6X8 Pentode/Triode Musings

[Eli Duttman]

Miles Prower mentioned the 6X8 pentode/triode in another thread.

I wonder if something could be made of this "oddball" type in a phono preamp. I see 2 ways around the common cathode connection: red LED bias and grid leak bias. I'm partial to grid leak biased 2nd gain blocks, as they yield better bass extension. Grid leak bias in the 1st gain block requires cap. coupling between the 47 KOhm cart. loading resistor and the pentode's g1. Given a 20 MOhm grid leak value, a transparent PTFE cap. will be affordable. Perhaps 1 of the time constants can be obtained by parallelling the 20 MOhm part with capacitance.

The $64 question is whether the noise factor of the pentode is adequate.

Comments please.

[Miles Prower]

I use these for voltage regulator error amps. You can make an unbalanced differential with the triode section as the noninverting input that receives the reference voltage. The pentode half becomes the inverting input that provides the gain. Connect its control grid to the voltage set/sense divider.

I was using 6KE8s for this, but these guys are a good deal more expensive, and 6X8s are quite common, nobody wants them, and I can get 'em dirt cheap.

[Eli Duttman]

Bump.

Anybody care to comment on the phono preamp concept?

[Miles Prower]

Originally posted by Eli Duttman
Anybody care to comment on the phono preamp concept?

Since this type was intended for use as a cathode coupled mixer/oscillator, it looks like the pentode section would be quiet enough for a phono preamp. It was designed for use in a typically low level part of the receiver. It also looks to have some very low internal capacitances as well. The triode section looks to have good linearity as well. You can always get around that common cathode by applying fixed bias, or adding a bit of positive grid bias to offset the cathode bias.

[Geek]

Originally posted by Miles Prower
...and 6X8s are quite common, nobody wants them, and I can get 'em dirt cheap.

Speaking of which, I have a big Ziploc full of the dang things. You want more?

Cheers!

[SY]

I'm partial to grid leak biased 2nd gain blocks

Eli, what does this do for headroom and overload performance? The second hole is where the signal is the highest...

[tubelab.com]

6X8s are quite common, nobody wants them....Speaking of which, I have a big Ziploc full of the dang things. You want more?

So did I. They were in the lot that I just sold though. I have too many other triode - pentodes that aren't joined at the cathode and have better specs.

[45]

Noise and microphonic effect are quite typical troubles with non-specialized tubes in this application.
If you don't need to buy anything except such cheap tubes I think the best response is trying all the possible solutions you mentioned.

Other possible cheap candidates you might consider are the 20EZ7 (basically a low noise and low cost 12AX7 with 20V filament supply and different pin-out) and the 5755.

IMO, the 5755 as input tube of a MM phono stage can really stand out.
120V anode voltage and 0.5 mA could be a good working point (the bias is -0.5V).

In my experience the best phono amps all had a passive split RIAA, with the 75 us network after the second stage. With some expedients (basically a design choice), such network can have a negligible insertion loss.

45

[Miles Prower]

Originally posted by Geek
Speaking of which, I have a big Ziploc full of the dang things. You want more?

Cheers!

Yep. I can use 'em.

[Miles Prower]

Originally posted by 45
Noise and microphonic effect are quite typical troubles with non-specialized tubes in this application.

Not necessarily. You never know until you try. The spec sheet for 6BQ7s make no mention of any sort of audio useage at all. Yet, when used to make a cascoded LTP with active tail load, these really rawk. More gain than the usual solutions (12AX7, 6SL7) none of that pentode nastiness problem mentioned with pentode LTPs. These, too, are cheap since they have no audiophool pedigree. I also discovered that the Sylvania and Motorola versions with the parallel connected heaters aren't microphonic at all. (Different story with the ones with series connected heaters.)

Besides, the fat cathodes score extra glowey bottle coolness points.

6BQ6GTBs also are "non-specialized" with no mention than anything other than TV horizontal deflection duty. Sounds as good as PP 6V6s, but with better than twice the output power. Given the need for pulling bigamps through the deflection coils, these require less impedance transformation, so that the OPTs are easier to design, and work better. Also, cheap with no audiophool pedigree. (But they've been going up lately. I guess the good folks at the VT store figured something was up when I bought a whole bunch at $1.30 a pop.)