Gibson GA-15 bias

[xtian]

Check my math, please. I have a Gibson GA-15 (no reverb or trem) with two EL84. I have 360v on the plates and 12.6v across the 150R cathode resistor (one resistor for the pair of power tubes).

AX84 bias calc suggests my 70% target is 23mA per tube.

12.6v / 150R = 0.084 / two tubes = 0.042mA per tube?!?!

Am I off by a factor of two?

BTW, the power tube sockets are both melted, suggesting the tubes were running *ahem* a little hot.

But the original cathode resistor spec'd by the schematic is 120R. So I'm suspecting my calculations.

[surfsup]

I think you are correct aside from some screen current (5-7mA per tube?) so it would really be maybe 37mA?

I have a 2xEL84 with 335 on the plates. 11.5 on the cathode 125R

this is about 38mA per tube, similar to yours and mine is fine. Your sockets are burned prob for another reason?

[xtian]

BTW, amp sounds good. VERY good. Tubes did not immediately red plate during about 60 seconds of playing at top volume, but they are radiating a lot of heat.

[Tillydog]

I have 360v on the plates and 12.6v across the 150R cathode resistor (one resistor for the pair of power tubes).

AX84 bias calc suggests my 70% target is 23mA per tube.

12.6v / 150R = 0.084 / two tubes = 0.042mA per tube?!?!

Calcs look fine to me - smokin'

[martin manning]

For a cathode biased amp ignoring the screen current you could stand to be 85-95% Pa Max if the load impedance is high enough, but you are >120%. Is it possible that your supply is just too high running on modern line voltage?

[ampfab]

Sounds similar to how matchless runs Their tubes. I'd let her rip. I know from my experimenting, they sound best that way.

[ampfab]

Matchless runs them even hotter, with a 120 ohm cath resistor.

[xtian]

Good info, guys. Martin, this is a modern, 90s GA-15, with PCB by Trace Elliot. Previous unknown tech had already replaced stock 120R cathode resistor with 150R.

[martin manning]

This seems like poor design practice to me. At least it's not melting the sockets anymore...

[xtian]

New ceramic sockets arrived today, so I swapped out the melted ones. Put in a new set of JJ EL84 tubes and turned it on. Sounded OK until I turned it up loud and hit a power chord, and BUZZTRANK--that's the sound of one of the EL84s arcing. I could see it happening. I only tried once more, to be sure.

Could be a bad power tube; I'll try swapping them and see if the same one arcs.

But could this be the high bias, as reported earlier in this thread?

[xtian]

I replaced the 150R power tube cathode resistor with a 250R, and this set the idle current at 29mA (366v plate voltage), or a little above 70% max dissipation at idle. Cranked it and hit some power chords, and no arcing.

Will stress test more tomorrow when house is empty!