Power scaling

[lowell]

I've implemented RGs amplified zener in my amp via a dpdt switch. One zener for CT plate voltage reduction, and one zener for the screens. I have it set at 100v reduction right now and I hear NO difference between "tweed mode" and full power mode. Here are the schems. I'd like to hear people's thoughts on why I can't hear any difference. I did check the voltages and it is indeed operating as designed. The 6v6 (cathode biased) tubes are reducing in bias current as well, the Vk reduces by 6v.

[d95err]

From your schematic, it seems like you have 464V on the plates to begin with, so 100V less would be a very small difference. The power output difference would be less than 50%, so it could very well be inaudible. Try 200V, that should make a bit more difference.

Also, I would suggest using the voltage drop on the whole B+ chain, not just to the poweramp, or at least to the PI as well. That should make the difference more noticeable.

[J M Fahey]

Hi Lowell. By

I hear NO difference between "tweed mode" and full power mode

I'd like to ask you: how are you using your amp?
If you are playing real loud, say above 6 or 7 , orverdriving your output tubes, you will hear a change, not so drastic but yet noticeable.
Now, if you are playing on, say, 2, it's the same that you have 360 or 460V available, you are not even approaching them and, besides the switch click, you'll hardly hear anything else.
I've spotted 2 mistakes in the schematics you posted, hope they are just typos:
In the OD tone stack, C7's left pin should go to the union C6/R10.
In the power supply sheet, you show *two* 6.3V VAC windings, in series, powering the filaments. It should be drawn as one center tapped 6.3VAC or two 3.15VAC in series.
I'd rather build the full powersoak circuit and set "by ear" the sweet spot.
Good luck on your project.
-

[lowell]

ok cool. I'm gonna try the 200v drop. Thanks for the tip on the tone stack - it has been sounding odd when I turn up the treble.

[booj]

I'd put a 500uf/150v cap acros each zener. I've tried both methods, and I like having the cap in there.
I'd leave the preamp voltages where they are and only reduce the plate/screen voltages. I've tried both in that area too and preferred not dropping the preamp voltages.
just my .02

[lowell]

Why the cap across the zeners? What affect does this have?

Side note, I've implemented the 200v zener and my 6L6 have completely cut off until I hit a big cord or play really hard. The 6v6 definitely have some crossover distortion. I've taken some some measurements and calculated Rb parallel (bias supply) and Rk parallel (cathode resistor). I'm now rewiring for 2 dpdt relays to control the 4 switching elements. Will report back.

[Merlinb]

Side note, I've implemented the 200v zener and my 6L6 have completely cut off until I hit a big cord or play really hard.

That is because you are not scaling the bias voltage along with the anode/screen. In a cathode-biased amp that all happens automatically of course, but in your case you need to reduce the bias by roughly the same percentage as the screen voltage, to maintain an equivalent operating point.
(E.g., if you halve the B+ you halve the bias voltage too)

[lowell]

Merlin, just got your book it is awesome. Yes that's why I'm wiring up the relays so I can control bias simultaneously w/ the plate/screen voltages.