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180.200.144.158
Those who haven't read the description of what I and my techie Bill did to the amps, can read it up over in reviews."Why do you ask, Timbo?"
Well I've been trying to follow the several threads about this over at Tube/Diy, led mostly by Triode/Kingdom and the headroom/overload/recovery issues. And especially bad old Pentode mode!
Way back when, I 'found' four NoS AWV and AWA Aus. mfd 6CG7s in the shelves of my soon to retire old techie and decided that they would make a good replacement for the 4 OEM 12AX7 driver tubes in the two - 2 - LEAK st20s that I'd been 'given.'
And that old techie nodded and agreed, and Bill then suggested a single CCS Fet each and LTP mode for the splitter-drivers. I made the mods because I instinctively? believed a higher-current lower-gain valve would be better in this role and overload more cleanly. And, we could allocate the NFB better - to the OPT stage mostly. There's a low-gain tt input tube as well.
The amps will soon be running from above and below 150Hz (line level crossovers). Still into ~91db/w sensitivity speaker loads.
Sooo, amp two - was used in triode-mode role driving an Fs trap, 3rd order HP crossover from 3.5khz and a big soft-dome tweeter It's now in UL mode (16wpc RMS 20-20khz) - and it will be working a LOT harder? Yes?
Further, the other chassis will be working a lot LESS hard, just from 150 Hz and down only and still in pentode mode (20W RMS 20-20K). Yes? The reflex tuning will still be the same QB3 and Fb @ 38hz. A slow, shallow initial slope. I'll run the two bass spheres in series - 16 ohm load - and change the OPT tap.
(The bass-duty amp will likely also have to drive a high impedance load for an active sub-woofer LP filter. Into a SS power amp and two or more sealed PP subs with LT eq. But it won't even blink at that.)
I've already had Bill put in the PLL 1st order filters on each amp's inputs as well as service them.
Soooo, what kind of tests for overload and recovery can I get Bill to do on the two amps in the workshop OR at home here running at 'turn that down' / 'Are you DEAF!!!!' loud? :-)!
How big's the room? The l-shaped lounge-dining room's longest dimension is ~ 35 feet, and has 8 ft ceilings.
Or do I just stop worrying, and forget about it? ;-)
TIA
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Edits: 09/04/14 09/04/14 09/04/14Follow Ups:
If by "examine" you mean measurements based on a controlled test, you'll need a test setup specifically for this. The easiest way to see what's happening is probably to drive the amplifier with an arbitrary waveform generator. Generally speaking, I would input several cycles of an "overload-level" sine wave and immediately follow that with a few hundred milliseconds of low-level signal. Analysis of the recovering signal probably must be done visually with a scope, as the short lifespan of the "recovery-in-process" condition probably prevents use of a distortion analyzer or other test gear. A simpler test might be to drive the stage into overload, then kill the input and monitor the DC drift on the grid with the scope, then repeat the test while looking at the cathode (if self-biased).
This issue might produce less obvious audible artifacts in a biamped system such as yours. The higher levels of THD and IM that I would expect to be generated during recovery are most likely to occur in the low frequency amplifier. That amplifier, however, is cutoff at a fairly low frequency, so much of the distortion won't be heard.
As for simply not worrying about this, I think it's worth noting that a number of respected designs from the '60s, including the Heath W6M and McIntosh MC60, used cathode followers to drive the final stage. Regardless of how modern audiophiles rate amplifiers like these, it's clear that driver impedance and overload recovery were on the minds of those earlier designers.
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Buy Chinese. Bury freedom.
On the AA website under 'reviews,' yes or no?
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
No, do you have a link?
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Buy Chinese. Bury freedom.
"can read it up over in reviews.' First line of my OP!? :-)!
Not that hard to find, really.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
I don't see anything in that post related to this thread. Is there a connection?
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Buy Chinese. Bury freedom.
When you say overload, are you talking about departing from class A?
Overload can be tough to test for, if you put a load on your amp and attempt to measure power/distortion, it will fall apart as it edges into class B, which is why cathode bias is a poor choice for such a design.
The best shot that you have is to design for the power you want in class A. You may potentially be able to put a voltage regulator under the cathodes of the output tubes to keep the bias from wandering all over the place, but that would be a topic of recommended research, not a solid suggestion.
yes, or no?!
I can't tell.
Warmest
Tim Bailey
Skeptical Measurer & Audio Scrounger
Your description was about the driver stage and the power supply.
The driver stage is not going to be the dominant problem with a cathode bias amp going from A to B, the cathode resistor/capacitor will be.
For the power supply, you really have two options.
1. A real choke input filter, which would provide adequate regulation for the wide range of currents demanded.
2. Tie the B+ feed to the output transformers from the first node of the power supply, which would minimize voltage changes from filtering impedance, but still isn't as good as above.
Do note that these power supply mods assume that you know to run the amp in fixed bias in the first place. With cathode bias, you need to stick to class A.
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