Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: phsyconoodler on October 08, 2019, 07:11:38 am
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I've got a Badcst classic cat on the bench that has one tube that redplates at idle. It follows one tube no matter which socket it's in and multiple sets. Thos amp has very high plate and screen voltages for a cathode based amp.
It has a 5.5k shared screen resistor and a 500 ohm bypassed cathode resistor fie each tube.
Ivectroed 3 sets of 6v6s and it does the same thing. One tube redplates.
This is frustrating and weird. Any insight would be appreciated!
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My simple little troubleshooters brain says...
shorted bypass cap?
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Have you checked the OT?
/Leevi
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It follows one tube no matter which socket it's in and multiple sets.
Both 6V6's are installed when you get these conditions, right?
So then, set A has one tube that red plates and that same tube still red plates in the other socket while the non-red plater doesn't red plate in either socket. Set B has one tube that red plates and that same tube still red plates in the other socket while the non-red plater doesn't red plate in either socket.
Put a piece of tape on the non-red plater (the blue masking tape that doesn't stick very well) from set A. Put a piece of tape on the non-red plater from set B. Now put the two tubes with tape on them into the amp and see what happens.
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After reading 2deafs reply I realized I had misread what you wrote.
For some reason my brain turned it into "stays with the socket", probably because "stays with the tube" isn't computing
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one tube that redplates at idle. It follows one tube no matter which socket it's in
Ivectroed 3 sets of 6v6s and it does the same thing.
My best guess is that "Ivectroed" translates to "I've tried". :icon_biggrin:
Seems to me that you have a bucketful of bad tubes. Or you have not properly described the symptoms.
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No it follows one tube not the socket. The amp does have unusually high voltage for a cathode based amp using 6v6s.
I find it hard to believe new tubes are that faroff. I did try a 10k screen resistor and 1k cathode resistors but it still does it.
This is rather perplexing
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If the tube is red plating in both sockets then it must be a bad tube.
What is the plate voltage? Is it much more than the maximum value of 6V6?
/Leevi
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Yes it's over 400v.
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What brand 6V6's?
JJ 6V6's are supposed to take higher dcv's than other 6V6's. :dontknow:
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JJ ~500V, NOS 6V6 less.
/Leevi
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JJs
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Did you ohm the screen n individual cathode R's without tubes in ?
did you ohm pin 3 at tube side of socket to CT of OT where B+ comes in?
those readings will be handy
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Each cathode resistor measures 500 ohms.
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I removed the bypass caps from the cathode resistors and the Red plating has stopped.
I'm assuming one of them is shot. I dont have any more to try but I will tomorrow and see what happens. Right now the cathode voltage is about 24v and the plates are 455v. Screens are 355. I am going to put the 5.5k screen resistor back in as well and see what I end up with.
At this point I'm thinking one of the bypass caps is shot and the voltage follows the tube that draws the most current and makes it worse.
Hopefully this is the problem
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So, what happened with "It follows one tube no matter which socket it's in"? You just testing us? :wink:
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Sluckey I'm just telling you what I observed. It only redplates one tube and it was not socket specific. Swap them and it followed the tube.
As soon as I removed the bypass caps it no longer redplated
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the reddish might still be lurking, by removing the caps I believe gain dropped :dontknow:
still would like to know the dcr for your OT primary windings. I had a bench amp I swore had bad tube, followed tube, and it was a bad (3 actually) tube, but the OT was 80% short on one side, so gobble tube move tube follows tube. worth the 2 minutes to eliminate
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the reddish might still be lurking, by removing the caps I believe gain dropped :dontknow:
+1
I suspect the redplate issue will return when you put install bypass caps.
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I cannot measure the cathode resistors with the caps installed which is odd. I have not had that issue with other cathode biased amps. I have never had to clip the capacitor to measure the resistsnce
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Ok. Update. I originally had to clip one end of the bypass caps to measure the values of the resistors. I knew thos was odd. With the resistors in place it measures .3ohms.
I just installed new capacitors and they measure 500 ohms across them. As they should.
One or both of these capacitors are the cause of this issue.
The amp is functioning perfectly. Original value screen resistor is back in. No redplating and it sounds great.
So yes,these bypass caps were the issue