Jon Yaeger wrote:

in article , John Stewart at
wrote on 3/15/05 7:37 PM:

Jon Yaeger wrote:

I'm putting together a Mullard 5-20 type amplifier around a couple of
vintage Dynaco A420 ultralinear transformers. (I posted the original
schematic on ABSE).

I'm going to deviate a bit from the classic circuit, e.g. I want to use a
triode input, probably an E88CC cascode, in lieu of the EF86 pentode,
because of pentode problems such as noise.

Contrary to what you have heard on this NG, the EF86 as a pentode is not noisy
&
certainly nowhere near what your preamp will do to the complete system noise
figure. Better stick to the phase splitter as used in the original 5-20. The
split load (concertina) will be hard pressed to provide enough drive for the
output bottles at low distortion. Furthermore, it provides little gain. Better
not try to reinvent a successful circuit, at least not until you have
considerable experience. You will be busy enough getting stability with your
NFB
hookup. JLS


Thanks John.

I've seen the EF86 used in a Heathkit preamp and it sounded OK to me, too.

I didn't mean to imply that the only choices were concertina or long tail
were the only two choices. I'm looking for ideas.

There are a couple of weaknesses with the original Mullard circuit that I'd
like to address:

1. The pentode has an output resistance of 100k, which drives 50 pF of input
capacitance from the phase splitter, giving a cut-off of 32 kHz.

So use a phase tweaker network, or use a medium U triode with Ra
about 10k.
Maybe you still need the phase tweaker.



2. The output stage has an input capacitance of about 30 pF, and the
cut-off of this stage is 100 kHz or lower.

So use a medium U triode, Ra from each anode is 10k,
not as high as 12AX7.



3. The driver stage has only 10 dB of overload capability.

That is 3 times the input voltage. Its OK.



4. I think some 30 dB of feedback is used

Yes, its a lot, and only possible with great OPTs.
Leak used a very similar circuit to the 520,
and with very poor OPTs, and they used 26 dB NFB.
Stability was a problem.



Your point on the concertina is well taken. However, I wonder if I could
follow that with a driver stage using a differential pair of triodes coupled
to 6SN7s as cathode followers?

OTOH, maybe the easiest thing to do would be to adapt the HK Citation II
driver circuit or some other stable, high-performance design.

Try maybe the circuit at
http://www.turneraudio.com.au/htmlwe...0ulabinteg.htm

With good OPTs, its difficult to get any better, imho.

Patrick Turner.



Jon

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