told him "you shouldn't do THAT either" (kids these days!). So actually I had only sprayed DeOxit Gold on the pins, and closer inspection (I can't read the tiny print without an electron microscope) reveals that DeOxit Gold is for treatment of gold contacts, but it is not an oxidation remover. I also tried some Radio Shack generic contact lubricant, that was a total mistake. Its main ingredient is MINERAL OIL gahhh. I figured in the future I'll use Olive Oil "Pam" on my pots because then at least I'll be lowering my cholesterol while dialing in my "tone" don't y'know.

So I will go by San Mateo Electronics and see if they have some DeOxit with the brush applicator. And if that continues to fail I'll getcha some before and after voltages.

For your good deed, you have saved this Forum from any SPICE analysis for the month of November!

Quote from Digital Larry on November 2, 2012, 10:59
OK, I got me a giant 5 oz. can of DeOxit from San Mateo Electronics. That is one OLD-SCHOOL electronics shop. Totally crammed, helpful staff, need a trained guide to find anything. They also had a wall display of tubes behind the counter. When I told the guy I was working on restoring a tube amp, he said "most of that's been up there for 40 years". Price on a 12AB5 was a little over $10. Tube Depot has them for a little over $6 so no contest there. Nobody appears to be making 12AB5's these days, so in spite of my nefarious plans it does not seem suitable to base a new design on.

I got home and sprayed the stuff all over the place (why they put this stuff in a spray can is a little beyond me - the brush applicator seems much better). I did manage to get some of it on the tube sockets. Put everything back together and did the old test. Worked pretty good for awhile, started cutting out, then worked pretty solid. So like an idiot I decided to try the other tube I had - back to square one. That's about the time the boys had to go to bed so I left it for another day.

To summarize this cliff-hanger, I'm pretty sure I'm dealing with a severely fouled socket in this 50-year old beastlet. If I can't get it reliable with a few more blasts of DeOxit I'm going to give up and drop in a new socket.

It may well be a bad tube socket. It is possible for the socket material to carbonize if there is excessive heat or current. At the high voltages that are in the circuit even a little leakage current could cause the problems you described. You might want to replace the 12AB5 socket on general principles to eliminate it as a problem. If you have an O-scope it would still be instructive to see what the waveforms look like at several points. Caig DeOxit is hands down the best contact cleaner ever made IMHO. Try cleaning the contacts inside and old flashlight using DeOxit, it makes it better than new!!! I keep DeOxit and all the other formulations in stock as spray, brush-on, nail polish style container and the felt tipped pens. I've been using it for over 30 years exclusively.

As Captain Beefheart said in Golden Birdies "And the panataloon duck white goose neck quacked Webcor, Webcor"