Calculating Output Impedance for a 6G15

[tubeswell]

That looks about right for the tail-end CF, but I'm not sure if the CF after the reverb recovery is going to add much - the tone control it feeds is one of those 'low current' suckers. Mind you, you would have enough juice to drive a TMB tonestack, or some other tone stack (- like in a 5E7, but without the local NFB?) - not that I know what any of that would sound like in a reverb unit.

[Sir Cuitous]

Your right - it may not add much, but could it hurt? Unless there is some "problem" with the CF after the recovery, I think I will re-draw my layout for this design.

Maybe one day I will build it with a TMB stack. I'm not sure If my layout has room for that this time. My guess is that it would sound quite similar to how it works in a guitar amp.

[tubeswell]

Well I doubt if it would do any harm and it probably would enrich the tone of the reverb side if anything

[Steve Conner]

Looks fine.

But if you added that other CF just to use up the spare triode, I'd suggest you just use both sections of the tube in parallel as the "tail-end" CF, and leave the rest of the circuit as a stock 6G15.

[tubeswell]

Hi Steve

Trying to work my way through the benefit(s) of your suggestion;

1) if you used both triodes in parallel, does that mean that if you use a (say 47k-56k?) common cathode resistor, would that mean that the heater-to-cathode voltage is lower than if you used one stage with a 100k cathode resistor? (I presume you should halve the resistance if you use one resistor for both triodes?) - Sorry if that's a dumb question

2) does two CF stages in parallel have half as much output impedance as one CF stage? (- That might be an even dumber question)

[Sir Cuitous]

Same questions as Tubeswell. Is this about right?:



What would be good resistor values? I suppose I could use a 12AU7 for this. I agree - parallel at the output makes more sense. I just couldn't envision it before.

[Steve Conner]

1) Nope, the heater-cathode voltage will probably be just as bad. In the original circuit, the CF always "follows" a DC level of half of B+, so Vhk is just about half of B+. If you halve the cathode load resistor, the idle current will (almost) double, but the cathode voltage will hardly change.

With Sir Cuitous's self-biased circuit the issue isn't so clear-cut: if you halved R4 but left R3 the same, it would cut the cathode voltage almost in half.

2) Yes, the output impedances combine in parallel just like regular resistors.

That schematic looks fine, except what's R2 for?

[Sir Cuitous]

Nix on R2 then. It was just there to betray my ignorance. If I have 250v on the plates, what would be reasonable values for R1, R3, and R4? And/Or what are some good books or manuals that reference this configuration?

[Steve Conner]

1 Meg, 470 ohms and 22-47k according to my magic 8-ball.

A great reference for tube circuits is RDH4, the Radio Designer's Handbook 4th edition. (This particular circuit is covered on page 321.) I have a paper copy, but you can get it free online somewhere.

[Sir Cuitous]

Steve, thanks for all your help (Tubeswell too). I believe I have enough to go back and redesign this thing. And I will get a copy of the Radio Designers Handbook.

I need to cut some metal and get some additional parts. I will create a post when I am closer to starting some real construction. These planning stages are important for me. I feel my projects have been successful because of it.

[Enzo]

Here is one place to download it:

http://headfonz.rutgers.edu/RDH4/

[Sir Cuitous]

Thanks Enzo, much appreciated. I have learned a lot from books by Morgan Jones and Richard Kuehnel, but I feel I need to go to the source (such as the RDH4).

What about the RCA Receiving Tube Manual? Which version? It seems like many versions available of this text.

[Enzo]

I am only familiar with RCA manuals RC10 and RC30 available as re-prints. RCA brought ot new editions all the time - for example I use RC28 in my daily work - but as far as I know only those two are in print.

You definitely want RC30. RC10 is much earlier and focuses on tubes we never use and doesn't include many we do.

The RC30 has some excellent tutorial chapters, but is nothing like the RDH in depth or scope. But I consider it an essential for any well stocked technical shelf. If I were starting over, it is the first book I'd buy.

When I was learning electronics in the 1950s and 1960s, I mainly learned from the RCA book and the Radio Amateurs Handbook from ARRL.

[tubeswell]

RCA manuals

Try the inks on this page (under RCA)

http://www.pmillett.com/tube_data.htm