biasing preamp tubes?

[cbarrow7625]

the bass pot is now 100k from ground instead of 25k

Hmmm...well then, Bruce's suggestion (I think) to change the Bass cap from .022uF to .047 uF is probably the right thing to do. You could go as high as .1uF (which I did on a marshall-like channel in my own amp to smooth out the low range and it worked really nice). Once you do that, you may find that you can go back to a 25k pot.

Double the value of the screen resistor if you split them (to 300 Ohm). Bruce's suggestion about an additioanl dropping resitor is a great idea too.

Bruce, thanks for jumping in on this and adding all kinds of really good ideas (keep 'em coming). I love to find out what other people like to do in these situations.

[daz]

I tried both a .047 and a .1 but theres little to no difference. the mid pot still boosts what i hear as treble, and it's bright and hard as he||. I keep staring at the wiring to see if theres a wiring mistake but it all looks fine. You should see this board....it's so sloppy and full of solder blobs and all from endless experimentation !Total slop fest....it's amazing it's not noisy ! I can't wait to nail the tone so i can get a new board and components and wire it all up new.

[Bruce / Mission Amps]

Hmmm...well then, Bruce's suggestion (I think) to change the Bass cap from .022uF to .047 uF is probably the right thing to do. You could go as high as .1uF (which I did on a marshall-like channel in my own amp to smooth out the low range and it worked really nice). Once you do that, you may find that you can go back to a 25k pot.

Double the value of the screen resistor if you split them (to 300 Ohm). Bruce's suggestion about an additioanl dropping resitor is a great idea too.

Bruce, thanks for jumping in on this and adding all kinds of really good ideas (keep 'em coming). I love to find out what other people like to do in these situations.

Well one thing I would do right now is dump the 100K mid pot.
I know I wouldn't like it or do it, But IMHO, the absolute largest value mid pot you could use is 50K.
Truthfully, I would just use a good old fashioned 25K audio taper pot for the mid control, 1M for the Bass and 500K linear for the treble with a much smaller treble coupling cap... as I mentioned before, 180pf to 220pF.
If you like more of that scooped out mid tone, use only two of the mid pot's lugs.
Wire the .022uF cap to the mid pot's wiper, like a 6L6 Fender Super Reverb amp, which also means at the junction of the mid pot wiper and bass pot far left lug, as viewed from the back of the pots.
And Daz, that mid pot would be wired as a variable resistor, again, which means as viewed from the back of the pot.... only the middle wiper is used and the left outside lug is grounded.
A 100K is really too large of a value as a mid pot in these and that 100K lifts the tone stack pretty far from ground, making all the tone controls less and less effective as you move further and further from 10K to 25K and on up.

[daz]

100k in series between the bass and mid pot works with the 25k giving it a smore linear transition and the sound of the 100k pot. however, still no meat when i turn the mids up. Marshalls always gave me the meaty part of the mids, not low treble. In fact, i may even say the mids are in the middle of the treble range while the treble pot is now pretty normal in it's center freq, maybe a tad higher.if i could center the mid pot at around 500 i'd be home. Amp is sounding well better than ever !

[cbarrow7625]

the mid pot still boosts what i hear as treble, and it's bright and hard as he||.

OK, maybe that's another clue. An alternate version of the standard tone controls places a .0047uF cap across the MR pot from the top of the pot to ground. This shunts all of the HF out of the MR circuit. Maybe that will help you.

[daz]

Oh my....that did the trick.......i had to stop playing it, as i got too darn excited !!! Thanks !!!

At this point i could stop and probably be happier with it than most any amp i've owned. But i think there are problems awaiting me if i use it to gig. Like i've said, i rarely do anymore, but i've been considering "coming out of retirement" if you wanna call it that due to the excitement this amp has given me. And now more than ever. But i believe at stage level, tho we never play very loud, it will be too much power amp wildness to go with mush preamp gain. i'd rather get more from the pre and just some compression from a power amp thats just starting to breath a bit. I've been debating whether to bother, but i figured once i got this right i'd build a new amp with the same pre and a couple EL34's ala DC-30, which i hear is a sort of "marshall 50 that sounds good low" sort of affair. My 50's were always justb sounding right at a tad over the level most bars would allow w/o pestering you all nite.

Anyways, this this is sounding freakin' great thanks so much ! I'll still likely have questions, but for now i think i'll just play.

[cbarrow7625]

You might want to try putting a post-PI master volume (especially easy since you have cathode bias). Just replace the power tube 220K grid resistors with a dual 250k Audio pot.

that way you get to hit the PI with more level & at least get it's tonal contributions in the mix, then tame it down right before the power tubes. Again, something I have done many times & really like. If everthing else in the preamp is done well, hitting the PI with more level should give you a "fuller" tone and simulate a little bit of what we all like in power amp distortion (after all the PI distortion is part of that sound).

Good luck.

[daz]

About week or so ago i got my hands on a dual 500k clarostat and did just that, but it sounded horrid. It was probably because of the preamp design tho. the PI was getting slaughtered and you could hear it. Theres still a lot of gain tho, so i'm not sure it would be any different.

[cbarrow7625]

I find that the trick is you also have to have the pream MV turned down a bit so that you don't hit the PI so hard. I agree that with the preamp MV on 10 it does sound horrid. Somewhere between 6-8 on the preamp MV I find a sweet spot in most of mine.

It just might not work for yours but I thought I'd mention it.

[daz]

Actually, i cranked it up just now and was surprised at how controllable it was. Whether it's loud enough i won't know till i play out with it. But if i have to build a new power section for the same pre i will. probably will anyways since it's rather fun hearing such awesome results.

[Bruce / Mission Amps]

Actually, i cranked it up just now and was surprised at how controllable it was. Whether it's loud enough i won't know till i play out with it. But if i have to build a new power section for the same pre i will. probably will anyways since it's rather fun hearing such awesome results.

OK sounds like you are making progress... now go back to your art work, or mine, or someone's ....and redraw it so we can see what it is you like.

[daz]

Ok, i'll try. I'm now home and the amp isn't here so i'll have to go by memory, which shouldn't be hard after all this !! LOL...

I'll post it in a while if i can remember everything well enough

[daz]

Here ya go. the filters may be reversed.... that is, i can't remember if it was the 16 or 20 in front and back. I think i had those values already and just used them rather than order more. would i benefit by getting larger filters or would that possibly kill the squishy sag i get and love?
as you can see i pretty much just used marshal values throughout V2 and the PI. The screen was 470 and changing to a 150 was one of two things that made a huge difference. The 150 made it tighter and smoother. The other thing was the extra resistor and cap in the stack, the resistor being my idea and the cap cbarrow7625's. it was that cap that changed the mid knob to a usable range, tho i may try different values to hopefully center it somewhere around 500-700HZ (guesstimate) where i like it.
No cap on second cathode....it just didn't need it. Tied it to a switch so i could A/B with and w/o it and the tone was just much better w/o. The tone is meaty yet not muddy. The lows can still stand some tightening, but not via adding more high end. I think the key may be in the P/S, possibly by lowering the screen to 100 ohm like it shows in many if not most 18 watt schematic. Going from 470 to 150 was great, so a bit more may be even better. heres the way it is now to the best of my recolection...

http://img245.imageshack.us/my.php?image=18rh2.jpg

EDIT: I was wrong about that 2nd stage cap....i DO need it, and badly. Don't ask how i thought i didn't....very long story. in any case now there is a 1uf bypass on the second stage, tho the value could change. I'm also going to experiment with the .0047 cap in the stack cbarrow7625 suggested.