Gassy WE 300b

toprepairman · 28th February 2011, 03:53 PM

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

Alastair E · 28th February 2011, 06:54 PM

Quote:

Originally Posted by toprepairman

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...

Knarf · 28th February 2011, 07:44 PM

Quote:

Originally Posted by Alastair E

--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.

Rod Coleman · 28th February 2011, 07:52 PM

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?

toprepairman · 1st March 2011, 10:57 AM

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

toprepairman · 1st March 2011, 11:02 AM

Oh and to add the getters are still silver but look poorish.

Rod Coleman · 1st March 2011, 11:18 AM

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.

toprepairman · 1st March 2011, 12:03 PM

Agreed!

316a · 1st March 2011, 12:37 PM

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

toprepairman · 1st March 2011, 12:48 PM

Already done (as mentioned in previous post)

28th February 2011, 03:53 PM	#1
toprepairman diyAudio Member Join Date: Nov 2010	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? Regards Henry

Thread Tools	Search this Thread
Show Printable Version Email this Page	Search this Thread: Advanced Search

Similar Threads
Thread	Thread Starter	Forum	Replies	Last Post
FS: for 300B (JE Labs 300B SE) builders	cartman	Swap Meet	2	29th October 2008 01:13 PM
Which 300B ?	JC951t	Tubes / Valves	33	1st August 2008 02:25 PM
300b	gianis	Tubes / Valves	2	9th November 2007 05:53 PM
* 35 watt * 300B SE = super 300B + Class A2	rick57	Tubes / Valves	4	13th July 2005 04:18 PM
Gassy Tube	morfeas	Tubes / Valves	22	31st March 2004 04:29 AM

New To Site?	Need Help?
Register to Participate Search Privacy Statement Contact Us	Frequently Asked Questions Did you forget your password? Mark Forums Read

28th February 2011, 07:52 PM	#4
Rod Coleman работник diyAudio Member Join Date: Jul 2004 Location: Warwickshire UK	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?

1st March 2011, 10:57 AM	#5
toprepairman diyAudio Member Join Date: Nov 2010	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

1st March 2011, 11:02 AM	#6
toprepairman diyAudio Member Join Date: Nov 2010	Oh and to add the getters are still silver but look poorish.

1st March 2011, 11:18 AM	#7
Rod Coleman работник diyAudio Member Join Date: Jul 2004 Location: Warwickshire UK	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.

1st March 2011, 12:03 PM	#8
toprepairman diyAudio Member Join Date: Nov 2010	Agreed!

1st March 2011, 12:37 PM	#9
316a diyAudio Member Join Date: Dec 2002 Location: West London	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

1st March 2011, 12:48 PM	#10
toprepairman diyAudio Member Join Date: Nov 2010	Already done (as mentioned in previous post)

Currently Active Users Viewing This Thread: 1 (0 members and 1 guests)