Cathode Resistor values for OD channel

[tubeswell]

I have done a cathode follower mod on V2 of my Classic 30 (Schematic attached). Now I want to up the gain in V2A.

I put in a 12AX7 which (understandably) does give it more crunch than the recomended 12AT7. I am aiming for that soft squidgy saturated crunch when the VR2 pot is turned up full, so I was wondering what to tack in as a substitute for R16. R16 is presently a 1.5k resistor in parallel with a 22uf25Vdc electrolytic cap. Can I go to a 1k or even 500Ohm resistor without causing too many problems?

Any discussion conflab on this welcome please?

[tubeswell]

This .jpg file might be slightly better resolution

[MWJB]

Can't read the schematic, it's a bit small, but generally reducing the size of the cathode resistor makes the tone more 'Marshally', I wouldn't go lower than 820ohms. If you get any unwanted side effects like the amp hesitating/cutting out when hit with a big chord go up to the next value that stops this happening.

[tubeswell]

Thanks MWJB

The link to the schematic is (off the Blue Guitar website) is here. I've given up trying to reproduce a quick readable version.

http://www.blueguitar.org/new/articl...y/c30twdbm.zip

Anyone else have other comments about suitable cathode resistor values for increased gain in V2A?

[tubeswell]

Or is a smaller Rk in the CF (i.e. in V2B) a better way of getting more distortion? I.e. would it be better to make the value of R17 68k (or 56k?) instead of 100k?

Or could I up the Plate resistor to V2A back up to 150k (stock value was 150k)? What might the differences give in the way of distortion characteristics? I am looking for distortion where I can still hear the underlying p'up tone, but but not so little that it sounds too clean. Furthermore if I up the plate resistor to V2A to (say) 150K, would I have to alter the Cathode resistor for V2B to counter-balance anything? I am a bit of a novice at this.

(I am using a 250kA pot for VR3 if that's any help - to attempt to spread out the volume increase on the dial a bit more)

I also have 4.7k resistors for R59 and R60, which I guess gives a slightly higher plate voltage than the 5k resstors recommended in the mod (but I couldn't obtain 5k resistors). Does that make a difference

[tubeswell]

I've put a 56k in as the Cathode resistor in the CF stage, which has added a bit more of what I'd call 'smooth' distortion and it sounds a mit more 'bassman-like' now - but possibly not quite enough. The Blue Guitar webiste C30 Bassman Mod has a 100k resistor in there and this value in plainly too clean for my tastes - so the questions (for general discussion) are:

1) How low could I go as a value for the CF Cathode Resistor (current value 56k) without the signal cuting out? and

2) Would lowering the GS Cathode Resistor compliment this (current value 1.5k in parallel with a 22uF electrolytic cap) (Somebody has already suggested 820Ω) ? and

3) Generally speaking, what kind of distortion effect to you get by increasing the Plate resistor in the Gain Stage (the current resistor is 100k)?

[hasserl]

I don't get the purpose of that mod. A cathode follower will provide a low impedance driver stage with close to unity gain. That CF is not driving anything, it goes to a volume control then on to the next gain stage. The net effect is to simply reduce the level of gain by removing that triode from the circuit.

I guess if you like the results that's all that matters. But I don't understand why Steve did that to begin with. I did not read the essay on the mods, maybe he addresses it there.

[tubeswell]

It sounds effing amazing! I've just got to get a bit more squishiness in the OD channel, and I suspect lowering the value of the Cathode Resistor and/or increasing the value of the Plate resistor in V2A (and/or further lowering the value of the Cathode resistor in V2B will do it for me), but as I'm a newbie at this, I was hoping somebody out there in cyberland would have a few clues for me. So far even with the limited amount of distortion I've got, it sounds as if its got the potential to be much better than stock. The distortion is now much less like an insect buzzing. Even that said, I'm sure Marshalls and fenders with a CF circuit get more distortion than what I've got at the moment, so there's gotta be a way.