Power amp tube situation. Tech advice needed

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purpledc
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Joined: Sun Jan 30, 2011 2:39 pm

Power amp tube situation. Tech advice needed

Post by purpledc » Sat Aug 06, 2011 5:00 pm

Hey guys, Has anyone had a peavey tube amp where one of the tube sockets consistently runs hotter than the other 3? My 6534 has one socket that runs about 5-7 ma hotter than the other three. This is the one furthest to the left when viewing from the back. I put new tubes in it and went to bias it and found this out. So To be safe I biased it via that one socket alone as I knew as long as I was in a safe range with it the other tubes would be well below melting point. I just found this odd. And I believe Peavey told me to bias the amp at 42.5-43 volts. At this setting that one hot socket would have chernobylized. Any ideas? I know its not the tubes as It didnt matter which of the 4 new tubes (or the old) I used this slot was consistently higher.

rogue777
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Re: Power amp tube situation. Tech advice needed

Post by rogue777 » Sun Aug 07, 2011 7:25 am

See if there is a grid resistor for that tube, if there is perhaps its changed value. Verify the B+ for all tubes is the same value on the plates, also check screen voltages, and check the screen resistor for that socket, for values compared the other three others. The screen current adds to the cathode current if your measuring that, so that might account for the difference. Good to try to swap tubes to be sure what you think - that is - that One socket does have a difference cathode current.

I don't think 5-7 ma is that much of a difference, but your point that one tube is hotter may have a real circuit cause for that,,,thinking resistor value. Don't really know what the schematic looks like.

KTB
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Re: Power amp tube situation. Tech advice needed

Post by KTB » Sun Aug 07, 2011 10:05 am

Could be a cold solder joint on one of the socket pads. I would at least reheat them and make sure.
The screen resistor current on 6L6's is so small that I doubt a different value would amount to even a few milli amps if not micro amps. Could be a bias feed or coupling cap passing some DC.
Monitor pin 5 at idle and see if it changes when the tubes heat up and start conducting current.

purpledc
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Re: Power amp tube situation. Tech advice needed

Post by purpledc » Sun Aug 07, 2011 12:22 pm

KTB wrote:Could be a cold solder joint on one of the socket pads. I would at least reheat them and make sure.
The screen resistor current on 6L6's is so small that I doubt a different value would amount to even a few milli amps if not micro amps. Could be a bias feed or coupling cap passing some DC.
Monitor pin 5 at idle and see if it changes when the tubes heat up and start conducting current.

Funny thing happened and I dont know why. I let the amp sit on for a little while (1/2 hour) then biased it again. No issues and the numbers are much closer now. Its possible I didnt let it warm up enough on the initial run. Funny thing again is biased it with a probe to Eurotube specs and then checked the voltage at the test points and it was no joke spot on at 42.5 Volts

KTB
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Re: Power amp tube situation. Tech advice needed

Post by KTB » Sun Aug 07, 2011 12:34 pm

purpledc wrote:
KTB wrote:Could be a cold solder joint on one of the socket pads. I would at least reheat them and make sure.
The screen resistor current on 6L6's is so small that I doubt a different value would amount to even a few milli amps if not micro amps. Could be a bias feed or coupling cap passing some DC.
Monitor pin 5 at idle and see if it changes when the tubes heat up and start conducting current.

Funny thing happened and I dont know why. I let the amp sit on for a little while (1/2 hour) then biased it again. No issues and the numbers are much closer now. Its possible I didnt let it warm up enough on the initial run. Funny thing again is biased it with a probe to Eurotube specs and then checked the voltage at the test points and it was no joke spot on at 42.5 Volts

Most techs and I included will bias the tubes up and let it sit at idle for 20 minutes as new tubes will drift after they warm up good. Then come back and rebias it again and you will usually encounter different readings across the board. Some tubes when matched or tested don't do this because they have already passed the burn in stage.

rogue777
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Posts: 301
Joined: Mon Sep 14, 2009 7:07 pm

Re: Power amp tube situation. Tech advice needed

Post by rogue777 » Sun Aug 07, 2011 12:43 pm

Well said KTB, thanks for the better reply. I was trying to put some ideas out there.
Isn't the screen current about 2 ma?

Hey my amp is broken!
Are you sure?
can't fix something that an't broke.
:|

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