Symmetrical SRPP vs Self Biased Cascode for V1 into Tonestac

[David Root]

I'm looking at these two circuits and have a few questions in the above application:

1) Best tube type for tone? Gain isn't really an issue! I'm not looking for pentode type gain here, only about 80-90.

2) On the tone, I hear that both circuits give nice tubey tone, but how do they differ, particularly with odd vs. even harmonics and IM distortion?

3) On tubes, the 6DJ8/6922 is designed for cascode use, (150V cathode to heater voltage rating), but NOS is $40-$60. The 6N1P Sovtek is $10 and fairly similar.

4) Microphonics and basic noise floor are issues in both circuits, because of the high gain involved.

5) Going into a tonestack the symmetrical SRPP has about one third the Z out of the self biased cascode, which might give it an edge other things being equal?

[jimipage]

check this out:
http://thetubestore.com/teslaecc88.html
$12

don't know if you've been to this site, but they have lots of great info:
http://www.valvewizard.co.uk/

good info here, too:
http://www.geocities.com/keirkegeerd/ca ... gdiyer.pdf

[mhuss]

I used the SRPP on my WattBox (a Hiwatt-ish take on the Hot Box), and it produces lots of gain, with a sort of 'pentode' character. It very slightly clips the positive going signal even at low levels, which is odd, but not a bad thing tonally.

If you raise the heaters to 70v or so, you can use any 'normal' tube you like. 12A*7s all work fine for me.

--mark

[David Root]

Thanx, Gents, for your responses.

I have seen the Tesla ECC88 on another tube site. Is it better than the Sovtek? I'm always concerned with current/recent production microphonics and noise floor.

Yes, I've been to Valve Wizard, that is an excellent site which I would also recommend to anyone.

The geocities site I don't know-thanx, will check it out.

Mark, on the SRPP, this clipping is probably what the Valve Wizard characterized as "distortion not suitable for hifi, but great for guitar amps". Does it clip top or bottom? Presumably it's clipping in the upper triode?

What tube did you use?

I have already allowed for possibly hiking the heaters, using the rectified 50V bias tap off the PT I've just ordered, so I can in fact use anything. I have also been looking at 12AX7, 12AY7, 5751, 12AT7, using TubeCad, and at the Russian 6N1P, which intrigues me. Its transconductance is less than half the 6DJ8/6922, its gain about the same, maybe slightly higher, but its max. plate voltage is 300! Cathode to heater voltage is only 100V "with heater at positive potential", but 250V "with heater at negative potential". Does this refer to heater RELATIVE to cathode, or to ground? Makes a difference.

[mhuss]

Clips on top.

I ended up using a silverplate EI ECC83. Different 12AX7s/ECC83s made a major difference in the sound (as did using 12AT7s or 12AU7s). I didn't try the 6DJ8 because I didn't want to rewire the socket.

I'm sure the spec is as stated -- heater to cathode. The tube has no intrinsic relationship to ground.

--mark

[David Root]

Thanx Mark. So, if it clips only on the top, then the tube operating bias is a bit on the hot side, right? Would this be a function of the tube type or the circuit design? I ask this because I'm reading up on the SRPP, particularly the fixed bias and the symmetrical variants, and so far I've not seen anything about essential distortion content other than the Valve Wizard's piece on SRPP. However, we are talking about a push-pull circuit here.

Thanx also for commenting on tube types and the heater/cathode point.

[mhuss]

I'm using 1K resistors in the SRPP because I liked the way it sounded better. The 820s (as shown on the valvewizard page) clip a little more softly, and I'm sure if you experimented you could find the "most symmetric" bias point.

--mark

[David Root]

OK, thanx, I think I have it now. Still interested in the cascode too.