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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Any advice on modifying a 2A3 SET amp to parafeed?


2008-02-21 9:04 pm
Hello all,

I've finally settled on a design to convert to parafeed:


This is a nice design as is, but I just can't resist the urge to play with it! What I'm planning to do is use a Hammond 125ESE for the output, a Hammond 193C (20H, 100mA, 155 ohms DCR) for the plate choke, and a 2uf series capacitor.

What do you guys think?

And BTW, thanks for all the advice I've gotten from members of this forum over the past few days...

What I'm planning to do is use a Hammond 125ESE for the output,

Is the Hammond 125ESE with airgap? You really don't need an OPT with an airgap on a parafeed circuit.

I am about to embark on a parafeed 2a3 project, with a 12at7 driver, and am planning to use a Talema mains toroidal transformer for OPT. (I built a 6080 parafeed with a Talema mains toroidal transformer for OPT and it sounds excellent).

Try a 230V primary, 9V secondary toroidal transformer for your parafeed OPT.

Please keep us posted on your project, am starting mine towards end of the month (March 2008).
Hi Zman

Have you measured the inductance of this choke over the audio frequency range (and beyound that)? I am asking because it is meant as a power supply choke and therefore optimized for the mains frequency, possibly presenting worse inductance in the audio frequencies, and therefore bad overall performance of the amp.

You can try it and report results. If it does not work you can always use the choke in a power supply, and go for the One electron Retard chokes, which are build for parafeed operation of SE amplifiers. They are available from tubesandmore.com



2008-02-21 9:04 pm
To Alexq:

I had never even thought of using a toroidal mains transformer for this. Very interesting idea! I was just going to use the 125ESE's because I happened to have them on hand.

It's funny you should mention this change in iron, because I just read very recently that the stock Bottlehead Paramount amps use a $4 70V output transformer.

To Erik:

I had no idea these chokes even existed... I'll certainly take a look!

Thanks guys!

Search for Shoog's work on this forum.

He has been doing a lot of experimental work with toroids as OPTs. I think he even did a push pull amp using toroidal transformer as OPT. As long as you can remove unbalanced DC on the toroidal transformer it will not saturate. I read that toroidal transformers have better frequency response than regular EI transformers.

I am just waiting for a pair of 2a3 and the 4 pin sockets and I will be doing a parafeed 2a3 using a Talema mains toroidal transformer with 230V primary and 9V secondary. I will let you know how it will sound. :)