Understanding "grid drive" to drive, 2, 4, 6 power tubes

[Old Tele man]

...or, simply use a "True RMS" capable meter, such as Fluke.

[nickb]

...or, simply use a "True RMS" capable meter, such as Fluke.

That doesn't help. You need something that can measure average DC. The problem is that DMMs are sampling devices and quite low frequency sampling at that. So, unless they have a low pass filter on the input the readings will not be steady or reliable.

[Helmholtz]

...or, simply use a "True RMS" capable meter, such as Fluke.

Measurement is made with Fluke 27 FM which is true RMS

I just checked the manuals of several "True RMS" handheld meters. None of them seems to be able to measure AC and DC voltage components simultaneously. Real RMS should comprise both. And unfortunately RMS of the sum of DC and AC voltages is not equal to sum of DC and RMS AC.
But AC and and DC power components of a resistor add up linearly, that's what I used in my proposal. Total RMS voltage (including DC) is not required.
For the AC voltage measurement across the screen resistor a "true(AC) RMS" meter is fine. It is actually necessary with distorted screen currents at clipping.

[nevetslab]

I just checked the manuals of several "True RMS" handheld meters. None of them seems to be able to measure AC and DC voltage components simultaneously. Real RMS should comprise both.

Another good application for a Fluke 893A Differential Analog Voltmeter (AC / DC), which allows dialing in the average voltage and see the min/max meter swing for eyeball averaging.

[Dave H]

I attempted to measure screen dissipation with my Picoscope but wasn't helped by low resolution (8 bit). The amp was 2 x EL84 15W cathode bias.

I connected Channel A and B scope probes each side of one of the EL84 1k screen resistors. Channel A (blue) to the EL84 screen pin and Channel B (red) to the screen B+ node. I set up a Pico math channel (black) to plot (B-A)*A/1000. i.e. Is * Vs = screen power and set up a Measurement to calculate the DC average of a whole number of cycles (4) of the (B-A)*A/1000 channel. The result is shown at the bottom of the first plot below. It has screen dissipation at 3.6W.
The second plot below shows the amp's clipped output across a 10 ohm dummy load.

Using a cheap DMM set to DC volts I measured 13.25V across the 1k screen resistor and 280V on the screen pin which works out at 3.7W screen dissipation.

[Helmholtz]

Thanks, this confirms Nick's statement that average DC measurements yield usable results.
But as mentioned earlier steady state close to clipping is not worst case, as screen supply voltage will be low.

[nsubulysses]

I have learned a lot about phase inverters and screen grid dissipation and many other things along the way. Thanks!

Could anyone care to speculate on this question?

SO I have an amp with 6x6CA7 and it does 180W at clipping. Power tubes clip at 110mA or so. OT primary impedance is 1.6K.
A 4 power tube amp with same power supply voltages and 1.9K OT primary does 144W and power tubes clip at 130mA or so.

If I order a 200W OT with 1.1 or 1.2K primary impedance will my power tubes clip at the expected-ish 130mA and will I achieve 200-210W at clipping?

I guess I am wondering if 1.1 or 1.2K primary will bump me up significantly, like 20-30W. Or would it be marginal, 5-10W whatever.

Everyone, please hold your comments how 180W vs 200W is not a worthwhile difference to care about.

Soon another experiment will happen and I'm excited to see how it goes. I will build a 4 power tube amp with a PV Roadmaster power transformer I got for free. the PT is formatted similar to the SVT where there are two seperate high voltage taps. It will have a tap for 600VDC for plate supply and 350V for screens. If it is called a 200W amp with 4xKT88 I expect it to do 180W with 4X6CA7. I wonder how different it will sound with this power supply arrangement vs the current one I use which is similar to Ampeg V4 or Traynor Custom special with high voltage for plate and screen, and a big resistor in place of choke to make the screen supply sag when amp is cranked?

[nickb]

SO I have an amp with 6x6CA and it does 180W at clipping. Power tubes clip at 110mA or so. OT primary impedance is 1.6K.

This doesn't add up.

3 x 110mA = 330mA. A 1.6k OPT is 400 ohms each side so power out =0.33^2*400/2 = 22W under these conditions.

What is your supply voltage?
How did you measure 110mA?
How did you measure power out?
Do you have the OPT specs?
What is the screen voltage?

[Helmholtz]

SO I have an amp with 6x6CA7 and it does 180W at clipping. Power tubes clip at 110mA or so. OT primary impedance is 1.6K.
A 4 power tube amp with same power supply voltages and 1.9K OT primary does 144W and power tubes clip at 130mA or so.

If I order a 200W OT with 1.1 or 1.2K primary impedance will my power tubes clip at the expected-ish 130mA and will I achieve 200-210W at clipping?

I guess I am wondering if 1.1 or 1.2K primary will bump me up significantly, like 20-30W. Or would it be marginal, 5-10W whatever.

According to 6CA7 data, a pair will deliver 70W into a 4k load with 475V plate supply, 400V screen supply and 150mA cathode current (125mA plate + 25mA screen) all at full power. Higher screen voltage tends to decrease optimal plate load. From this I conclude that for 6 tubes an OT primary impedance of little below 1.3K should be optimal and should give around 210W. Higher than optimal load resistance generally decreases max. cathode current and output power before clipping.
Final result will depend on OT losses (5% difference means 10W) and power tube quality (especially saturation voltage).

[Helmholtz]

power out =0.33^2/400/2 = 22W

Don't understand this equation. I would think something like P= (0.33^2) *400*4 = 174W.
And this would be primary power without considering OT losses.

[nickb]

Don't understand this equation. I would think something like P= (0.33^2) *400*4 = 174W.
And this would be primary power without considering OT losses.

Sorry, I had a simple typo. It should read (0.33^2)*400/2 =22W.

I'm taking the 330mA figure as the peak current but I'm wondering if it was supposed to mean something else? Taking the datasheet example of 125mA the peak current is more like 400mA. Hence the questions for clarification above.

[Helmholtz]

Specified plate current in datasheet is most probably average DC, so not suitable for power calculation.

[Old Tele man]

Data sheet numbers are usually for BOTH tubes at idle, but RMS for one tube, in Class AB operation.

[g1]

SO I have an amp with 6x6CA7 and it does 180W at clipping. Power tubes clip at 110mA or so. OT primary impedance is 1.6K.
A 4 power tube amp with same power supply voltages and 1.9K OT primary does 144W and power tubes clip at 130mA or so.

If I order a 200W OT with 1.1 or 1.2K primary impedance will my power tubes clip at the expected-ish 130mA and will I achieve 200-210W at clipping?

You probably mentioned PT type earlier, but this question assumes that the PT has more output available and it is not the limiting factor. Could you refresh us on which PT is in use?

[nsubulysses]

Custom heyboer PT 840VCT. Not sure of the current handling, probably about 800mA. I said, "hey what should it be, 800mA or so?" they said, "yeah somethin like that". The 4 power tube version is based on Hammond 278CX which is 800VCT at 535mA. (Heybeor PT ordered as 840VCT because the primary is 120V. Hammond PT has 115V primary so you get a higher B+ even though it's only 800VCT)

current draw pwer power tube is measured over a 1 ohm 1% precision resistor in cathode.