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This is my father’s amp. I’m not sure if he built it in the 30s or 40s. I haven’t been able to find a data sheet for the power tubes (6B5). What I know is they are dual triodes with the cathode of the first direct coupled to the grid of the second. This makes them self biasing, although he used a bias resistor.
The preamp and PI tubes (6SC7) have a unipotential cathode. That is a new term to me I don’t know if it’s one cathode with a plate and grid on either side or two cathodes connected internally.
First I replaced the filter caps in the PS and brought the B+ voltage up from 238 to 480.
I traced the schematic and started with a plan to just replace some caps, but found a lot of resistors out of tolerance. The 50K resistor in the PS, measures 15K. That might explain the 282V on one of the plates in the PI, which exceeds the 250V max. The 105V on the other I assume is due to a leaky coupling cap. I was about to replace the coupling caps when I came to my senses. With this point to point (rats nest) wiring changing one part at a time was pointless and I would have to gut the whole thing and start from scratch.
The primary on the OT is 8600ohms. The possibility of using the chassis and transformers for a Princeton reverb clone is a possibility for the future. Although the voltage on the PT is a little high for a PR. For now I want to work with the original tubes and transformers.
My questions are,
Has anyone seen a tone circuit like this? It works well, but has the limitation of only 4 positions. I’m thinking of perhaps a tone stack between the first and second preamp stages when I’m done.
The choke seems to be in an odd place. Could it be that the choke along with the 16uf caps and 52K resistors form a 120HZ filter? Or should I move the choke to a conventional location in the circuit.
The amp sounded pretty good with guitar before and after the caps were replaced. A lot cleaner than I remember it when I last used it in the 90s. The last thing I did was plug in the old Turner mic. That didn’t work but it might be the mic. It hasn’t been used since the 70s.
I’m open to any suggestions, comments, or smart ass remarks. And thank you in advance for your help.
The preamp and PI tubes (6SC7) have a unipotential cathode. That is a new term to me I don’t know if it’s one cathode with a plate and grid on either side or two cathodes connected internally.
First I replaced the filter caps in the PS and brought the B+ voltage up from 238 to 480.
I traced the schematic and started with a plan to just replace some caps, but found a lot of resistors out of tolerance. The 50K resistor in the PS, measures 15K. That might explain the 282V on one of the plates in the PI, which exceeds the 250V max. The 105V on the other I assume is due to a leaky coupling cap. I was about to replace the coupling caps when I came to my senses. With this point to point (rats nest) wiring changing one part at a time was pointless and I would have to gut the whole thing and start from scratch.
The primary on the OT is 8600ohms. The possibility of using the chassis and transformers for a Princeton reverb clone is a possibility for the future. Although the voltage on the PT is a little high for a PR. For now I want to work with the original tubes and transformers.
My questions are,
Has anyone seen a tone circuit like this? It works well, but has the limitation of only 4 positions. I’m thinking of perhaps a tone stack between the first and second preamp stages when I’m done.
The choke seems to be in an odd place. Could it be that the choke along with the 16uf caps and 52K resistors form a 120HZ filter? Or should I move the choke to a conventional location in the circuit.
The amp sounded pretty good with guitar before and after the caps were replaced. A lot cleaner than I remember it when I last used it in the 90s. The last thing I did was plug in the old Turner mic. That didn’t work but it might be the mic. It hasn’t been used since the 70s.
I’m open to any suggestions, comments, or smart ass remarks. And thank you in advance for your help.
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