Push-Pull 6V6 Speech Amp / Driver Problem Solved

[W9ZSL]

I've spent the better part of a week cleaning the shop, sorting parts and gathering for all the various projects.  I've been going round in circles about a speech amp / driver which could be "universal" for both the 811A modulator and a smaller 5933 50 watter.  I think I found the solution by accident.

By chance I decided to sell a Merit modulation transformer used in a 12 watt unit and discovered it was an "all purpose" circuit for QRP plate or screen modulator OR A DRIVER!  The Merit iron is used in the diagram.  

I picked up a universal Stancor driver iron 10K to P/P class B grids to replace the Merit exactly so when assembled, I'll have at least 10 watts available for drive.  Best of all, this uses a very unique phase-inverter.  I need a handful of parts for it which are on order.  All resistors are 5%.  This is a complex circuit but is touted as being very linear.

Meanwhile I've gathered all the parts I need for an all-band 6AG7 / 6V6 exciter using a choice of 2 oscillators, a 90-100 watt AM/CW Amp with Parallel 5933's, a modulator using 5933's, a modulator using 811A's and an amp with an 813. Six projects total. The gathering itself jacked me up to do something and running up and down the basement steps has been just the exercise I need.

I pirated some parts from my original single 6V6 into a UTC S-8 for the P/P driver so that particular amp and its parts will be surplus.  Anyone need an NOS UTC S-8?  That design would be really huffing and puffing to do more than 5 or 6 watts.

The P/P job is good for around 12.  Here it is.  I'm going to use a .05 in place of the mic preamp .001 and add a line input switch.  I agree that the circuit is really bizarre but I'm not about to mess with it much.  It eliminates the interstage transformer, has plenty of power for driving a pair of 811A's (10 watts output) and just feels right!  

Thanks to Bella the Cat, I'm losing an hour of sleep every night.  Getting up an hour early is an incentive.  If you think we're confused by DST, cats aren't.

[N2DTS]

Sounds like you are having fun now!

[Pete, WA2CWA]

Why is R17 in the schematic, one side connected to the high voltage supply and the other end connected to the cathodes?

[WB3JOK]

Maybe for additional cathode bias (without increasing the tube current)?
So the cathode resistor can stay small, it's only 270 ohms.

[W3RSW]

R17's part of the voltage divider to insure stability of bias.  Without it you'd have normal cathode bias which is not as stable as having a separate bias supply of say -25 volts on the grids.

It's action appears to be between a stiff, separate bias supply and simple cathode resistor bias.

If the 6v6's are class A then the simple cathode resistor bias would be relatively stable, --haven't done the calcs. But this amp appears to be somewhere between AB1 and 2.

I'm also intrigued by the common cathode phase inverter with g1 of lower tube half grounded. Notice that the common cathode resistor is not bypassed. The bottom tube half mirrors the top but out of phase. It appears to be self balancing.

[W9ZSL]

The pair of 6SJ7 tubes operate as a "common-return" type of phase inverter to feed the grids of the 6V6 tubes.  No bypass capacitor is used in conjunction with the common cathode resistor and the common screen resistor of the two 6SJ7 tubes of the inverter.

Since the signal is fed only to the grid of the first tube of the inverter, and the grid of the second tube is grounded, the signal voltage which produces the output from the second tube of the inverter is that which is developed across the common cathode and screen resistors.  Since this procedure always would result in slightly less voltage output from the second tube as compared to the tube which is fed directly by the signal, degenerative feedback from the plates of the 6V6 tubes is coupled back to plates of the two 6SJ7 tubes in the inverter.  The feedback resistor for the second tube of the phase inverter is greater in value than that connected to the plate of the first tube by an amount sufficient to permit both tubes to deliver the same signal voltage to the 6V6 grids.

In addition to equalizing the output voltages from each side of the phase inverter, the feedback serves to improve the linearity of frequency response and affords greater reduction of harmonic distortion in the output stage.  Also, since the effective internal impedance of the 6V6 output stage is reduced considerably by the degenerative feedback, the output voltage regulation with varying load is improved.  That is key if a driver transformer replaces the modulation transformer.  Semi-fixed bias on the output tubes is obtained by passing the bleeder current through the cathode resistor for the stage.

It's really ingenious and versatile.  Those Old Timers didn't miss a beat!

[W9ZSL]

Having fun?  It's about damm time!

[N2DTS]

That is a lot of projects, are you going to build them all at ones or one at a time?

The part that takes me the longest when building something is finding a design that will do what I want with the parts I have on hand or can be got cheap, then gathering everything together.
Lights, meters, knobs, switches, nice tube sockets, tubes, terminal strips, etc.
Then I play chassis chess a while, and it comes together or it does not and the parts all go back into their hiding places.
Sometimes things work out really well, almost like I knew what I was doing, sometimes not so much but it works, and sometimes its just a bodge and gets scrapped.

Its somewhat easy to build things large, its very hard to build something small nice and neat.

I do not hear a lot of homebrew rigs on the air anymore.
I hear a few, but not as many as I used to.

[W9ZSL]

It's going to be one at a time.  I'll probably start with the modulators which will be simple; just the mod iron and the tubes.  Everything will be modular.  The 100 watt AM amp is also simple...from the '57 ARRL handbook.  One speech amp / driver for both.