two questions on Parafeed amplifier

[croccodillo]

hello,

I have some questions for you.
I'm thinking to build a Parafeed amplifier, using four KT88 tubes.
In the attached drawing, THIS, you can see, on the left, the "traditional" approach for this topology.
On the right, instead, you can see my idea: having a voltage equal to half the B+ voltage in manner to reduce at the minimum the stress for the output capacitor.
Moreover, having an active regulator (the two mosfets on the right) we can have exactly the same voltage we have on the V2 plate on the opposite side of the OT: in this case should be possible to remove the output capacitor and connect directly the plate to the OT.
Using a simple comparator and a long-constant filter, we can exactly follow the plate idle voltage.
What do you think, should it work?

Next: If I have a PP OT, dimensioned for KT88 (2500Kohm), could I use it for a parafeed output, simply connecting half of the primary winding?

Thanks in advance

Ciao,
Giovanni

[tubelab.com]

There is no theoretical reason why this won't work. I have briefly experimented with a split supply and a totem pole amplifier using KT-88 in the past, and it worked well.

It should be noted that if the servo loop becomes unbalanced for a period of time (due to a transient that upsets the operating point of the output tube) there will be saturation related distortion due to current through the OPT. Saturation distortion sounds particularly gross.

Regarding use of half or the OPT, it might work, or it might not. The only way to tell is to try it. I have tried this, and sometimes the unterminated end will cause a resonant peak in the frequency response, although it may fall outside the audio band. Sometimes it can be fixed by connecting a resistor across the unused winding half. I prefer to use the whole primary and mis-match the secondary. In this case you may not be able to do that unless you have 4 ohm speakers (on the 16 ohm tap). Try it both ways and see which provides better sound.

[Shoog]

One suggestion. Return the primary to the top of the cathode resistor. This will minimise the sonic effect of the cathode resistor. I think this is called "ultrapath".

The only issue I have seen discussed about using a PP transformer (apart from the one which Tubelab mentioned) is that interwinding capacitance may be a bigger issue.

I am currently thinking about reconfiguring my parafeed amp. It currently uses 807's as outputs and TT21's (KT88) as a CCS. I just carn't live with the appalling inefficiency. I am going to investigate the suitability of using microwave oven transformers as a plate choke.

Shoog

[croccodillo]

I am currently thinking about reconfiguring my parafeed amp. It currently uses 807's as outputs and TT21's (KT88) as a CCS. I just carn't live with the appalling inefficiency.

What do you mean with "appalling inefficiency"?
I'm sorry but my insufficient english can't help me understand your phrase...
Do you mean your amp does not sound well? This is a bad news for me, cause I have 4 KT88 and my idea was to use two of them as CCS and the other two as SE amplifiers.
The KT88 used as CCS will be driven by a solid state CCS, connected through the cathode and the G1.

I am going to investigate the suitability of using microwave oven transformers as a plate choke

I work for a company that develops and sells industrial microwave ovens, and I'm a senior R&D engineer (look at www.milestonesrl.com, histopathology division): unfortunately, microwave HV transformers are nothing else than normal transformers, without any gap, and thus cannot be used with DC currents (or at least they can handle really little DC currents).

Ciao,
Giovanni

[Shoog]

It sounds fine and I like it very well. However the relationship between power in and power out is very poor and wasteful. That is what I mean by appalling inefficiency.

I am using a microwave oven transformer secondary winding as a power supply choke passing nearly 0.5A. It works well (though it runs hot because of it high resistance). I have read on this forum of at least one person using a microwave oven transformer as a plate choke. I cannot say with any certainty whether it will work but it will cost me nothing to find out. Your engineering experience may be able to save me the trouble of experimenting.

Shoog

[Shoog]

I just modelled my power supply with the microwave choke. Replacing the choke with a simple 100R resistor. It could very well be that the choke is simply performing as a big 100R resistor. Without taking some measurements its hard to say.

Shoog

[croccodillo]

Well, a kitchen microwave oven transformer is usually rated at 1300 VA peak; that means a primary current, fully loaded, of about 5,6A@230V or 11,3@115V (usual voltages here in Europe).
You said you are running the MW trafo at 0,5A.
I have no experience in using a transformer with DC current (it isn't a normal situation...), but it looks like it can work, at least if you're using a 115V primary transformer, cause your current (0,5 A) is quite small.
Try it and let me know!

Ciao,
Giovanni

[Shoog]

When I have been to the recycling depot and got my hands on a transformer I will lash up a parafeed amp to test. When I have done that I will report back on my results.

Shoog