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Old 28th February 2011, 03:53 PM   #1
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Default Gassy WE 300b

Servicing a Audion Silver Knight stereo single ended amp, I find that BOTH the Western Electric 300B's have gone gassy. Run the amp up on the variac to about 160/170 input volts and it starts to run OK, until you wind the input signal up and off it trips and the control grid climbs up toward the anode. I'm just surprised to find that both valves are doing this in very similar fashion. Anyone else had this?
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Henry
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Old 28th February 2011, 06:54 PM   #2
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Originally Posted by toprepairman View Post
Servicing a Audion Silver Knight stereo single ended amp, I find that BOTH the Western Electric 300B's have gone gassy. Run the amp up on the variac to about 160/170 input volts and it starts to run OK, until you wind the input signal up and off it trips and the control grid climbs up toward the anode. I'm just surprised to find that both valves are doing this in very similar fashion. Anyone else had this?
Regards
Henry
Hmm, It IS a little odd that BOTH the valves have gone down to air....

--In fact--its quite unlikely, but Possible

Have you confirmed by changing them for a known good pair of 300B...?

Worth checking the coupling-caps while you're in there--Best by substitution and try again. They dont have to be some daft gold plated, diamond-studded whale-oil audiofool cap, a good Polyprop or Russian K40Y-9 will do for testing out, and sound just as good...
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Old 28th February 2011, 07:44 PM   #3
Knarf is offline Knarf  Denmark
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Originally Posted by Alastair E View Post
--In fact--its quite unlikely, but Possible
I have seen that in a pair of power tubes. Once. Pair of 6146B beam power pentodes run in parallel in a ham radio transmitter, had been *badly* abused and overheated by the user.

Not sure though how you'd manage to do that in a pair of 300Bs in two different channels of a class A audio amp...?

- Frank.
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Old 28th February 2011, 07:52 PM   #4
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Henry, Is the amp fixed bias? If so, does it respect the WE specification for a 50K grid leak resistor? Needs 470n+ couplers to maintain LF with 50K pulldowns.

If the resistors are OK, the coupling caps are a good suspect, I agree.

Do the getters look OK?
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Old 1st March 2011, 10:57 AM   #5
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Hi Guys, thanks for the replies, all very sensible.
Had already gone for the coupling caps first, replaced then with 1000v .47uF. They were .33uF 400v so were open to suspition.
Unfortunately I don't happen to have any 300b's floating around so can't check by substitution.
Knocked up an adaptor and tried them in my curve tracer but it only goes up to 270 volts and they performed fine.
The amp has cathode auto bias, and with 120K grid resistors. I added 82K resistors across them to bring them down to the 47K region but made only a marginal difference.
This amp is a 220 V 'export' model. Running on the variac at around 160 in, it puts out around 400 v HT., impyling a final HT of around 550v which seems a bit high. The cathode resistors are 1K, (one of them was blown to bits).
Really could do with an original circuit diagram with voltages on it if anyone can track one down?
Got to say the build quality is a bit crap.
Any more ideas?
Regards
Henry
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Old 1st March 2011, 11:02 AM   #6
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Oh and to add the getters are still silver but look poorish.
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Old 1st March 2011, 11:18 AM   #7
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Does sound like the 300Bs are being roasted.

I use 1K cathode resistors with a 420V supply, and they draw about 70mA like this, whichever maker's 300B is used. That's a comfortable 25W on a 40W anode. If the on-load supply gets much over 500V, the current will approach the 100mA limit, and the 40W limit also encroached:
425V x 90mA is 38.25W !

I think the 300Bs will have a short life at that operating point.
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Old 1st March 2011, 12:03 PM   #8
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Agreed!
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Old 1st March 2011, 12:37 PM   #9
316a is offline 316a  England
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It could be the couplers . I was presented with an Audionote amp to repair , both the Audionote/Jensen PIO caps were leaky . This can easily be determined by disconnecting the couplers and measuring the cathode voltage to ensure it's within spec and stable

316a
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Old 1st March 2011, 12:48 PM   #10
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Already done (as mentioned in previous post)
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