Danelectro Twin Twelve

[Zee]

SO how does it compare to the Twin 15 schematic at Schematic heaven?

http://www.webphix.com/schematic%20h...win15_1961.pdf

They seem to be pretty close. The Twin 15 doesn't have reverb, the power supply and power tube sections are different, but, other than that, it looks to be the same.

Hey, I finally got attachments to work. Not sure what changed, but here's my version.

[Bruce / Mission Amps]

Zee, where's the bias supply in that schematic? I see the +25v in the other schems. Is it trying to use the 12AX7 filaments as DC bias?

[Zee]

Is it trying to use the 12AX7 filaments as DC bias?

From what I've read it's an old trick used to cut costs and possibly reduce hum. There are two or three Danelectro/Silvertone amps that do this. My formal education was a long time ago and my vacuum tube experience is fairly limited, so I cannot comment on any inherent benefits or drawbacks.

[52 Bill]

It looks like the 12AX7 filament is being powered by the dc bias voltage of the output tubes.

The question that I have, is the value of the cathode bypass cap on the output tubes correct? I can't believe that they would use a 450 volt cap there.

[Zee]

The question that I have, is the value of the cathode bypass cap on the output tubes correct? I can't believe that they would use a 450 volt cap there.

I looked at the original print and it lists the value, but not the voltage. I couldn't tell from my pictures, but I will double check that. I believe it is correct, though. I have to open it up again anyway to check one of the resistor values.

I was doing this slowly and carefully, but I certainly could have made a mistake. The original schematic isn't 100% accurate, either. For example, the choke across the speakers is missing.

There are a couple caps listed on the original as 25 volts, whereas mine are 50 volts. No big deal there. In the reverb circuit the original shows two 2.2 K resistors and I have a 2.2K and a 4.7 K. Maybe that was an assembly error.

In any case, I compared my schematic to the original line for line and noted what was different. When I get a chance, I'll double check my drawing and make corrections as needed.

[jrfrond]

It looks like the 12AX7 filament is being powered by the dc bias voltage of the output tubes.

It sure is! Interesting arrangement in that a failed 12AX7 heater could disable the power tubes. Are they saving money on the PT heater winding? Or just trying to avoid using a big resistor? Or both?

I really question the integrity and reliability of this. If it were my amp, I'd put the heaters where they belonged. Then install a proper cathode resistor and bypass cap.

[Prairie Dawg]

That was something from the mind of Nat Daniel and Danelectro did it on a lot of their amps from the late forties and early fifties. My Sherwood 84-22 a/k/a Silvertone 1303 a/k/a Danelectro Vibravox a/k/a Noble etc etc etc uses the filament of the first preamp (12SJ7) and a 40 ohm resistor to do the job of biasing a pair of 6L6GCs. The 12SJ7 tends to run at a higher filament voltage which makes it kind of gainy and out of character for an amp built in the late 1940s. I've got a Pioneer 25 in a state of dishabille, so to speak, that uses a pot in there as well. It takes quite a while for the amp to become fully operational but once it does, it's very pleasant and a good performer. Nat was never known for a conventional approach to anything I do not think. I woulda liked to have bought him a few beers and listened to his stories.