Question about a Sear Silvertone

[pula58]

What is going-on at the output tube cathodes here in this schematic?

Looks like a 39 ohm Resistor to bias the 6L6 cathodes, but, wait, what's the 220ohm resistor and the connection to the 12AX7 heaters at one end of the 39 ohm resistor? At first I thought that possibly the heater output center tap is being elevated (for hum reduction), but, there is an artificial center tap (using two 68 ohm resistors) of the power trannies heater output(see schematic, lower right hand corner).

http://www.el34world.com/charts/Schemat ... ne1474.pdf

[Phil_S]

The 220z is in series with the 39z, making 269z as the bias resistor. The use of two resistors provides a voltage divider where the designer determined to elevate the heater circuit ground.

The heaters, lower right, have a faux CT made with a pair of 68z. I suppose the 39z is somehow makes up the difference from the typical 100z pair.

I think we see 220+39 because that's as close as we get to the targeted 270 with common values and still provide a little extra for the heaters. I suppose 250+22 would have worked just as well, but maybe, back in the day, finding a 250 wasn't so easy? I don't know.

[Firestorm]

It sure looks like what the schematic is trying to show is the first two 12AX7s running in series off the 27VDC from the 6L6 cathodes divided down to 24V.

[Alexo]

yes... Andy Le Blanc posted about this a while back... the AX7 heaters are being used as a sort of constant current source (looks like only 12V6 on top of two triodes in series). You've got approx 150ma flowing through the power tubes' plates, into their cathodes and then through the AX7, powering its heater circuit.

Tried this in one amp - worked OK, and was nice to have a free DC heater supply, but that one AX7 glowed meekly at idle and bright under signal as more current passed through it. Very unsettling! so I took it out...

[LOUDthud]

The 12AX7 heaters are used as a cathode bias resistor for the 6L6s. The 220 ohm and 39 ohm just adjust the bias slightly. A 12AX7 wants 12.6V at 150mA, a little Ohm's law gives you 84 ohms. So the effective cathode resistor is 39 plus the parallel combination of 168 and 220 which is 134 ohms. Note that the schematic specifies 6L6-G. That's an old version of the tube that is only good for about 30W per pair.

[ampdoc1]

Long time since I've looked at that. My first amp repair!!! A friend gave it to me...wasps nests inside, that "pressed-board" cabinet, and output trannys each about half the size of a cigarette pack.

But it sounded remarkably good for 70's rock, except for the tiny reverb unit.

[Prairie Dawg]

That was a feature of a lot of Danelectro made amps. I just finished resurrecting a Silvertone Two-Twelve. The first preamp tube was mounted on a slab of phenolic in the upper chassis and it had been used as a latrine by mice. Every last one of the green Sangamo capacitors in the amp had to be replaced, but did it ever sound good when it was done.