If you had 2 choices...

[iknowjohnny]

....on how to configure a 2 12ax7 (not counting the PI) fairly high gain preamp in a amp that would be dynamic and clean up well with the guitar volume and go to smooth tight distortion when up, which would you choose

1)-4 regular gain stages into a plate fed tone stack
2)-3 regular stages and a cathode follower fed tone stack

Just curious, as i have yet to try 4 gain stages and i hear for high gain thats the way to go. Thos i wonder how that will affect guitar volume and playing dynamics.

[Cliff Schecht]

If you run the math, 4 stages is overkill. The amount of clipping you can get out of just 3 stages is enough to satisfy even the biggest metal head.

You can squeeze not only a lot of gain but a lot of character out of just 3 stages and a cathode follower. I think the advantage of a fourth gains stage would be higher bandwidth because you aren't overly reducing the gain-bandwidth by pulling too much gain out of each stage. Then again, I use GB intentionally to set the overall character of an amp (bright, dark, warm, etc..).

There are lots of subtle design tricks that you can use to get the most out of three gain stages though. Even the often ignored cathode follower plays a big part in the sound. A smaller cathode resistance invokes asymmetrical clipping which can sound great up to a certain point and flabby past that point.

For dynamics, look at the recent discussions on large B+ resistors to get more dynamic stages. Larger resistors drop more B+ when more current is drawn from the preamp and increase distortion. Playing softer drops less B+ and gives more headroom.

[iknowjohnny]

Thanks. I never thought about the dropping resistors in that way, but i believe that would indeed be the case. And to think i just lowered mine to get more voltage to the preamp ! I'll not only try going back but maybe even bigger as i have in the past. May have been one of those things that worked well but i didn't quite get it at the moment and changed it. You know how that happens....

[tubeswell]

I did this with a C30 a couple of years back and got a pretty good overdriven sound out of it - in hindsight the only thing I would do for even more gain is strip the extra 1M grid resistor in front of V2A. The nasally clean is pretty much what you get when you shoot for a decent middy-sounding OD

[tubeswell]

Here is a soundbyte of the above mod (with just a strat)

(The amp is basically this pre-amp with a cathodyne concertina PI and 4 x fixed bias EL84s in PP)

[paulster]

I'd do 3 gain stages plus CF Komet-style with minimal attenuation or 4 gain stages with appropriate attenuation to use each stage as a voicing stage and get the distortion to hit on stages 2, 3 and 4 in very close sequence to get a really good clean to scream dynamic.

You really only need 3, but 4 offers some additional voicing options which could be useful if chosen very carefully.

[Phil_S]

Thanks. I never thought about the dropping resistors in that way, but i believe that would indeed be the case. And to think i just lowered mine to get more voltage to the preamp ! I'll not only try going back but maybe even bigger as i have in the past. May have been one of those things that worked well but i didn't quite get it at the moment and changed it. You know how that happens....

Recall that discussion you and I just had about the dropping resistors. We focused on supplying more voltage to the preamp tubes, but the math also forces you to look at current. There is an inverse relationship between voltage and current -- when one rises, the other will fall and vice-versa. When you are dealing with a preamp tube like a 12AX7, it will draw up to about 3mA per side. The actual current draw depends on a variety of factors, including how you biased it (grid leak, cathode, and plate load resistors all play into this), and how much voltage you hit it with. The dropping resistors are also going to have a say in current flow to some extent. Ohm's law is a "law" for a reason. V=I*R holds everything you need to know about the basics. If you measure the voltage drop across any resistor, then you know how much current is flowing at that point.

When you increase the dropping resistors in the B+ supply, plate voltage will drop. Along with that you'll experience decreased clean headroom. So, think about whether you want to change the tube bias or the plate supply. For tube bias, you really should draw a load line -- visit the Valve Wizard's web pages and I'm sure you'll find something there on load lines. I'm attaching a page out to the RCA manual showing some standard configurations and the expected gain factor. The RCA manual will give you a hint as to how operating conditions can be changed and the expected result.

[Darkbluemurder]

....on how to configure a 2 12ax7 (not counting the PI) fairly high gain preamp in a amp that would be dynamic and clean up well with the guitar volume and go to smooth tight distortion when up, which would you choose

1)-4 regular gain stages into a plate fed tone stack
2)-3 regular stages and a cathode follower fed tone stack

It depends on what you define by "fairly high gain". Given that you want "clean up well" makes me conclude a 3 stage preamp would suit this goal better. 4 gain stages are preferable for "chugga-chugga" and "wideli-wee".

What kind of power amp are you planning to use with this set up? E.g. if you want to use EL84 tubes I would take a look at the Phaez Daisycutter. The schematic is on the AX84 site. It's a 3 stage preamp into a cathodyne and 2 cathode biased EL84 tubes. The designer wanted a high gain amp which cleans up well. I have not built this but it looks interesting.

Cheers Stephan

[Jana]

4 gain stages with a cathode follower driving the tone stack. You better be good at lead dress though.

[iknowjohnny]

Wow, a lot to think about. I will have to read this when i get home tonite and take it all in. good stuff tho....i actually understood more than usual !

[jaysg]

If you must have a cathode follower, you can sub an IRF820. It works fine.

[John_P_WI]

I remember reading a comment from KOC that Doug (Roccaforte) bought the TUT books, so there's no surprise that there are similarities.

KOC's circuit is great and will give instant Zakk Wylde tones and can be highly modified. I would suggest moving / adding another "drive or gain" pot for more flexibility (between stages 1 and 2, 3 and 4).

You can dump high freqs with caps and get Splawn tones too.

Another trick is to reconfigure to have three gain stages, plate driven tone, then 1 gain stage. 3+1. The last gain stage will add harmonics.

As Jana says, you better have good lead dress and good filtering / grounding too.

[iknowjohnny]

If you must have a cathode follower, you can sub an IRF820. It works fine.

Can you clarify? I have no idea what to do with a SS device in the CF circuit. And also why?

[jaysg]

Can you clarify? I have no idea what to do with a SS device in the CF circuit. And also why?

A cathode follower has three connections:

1) grid to previous plate -- MosFet Gate
2) plate to B+ -- Mosfet Drain
3) cathode to (typically) a 100K to ground -- MosFet Source

There's an article called Mosfet Follies at www.geofex.com which explains a few things you can do with them. The answer to 'Why' is that you ran out of triodes.