6v6 R-C stage

[Andy Le Blanc]

I've been noodling the last few days on a reverb circuit.
Following a similar notion to an old ampeg reverb-a-rocket at first.
Using a capacitor to couple to a reverb tank, after a gain stage.
ampeg, magnatone, traynor used this in many production amps.

Its works... I was never enthralled with the tone in an old traynor I used to noodle with however.

So I figure I'd try it and explore a couple other circuit examples that caught my interest along the way.

first I tried a 6f6 stage I found in a HP 220A, and to my surprise it worked
as a driver in a verbarocket circuit.
It wasn't spectacular, and the screen voltage was too high for my liking, getting into virtual cathode land.

so next I tried a stage suggested by an article is glassxpress by Micheal Borrows aug 2002 "6B4G Stereo 90 Power Amp"
He uses a 6l6 R-C stage to drive the power side of his build

After a bit of noodling I got a similar circuit with a 6v6, with the screens voltage in an acceptable region.

And it seems to work very well as a verb tank driver, replacing the typical tube and transformer with a single tube.

I have a feeling that the same approach will work with a variety of power pentodes.

The build I'm working towards is an all octal amp, nice to have a single octal tube driving the verb tank as well.

[Colossal]

Very cool, as usual Andy. I always enjoy your musings.

[RWood]

How 'bout some details?

[Phil_S]

Andy is the master of the slow tease. Experience shows he'll tell us when he's ready. Patience.

[Andy Le Blanc]

jeeez.. thats too funny.

its just an R-C stage, the same as any other...
except for the resistor network that makes up the screen supply

1K bypassed cathode

The plate resistor(s) are 10k and 15k series

the screen supply is tapped at the junction of the two plate resistors
and is a further resistor network with a 120k then a 220k bleeder to ground
the screen voltage is derived at the junction, then you can use an additional screen grid resistor here if oscillation or RF issues crop up
I set it up so that under zero signal conditions the screen is about 20v negative with respect to the plate
when measured as a difference between the plate and screen at the tube socket

the coupling cap is huge .5 to 2uf... the larger the value the larger the phase delay and apparent dwell

The plate resistors compromise a ultra-linear tap, you can play with the ratio if you want

I got a bit over 3v ac at the output... the accutronics nomagraph for for over load gives a 3.5 nominal figure

I was surprised, probably do a bit better with more plate voltage,
but for my application I can peg the amp with out apparent distortion in the verb