Audio drive to g2 connected to g1 via 15K0

[richard8976]

The KT88's I have were in a modulator for a transmitter.

Audio drive from a centre tapped transformer, was fed to g2 on the KT88's, and g2 was connected to g1 via a 15K0 resistor.

What connection type is that? Thanks.

[DF96]

That is zero bias Class B screen grid drive. If g1 is positive then the valve is turned on hard, but the current flow is determined by g2 voltage. The tetrode is being used as a low mu triode. The 15K resistor restricts g1 current, as it is not designed to dissipate much power. g1 only goes a few volts positive. As g1 is much nearer the cathode than g2, if they had the same potential then g1 would take most of the current and get very hot.

The aim here is efficiency rather than ultimate fidelity. When the KT88 first came out it was just another big beam tetrode, and not so astonishingly expensive as it is today, so it was used for all sorts of normal everyday applications.

[richard8976]

Yes, I see no bias arrangement at all on the circuit diagram that someone drew out.

I'll probably rebuild the modulator but not with the GEC KT88's - a bit too good for the purpose. Possibly a modern KT88 pair.

Anyway, thanks, I was almost not believing my eyes as to the connection, not ever having come across that before.

[tubelab.com]

What connection type is that? Thanks. .......That is zero bias Class B screen grid drive.

I have seen a similar circuit using 807's. I built it and played around with it several years ago. The value of the resistor affects the distortion a lot.

The tetrode is being used as a low mu triode.

Actually the Mu is rather high. My experience reminded me of an 811A. I have seen some circuits like this from the WWII era for modulator or PA amp applications. A few of us have experimented with circuits like this using solid state components to improve the distortion. No highly successful designs have resulted yet though.

G1=G2/mu Scaled Drive Strawman Design

[richard8976]

Looking at the instructions I have for the AM transmitter, I see it does mention that the tubes are working "in class B zero bias". Not seen that before.

And curiously, the tubes mentioned are 807's.

But, the modulator deck had a pair of KT88's instead of the 807's. Maybe someone just put in KT88's for the 807's.

[Miles Prower]

Originally Posted by richard8976

Looking at the instructions I have for the AM transmitter, I see it does mention that the tubes are working "in class B zero bias". Not seen that before.

Paralleling G1 and G2 (and G3 for pents with actual suppressors) is something that is done a lot to make high-u, RF, pseudotriodes. It helps uncomplicate designs by eliminating the screen supply. I've seen it done with some of the bigger horizontal deflection VTs as well. In that case, distortion is not a factor since you eliminate the nasty harmonics with one or more LC tuners, bandpass filters, antenna tuners/"matchboxes", and the like.

It's not something I'd do for an audio amp where the consideration was fidelity as opposed to efficiency and lotsawatts. You have all the same problems that the proper, high-u RF triode finals have: a grid current drive problem, the high r(p) that reduces speaker damping, the possibility of increased h3 and higher order harmonics.

I'd use the KT88's (or 807's) the way they were intended: as pents, and clean up the harmonics with local NFB and/or gNFB. If you want triode audio finals, then consider types like the 6CK4, 6BX7, 6AS7, or one of the other vertical deflection or series pass types. If you need more output, consider 845's.