Power Scaling...again

[mooreamps]

Again. I agree. But does the best tone for a low voltage operated pentode happen closer to cutoff?


Chuck

Well, that's the thing. Perhaps, if we can say just for sake of discussion, there are "two" basic forms of electronic power scaling.

Let us assume one is the form of the variable plate voltage regulator, which would cause us to ask the question, what is the tone for a low voltage operated pentode ? Well, you may hear different opinions on that one.

Then, let us assume the other is the circuit that I use, which leaves the plate voltage at a constantly high level, and only varies the plate current that flows through the tube. I prefer this method because I believe pentodes only sound good with high plate volts. I've ready heard hear a few "choice" opinions on that one. But the guys who play though my amps here in the bay area seem to like getting those cranked sounding tones between, I'm guessing between 3 to 4 watts when playing on stage.

-g

[booj]

I had my amp set up for both variable screen, or variable plate/screen supplies. I ran into problems with crossover distortion when I adjusted just the screen supply. That was with 2 JJ 6V6-S's and an Eico 18 watt output tranny. I never went below 7 watts in screen mode.
I've switched to a Marshall 50 watt o.t. and 6550 outputs, and am finding that if they are cathode biased just a little cold with full plate/screen voltage, I will get some crossover distortion, but if I move the impedance switch to 1/2 of what I'm using (i.e. 8 ohm load, 4 ohm tap), I get good, fat waveforms with no problems with crossover distortion.
Sometimes I hear the wheels of the universe... grinding....

[mooreamps]

I had my amp set up for both variable screen, or variable plate/screen supplies. I ran into problems with crossover distortion when I adjusted just the screen supply. .

Well, you certainly can not run the screens higher than the plate. I don't know just how you have you power scaling setup. But , for now, I keep the plates up high, and just regulate the plate current through the screen grid.

-g

[booj]

The screens were about 50 volts below the plate voltage to start. (plates at 450, screens at 400) Then I decreased the screen voltage from there. I got usable results, but there was earlier crossover distortion when the output was pushed into clipping. That phenomena got worse when I switched transformers. So I removed the screen control feature and just use plate/screen control.
I'm sure it has a lot to do with the OT vs output tubes vs plate voltage. (OF Course!) That's just my experience.

[Borg54]

I just completed a MOD that I like better than any power scaling scheme that I have ever tried. It's only five maybe six parts at the most. I read all about it over here.
http://www.el34world.com/Forum/index.php?topic=4401.0

This last week I installed this power mod on 5 different amps with absolutely amazing results every time. Do your self a favor and hook it up with the pot and just Dial in your reduced setting. The whole block will thank you for this mod.

It's also been a hot topic over at 18wattdotcom
Topic: new lite with 1/4 power switch
http://18watt.com/modules.php?name=F...wtopic&t=20058

[mooreamps]

The screens were about 50 volts below the plate voltage to start. (plates at 450, screens at 400) Then I decreased the screen voltage from there. I got usable results, but there was earlier crossover distortion when the output was pushed into clipping. That phenomena got worse when I switched transformers. So I removed the screen control feature and just use plate/screen control.
I'm sure it has a lot to do with the OT vs output tubes vs plate voltage. (OF Course!) That's just my experience.

That's strange. I've just never seen that. I have my brake running in about 8 or 9 amplifiers, different layouts ; 10 watt, 18 watt, 36 watt. I've never had any issues with crossover distortion.

-g

[BackwardsBoB]

The screens were about 50 volts below the plate voltage to start. (plates at 450, screens at 400) Then I decreased the screen voltage from there. I got usable results, but there was earlier crossover distortion when the output was pushed into clipping. That phenomena got worse when I switched transformers. So I removed the screen control feature and just use plate/screen control.
I'm sure it has a lot to do with the OT vs output tubes vs plate voltage. (OF Course!) That's just my experience.

Are you scaling the bias voltage along with the screen voltage?

[booj]

It's a cathode biased circuit, set up so at max. power for that (screen-adjust) set-up it doesn't cook the tubes and still operates acceptably at min. power. I see now that to make it work properly the screen and grid-cathode voltage would have to be adjusted simultaneously.

[mooreamps]

It's a cathode biased circuit, set up so at max. power for that (screen-adjust) set-up it doesn't cook the tubes and still operates acceptably at min. power. I see now that to make it work properly the screen and grid-cathode voltage would have to be adjusted simultaneously.

No it doesn't. If it's self bias, the self bias will drop all by it's self as a function of plate current. Maybe there is something else amiss that's giving you the cross-over distortion.

question. What value of series grid stopper you running on your power tubes ??

-g

[booj]

From the plate supply through 2k to 50uf. That's the screen supply. Then through 2 1k's to the screens.

[mooreamps]

From the plate supply through 2k to 50uf. That's the screen supply. Then through 2 1k's to the screens.

You don't understand.
Not the screen grid. The "control" grid.

-g

[booj]

Oops! Sorry. 5.6k, and not mounted at the socket. Mounted on the "experimental circuit board". I realize now that at the socket may work better.

[mooreamps]

Oops! Sorry. 5.6k, and not mounted at the socket. Mounted on the "experimental circuit board". I realize now that at the socket may work better.

It should be close enough. If so, not sure why you would be seeing the cross-over distortion mentioned earlier.

-g

[booj]

My first guess would be the output transformer possibly not matching the tubes. An Eico 18 watt output seemed tolike 6V6's and 6L6's, and crossover with EL34's and 6550's.
With the CE Dist. Marshall OT, 6550's don't work unjless the impedance is set to say, 4 ohms for an 8 iohm load.
And now the subject just diverges.
Experimenting in the real world has it's advantages, untill the head just automatically starts diving for the table.

[mooreamps]

My first guess would be the output transformer possibly not matching the tubes. An Eico 18 watt output seemed tolike 6V6's and 6L6's, and crossover with EL34's and 6550's.
With the CE Dist. Marshall OT, 6550's don't work unjless the impedance is set to say, 4 ohms for an 8 iohm load.
And now the subject just diverges.
Experimenting in the real world has it's advantages, untill the head just automatically starts diving for the table.

Sounds like there is more investigation to be done....

-g