Help with Parmeko AP32047 2 Watt Tube Amplifier - made for Navy ship intercom

[Maverick616]

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.

 

[ruffrecords]

The plate resistors of the 12AX7 look OK to me. Fairly typical values but the first cathode resistor is rather high. This will cause some local NFB which will limit the gain of this stage to about 5 times u it will also raise the input impedance, presumably to minimise the loading on the 40:1 input transformer. For a line input you can probably go straight into the 1M pot.

The output transformer is probably designed to work at what used to be called 70V line or 100V line. It is basically a high voltage drive designed for use with long cables with minimal loss. It is still in use today for output PA circuits for example.

https://www.doctorproaudio.com/content.php?2662-constant-voltage-70v-100v

I have no experience of these but the above article may help you understand them better.

Cheers

Ian

 

[Maverick616]

Thanks Ian, all really useful info here! I've had a read through that constant voltage page and it does indeed seem like the transformer on it won't be appropriate for much else. It's got me wondering if I could remove V2 from the circuit and run V1 straight into something like a 4:1 Carnhill, then get the CFB from the secondary winding (with a resistor)?

Thanks,
M

 

[ruffrecords]

Thanks Ian, all really useful info here! I've had a read through that constant voltage page and it does indeed seem like the transformer on it won't be appropriate for much else. It's got me wondering if I could remove V2 from the circuit and run V1 straight into something like a 4:1 Carnhill, then get the CFB from the secondary winding (with a resistor)?

Thanks,
M

12AX7 most definitely does not have the cajones required to drive a transformer.

The 6BW7 is a meaty beam tetrode. I first used one in a (cough) pirate radio transmitter back in the 60s. You could reconfigure it as a cathode follower and then it could drive a 4:1 Carnhill with ease.

You might like to take a look at the Motwown DI box thread elsewhere on groupDIY because you have all the elements there that you would need to build one of those.

Cheers

Ian

 

[Maverick616]

Good shout on the motown preamp, it looks like a great reference and I'll likely build that if I can't get much use out of the current topology. Also really good to know the tetrode will be suitable for driving a 4:1, I think I have an ungapped one laying around somewhere.

Say I wanted to retain most of the design around V1 (for curiosity mainly), I still need to solve the cathode feedback on the second stage. Please correct me if this isn't right but could we just use an appropriate cathode resistor to ground instead, and forget about the transformer feedback winding? I understand that cathode followers have excellent local feedback, so maybe it won't be too unstable?

Cheers,
M

edit: the Motown thread was an informative read but I didn't spot that the initial suggested circuit with the CF tetrode is unverified. It's lead me down the rabbithole of Pultec-style cathode follower output stages though which seems appropriate - lots of reading to do.

 

[Maverick616]

Here's a few photos as promised, ignore the garage carpet haha. Annoyingly I haven't had much time to play with this yet but I've got some new tubes ordered as both had failed vacuums, and I've started building replacement rectifiers for the seleniums.

It weighs an absolute tonne. Everything is kind of tetris'd in place so I'll need to strip most of it down to get the output transformer out.


Surprisingly takes standard sized UK mains fuses (holder removed).



Still not too sure how to approach the feedback in the second stage. Thinking of maybe grounding the cathode through an appropriate resistor (and bypass cap?) and the VTB2290 on the output as a starting point unless anyone has any other suggestions. How common were feedback windings on OPTs in amps of this era? I wonder if isolating the feedback had some benefit with it being part of a huge system.