Smartest way to 200W?

[trobbins]

I think your loadline would be close to link. I think there are only up to 300V screen curves in datasheets. If you get peak current per tube up to 450mA, then simple power calculation is:
(Ipk)2 x Rpp / 8 = 0.9 x 0.9 x 2k / 8 = 200W

You'll need to modify the curves in your head for higher screen voltage at Vgrid=0V - but effectively you'll need to keep screen and B+ high to get to that peak current point. And of course you battle surging screen current, and other parasitic voltage drops come in to play such as DC resistance of OT primary half winding. Did you measure B+ and screen voltage with an rms meter or from an oscilloscope waveform?

Ciao, Tim

http://dalmura.com.au/projects/2k%20...V%20screen.pdf

[Gaz]

I think your loadline would be close to link. I think there are only up to 300V screen curves in datasheets. If you get peak current per tube up to 450mA, then simple power calculation is:
(Ipk)2 x Rpp / 8 = 0.9 x 0.9 x 2k / 8 = 200W

You'll need to modify the curves in your head for higher screen voltage at Vgrid=0V - but effectively you'll need to keep screen and B+ high to get to that peak current point. And of course you battle surging screen current, and other parasitic voltage drops come in to play such as DC resistance of OT primary half winding. Did you measure B+ and screen voltage with an rms meter or from an oscilloscope waveform?

Ciao, Tim

http://dalmura.com.au/projects/2k%20...V%20screen.pdf

Tim, thanks for the link! This is Alex G from AX84, so you're already familiar with my project a little bit I measured voltages with a true rms meter while observing clipping on the scope.

[trobbins]

Hi Alex,
A meter for B+ is fine for estimating anode-cathode voltage, but it may give you a significantly false screen voltage reading if you are pushing to Vgrid=0 as the screen current peaks for only a small portion of the waveform (unless you're cranking it to near square wave). Looking at the KT88 curves indicates perhaps approaching 100mA peak through 1k, so you may be getting towards 100V less than what you measure.

Maybe redoing your testing with 100R screen stoppers may give a few more watts.

Ciao, Tim

[J M Fahey]

100 mA peak screen grid?
Don't doubt your post, too sleepy to check curves, but that would certainly give me the creeps !!!!
I bet the screens would light like a Christmas Tree !!!
Even if a small portion of a just clipping sinewave, output squarewaves are practically *guaranteed* by Rock players.

[trobbins]

Which is consistent with the common demise of many tortured screen stoppers in early guitar amps.

How close the screen current would get to something like 100mA can probably only be measured by a cro. The datasheet typical value is rising rapidly through 100mA at Vg=0V if screen is held at 300V and anode saturates to 25V. But screen will droop to well below 300V, so peak current wouldn't reach 100mA with 1k stoppers.

[R.G.]

As a practical matter, my pocketbook gives out on tube amplifiers somewhere beyond 100W.

[Gaz]

As a practical matter, my pocketbook gives out on tube amplifiers somewhere beyond 100W.

R.G., we all know you're the 1%...

...of guys that will put MOSFETs in their tube amps!

[Gaz]

I paralleled the 1K screen stoppers with 100R resistors (for about 90R), and the amp will now do 210Wrms clean! Only problem is screen dissipate just under 12W with max signal - the JJ spec sheet claims 8W max. I tried 500R stoppers too, which yielded 170Wrms, but still max dissipation on the screens was around 10W. With the 1K's the g2 dissipation was safe around 7W.

This may be a stupid question, and just humor me here, but would it be correct to assume that the voltage I'm measuring at clip on the screens with the 100R stoppers would be the voltage I'd need to see with the 1K's as well to get 210W? I'm thinking it may not be such a linear relationship as I'm presuming.

[J M Fahey]

Tubes are voltage controlled valves, so all electrons know (among other voltages) is screens are at such and such voltage at a certain moment.
They don't know or care where that voltage came from.
Raising screen voltage is excellent for tubes to pass more current even when saturating (because screens "cheat" electrons attracting them with higher than plate voltage), only problem is that screens pass current themselves and may overdissipate.
So there you have a limit as to how much power you can pull from a tube.
In this case, getting 210W by overdissipating screens 50% beyond absolute maximum is a dangerous move.

