Half power switch idea... crazy?

[jjjtttggg]

Hi there,

I'm planning my second amp build.  First was a variation on 5e3 with a different tone stack, switchable extra gain stage/master volume, switchable built in tube screamer, and an effect send/return loop.  OK, so not much 5e3 left other than concertina PI and cathode biased 6V6s output stage, but it's a great little gigging amp.

Here's my question... I want to play around with switchable power levels and I've read about a number of ways of doing it.  I haven't seen this idea (probably for a reason), so I'm interested in opinions on whether it would work.

Could I switch a large valued capacitor across (ie. in parallel with) the lower resistor of the phase inverter, thereby turning "off" one of the 6v6 output tubes?  I'm not sure of the impact of this "idled" output with the active one as they share flux in the transformer, but otherwise, the remaining portion of the output stage looks an awful lot like a single ended Princeton output stage.  It would have a grid stopper that the Princeton doesn't have, which I don't think hurts anything, and would have half the gain, (and less headroom) in the stage just ahead of the output valve due to 56k vs 100k output resistor, and input biased up due to the ac shorted lower 56k resistor, but otherwise it seems it should work.

Thoughts?

-J

[PRR]

Welcome.

Just try it.

If I understand, bypassing the cathode of a concertina will change the gain to the plate from like 1 to like 50. Massive excess gain. Probably not what you want.

I have seen a "volume pot" to one of the output tube grids.

[jjjtttggg]

Thanks for the reply PRR.  You are right, I probably should just try it.  Maybe I'll build it on a piece of plywood to try it out.  If it doesn't work, my alternate plan will probably be something totally different, so I don't want to commit to anything in my chassis layout. 

You are right, it would definitely turn the concertina into a gain stage, but that would make it look exactly like the second stage of a single ended Princeton (but with half the gain due to a 56k output resistor rather than 100k, and less headroom due to the input being biased high).

Thanks again.
-J

[jjjtttggg]

PS - any feelings about whether the output transformer would care about the "idled" half of the output stage?

[sluckey]

Should be good.

[frus]

I have seen a "volume pot" to one of the output tube grids.

I think Peavey even has some kind of patent related to that
For instance, Valveking has a "texture" pot, that is essentially exactly what PRR suggests


see the last page of this:
https://ultimate-guitar-valveking.wikispaces.com/file/view/VK_112_Circuit.pdf

[PRR]

> Valveking has a "texture" pot, that is essentially exactly what PRR suggests

The pot is "backward connection". Input to wiper, output off top. It does tend to increase top-drive while it reduces bottom drive, which may avoid a let-down when dialed off full power mode.

If you are not building a million, "imitating" Peavey's details is often a good safe path.

[tubenit]

Kevin O'Connor's  Soma-18 design does the same thing.  From what I've read on posts on other forums where individuals try it is that it does not sound very good.   There is a brief thread discussing this here:

http://el34world.com/Forum/index.php?topic=11667.0

With respect, Tubenit

[terminalgs]

from the Valveking owner's manual:


"As the TEXTURE control is rotated counterclockwise, the effect of one half of the 6L6GC power tubes is progressively subtracted from the circuit, while the gain of the driver tube is slowly increased. The driver's low-frequency response is also altered along with the gain, resulting in more even-ordered harmonic distortion from your power amp, even at lower-than-stage-volume settings. Finally, with the TEXTURE knob in the fully counterclockwise position, the result is a real single-ended power amp section that operates and responds exactly like a true Class A power amp, driven by a real single-ended high-gain tube stage."

[kagliostro]

while the gain of the driver tube is slowly increased

A double pot single shaft where one pot manage the signal from 1/2 of the PI to the grid of one power tube

and the other pot is in series with a cathode bypass capacitor on the gain stage that precedes the PI

so rotating the shaft the resistance in series with the capacitor decreases and "gain" increases ??

Franco