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24.27.41.140
In Reply to: RE: Losses in Output Transformers posted by Paul Joppa on September 15, 2021 at 21:49:09
Paul, the transformers are specified as 6.6K, and that's what I measured on the bench in testing last year. There's a small difference between the 4-8-16 ohm taps. Based on the voltage drop I mentioned earlier, which occurs at roughly 45 mA idle per tube, I believe the primary DCR is 250-300 ohms. These are not as large as ST-70 transformers.
I worked on the amp today to further optimize the feedback and response. At the end, I made another power measurement. With +430V on the CT of the OPT and +350 on the screens (full output, one channel driven), maximum output is about 30W. The tubes are set to idle at 45-50 mA. Current at full output is about 85 mA per tube, measured with a DVM across the 1 ohm cathode resistors.
The original outputs used with this transformer set (PT and OPTs) were 7591. The manufacturer rated the amp at 35W/ch, probably with one channel driven. I changed the outputs to 6L6, then designed a new driver with higher gain. Open loop gain of the driver is about 350, more than twice that of the original 7199. Output headroom at the 6L6 grids is also more than 6dB, relative to the 60V pk-pk required for full output.
Again, this isn't the first time I've encountered this. I'm becoming somewhat skeptical of power tube ratings in terms of the "typical operating conditions" presented in the sheets. It's also not clear to me how that data is acquired by the manufacturers, and exactly what it represents.
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Buy Chinese. Bury freedom.
Follow Ups:
So far, I've never seen an accurate
tube characteristics chart-- or plate-
curve sets that were anything better than a set
of suggestions, and nothing more.
Even those figures vary considerably. A good
example is the 12BZ7A. Some charts look as
if the tube isn't linear enough for audio.
Charts from another MFR. looks just like a good 6SN7.
SO__ what gives?
What gives is WHAT DOES IT ACTUALLY DO?
TWO ways to find out for sure: (1) run your
own plate curves. (2) LISTEN to what it is
doing, using different voltages & currents.
I do both. Because you have to do it.
-Dennis-
I have, more than once(!), found that my gut feel for "close enough" is not always in fact close enough, and I have to start over with more attention to more details.
In the initial post, you drew attention to a specification for 450v/350v plate to cathode, using 6L6GCs into 6000ohmsCT. But you are measuring something like 415v/350v (after subtracting transformer DCR losses at max power) into 6600 ohms. All these little differences can add up fast ...
My two cents.
Yes, small differences can add up, but I wasn't expecting to see the full 50W. The original spec for the amplifier using this iron was 35W RMS output, and that's relative to even lower voltages. The original circuit only indicates +420V on the CTs and +410V on the anodes, and that's at idle. The higher numbers I posted were measured at full output after the supply sags.
In addition to the original specifications, I've searched for other analogous operating conditions. Several manufacturers show 6L6 variants operating in AB1 at lower voltages than this producing more than 35W. The RCA 807 sheet shows +400V anode / +300V screen into a 6.8K load will deliver 36W. The Sylvania 7591 shows +400V anode / +350V screen into 6.6K will deliver 37W. Are these theoretical numbers? Do they include the losses of test transformers? I don't know, but the amp on my bench is operating at higher plate voltages than this, and I expected to see more.
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Buy Chinese. Bury freedom.
Here's the chart from Sylvania. My amp clearly clearly falls in-between the two columns that are outlined. But just for the sake of argument, let's say it coincides with the lesser of the two, the one on the left - +400V anodes and +300V screens. According to Sylvania, that should produce 34 watts (nearly identical to the 807, at 36 watts). So why is it, even with higher voltage on the elements than shown in that column, the tubes aren't producing this much output power? Does anyone here know how manufacturers derived these output numbers for their power tubes? Is there a technical text somewhere that describes this rating system?
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Buy Chinese. Bury freedom.
RCA tube division production test method
I'll spend some time on that.
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Buy Chinese. Bury freedom.
and this book will provide more color.
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