C3g driving 2a3: Triode or pentode?

[Klimon]

Hello,

Can anyone comment on the particular benefits of pentode/triode as single driver? I want to drive a R120 (very similar to 2a3 but only 70Vpk-pk driver requirement at 250V/50mA and IDHT) as single-ended output.

Linearity of C3G looks excellent both as triode and as pentode (judging from loadlines: pentode at 19mA / 170V / -1,5V / 10K plate resistor).

Rp is much lower for triode but pentode has much higher voltage and current swing.

The pentode solution sound attractive as it doesn't benefit from extras like IT or plate chokes (which I can't afford) AND pentode as single driver for 300B and the likes has won praise of various builders. If possible I'ld DC-couple C3g to the output tube to reduce parts count even further.

Thanks in advance

Simon

[pmillett]

I am a pentode driver fan myself.

I didn't include the C3m/C3o/C3g in the tests but you can draw some conclusions from similar tubes in this data:

http://www.pmillett.com/pentodes.htm

Rp doesn't matter much, the Zo will be dominated by the plate load (10k in your case).

The R120 isn't particularly low distortion in SE, but it does sound nice. So whatever driver you use won't have a huge effect on linearity (within reason!).

I have an amp that uses R120's being driven by a combo of a 6J7 pentode and 7N7 cathode follower. The CF will actually drive a bit of grid current into the R120 and soften clipping a bit. I think a direct-coupled high-Gm pntode would do that as well.

Guard those R120 tubes with your life! Last time I saw some sold they went for $300-400 each!

Pete

[Klimon]

Hi Pete,

Thanks for the input and even more so for your testing that lead me towards the pentode driver! C3g pentode dc-coupled it will be than (certainly with the extra benefit of grid current!)

On ebay r120 've gone from around 80$ to around 150$ for a matched pair solid plates in a years time... but still a long way from the prices some retailers ask.

I hoped they'ld be the better of the middle-class 2a3 (as an 'exotic', single-plate variety, your remark seems to imply they have quite some distortion though (2nd order?)..

Simon

[pmillett]

Yup, almost entirely 2nd order distortion. Not all that different than a 2A3, maybe a little more distortion (at least at 1W out).

I'm really not sure why they sound so good... I did notice that the distortion level increses very slowly and linearly with output power, more so than most tubes. It maybe even has more distortion than 2A3 or 300B at very low signals.

I really wanted to do push-pull with them - I suspect they would be very linear in P-P with maybe 8k plate-to-plate - but I only have 4 tubes (RT, new in box with sequential serial numbers!) and don't really want to spend the money to get any replacements!

Pete

[wagrubbs]

I have three matched R120 tubes and would like to build a three channel amp for my K-horns and LaScala.

Does anyone have any suggestions for the circuit? I have the power supplies ready to go.

Thanks in advance?

[Klimon]

'Tossie' published a few schematics with R120 SE (google "Valves' world" (sic)); nothing special though IMO. I remember a diyer recommending the datasheet operation point (250V / 60mA IIRC) over higher voltage alternatives. I've got a schematic with C3G pentode as driver in my head but still have to build it. Good luck and keep us posted on your progress.

Simon

[ErikdeBest]

Hi Simon

What are your plans to achieve DC coupling? A very large (bypassed) resistor in the cathode of the R120's? Maybe a dual (stacked) supply?

Cheers, Erik

[Klimon]

Hi Erik,

I'll use RC-coupling because I want the amp to run both R120 and 6S4S, at different operation points. A switch-connected first cap in the output stage PS should allow me to flip the B+ to the required voltage.

Simon