Ruby 6L6GCM-STR bias

[mresistor]

Hi all..  I've got a blackface showman amp here and I installed a new matched quad of Ruby 6L6GCM-STR tubes in it. Normally in this amp after checking and setting the bias for 60% of max plate dissipation, the bias voltage will be around -52 to -50 volts. But with this quad after setting bias the bias voltage is -45.


Plate voltage 436 / plate current 41 milliamps -- shooting for 17-18 watts or 60% of max per tube.


Should I be concerned about this set of tubes? This doesn't seem normal. I installed an old Fender set made in the USA and they bias up normally ( to me ) and the bias voltage is around -50, and these are old used tubes. It just seems odd that these 6L6GCM-STR tubes bias like this.


Also this amp runs HOT!  With the tubes so close to the PT I guess this is the way they are, but dang it gets hot. After about 40 minutes of playing the PT is pretty hot to the touch. I can put my hand on it for a least 5 seconds though. So it's not scalding hot, but I bet when the chassis is in the cab it gets pretty toasty .

   

[dude]

I have had voltages on the plates vary as much as 20 vdc with different matched tubes, so the milliamp settings will vary a lot too. I think it's normal and those 6L6's want to see that voltage. You can always do the math for dissipation.


As far as the heat, no red plating, should be ok. I too have had tubes biased at 70% dissipation and seemed excessive heat was coming off the tubes but no red plating. Someone else will chime in... I'd be interested in what they say.


al   

[PRR]

Grid voltage at low currents varies a lot from tube to tube. 20% maybe. So "normal" say 51V could be 41V or 61V. A specific tube biasing "happy" at 45V is not alarming. That's why (some) amps have bias trims.

I suspect a Showman "could" be biased quite cool and still do what it was made for (play loud). Try setting to -50V and see if very soft sounds get hoarse (and if you would ever play a Showman that soft). It will idle cooler, which is not essential, but nice for longevity.

[mresistor]

Thank you     I will try it.. 

[mresistor]

I tried these tubes in a Super Reverb..  and to get them biased up to near 18w I have to adjust the bias pot to the limit of one end (least negative). I've never seen 6L6GCs that failed to bias within the adjustment range of the bias adjust pot on a vintage Fender amp.