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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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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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