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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Yes, but... posted by Engineer on November 27, 2000 at 05:37:29:
What the...
Hunh?
Grid resistors on output tubes are NOT for suppressing parasitic oscillations, but to what? Improve transient response?
Boy, that's a fresh one on me. I've read plenty of electronics texts from the tube era, many with darn good sections on audio design & I've NEVER heard of this one. And I've been heavily into designing, repairing & reading about tube amps for 20 years!
Mr. Engineer, could you please point us in a more specific direction to that particular 1955 AES paper?
I don't have the schematic to the HF-87, as least not handy, but I'm going to assume that this "cathode-coupled inverter" is what is also known as a "long-tailed pair".
This inverter would not have the ability to drive the grids of the output tubes very far into positive territory, if at all. We couldn't be talking more than 3 to 5 volts here.
And how is "charging" the coupling capacitors positive going to cause problems? They are "charging" all the time, anyways! The grid resistance will pull down the voltage quickly enough, I've never heard of this problem before!
And are they really "charging"? I don't think so. They are there to block the high voltage DC from the inverter (assuming a normal long-tail pair inverter capacitor coupled to the output tube grids). Unless they are leaking, no voltage changes on the output tube grid side of them can affect bias. At all.
This asymmetrical clipping problem that freddyi was talking about seems to be more an artifact of the simple splitter inverter, where the output impedance is different on each phase's output (the cathode fed phase is obviously a cathode follower, the anode fed phase obviously an anode follower!). A long tailed pair would be exempt from this problem, even if it wasn't exactly symmetrical in its output. It has the pretty much the same output impedance on each phase regardless of exact symmetry.
Given the ratio of difference between the grid bias resistance (minimum 50,000 ohms for common audio pentodes) and what you are recommending as a typical range of series grid resistance (1000 to 4700 ohms), I fail to see much current being stopped from going into the grid & being diverted in to the grid bias resistance instead.
There would have to be only the most modest improvement in clipping behaviour, assuming this distortion mechanism that you're describing even exists! The only amp that I've ever heard that clipped really badly was the Fisher SA-300, which had the stupidest coupling networks from the phase inverter to the output tubes that I've ever seen. It was a form of mild direct coupling (fixed-bias grid-leak, you might say. A 2.2M resistor was in parallel with the coupling caps), and it made the amplifier "stick" when hard-clipped. For those who don't know what "sticking" is, it means that the amplifier goes into a state of prolonged oscillation when overdriven, "sticking" in that state & refusing to respond to any input signals or recover to a stable state. Simply removing those resistors eliminated the problem. Completely!
Can't say in my experience that ever noticed an audible difference in clipping characteristics in amps related to the size of their output tube grid series "stopper" resistors. I agree with your arguments against making the value too large. Just not about making them too small. And any references to this resistance that I've read has ALWAYS referred to its function as suppressing tendencies to oscillate.
And this talk of "maxing out the power supply when clipping"...of course you're maxing out the power supply! Why is adding another cap, even if one is gone, going to make a difference here? Of course the HT rails sags under load! And in old vintage amps, the series chokes, resistors & valve rectifiers are far more of a problem than just adding a capacitor can fix. Assuming that HT rail sag causes these kinds of output stage anomalies, which is also very much open to question. It probably causes all sorts of other anomalies as well, and some that are probably far worse...
Sorry Mr. Engineer, but I think you are a very creative thinker with a strong lateral group of tendencies.
All this talk of DC bias pushing things into Class B (the DC bias is going more negative at clipping? What? This sounds like so much goop...)
I'm just a boy from Missouri, so please provide some references & proof please!
Other opinions, anyone?
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Follow Ups
- Re: Yes, but... - Joe Rosen 12/2/0004:39:49 12/2/00 (2)
- Re: Yes, but... - sajti 12:37:26 12/3/00 (1)
- Re: Yes, but... - Joe Rosen 13:04:06 12/3/00 (0)
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