Collins 26C Analysis

[DaveP]

Whilst looking for an alternative to the 6C8-G, I looked into the history of the this tube and the 6N7 which has similar characteristics.
Comparing the two:-
6C8-G 6N7
Vp 250V 250V
-Vg 4.5V 5V
Ip 3.2mA 6mA
mu 36 35
rp 22.5k 11.3k
gm 1.6 3.1
fA 0.3A 0.8A
The 6N7 came first in 1936, mainly for making the most of battery output stages (before universal mains power)
The 6C8-G was introduced by Ken Rad in Jan 1937, it must have very similar geometry because the mu is practically identical.
The main difference is that the cathode heating is much less, which results in half the gm and double the rp.
This got me thinking that it might be possible to modify the characteristics of a 6N7 by inserting a resistance in the heater supply of the 6N7?
best
DaveP

 

[DaveP]

The inaccuracies of the tangent method make it difficult to estimate the resistance of the 6C8G at very low currents near cut-off. These are the parameters supplied by Tung-Sol courtesy of Frank:-

I managed to find a formula which predicts the ra/rp curve, it looks like this:-
Where x = current in mA and y = resistance in kohms.
y = 41*√1/x + (x^(e))/180. (If there are any maths specialists out there who can improve on that I would be grateful!)
Anyway, this has allowed me to draw a nice curve into the cut-off area.

You can see that the yellow trace does not have the bumps and sudden changes of direction from the curves at the limits.
It follows the actual curves quite closely until about -5V, then the actual curves are too difficult to measure accurately.

My conclusions are that it is not realistically possible to substitute the 6C8-G for another tube. The resistance characteristic of starting at 26k and increasing to 120k at cut-off cannot be found on other tubes and are almost certainly essential for the shape of the compression curve which is at the heart of the sound of this compressor.

I hope you have enjoyed this thread as much as I did and thank you for your support.
best
DaveP

 

[emrr]

Bravo Dave! Thanks for the exploration!

 

[Disco Volante]

Bravo! I'm learning so much here.

 

[chrispbass]

Great thread Dave…thank you

 

[DaveP]

I must admit I am very tempted to make one of these again, I will wait until the end of the month when I have a scan to see if I have the all-clear.
Thanks again to you guys
best
DaveP