Strange preamp tube failure on Peavey 6505+

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chikov
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Strange preamp tube failure on Peavey 6505+

Post by chikov »

I was asked to look into Peavey 6505+ 100W amp head. It has four 6L6 output tubes ( only two were present) and six 12ax7 preamp tubes. The problem That I ran into reightaway was all 6 preamp tubes have failed (I checked on my tube tester). I have never seen ALL preamp tubes fail and the owner told me that he recently replaced all of them ( he used Tung-sol brand). Does anyone know what could take all of them out like that and what should I watch for in order not to ruin the new set of replacement tubes? Thanks.
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xtian
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Re: Strange preamp tube failure on Peavey 6505+

Post by xtian »

Are you SURE the tubes are dead? Try one or two in a known good amp.

Start by checking heater voltages in the Peavey, obviously. The preamp heaters are DC rectified, separate from the power tubes, so you should check out that circuit: http://www.thetubestore.com/lib/thetube ... ematic.pdf

It would be interesting to autopsy the dead tubes. Measure the heater conductivity--should be a couple ohms. Measure conductivity between the triode elements and the heaters--there should be no conductivity.
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chikov
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Re: Strange preamp tube failure on Peavey 6505+

Post by chikov »

Small update. I checked all filter caps , solder joints, voltages. Next, I installed the good tubes and turned the amp on... well, it started working fine... I wonder if the owner got the failed tubes to begin with. I will message him about it...
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Re: Strange preamp tube failure on Peavey 6505+

Post by chikov »

xtian wrote: Sun Feb 18, 2018 4:15 am Are you SURE the tubes are dead? Try one or two in a known good amp.

Start by checking heater voltages in the Peavey, obviously. The preamp heaters are DC rectified, separate from the power tubes, so you should check out that circuit: http://www.thetubestore.com/lib/thetube ... ematic.pdf

It would be interesting to autopsy the dead tubes. Measure the heater conductivity--should be a couple ohms. Measure conductivity between the triode elements and the heaters--there should be no conductivity.
Checking heater voltages is a great idea - I will do it right away. As far as knowing for sure if tubes are dead - I have a good tube tester “ Dyna Jet 606”. It is very reliable, had only one owner since the 70s.
chikov
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Re: Strange preamp tube failure on Peavey 6505+

Post by chikov »

It is an unusual to me design. Four out of 6 tubes have 6VDC on filaments and two have 6AC... I did notice that the amp started making random popping noises, but I will poke around with my bamboo stick first, the problem might be in contacts or one of my replacement tubes is so old that it is dying (I did check them on my tester as well, but most of them are not new). I will keep you posted.
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Re: Strange preamp tube failure on Peavey 6505+

Post by pompeiisneaks »

I'd also check that none of the power tubes have shorted from pin 3 to 2, I had that happen on an EL34 and it took out the heaters on all tubes in the amp.

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chikov
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Re: Strange preamp tube failure on Peavey 6505+

Post by chikov »

Hmmm... it does not have any obvious shorts but after I replaced all the preamp tubes the amp started working almost as it should BUT after you play for a few seconds it starts popping. I recordea video of this problem... i will attach it. I know for sure it is not tubes and is not the guitar cable... what might cause this type of a problem?
https://youtu.be/uGFmd-DiUrE
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Littlewyan
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Re: Strange preamp tube failure on Peavey 6505+

Post by Littlewyan »

Measure the HT Voltage to make sure it hasn't gone up too much because of the two missing power tubes.

Also tap each tube to make sure none are microphonic. I use the rubber on the end of a pencil.
chikov
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Re: Strange preamp tube failure on Peavey 6505+

Post by chikov »

Littlewyan wrote: Mon Feb 19, 2018 12:15 pm Measure the HT Voltage to make sure it hasn't gone up too much because of the two missing power tubes.

Also tap each tube to make sure none are microphonic. I use the rubber on the end of a pencil.
I tried all 4 tubes (6L6), but it did not help. the sound is getting even worse, it is quieter now and crackling even worse. I hope my replacement 12ax7 are ok. As soon as I have the time, I will dig a little deeper. Your help points me to the right direction where I could look into and take it from there... thank you... Please share more ideas....
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Re: Strange preamp tube failure on Peavey 6505+

Post by sonicmojo »

Check your send/return jacks if the amp has them or run a jumper cable to b sure. I've had similar signal problems when those are dirty. Just a thought.
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chikov
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Re: Strange preamp tube failure on Peavey 6505+

Post by chikov »

sonicmojo wrote: Tue Feb 20, 2018 5:24 pm Check your send/return jacks if the amp has them or run a jumper cable to b sure. I've had similar signal problems when those are dirty. Just a thought.
That was a good idea that I forgot about. I sprayed some contact cleaner (Deoxit) inside the jacks and inserted 1/4 cable in there. unfortunately the problem remained. I noticed that on clean channel my volume dropped too. I tested all my preamp tubes and the V1 has a short in it. In my tube tester it showed a short in the position 6 and 7. Something is killing my tubes and I intent to find out what that is...
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Reeltarded
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Re: Strange preamp tube failure on Peavey 6505+

Post by Reeltarded »

Popping like the output tubes are flashing? Arcing output tube...
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chikov
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Re: Strange preamp tube failure on Peavey 6505+

Post by chikov »

Reeltarded wrote: Thu Mar 01, 2018 4:55 pm Popping like the output tubes are flashing? Arcing output tube...
I did the magic bamboo stick test and discovered that the popping happens when I push on the main circuit board. The hard part is to find where the weak connection is. I propped the board up and would touch the board in different places and would get a popping but the problem is that does not matter where I touch the board, the whole thing moves anyway... so I need to think of a way to icolate the problem but it is hard...
chikov
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Re: Strange preamp tube failure on Peavey 6505+

Post by chikov »

Reeltarded wrote: Thu Mar 01, 2018 4:55 pm Popping like the output tubes are flashing? Arcing output tube...
Yes, the output tubes are flashing! Every time I wiggle the board both tubes flash the blue light from the middle... what causes for both of them to flash in synchronized manner?
I tried the two other good 6L6 tubes and it does the same thing - every time it popps I see a blue faint light flashing from inside of the plates...
And what is interesting that the popping does not start right away... after all the tubes are warm and then 5 more sec plus. As if something gets to a certain temp and becomes unstable...
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Re: Strange preamp tube failure on Peavey 6505+

Post by chikov »

Quick update... I think I found the problem... I found a cold solder on one of legs of the capacitor ( more on that later). Once I resoldered it the popping stopped. Could it have killed all the preamp tubes? Let me study it little more...
Too bad I do not have exact schematic...
On my PCB this cap is C53. On the schematic link above ( not exact but close) C53 is a coupling cap between phase inverter and the output tube... I guess a weak connection can cause the voltage surges that could kill a tube or two...
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