DHT Topology Comparison Question

[dsavitsk]

I am working on a line stage that uses a 01A to start, and probably some other tubes later. The plan is to use a tansformer to couple it such that it provides a small amount of gain and a reasonably low output impedance. For the purposes of experimentation I have some cheap Edcor 10K:600 transformers to use.

I am considering a couple of different topologies (attached) and thought I'd try to elicit some comments about each. The first, the parafeed, seems to have the advantage of a smaller, and thus higher quality cap in the signal path, though that does seem likely to add $80-100 to the build cost. The disadvantage is the solid state CCS which in other circuits I have used it in has induced a solid statey sort of sound, though that might not be an entirely bad thing. It could be replaced with a tube CCS, but for now that is more ambitious than I want to be. I am without a preamp so I'd like to get the basic circuit working first.

The second has the advantage of simplicity, and the disadvantage of the electrolytic in the signal path.

The third has the advantage of no caps in the signal path, but the disadvantage of more hum, and thus a beefier power supply, perhaps with some regulation, to take care of that.

There is also an additional wrinkle. Magnequest makes a parafeed transformer that will work in the first circuit for about $200. Electra-Print makes a transformer for about $160 that will work in the second two. Now, if the electra-print will also work in the parafeed arangement then that might be a good choice, but if using a transformer that is intended to have DC across it is not optimal in a parafeed circuit, then I'll need to choose a topology first before buying the transformer.

So, any thoughts and opinions (including modifications to the partial schematics) would be very appreciated. Oh, also, where it shows the CCS, that might be the IXYS chip, or a bottlehead style circuit, or perhaps something out of MJ's book. I'd probably try a bunch.

Thanks

[Gluca]

Parafeed transformers come with no gap while series feed transformers are gapped to allow a certain DC (your plate current) to flow.

You don't want to use a SF transformer in a PF topology (you would not be taking the advantages parafeeding offers) and you don't want to use a PF transformers in a SF topology (the core would sature after the DC current).

Me, I would pick up option 1 (that's what I am going to do with my line stage actually).

Ciao
Gianluca

[AndreasS]

Originally posted by dsavitsk


The second has the advantage of simplicity, and the disadvantage of the electrolytic in the signal path.

The third has the advantage of no caps in the signal path, but the disadvantage of more hum, and thus a beefier power supply, perhaps with some regulation, to take care of that.

The second and third haves capacitors in the signal path:
in the second the last capacitor of the power supply and the bypass cap are in the signal path;
in the third the "bypass cap" of the cathode resistor is the last cap in the power supply in series with the cap from the cathode to B+

regards Andreas

[Eusebius]

How big does the cap have to be in the second option? If it's something like 47 to 100 uF you can use a motor run or other polypropylene.