Phase inverter

[ThSpeakerDude88]

I have been wondering how I can rework that input stage by the way..

I know a lot of people use a 12ax7 and a 12au7, but that causes a few problems such as cross talk is greater, and the need for perfectly matched tubes to obtain true stereo balance.


I have some good matched 12au7 side "D" Ghetter cleartops, each tests @ 81/81 on my B&K 700.

How should I modify the circuit to accept these?

[SY]

Crosstalk is close to a zero issue. The section-to-section coupling could possibly degrade the 20kHz separation by a few dB, which is not going to be audible.

The less said about using a 12AU7, the better.

[ThSpeakerDude88]

Originally posted by SY
The less said about using a 12AU7, the better.

why is that?

[Miles Prower]

^^^^
The 12AU7 isn't the most linear triode out there. You see it used as an RF amp, or in quasi-digital applications such as multivibrators and related devices such as RS gates, or frequency dividers. It's not real good for audio, though it looks better than the 12AV7. The 12AU7, however, seems to work pretty good as an SRPP or Mu-stage, or as an LTP with active tail load.

[SY]

Originally posted by ThSpeakerDude88


why is that?

Compared to similar triodes (6SN7, 6FQ7/6CG7...), the distortion is high, and high in odd-order stuff. Bleh.

[oshifis]

This is because the 12AU7 is a variable mu triode. Such tubes were used in AGC circuits.

[ThSpeakerDude88]

would it be best to convert the sections to 12ax7's then? I do seem to see a lot of preamps however, with 12au7's in them...

[Johan Potgieter]

SpeakerDude88,

No.

12AX7 has a too high mu and rp to make it useful for much of a driver for power tubes, etc. Also the anode does not go low enough at Vg1=0V. Try to keep to something with low rp and mu around 40-50 to avoid too much Miller cap. problems. Tubes like 12AT7, ECC88 etc. are quite good.

The 12AU7 in a pre-amp is not bad as the voltage (current) swing is low and mu not too variable over that small swing. But I would not prefer it even there.

I read a while ago that the zero gain of the cathodyne was mentioned as a factor. But if one counts that an ltp uses two triodes at half full gain, then the cathodyne actually has more gain, using the other triode as a "full" voltage amplifier, as is usual.

[ray_moth]

I'd avoid 12AU7 if I were you. It was not designed as a low-distortion tube for audio use and it is quite poor in that role. You would be better off with 12AT7, as Johan says, or 6SG7 or 6FQ7, which are 9-pin equivalents of the 6SN7.

[ThSpeakerDude88]

I didn't think of a 12AT7....And now that you mention it the 6SN7 equivalents either, as I know a lot of hi-fi amps use that as a driver/pre. I have heard of the 6FQ7 but not the 6sg7.

I would like to be able to use a tube that can do both the pre and the driver side of the 12dw7...short of just buying a 12dw7.. btw is there anything wrong with that as long as I get them matched? JJ has current production.