Getting more clarity from 2204 type PA?

[iknowjohnny]

I have a home brew thats from the PI to the speaker is pretty much the same as a 2204 with 2xEL34 and all the typical PI values (100k/82k, 470R/10k, 1m/1m) but with a variable NFB knob. But other than the variable NFB (250k pot/33k set resistor in series) pretty much JCM 800 single channel, 2204 from the PI back.

The preamp is perfect. I absolutely do not want to change that for many reasons too long to go into. But the tone while just great sometimes doesn't have enough clarity in a mix without turning the treble and/or presence up too high, making the time thin. I have about 10 tune types including my fav, winged C's.

Anyways, I'd appriciate any ideas on how to get more clarity from the PA. I guess you could call clarity more treble, but whatever you wanna call it i just want the PB to add clarity/articulation without making the tone too thin. I've tried a variety of presence caps but no go.

[M Fowler]

Maybe there is just too much gain in the PI.

The 470R PI bias could be increased to 680R or 860R and increase the tail resistor to 24K or 47K.

[iknowjohnny]

I've tried that in the past, tho not for this reason. don't recall it becoming clearer, but i'll try it again, what the heck. I also have just 20uf filtering on the PI. It hit me I should try much more, as more filtering usually has a clarifying effect. I can just try paralleling a 32uf to get about 50.

[Phil_S]

Have you tried a 5751 for the PI? That should shave a bit of gain off it without substantially changing other characteristics. If you are experiencing a problem with the PA and it is built to correspond to the schematic, you may just be hitting it with too much signal. Since you state that rebiasing the PI isn't doing it for you, that's why I'm suggesting the 5751. Or you might build a noval-octal adapter and try a 6SL7 which will shave off a bit more than the 5751, but is still a close first cousin to the 12AX7. I think a 12AT7 with its much lower internal plate resistance may be too much of a change even though the gain potential on it is similar to the 5751.

[iknowjohnny]

I'm not saying theres a problem, just that i'd like more clarity from the PA. The preamp is just slightly darker than typical so i want to get some clarity elsewhere w/o toughing the pre because it sounds and feels so perfect and anytime i try anything to change it it's never as good. I'm just looking for some ideas on circuit tweaks that might brighten up the PA a bit so as not to have to change the pre.

[M Fowler]

Add a cut control.

[Firestorm]

Try changing the grid return resistors from 220k to 150k. Subtle, but clearer IMO.

[iknowjohnny]

Add a cut control.

A cut STARTS where it is now and gradually from there cuts highs and therefore clarity.

[iknowjohnny]

Try changing the grid return resistors from 220k to 150k. Subtle, but clearer IMO.

hmmm.....i'll give that a shot. Again something I've done it before but I didn't do it for this reason and didn't care for whatever it did do. But what the heck. I'll parallel 470k's just to see. (result is 150k)

[Reeltarded]

Put trimmers both PI grids.

Lose the NFB and put a cut control in it.

[iknowjohnny]

I've done the cut + no NFB before. At it's bightest it's basically what i have now with the NFB pot cranked for low NFB. With 250k pot plus 33k set R for nfb, turning it all the way is very close to no nfb at all and thats where i use it most and still want more clarity.
Why trimmers on PI grids? When i use grid stoppers on any 12ax7 grid it always reduces highs so i don't understand how that would help.

[Phil_S]

I didn't suggest reducing the grid stoppers to 150K, figuring that is a way to increase headroom, but I didn't think that's what you are seeking. I admit, I could be way off on assuming what you think you are looking for.

Maybe you really want a Fender Twin but just don't know it yet? The question is a bit of a prank on you. The serious question is, how do you determine the preamp is so perfect for you? What I'm getting at is that you've made some assumptions about the pre and PA. What are you thinking here? If you need the PA to hear the pre, how can you be so sure about the pre?

[M Fowler]

If you want more treble increase the PI bias resistor to 1k2