audio driver V2-cdc

[WA1GFZ]

I'm playing with the idea of mounting an L bracket between the chassis and case of my V2-CDC to hold a pair of sockets for a push pull audio driver. I'm thinking of a pair of 6AQ5s maybe triode connected. This way I don't have to modify the chassis or the present audio line up. The extra gain would allow me to do some negative feedback.
Expert thoughts welcome?? gfz

[WD8BIL]

WOW Frank..... your gonna drive those 807s right outta their sockets !!! That's a lotta drive.

I think the 12AU7 phase inverter is primo to do what you want.
For the feedback you can originate it from the 807 output.

This shows 6550s but works well with 807s.

[WA1GFZ]

Bud,
My present driver transformer has a 10K output Z. Your circuit is a lot higher Z so I think it is going in the wrong direction. I have been playing with a simulation using FETs. I have a nice circuit with an inverter and two source followers. The CDC has triode connected second stage so easy to apply feedback. just eliminate the cap across the cathode resistor. I need a bit more gain and don't really need a lot of driver power so I can run it pretty cool but it gives me the option to triode connect the 807s. The positive peaks are not above 100 % as it stands now. This may take a different modulation transformer or different modulator voltage. The high voltage is 760V and sags about 30 volts when I nail it with modulation.
The present driver brick walls at 0 volts on the grid. I need to look at the saturation voltage of the plates again and the effect as it goes below the screen. Maybe I just need to drop the final screen voltage and reduce the output power. I have 25K series dropping and about 218v on the final screens. Dip is at about 230 ma with all the loading caps dialed in so running a bit high. Plenty of grid drive. I see you have 30 K screen dropping and bleed some to ground so interesting schematic. TNX

[WA1GFZ]

Bud,
Just checked 6146 spec again. I'm too high on the screen. I should be around 160 volts. This explains why it wants to crank out power and low modulation peak.
I'm lightly loaded which increases screen current.
I was looking at maximum and didn't notice typical. Max is 250V so figured I was fine at 218.
Da dis tube stuff is so hard..fets are so much easier.

[The Slab Bacon]

I tried a similar type fuzz inverter in a ranger that I was modding a while back, found that it just didnt sound the way that I liked, the audio was a little on the harsh side.
So I went to a more conventional style of fuzz inverter with the additional resistor under the 68k cathode follower, and plate powered from the low b+ instead. It worked very well with 6550s. those tubes are designed to run between class A and AB1. All they need is a little audio to tickle the grids and they really swing the monkey! They dont very much drive to be happy (6550s)

However if you are running 807s (or 6L6s) or anything else in AB2 you have to be able to push them into a little grid current. A straight fuzz inverter wont do it. You will need an additional stage between the inverter and the AB2 grids to do it or go back to transfoma coupling. A pair of cathode followers would prolly do it (or if you must)
Frank, get ur fets out. A nice triode and a GOOD interstage transfoma would definately be easier

                                                The Slab Bacon

[Bacon, WA3WDR]

Frank - consider building a screen regulator for the modulators in the CDC and adding a resistor of maybe 1 or 2K in series with each screen grid.  I got lower distortion that way in my Viking II.  I raised the screen regulator voltage about 50V to allow for peak voltage drop in the resistors, and I made control grid bias more negative to hold down the resting current.  That was with a pair of 8417s in the modulator.  The resistors were only 820 ohm 2W, but I did the same thing with a 6L6 audio amp and actually looked at the X-Y on a scope.  That amp has about 300V plate and screen, and I got minimum distortion with about 5.6K in series with each screen grid.

I didn't try other values of screen resistor in the 8417 Viking II modulator.  All I know is that the 820 ohm resistors audibly reduced distortion.

   The other Bacon, WA3WDR

[WA1GFZ]

Bacon,
I built a Zener regulator for the CDC with 30 K 20 watt off the high voltage and a pair of 10 watt 170 volt zeners. The screens are solid now. The resting current is a bit 70 ma. high so plan to zener regulate the bias at a higher voltage.
I can barely hit 100% peak modulation and see the distortion near minimum modulation.

I wonder if any of you guys know if you can put the screen winding in series with the output winding of an ART 13 mod transformer to get more monkey swing out of the modulators. I have a couple ART 13 mod transformers and may give them a try.
Anybody out there ever see the wire size of the screen winding compared to the primary and secondary. I can measure it but it will become a science project. I know someone has taken them apart. gfz