Common cathode circuit values

[santonel]

Is there are a definite step-by-step sort of process for determining Ra and Rk and the caps in a common cathode for any given audio tube? I can't seem to wrap my head around it the way it's explained on most of the sites I've found. I'd like there to be just checklist of steps or at least a simple concise explanation in point form.

[Arnulf]

Of course.

1: Any subsequent load and Ra determine the loadline, B+ and Pa limit the area in which the loadline must reside

2: Quiescent point is chosen on the loadline with help of dynamic characteristics curves

3: Quiescent point Va and Ia values determine Vg

4: Vg and Ia (Ik in the case of tetrode or pentode) determine Rk (Ohm's law)

5: Rk and desired corner frequency (Fc) determine Ck

[Merlinb]

Originally Posted by santonel

Is there are a definite step-by-step sort of process for determining Ra and Rk and the caps in a common cathode for any given audio tube? I can't seem to wrap my head around it the way it's explained on most of the sites I've found. I'd like there to be just checklist of steps or at least a simple concise explanation in point form.

You can use formulae to find suitable values, but using load lines is much more intuitive, and you can see at a glance what the limiting factors are (headroom, approximate THD, dissipation etc. The Valve Wizard
The cathode cap is often not calculated at all- just use an arbitrarily large value. However, it can be calculated if necessary: http://www.freewebs.com/valvewizard2...BypassCaps.pdf

[Miles Prower]

Originally Posted by santonel

Is there are a definite step-by-step sort of process for determining Ra and Rk and the caps in a common cathode for any given audio tube?

No, not really. Hollow state design isn't like solid state design. With VT's, you have three or more separate elements just hanging there in free space (or a reasonable approximation) all doing their own thing. It's not like a transistor where the emitter/base/collector/source/gate/drain are all connected, and where you can expect fixed relationships such as Vbe= 0.6Vdc (Si BJT) and so forth that make it easy to design by formula. Another thing that makes solid state design so easy (to get really, really wrong) is that it very often doesn't matter exactly what transistor you wire into the circuit. It's not like the difference between a 12AX7 and a 12AU7: put the wrong one in the wrong hole, and, believe me, you will notice.

The best way to design is from Loadlines (right-hand column, first table) for your type of choice.

[santonel]

Thanks. I didn't mention it but it's probably obvious I'm a newbie. I was looking at the valve wizard's site and it's a great resource I'm just slow at picking it up I guess. It doesn't seem to be sinking in completely but I'm getting there. Hope you don't mind me asking really dumb questions!

There really is a lot of additional factors to consider in ac circuitry! Plus in the area of tube electronics I've seen lots of terms and symbols for the same thing (B+ and HT supply for example) and I've always had trouble with terminology.

Basically what I want to do is rebuild my old solid state practice cube guitar amp with a tube circuit using the spare parts I have. Awesome sound quality, good parts aren't expected. I'll be excited as hell if it works! The idea is to build a really simple SE guitar amp that I can experiment with. Nothing fancy just one 12AX7 preamp tube and one 6CH6 power tube.

So I think I've got the heater circuitry pretty much down and maybe the B+ supply too. With the transformers I have on hand I have them rigged up so that I can get 172 Vac which is ~240 Vdc after rectification right? (you just multiply by root 2, no?). I figure if I can wire it in a "boost" configuration I can get 190 Vac out of it (~270 vdc I think).

Right now I'm just trying to work out all the values for my preamp stage. As I understand it, it's a matter of picking out a certain value for Ra (by means of an educated guess and looking at the plate curves) and then you can fill in all the other values, right?

At 240v B+ using 100k for Ra the load line looks kinda steep and it looks like it goes below the linear regions past about the -2v curve assuming a bias of -1v. I do love tube distortion but I would like to get some cleans out of it too. I guess it depends if the input signal forces the grid that negative. I don't have a scope but roughly what can I expect to be the peak-to-peak voltage of a signal coming from a standard humbucker pickup?

I'm wondering if a higher B+ voltage is in order or a different Ra value and what the reasons are for going with one or the other.

[Miles Prower]

Originally Posted by santonel

Plus in the area of tube electronics I've seen lots of terms and symbols for the same thing (B+ and HT supply for example) and I've always had trouble with terminology.

Yeah, that was way before the IEEE standardized terminology, and it can be a bit confusing at times, especially when perusing older schemos.

Basically what I want to do is rebuild my old solid state practice cube guitar amp with a tube circuit using the spare parts I have. Awesome sound quality, good parts aren't expected. I'll be excited as hell if it works! The idea is to build a really simple SE guitar amp that I can experiment with. Nothing fancy just one 12AX7 preamp tube and one 6CH6 power tube.

Sounds just fine for a first project. Go for it.

With the transformers I have on hand I have them rigged up so that I can get 172 Vac which is ~240 Vdc after rectification right?

Not exactly, as this doesn't take into consideration voltage averaging at the first reservoir capacitor, or load current voltage drop. It's still not a prob since VT circuits are a good deal more "forgiving" than solid state.

At 240v B+ using 100k for Ra the load line looks kinda steep and it looks like it goes below the linear regions past about the -2v curve assuming a bias of -1v. I do love tube distortion but I would like to get some cleans out of it too. I guess it depends if the input signal forces the grid that negative. I don't have a scope but roughly what can I expect to be the peak-to-peak voltage of a signal coming from a standard humbucker pickup?

You might also want to get familiar with Frank's site -- lots of information concerning VT types. Given that the nominal Rp for the 12AX7 is 60 -- 90K, a 100K plate load is inadequate. For triodes with passive plate loading, you need a plate load that's at least twice the nominal Rp for the type. For a 12AX7, that means 180 -- 220K is more like it. With a Vpp= 240Vdc, that will give a very low plate current, but that's OK as the 12AX7 was designed to operate with very low plate currents so that you can get your voltage gain with decent linearity without requiring some hellacious DC rail voltages.

If you're going to be using this as just a guitar amp, then you can still use the 100K since your input voltage will never come close to the -2Vdc level and the resulting cutoff nonlinearities. I don't believe gee-tah pick-ups produce much more than 10mVp of voltage output.

[Merlinb]

Originally Posted by Miles Prower

I don't believe gee-tah pick-ups produce much more than 10mVp of voltage output.

Hmm, I think you've been exposed to disc cartridges for too long! A modern single-coil pickup will give you about 0.5Vp-p, and a humbucker more like 1Vp-p with normal strumming. A high output humbucker can peak at 4Vp-p under heavy strumming!
Fortunately this doesn't matter, since guitar amps sound best when clipping!

Santonel, in guitar amps almost anything goes. There are no 'correct' values, and not even many 'standard' values either! Pretty much everyone starts off by copying a Champ circuit or similar (e.g., Epiphone Valve Junior)