Hi All
I have come to an impasse in my work on this and am reaching out for suggestions on ways forward. Long intro with hopefully enough to start a conversation:
This is an F-2 Rock-Ola amplifier from a 1939 jukebox. It had already been restored to working order by the previous owner replacing all caps and resistors, leaving just the original field coil speaker and the transformers. It has a single 76 triode driving 2 5881 output tubes in PP cathode bias via an interstage transformer PI. It has a .0001 cap and a 50K resistor across the output plates which as far as I can tell is aimed at reducing hum from the PI transformer. The PI tx is situated as far from the PT as possible in the chassis. There are 3 choices of secondary tap for the OT which I intend to put on a switch for adding a second permanent magnet speaker.
The original tone controls were removed which were actually some different capacitors on an off/on/on switch for both bass (Rumble) and treble (scratch) designed for making 78 records sound a bit better as they became worn. I tested this as acquired by feeding it with Spotify from my laptop through a stereo-to-mono box a la Skip Simmons design (just a couple of 82K resistors on the hot leads of the stereo lead into a "1/4 jack). It worked pretty well, loud enough for a living room at full volume.
My idea is to add a one-tube channel and make something akin to a Gibson EH-150 for guitar input. The input for the cartridge is the same size as an octal tube so I elected to go with a 6SL7 dual triode (equivalent to 5751) wired like the preamp of a 5F2A with vol and tone. Looking at amps like the 5C3, I think this should be enough to push the output stage, albeit fairly gently.
I only added bypass cap 25uF/50vat the first stage for now, from my reading, the 100K load resistors and 1.5k cathode resistors should work well with the 6SL7. Voltages all look ok across the amp - kind of on the low side for later fenders, but I think earlier tweed ran fairly low voltages.
Having tacked it all together, I get sound through the speaker, but at VERY low volume. A lot lower than the single 76 triode stage. I've triple-checked my wiring and can not find a mistake so I'm at a bit of a loss. The soldering is good - a bit lightweight as I am still fiddling with layout and lead dress, but all tests good for continuity. Am I just barking up the wrong tree here? There's no real originality value left in the amp so I'm quite happy to consider any circuit changes, including converting to a tube PI but I can't see why it wont work as is. I can only conlude there must be a dumb error in there somewhere.
Some pictures to look at. My schematic includes a possible NFB loop which I haven't implemented yet. I am also adding grounded cord and fuse, currently running off my variac at 110vac.
My Schematic
Voltage chart (need to confirm B+)
Output stage
76 Triode stage
overview
6SL7 stage
Top view
I have come to an impasse in my work on this and am reaching out for suggestions on ways forward. Long intro with hopefully enough to start a conversation:
This is an F-2 Rock-Ola amplifier from a 1939 jukebox. It had already been restored to working order by the previous owner replacing all caps and resistors, leaving just the original field coil speaker and the transformers. It has a single 76 triode driving 2 5881 output tubes in PP cathode bias via an interstage transformer PI. It has a .0001 cap and a 50K resistor across the output plates which as far as I can tell is aimed at reducing hum from the PI transformer. The PI tx is situated as far from the PT as possible in the chassis. There are 3 choices of secondary tap for the OT which I intend to put on a switch for adding a second permanent magnet speaker.
The original tone controls were removed which were actually some different capacitors on an off/on/on switch for both bass (Rumble) and treble (scratch) designed for making 78 records sound a bit better as they became worn. I tested this as acquired by feeding it with Spotify from my laptop through a stereo-to-mono box a la Skip Simmons design (just a couple of 82K resistors on the hot leads of the stereo lead into a "1/4 jack). It worked pretty well, loud enough for a living room at full volume.
My idea is to add a one-tube channel and make something akin to a Gibson EH-150 for guitar input. The input for the cartridge is the same size as an octal tube so I elected to go with a 6SL7 dual triode (equivalent to 5751) wired like the preamp of a 5F2A with vol and tone. Looking at amps like the 5C3, I think this should be enough to push the output stage, albeit fairly gently.
I only added bypass cap 25uF/50vat the first stage for now, from my reading, the 100K load resistors and 1.5k cathode resistors should work well with the 6SL7. Voltages all look ok across the amp - kind of on the low side for later fenders, but I think earlier tweed ran fairly low voltages.
Having tacked it all together, I get sound through the speaker, but at VERY low volume. A lot lower than the single 76 triode stage. I've triple-checked my wiring and can not find a mistake so I'm at a bit of a loss. The soldering is good - a bit lightweight as I am still fiddling with layout and lead dress, but all tests good for continuity. Am I just barking up the wrong tree here? There's no real originality value left in the amp so I'm quite happy to consider any circuit changes, including converting to a tube PI but I can't see why it wont work as is. I can only conlude there must be a dumb error in there somewhere.
Some pictures to look at. My schematic includes a possible NFB loop which I haven't implemented yet. I am also adding grounded cord and fuse, currently running off my variac at 110vac.
My Schematic
Voltage chart (need to confirm B+)
Output stage
76 Triode stage
overview
6SL7 stage
Top view
