Tube DIY Asylum

Do It Yourself (DIY) paradise for tube and SET project builders.

Use this form to submit comments directly to the Asylum moderators for this forum. We're particularly interested in truly outstanding posts that might be added to our FAQs.
You may also use this form to provide feedback or to call attention to messages that may be in violation of our content rules.

You must login to use this feature.

Inmate Login

By default, logging in will set a session cookie that disappears when you close your browser. Clicking on the 'Remember my Moniker & Password' below will cause a permanent 'Login Cookie' to be set.

Moniker/Username: The Name that you picked or by default, your email. Forgot Moniker?	Examples "Rapper", "Bob W", "joe@aol.com".
Password:	Forgot Password?
	Remember my Moniker & Password ( What's this?)

If you don't have an Asylum Account, you can create one by clicking Here.

Our privacy policy can be reviewed by clicking Here.

Inmate Comments

From:
Your Email:
Subject:

Message Comments

Original Message

RE: Broskie is correct on this one.

Posted by cpotl on June 15, 2021 at 13:32:00:
"The difference in gain between the two outputs is given by: A1/A2 = 1 + [(Ra+ra)/(Rk(mu+1))]"

This is consistent with the answer Crowhurst would have got if he hadn't made an algebraic error. He has transcribed his formula from Chapter 12 for the input impedance of a grounded grid amplifier incorrectly. The correct formula from Chapter 12 would say Rin = (RL2 + Rp2)/(1 + mu2), but first of all this has ended up in the garbled form appearing in the line above eqn (96), and then he has evidently interpreted this, incorrectly, as (RL2 + Rp2)/mu2. This gives him a wrong formula (96) for Rx, and then in turn this gives him a wrong formula (97) for the required ratio RL1/RL2 of anode load resistors for getting balanced outputs.

His formula (97) clearly cannot be right because if you take the limit when Rk goes to infinity (the ideal CCS limit), it gives RL1/RL2 = 1+ 1/mu, which would say that the anode resistors should be unequal even in the ideal CCS limit. As has been discussed extensively already, this is simply impossible.

Correcting his calculation, one instead gets

RL1/RL2 = (1 + mu) Rk/ [RL2 + Rp2 + (1 + mu) Rk],

and this does indeed imply RL1/RL2 becomes 1 in the large Rk CCS limit. It is also consistent with Ralph's formula (his (A1/A2)^{-1} is the corrected expression for RL1/RL2 above).

Out of curiousity I tried the repeating the (corrected) Crowhurst calculation in the case where one does not make any assumptions about the two tubes being the same. So I allowed them to have different mu values mu1 and mu2, and different internal plate resistances Rp1 and Rp2. If I did my sums correctly the required ratio RL1/RL2 for getting balanced outputs is now

RL1/RL2 = (1 + mu2) Rk/ [RL2 + Rp2 + (1 + mu2) Rk]

In other words, the characteristics of tube 1 do not enter at all into the formula for the ratio of load resistors one needs in order to get balanced outputs! This seems a bit remarkable, but I think it is correct; at least, if Crowhurst's other formulae are correct. It happens because Crowhurst's eqns (92) and (93) imply Ep1 = RL1 Ek/Rx, and so when one equates Ep1 to Ep2 and solves for RL1, the Rp1 and mu1 parameters characterising tube 1 do not enter.