MV, Powerscaling in 50W, 100W Amps

[Gregg]

This upload site is obviously screwed. Here's another link to the schematic:

DepositFiles

Choose Free download, then "Download file in regular mode by browser". These sites are pissing me off, honestly. Any other similar sites recommendations that wouldn't require DNA samples?
Let me know if it's OK so I can repair other links as well.

But how do they do it?

How do they do what?

[Alan0354]

This upload site is obviously screwed. Here's another link to the schematic:

DepositFiles

Choose Free download, then "Download file in regular mode by browser". These sites are pissing me off, honestly. Any other similar sites recommendations that wouldn't require DNA samples?
Let me know if it's OK so I can repair other links as well.



How do they do what?

Thanks, I got the schematic with this one. You are of most helpful.

I was asking about how they do power scaling, the schematic answered the question. It controls the screen voltage only instead of both the main high voltage at the center tape of the OT and screen.

Thanks

[Jazz P Bass]

Take a look at an old Univox amp.
Like the U1061.
650 (700 nowadays) Vdc on the plates of 6L6GC output tubes.
Say What?
Now look at the screen voltage.
321Vdc.

[Alan0354]

Take a look at an old Univox amp.
Like the U1061.
650 (700 nowadays) Vdc on the plates of 6L6GC output tubes.
Say What?
Now look at the screen voltage.
321Vdc.

Screen voltage is 1/2 the Vdc. Why do they want to do that? Jacking up the voltage at the plate so high and power down using the screen?

[Jazz P Bass]

My guess: Clean headroom.

[Alan0354]

My guess: Clean headroom.

From my experiment, that does not give more clean head headroom. It just lower the power exactly like if you lower the +B and screen together. They distort!!!! At least in my amp!!!

And that 650V on the plate, that's a lot of voltage for 6L6.

[g1]

Some Musicman amps run 6L6 at even higher voltages than 650V. With the lower screen voltages they can get away with it.

[J M Fahey]

Screen voltage is 1/2 the Vdc. Why do they want to do that? Jacking up the voltage at the plate so high and power down using the screen?

It does not work that way.
1) they got screen voltage as high as possible, beyond which tube becomes unreliable.
In this case= 350V.
2) they got plate voltage as high as possible, in this case 650V (or whatever)>
3) they plotted load lines, came out with a proper plate load, made the OT which would fit.

[Alan0354]

It does not work that way.
1) they got screen voltage as high as possible, beyond which tube becomes unreliable.
In this case= 350V.
2) they got plate voltage as high as possible, in this case 650V (or whatever)>
3) they plotted load lines, came out with a proper plate load, made the OT which would fit.

I don't understand, most amp with 6L6 have the screen the same voltage as the plate, which is about 450V. Why do they want to jack up the plate to 650V and reduce the screen? Any special reason for that?

We are talking about reducing the screen voltage to do power scaling, of cause you reduce power by lowering the screen voltage.

[Gregg]

It's no a power scaling thing but more like the opposite, just another operating condition. You can see the same thing going on in Ampegs with 6550/KT88 type of tubes. Check out some 6550/KT88 datasheet for operation examples.

[Gregg]

I dug up this old thread where Ray experimented with Paul Ruby zener mod:

http://music-electronics-forum.com/t255/

Down the thread (replies #14 and #15) an idea of "zener scaling" is proposed. I tried to draw a schematic but I'm not quite sure I got it. Please check it and let me know what corrections should be made:



However concerning the comment in #23 I don't know of any MOSFET that is internal diode free.

[Alan0354]

It's no a power scaling thing but more like the opposite, just another operating condition. You can see the same thing going on in Ampegs with 6550/KT88 type of tubes. Check out some 6550/KT88 datasheet for operation examples.

I know it's not power scaling as there is no adjustment. I am just wondering why they do that.

[J M Fahey]

They get much higher power than what the 450+450V guys can ever dream of, that's why.

As in 65/70 real watts versus just reaching 50W RMS.

That's why Univox/Musicman/Peavey/Hiwatt use double tier rails.

It's more expensive and complex that way, of course, so most go with single rails.

[Gregg]

Some updates on my experiments: I tried the dual rail bias (with silicon diodes only - upper part of the schematic above, lower part is obviously wrong) as suggested in the other thread and it worked. I did also some A/B tests between the "regular" zener mod and dual rail bias and they sound pretty much the same. Actually the spectrum is not quite the same but it affects higher order odd harmonics which are at around -30dB or less. If you listen very carefully maybe you'll be able to hear the difference but as a whole it's a pretty good match for the regular mod. Maybe I'll do later some samples.
In some forums Santiago (the designer of the AFD/YJM amps) commented that level is controlled digitally inside the amp. My guess is they lower the signal from the PI not allowing the power amp to clip very hard which is one way to avoid crossover (users reported no crossover and/or fizziness).
If you look at the YJM100 guts below you'll notice 4 pcs of what it looks like big ass MOSFETS near the power tubes sockets. I guess it has something to do with the 50/100W option maybe switching tubes off which is connected to the feature of the YJM has in case of tube failure to "choose" which two power tubes to run. The manual says you don't have to change the load in that case (?).

http://www.tonymckenzie.com/Site/ima...ole-chasis.jpg

[GainFreak]

Recently the EVH5150III combo schematic came to my attention. On page 5 you can see another take at power scaling (the screen type so to speak). Nothing new as concept except that the "high voltage on pot" problem is avoided and the circuit is driven by opamps.
As you know because of the poor regular dual pot tracking and matching the most simple designs suffer from non linearity which especially at low levels results in crossover distortion and other artifacts.
In this the POWER pot is a dual gang one with the Master which is made maybe to prevent getting that nasty crossover distortion at low levels when the PI is hit with stronger signal. Something similar is going on in the YJM100. No variable feedback like in YJM100 though.
I'm wondering how linear this design would be provided there's some feedback going on from the current sensing resistor (BIAS CURR). Looks like a project for the weekend.
Maybe someone who actually used the amp would be able to tell us how it works.

EVH_2x12.pdf