Garnet Gnome G15T Bias

[Aerostoon]

I am working on checking the bias on my late 70's Garnet Gnome G15T. I have attached an image of my reading. The plate dissipation seems very low for a single 6L6GC. The max wattage for the 6L6GC is 30. Would 25 watts be a good target?

[Aerostoon]

Here is the schematic.

[nickb]

This amp is self-biased by the the 470 ohms and doesn't need to be adjusted. Your 6L6 will last forever. If you were to set it higher you risk saturating the output transformer and overstressing the power supply.

[Justin Thomas]

While it certainly wouldn't kill a 6L6GC, you COULD go higher. But if he amp SOUNDS fine, AND you like it, why needlessly kill and cook the power tube? Maybe you can actually afford a really nice NOS 6L6GC and let it last for YEARS or maybe DECADES in YOUR amp, instead of needlessly killing valuable tubes? Or, you could probably just plug a 6V6 in that amp... but no, there is no reason to intentionally kill tubes if it sounds good. It might hum more if you bias it hotter, too...

The reason so many Ampegs still run and sound great with the original cathode-biased 7591As is because Ampeg didn't insist on running them at their utter limits. Nowhere near it, actually... I for one am glad that all my original 7591As are still going.

Justin

[Aerostoon]

Thanks guys. I had to fix up some of the wiring and the components to return it back to the factory circuit. Someone had modified it to add the unused gain stage of the tremolo 12AX7. To me the amp sounded very bright and I didn't think the mods had improved the sound. I used the wiring schematic and I had a completely original amp chassis to compare mine to.

[Aerostoon]

[Aerostoon]

Do my calculations seem correct:
Bias Resister Measured Value 494.8 Ohms
Voltage reading across cathode resister 23.14 VDC
Plate current calculation 23.14 divided by 494.8 equals 0.0467 Amps (PC)
Plate voltage pin 3 to ground is 293.4VDC (PV)
Watts equals PV X PC
293.4 times 0.0467 equals 13.7 Watts (Plate dissipation).

[Justin Thomas]

Nothing wrong with the math, but to be entirely technically correct, you'd subtract the cathode voltage from the plate voltage, THEN multiply the remainder times the current. But I think it ends up being about 12.5W instead. Not really enough watts to make a difference. And, how do you like the sound of the amp? If it ain't broke, don't fix it.

And is that a Peavey T-60? Or T-something...

Justin

[Aerostoon]

Thanks Justin! I haven't tried the amp yet. I had it connected to my dummy load. I will connect the speaker and try it out.
Yes, that is a T-60 I used to own. My current T-60 is the black poplar bodied one. Much lighter and the newer bodies were contour cut so I find them very comfortable to play. Now to reassemble the amp

[mac dillard]

FWIW: All of the single ended amps I have built and looked at were biased around 90% of Pmax. Some of the Fenders at 100%. Your amp is biased really cold for a 6l6 tube in single end operation. Put a 6V6 in it and see what it sounds like.. Betting you will like it. I'm guessing that if that circuit was designed for a 6L6 tube the cathode resistor would be around 250 ohm.

[Aerostoon]

I agree, the voltages and resistor value seems suited to a 6V6 instead of a 6L6. Is there any risk of trying a 6V6 on there? I'll check the bias for that tube as well.

[mac dillard]

With an anode voltage of about 300 vdc and a cathode resistor close to 500 ohms I think you will still be biased on the cold side for the 6V6. I would expected a cathode resistor of around 450 ohms. You should not have a problem with a 6V6. Probably should check the screen voltage.

[nickb]

I agree, the voltages and resistor value seems suited to a 6V6 instead of a 6L6. Is there any risk of trying a 6V6 on there? I'll check the bias for that tube as well.

You can certainly drop an 6V6 in there without changing anything. It'll run Pd right around 9W. No problem at all and still quite conservative. As designed for a 6L6 I estimate the plate load impedance around 5.5K ohms for max power out of 5.3W or so. With a 6V6 you'll drop to about 3.4W and that may suit you well - it depends how loud you need. The whole amp will run cooler due to lower dissipation.

If you drop the 6V6 cathode resistor to 270 ohms 2W then you could get back to the original power without stressing anything else above the original. But what would be the point as you've now bought a 6V6 that won't last nearly so long as the original design did with the free 6L6?

Anyway it's fun to play.