Silvertone 1334 Setting Bias

[DrGonz78]

So I got this 1954 Silvertone 1334 amp and I am trying to understand how the amp sets the bias for the pair of 6L6G tubes. I don't have the 1334 schematic but I am pretty sure the 1335 is almost identical. So I measure 333v on one plate and 329v on the other one of the 6L6G sockets. Measuring resistance from CT of the OT to the each primary plate connection I get 212.9 ohms and 232.5 ohms. I get a respective voltage drop of 12.6v and 16.5v. Which if correct measures out to be 12.6 / 212.9 = 59ma and 16.5 / 232.5 = 70ma (really 71). So it would seem that the 59ma tube is pulling 103% (19.6 watts) dissipation and the 71ma tube is idling at 124% (23 watts) maximum dissipation. So the bias seems pretty hot right now as these tubes are rated at 19 watts. Note that I have installed a 3 prong power cable and the four main filter caps have been installed too.

Looking at the schematic I see 19v at the cathode which goes to ground but first is connected between two 12ax7 heaters. Still can't make sense of what type of bias this amp uses and I am sure there is a good explanation. Then there is the question of how to adjust the bias on the amp. Any help would be greatly appreciated thanks!

[DrGonz78]

Okay actually am I starting to understand better what I am looking at. The 12ax7 must be wired in parallel and that is the voltage you see at the cathodes on the power tubes. So it is cathode bias through the heaters of the tubes. I am going to have to think this out again. Any pointers would be helpful too thanks.

[Enzo]

Cathode bias. Your choices are fixed bias or cathode bias. There is no bias supply here, so it cannot be fixed. The fact they wired a tube heater in the cathode circuit is irrelevant. The cathode gets its voltage from the current through the cathode circuit.

The tube heaters need about 150ma, or about 75ma per power tube. Your numbers are not far off that./

To get the dissipation of the tube you need the current through it and the voltage across the tube. Not just plate voltage to ground. Much of the time our cathodes are grounded so they are the same. But here the schematic says 19v, so subtract that from the plate voltage.

333-19=314 But how much voltage do you actually have on your cathodes? Use that figure.

Those tube heaters? If the power tubes were going to have the current flowing anyway, might as well put it to use heating the tubes. So they did.

High dissipations like that are consistent with Class A operation.

[Enzo]

Your drawing shows the heaters wired pin 4 to 5. So they are on 12v power. Looks like two tubes in series. so we need 20-24v for them. Close to the 19v on the drawing.

[DrGonz78]

Your drawing shows the heaters wired pin 4 to 5. So they are on 12v power. Looks like two tubes in series. so we need 20-24v for them. Close to the 19v on the drawing.

I meant to type the heaters are wired in series but instead mistyped parallel. I will measure cathode voltages tomorrow night to check on that too. Thanks Enzo for the great explanation. So by designing the amp this way the cathode voltage would be DC and thus the 12ax7's heaters are powered by the cathode circuit, so less chance of hum on those tubes right? That is pretty cool I do declare. I had looked at this same circuit in the back of my Jack Darr book a while ago. I remember scratching my head even back then looking at those heaters in series from the cathodes to ground. I think the version in the Jack Darr book used a 6AU6 instead of the 6SJ7 tube for the tremolo section. Just looked it up and it was the Wards Airline Model GDR-8516A.

[J M Fahey]

These were most inexpensive amps so every cent counts.
They saved on one big wirewound 6L6 cathode resistor and some 12AX7 filament current so thinner wire ... not bad.

One buck saved on a Million amps means: