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
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...
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.
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
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
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.
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.
I'm assuming these are modern WE 300B production, not tubes made 30 or more years ago.
I used these tubes in some of my commercial SE amps and they were not as reliable as I would have hoped. In a couple of cases they failed at pretty low hours and were apparently gassy. These were operating at 400V with fixed bias at 60 - 80mA of plate current depending on the individual client. This was over 10yrs ago now so should be taken as anecdotal.
Not being as well healed as some of my customers my amps got JJ 300B instead and these have lasted over 5000hrs under the same conditions of use and are still going strong today. (10yr old tubes now)
You'll need to get your hands on another set of tubes in order to figure out what is going on with these amps. FWIW I'd certainly not recommend running beyond 80mA of plate current with 400V plate to filament.
I used these tubes in some of my commercial SE amps and they were not as reliable as I would have hoped. In a couple of cases they failed at pretty low hours and were apparently gassy. These were operating at 400V with fixed bias at 60 - 80mA of plate current depending on the individual client. This was over 10yrs ago now so should be taken as anecdotal.
Not being as well healed as some of my customers my amps got JJ 300B instead and these have lasted over 5000hrs under the same conditions of use and are still going strong today. (10yr old tubes now)
You'll need to get your hands on another set of tubes in order to figure out what is going on with these amps. FWIW I'd certainly not recommend running beyond 80mA of plate current with 400V plate to filament.
Last edited:
Hi All and thanks for the continuing suggestions.
I still found it difficult to conedemn the 300B's especially as they looked good on the curve tracer.
Here's an interesting tale as to how a variac can be a double edged sword.
Winding the mains volts up and monitoring the various points with a scope usually avoids nasty disasters, but in this case came up with a red herring.
By not applying any signal I gingerly wound up the mains volts, expecting the valves to suddenly gulp lots of current as the gas ionised.....but it didn't happen. Then add signal and all was well.
Go back to 160/170 volts and wind the input signal up and the sudden jump up of grid volts resumed, it was completely predictable and repeatable.
Turns out this peculiar phenomenon must be down to the low filament power. The fils are DC fed so even though the mains volts are reasonably high, the fils are barely running (the input valves are perfectly happy) so this gassy effect is able to operate, but as the volts go up the fils start to work properly and the valves conduct correctly.
I've never had this happen with indirectly heated valves so it's a 'funny' of direct heated valves.
One to remember!
Cheers all.
Henry
I still found it difficult to conedemn the 300B's especially as they looked good on the curve tracer.
Here's an interesting tale as to how a variac can be a double edged sword.
Winding the mains volts up and monitoring the various points with a scope usually avoids nasty disasters, but in this case came up with a red herring.
By not applying any signal I gingerly wound up the mains volts, expecting the valves to suddenly gulp lots of current as the gas ionised.....but it didn't happen. Then add signal and all was well.
Go back to 160/170 volts and wind the input signal up and the sudden jump up of grid volts resumed, it was completely predictable and repeatable.
Turns out this peculiar phenomenon must be down to the low filament power. The fils are DC fed so even though the mains volts are reasonably high, the fils are barely running (the input valves are perfectly happy) so this gassy effect is able to operate, but as the volts go up the fils start to work properly and the valves conduct correctly.
I've never had this happen with indirectly heated valves so it's a 'funny' of direct heated valves.
One to remember!
Cheers all.
Henry
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.