Bad 6L6GC tube (short between grid and cathode ?)

[saint gillis]

Hello, I've bought 4 matched 6L6WXT Sowtek tubes, new, for a Fender Bassmann 100.
I've replaced old caps, HV resistors, cleaned, really renewed everything, so the circuit and wiring has been triple checked and is reliable.

One of the 6L6s makes the bias voltage (~ -55V) drop to about -1,2V, even when the amp is in bypass so there's no high voltage on the plate.

Is it possible to get a bad new tube like that? has anyone ever experienced that?

 

[mjrippe]

Have you tried it with other tubes? Or moved the tubes to different positions? Yes, it is possible to get a bad new tube, but on an amp that has just been re-worked I always test with a known good set first.

 

[Tubetec]

Definately dont switch the amp out of standby with the grid volts so low , it will draw huge plate current and you could blow a HT fuse or damage the transformer .
You can try it with a pair good tubes just to make sure all is well with the amp , Id say try getting them to replace one of the pairs , often in this case where a tube is defective they'll just send you out a new pair . You could also show a pic of how the bad tube drops the bias voltage on the multimeter , or in the case of a dead short between electrodes a resistance measurement should suffice .

 

[CJ]

pull the suspect and check pins 5 to 8 with ohm meter.

and/or use regulated power supply to apply volts across 5-8 and see if the ammeter pegs.

is the tube oriented correctly in the socket? of course it is.
if you were skilled enough to do a cap job then you are not going to make that mistake.

yes new tubes are a fright, had a brand new JJ go south, right out of a matched quad set.

then there was that time when JJ shipped a huge batch with undersized pins, can you say shake rattle and blow?

 

[saint gillis]

Yes I have tried different combinations as it is a matched quartet. And it was always the same tube causing the trouble whatever the socket it was plugged in.
A strange thing is that it can work correctly for a long period of time (like 1 hour), then you switch off the amp, and turn it on again like after 3 minutes, and the same tube will cause the problem again..

 

[CJ]

what is your plate voltage?

 

[saint gillis]

Something like 440VDC IIRC

 

[Gus]

Have you checked the output tube grid coupling cap(s) leakage at working voltage?

 

[CJ]

28 watts plate disp (derate from 30 due to NOS being stronger) x .65 = 18 watts

18/440 = 41 ma

set bias to 41 ma, xfmr shunt method 82 per side, be careful! use clip leads on ammeter, place meter from B+ red to blu, then red brown

or shunt stand by sw, set bias to 164 ma

look for mismatch just for the heck of it

note down drop in B+ due to current demand

 

[saint gillis]

I had the same problem even with very cold bias (30mA)
Now the tubes are on the way back to the supplier.

 

[saint gillis]

Have you checked the output tube grid coupling cap(s) leakage at working voltage?

Yes