Mains toroid as IT with DC balance?

[Michael Koster]

All this talk of mains toroids and DC compensation gave me an idea
for an interstage coupling circuit.

It's basically an inverting interstage transformer with CCS bias
on the secondary to act as both DC cancellation of the driver
current and current supply to a shunt bias regulator for the
output tube.

This could be great for A2 drive also, since the driver will be at
it's low-Rp, high current extreme when driving the grid positve.

Just the thing to motivate building a SE project.

Cheers,

Michael

[Sheldon]

Don't go getting all distracted. I want to see how this one comes out: Spud-Assist: Totem-Pole Current-Mirror PP Hybrid

Specially this version: http://www.diyaudio.com/forums/showt...59#post1599359

Sheldon

[ErikdeBest]

Hi Michael

Interesting idea, I saved the thread for whenever I have time to try it out, but I already have one question about the interstage transformer. You define it as a 1:1 at 15VA. Do you know where one can get such a trafo? For small power units I only found toroids with the secondaries with much lower voltages (like 2x115V on primary and 2x15V on secondary).

Many thanks, Erik

[revintage]

Great idea! Have you measured the inductance?
Must try to find a 115+115:115+115.

[revintage]

Found it!

http://www.antrimtransformers.com/products.php?sec=98

[ErikdeBest]

Good found Revintage, the prices though aren't exactly 'surplus' style (what I am used to pay for iron, normally)

Maybe a 2,5W 15k:15k or 10k:10k, 13 dollars costing non-gapped interstage transformer from Edcor could be used in this configuration?

Erik

[revintage]

Erik:

! Can admit you almost get a halfdescent dedicated audio IT for those money. Realized it was GBP not USD !

[Michael Koster]

This is true. I realized as I was writing it up that it might be best to
use a purpose-built IT ungapped or with a small gap for ~5mA to
handle some drift. I do have some of the Hammond isolation toriods
(182D110 and 1182E117) but they are not cheap to buy at $40US)

I should get around to measuring the inductance but I will likely
make in-circuit measurements on my little spud amp with different
transformers clipped in.

Sheldon - I'm currently building an amp based on the parafeed
circuit, waiting for a pair of 6S45pi to arrive from Russia to try
also. Maybe I can abuse the 5842s some more in the meantime.

Stay tuned on the push-pull circuit. I'm thinking about a self-bias
version that might have good DC stability. I don't want to add
any parts, though!

Michael

[revintage]

Hi Michael,
Hope you don´t mind, me using your schematic to propose a DC-free transformer solution. Did this when I realized that getting the toroid was both hard and expensive.

[kenpeter]

Rev, you might want to tap that FET at the source (for Mu follower
with lowest drive impedance). Or if you are going for MJK inspired,
design, halfway up the current sense resistor. At the "Anti-triode"
balance.

Its a good thing Michael hasn't discovered the Hall Effect yet.