[jazbo8]

Is this one of those trick questions? R=V/I, so the relationship is linear, but you cannot get the exact voltage at the screen just by changing the screen resistor and expect the same output power... Did I get it right?

Jaz

[Gaz]

Tubes are voltage controlled valves, so all electrons know (among other voltages) is screens are at such and such voltage at a certain moment.
They don't know or care where that voltage came from.
Raising screen voltage is excellent for tubes to pass more current even when saturating (because screens "cheat" electrons attracting them with higher than plate voltage), only problem is that screens pass current themselves and may overdissipate.
So there you have a limit as to how much power you can pull from a tube.
In this case, getting 210W by overdissipating screens 50% beyond absolute maximum is a dangerous move.

I wasn't planning on over-dissipating the screens, I was thinking perhaps I could raise the screen voltage and keep the 1Ks to limit current safe levels. I'm not sure if I'm missing your point and my logic is flawed.

[trobbins]

Alex,

Were you measuring the rms voltage at the screen, and the rms current from the voltage across the screen stopper? If so, then you are just measuring VA power, and need power factor to determine the rms Watt dissipated in the screen, as it isn't just volts x current, because the waveform will likely be getting quite distorted at max output, and real power dissipated may be significantly lower than your suggested 12W. Without some exotic measurement equipment then a waveform of voltage and current may be an indicator.

Ciao, Tim

[J M Fahey]

I wasn't planning on over-dissipating the screens

Only problem is screen dissipate just under 12W with max signal - the JJ spec sheet claims 8W max.

Am I missing something?

As of:

I was thinking perhaps I could raise the screen voltage and keep the 1Ks to limit current safe levels

the answer is:

all electrons know (among other voltages) is screens are at such and such voltage at a certain moment.
They don't know or care where that voltage came from.

which in good English means:
*suppose* your KT88 need 300V on screens to produce 210W, and under those conditions they pass 25mA average, meaning they dissipate 12W average (this is just an example); they "don't care" whether those 300V come from a stiff regulated 300V source with 0 ohm resistors, or from a 350V one and 100 ohm ones or from a 400V one and 1K ones ; *they* will always pass said 25mA and drop 300V=12W.
Values are made up, just to explain the concept.

[Gaz]

Am I missing something?

As of:

the answer is:
which in good English means:
*suppose* your KT88 need 300V on screens to produce 210W, and under those conditions they pass 25mA average, meaning they dissipate 12W average (this is just an example); they "don't care" whether those 300V come from a stiff regulated 300V source with 0 ohm resistors, or from a 350V one and 100 ohm ones or from a 400V one and 1K ones ; *they* will always pass said 25mA and drop 300V=12W.
Values are made up, just to explain the concept.

You're not missing anything, I was just was not planning on leaving the 100R resistors in, it was just a test.

Thanks for the really clear further explanation, I do understand what you are saying!

[Gaz]

Alex,

Were you measuring the rms voltage at the screen, and the rms current from the voltage across the screen stopper? If so, then you are just measuring VA power, and need power factor to determine the rms Watt dissipated in the screen, as it isn't just volts x current, because the waveform will likely be getting quite distorted at max output, and real power dissipated may be significantly lower than your suggested 12W. Without some exotic measurement equipment then a waveform of voltage and current may be an indicator.

Ciao, Tim

Here's what I did to measure the screen dissipation (and I had read this somewhere (Aiken's site maybe?): measured voltage drop across the screen resistor to find screen current. Then I multiplied screen current by the sagged screen voltage when the measurement was taken. Is that what you're describing, Tim?

If so, it sounds like you're saying my measurements are pretty useless. Is there any other clever way to do it with out the the new-fangled fancy testin' machines?

Thanks for the hand-holding guys!