Maverick616
New member
- Joined
- Apr 25, 2018
- Messages
- 4
Hi all, I’m soon to be in possession of an obscure 2watt tube amp, model AP32047, designed by Parmeko in England and dated early 1950s. This would've been used as part of a Navy ship's intercom which is cool in its own right. I’ve luckily managed to find a document containing the schematic and OPT spec - a big thanks to Comms Museum for allowing me to share here. My goal with this project is to turn this into a line(ish) level preamp/saturation box - I'm not after crystal clear performance here, but something useable. I'll add some pics of the unit later in the week. Here's the schematic & OPT:
A couple of initial observations:
-Input transformer is 1:40 25ohm pri.
-Output transformer is 3.53:1 400ohm sec.
-Has 'line' and mic inputs
-Output designed to drive a constant voltage loudspeaker at 31.6v according to the document.
-CFB is sourced from a tap on the OPT.
-V1 is a 12AX7, and V2 is a 6BW6. V1's plate resistors look rather high but this seems fairly normal for amps of this era?
I’m currently at a bit of a loss with how I can adapt the circuit to output at line level given that the OPT is designed to drive a loudspeaker. I suppose the main question here is: could the OPT be configured for a line level output, maybe just coupled to stage 2 of V1? assuming we have to replace the OPT, what can we do about the CFB? I've thought about padding and loading the OPT but I don't know how this would work with the high voltage output.
What are people’s thoughts on the topology in general? To my inexperienced eyes, it looks like a cheap design that was sufficient for its use but going to struggle outside of the OPT's listed frequency response. Regardless, I’m very keen to do what I can to get a 'useable' sound out of this. Please throw anything at me, I’m hoping this can be educational as tube design is something I’ve wanted to properly dive into but had little opportunity until now.
A couple of initial observations:
-Input transformer is 1:40 25ohm pri.
-Output transformer is 3.53:1 400ohm sec.
-Has 'line' and mic inputs
-Output designed to drive a constant voltage loudspeaker at 31.6v according to the document.
-CFB is sourced from a tap on the OPT.
-V1 is a 12AX7, and V2 is a 6BW6. V1's plate resistors look rather high but this seems fairly normal for amps of this era?
I’m currently at a bit of a loss with how I can adapt the circuit to output at line level given that the OPT is designed to drive a loudspeaker. I suppose the main question here is: could the OPT be configured for a line level output, maybe just coupled to stage 2 of V1? assuming we have to replace the OPT, what can we do about the CFB? I've thought about padding and loading the OPT but I don't know how this would work with the high voltage output.
What are people’s thoughts on the topology in general? To my inexperienced eyes, it looks like a cheap design that was sufficient for its use but going to struggle outside of the OPT's listed frequency response. Regardless, I’m very keen to do what I can to get a 'useable' sound out of this. Please throw anything at me, I’m hoping this can be educational as tube design is something I’ve wanted to properly dive into but had little opportunity until now.
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