Odd tube program amp

Just surfing around found this (below) schematic. I've never tried a grounded grid circuits. Can anyone comment on the gain of this circuit? Output tx is UTC A-25 (5:1)

Looks like an attempt to adapt RF techniques to audio. The problem is that the source is meeting a low impedance defined in large part by the 500r cathode res; there may be a significant advantage in this circuit if the input Z was dominated by the cathode impedance (approximately 1/Gm). 12AX7 doesn't seem a good candidate in that respect.
Common-gris circuits are good at eliminating Miller capacitance, which is moot in the context of audio. The capacitive part of the input Z is a few pF, which makes the configuration distinctive above 100MHz.
A number of designers adapted this technique to solid-state, without any significant improvement over the more common common-emtter topology.

Would this work as a (relatively) low impedance mix bus input?

Michael

Michael Tibes said:

Would this work as a (relatively) low impedance mix bus input?

Michael

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I guess it was what it was intended for; it is labelled as a "program amplifier", but its gain is excessive in that position. It could be a mic preamp, not particularly good at that.
As a summing + line out amp, it would work somehow but I think the S/N ratio would not be something to write home about...  Looks like something out of a 1940's mixer for AM radio, where the medium's dynamic range does not require outstanding performance.

One limitation with tubes is noise. To obtain a good signal to noise ratio at low source impedance the usual method is to use a common cathode circuit preceded by a step up transformer. An alternative method of obtaining a low input impedance is to use a common grid circuit as in this example. Unfortunately, the noise at the input of this circuit is the same as at the grid of a common cathode circuit, and the gain is also the same,  but we do not have the advantage of having the signal stepped up by a transformer and so the noise performance of this circuit for audio purposes is quite poor.

Cheers

Ian

certainly never seen this before so that is cool right there.

try it and see, if you get a different sound out of it then you have a tool for creativity,

CJ said:

try it and see, if you get a different sound out of it then you have a tool for creativity,

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Yea if the professors gave it higher marks I might have given it a shot...

ruffrecords said:

Unfortunately, the noise at the input of this circuit is the same as at the grid of a common cathode circuit, and the gain is also the same,

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So if the source was your typical 600 ohm secondary from most equipment back then, it would most likely be noisier than a typical say...1:4 transformer input tube amp because you don't get the noiseless voltage gain? It just looks to me like they were just trying to avoid buying the input transformers. But it seems there has to be some kind of advantage over simply going into the grid with a 500 resistor to ground. Must be a bit quieter? A 100uf capacitor wasn't the cheapest thing back then.

Seen this one floating around, might even been talked about here before but its peaked my interest. Looks like you would definitely want some input caps. A better output stage that could drive a little more with a bit more gain would be nice. Always liked the idea of using a low plate resistance 6AS7/6080 for an output tube, only capable of 6db gain but can drive 600 from the plate no problem. Then what always stops me is the heater current (2.5A) 

http://wooaudio.com/docs/tube_data/6AS7GA(6080).pdf

bluebird said:

Seen this one floating around, might even been talked about here before but its peaked my interest. Looks like you would definitely want some input caps. A better output stage that could drive a little more with a bit more gain would be nice. Always liked the idea of using a low plate resistance 6AS7/6080 for an output tube, only capable of 6db gain but can drive 600 from the plate no problem. Then what always stops me is the heater current (2.5A) 

http://wooaudio.com/docs/tube_data/6AS7GA(6080).pdf

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With the 2.7k +500r cathode res, it makes a little more sense, but it's just a curio, with no real advantage.