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    Building, troubleshooting and testing of these amplifiers should only be
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AC Characteristics of Glow Tubes?

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Joined 2003
If you download the 85A2 data sheet from Frank's site, you will find that although it doesn't give the slope resistance explicitly (it does on my Mullard data sheet), it gives a graph of Va against Ia, allowing you to measure the slope at a given current to determine the slope (AC) resistance. It also gives a figure for the noise.
That explains why bypassing a glow tube that is in the signal path with a small cap helps. I tried it without the cap, and it sounded okay, but was a little flabby in the LF response. I am not sure why the small cap helps with low frequency.

I am using this schematic

An externally hosted image should be here but it was not working when we last tested it.

and I think I am not sure how to think about the impedance that the cap sees. Obviously, the cap can't be larger than 0.1uF. With an ultrapath cap, it is typically a couple of orders of magnitude larger, but I am not noticing any low frequency roll off even with a 47nF cap -- maybe I just can't hear. I still need to take some measurements, but I could use some help in determining this.
I figured about 300, but, whether 100 or 300, this is why I am confused. I am assuming that this impedance, whether 100 or 300, is going to be multiplied by mu and added to the rp of the tube. And this I would think would account for the flabby bass as mu is pretty high. But it wouldn't account for a 100nF cap seeming to fix the problem. At any rate, I think it is time to do a few measurements -- a good excuse to buy a new sound card I suppose.
my Audio Cyclopedia mentions using gas tubes in place of capacitors -

"...Instead of the usual coupling capacitor, the control grid of the second stage is fed from the plate of the first tube through a small neon tube. Voltage regulator tubes, as a rule, are not satisfactory because of their high operating potentials. The purpose of such coupled amplifiers are to reduce the phase shift generally associated with resistance-coupled amplifiers at the extremely low frequencies. Gas coupling is only practical at fairly high signal levels because of the internal high noise level of a gas tube. The frequency response and stability may be improved by the use of negative feedback by eliminating the usual cathode-bypass capacitors." - Tremaine / 1975

I know - not very useful for what you are looking for!
Maybe this will help think about this -- this circuit should work the same. In fact, maybe it will work better and I should just do this instead. I have not figured out what the second tube should be yet. It could just be the same tube, but I don't see why it would need to be ... on the other hand, have I just drawn up a primitive push-pull amp with single ended iron?


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