Real McTube and Transformer Math (Gain & Impedance calculations...HELP)

[Blue Jinn]

Hello again!

Came across this http://www.dogstar.dantimax.dk/tubestuf/mctube2.pdf

and a brief discussion here:

http://www.groupdiy.com/index.php?action=printpage;topic=26848.0. Tonepad.com has a PCB, so this seemed a bit easier to start out with before tackling a p2p design. The design seems about a straightforward as is possible.

However, I already started thinking about some mods, Edcor has a nice 200v transformer for about $20, which seemed OK to use, and then toy around with the plate resistors using guidance from the resistance coupled amplifier section of the RCA handbook. That part seems easy enough.

Here is where I need the math help: I planned to eliminate the output pot, becuase, again if I'm understanding things correctly that would increase the output impedance to somewhere around the value of hte pot. If I'm doing the math correctly, it has an output impedance before the potentiometer of about 57k.  Using the plate resistance of the 12AX7 at 250v = 62.5k. (62.5k*680k)/(62.5k+680)

I want to use this with the line in on the board, not an amp, so 57k is too high. So I thought to use the Edcor 4:1 (10k:600.) If I'm understanding the other transformer threads I've looked at I'm calculating this xfmr should lower the signal by a the voltage ratio, (2400/4 = 600 or 10dB or so) (which is prob OK given the high gain already) as well as lowering the impedance to about 3.42k. 57k/x 10k/.6k x=3.42

Also, I can't figure how gain is calculated.  Using Av=(mu*Rp)/(Rp+ra) (Rp=plate resistor, ra= plate resistance of tube) I come up with (100*330k)/743k = 44 = 32.9dB for first stage and (100*680k)/743k = 91 = 39.2 for second stage. Do you just then sum the two = 72.1? This doesn't seem totally accurate, and what and where do you figure the load on the first stage? (Is it RV1 in the stock example? Mr Nachbaur indicates a gain of 2400 or "over 65dB" This part escapes me.

I am pretty sure it is OK to use the higher B+ and the output transformer, but I'd sure like some help with the math part!



Thanks

 

[Gus]

It is just a two stage triode amp.
Why don't you define what you want and design it?  There are few preamp designs at this site.

How much gain do you need?

What input level and/or wanted input resistance?

What output resistance needed?

Balanced or unbalanced output?

transformer out?

 

[Blue Jinn]

I want to use an existing design/PCB and learn from it. I've learned a *TON* just perusing this forum and reading the help threads. But the math part is what I don't yet fully grasp. I had also thought of using a cathode follower rather than transformer. But mostly I'd like to understand how the calculations work. I did come across another thread here, with some guidelines on impedance.

This circuit has ample gain and input impedance seems plenty high for instrument use. As for the specifics, unbalanced out, but need a fairly low impedance out to drive a line in on a mixing board. (50k input impedance I think.) Looking at the PCB it seems relatively simple to cut a trace or two for a cathode follower, but I'd rather try the xfrmr first.