PI grid leaks

[iknowjohnny]

I got a chance to try the cap across the tone stack. This is a very good mod that i could have used back when the amp was quite bright. But at this point it's a bit too soft sounding. But i really like it as far as what it does to the mids especially. I may try and get the amp back to where it sounds brighter and harder and then see how this mod sounds because sometimes you find more than one way to accomplish something and one way tends to sound best. I believe the main thing i did to soften up the hardness and brightness is drop the supply V at the preamp. Of course Jana, you just had me do the opposite at the PI, but i compensated at the first stages so as to not lose that tone. But i may try upping the voltage there and then this mod. Otherwise i may put it on a switch and dump the cut control because this seems to do a similar thing but w/o cutting volume so much. Anyways, i will experiment with it and after wards give the other changes a go. I will certainly try them all because so far your suggestions are very good. Your years of building certainly show in your suggestions. I'd say thanks again but i don't want to pi$$ you off.

[tubeswell]

I tried that and added a 47k grid stopper at the PI input and changed the coupling cap to .01 from .022.

Going from a .01 to a .022 will make blocking distortion worse (if it is already present - .02 is twice as much capacitance as .01). Also 47k might not be high enough, try 68k or 100k, or lose some bass earlier in the signal chain

[iknowjohnny]

you misread....i was saying i went TO an .01 from a .022. But that said, i'm really not concerned about this because it has no bearing on the tone unless the amp is nearly all the way up which is far far louder than i would ever use it. I only mentioned it out of curiosity to see if anyone knew why because it's always had me scratching my head.

[tubeswell]

Oops sorry, my bad

[John_P_WI]

Iknowjohnny,

You did not pi$$ her off, Jana is a lady...... No offense by saying thanks.

[iknowjohnny]

Iknowjohnny,

You did not pi$$ her off, Jana is a lady...... No offense by saying thanks.

OHH !!! Sorry Jana ! I had no idea. It's just that in europen countries there are a lot of male names that sound like female names to americans, and i just figured that was the case. (didn't notice "Minnesota" till i just now looked) And you don't hear of many women who build guitar amps ! So please accept my appologies. Anyways, your advice is great. I have been messing with all the things you have suggested and i am making more progress than i thought this amp was capable of. I'm finding that i will likely use many or most of them, but i now have the task of determining values and what to do with other aspects of the circuit that may now be obsolete value wise because of these other changes. You have put me to work big time !

By the way, a buddy of mine that was in the band with me for years just came out here for a visit and should be here today......from Minnesota.

[M Fowler]

Minnesota Rocks and so does Jana.

But I am still vacationing in Phoenix, nice!

[Jana]

Thanks Mark!

Yup, Minnesota rocks, although a bit slowly this time of year.

[iknowjohnny]

My poor ol buddy Joe came out thinking he was getting away from the brutal cold up there in minny only to arrive in california on the very day that started a full weeks worth of the worse rain storms we've had in years thanks to the El Niño ! Oh well, to him i'm sure this will seem like summer.

[iknowjohnny]

hey guys, maybe someone can explain this to me. First let me say that the amp, like most amps i've had to one degree or another sound harsher and less smooth as you turn the master past a certain point. I doubled the value of the cathode resistor on the cathode follower stage to 200k. granted, i don't know a lot when it comes to circuits, but i usually have some idea why something might work. This time i really didn't and i just tried this on a whim since i have only tried lower values there before. It may even give a clue as to the harshness the PI displays when using a post PI master. (which i am NOT anymore, it's pre PI) Anyways, i doubled it and the tone improved quite a bit becoming smoother with better mids. This was especially apparent with the master up to the point where it normally starts losing smoothness and gets harsher. Seems like it would do the opposite if it were a normal gain stage, but i really don't understand how a CF works so i'm not sure why this worked. If you can, laymans terms....why did that improve the tone so much?

[Jana]

You're flirting with disaster with a 200K on the cathode resistor on the CF. Did you measure what the DC voltage is on that cathode? Have you checked what the max DC heater to cathode voltage is for an AX7?

Whatever you do, DO NOT use a tungsol reissue tube in that position. lol.

[iknowjohnny]

It was about 35 or 40v more than with 100k. So i assume 100k is the max value to ever use there? How about this...the fact that altering the CF caused a change greater than i have gotten with most any other tweak, i then must wonder if that is where i should focus more of my efforts. What i am getting at is that you said to try a 220k in place of the 100k plate and changing the cathode from the 820 there now to 2.7k on the stage before the CF. I realize this is the stage before the CF, but being immdeiatly before it will this have a similar effect ? Or if not, what was your intention with that tweak?

EDIT: I tried it. It seems like it does a very similar thing, but the gain of the amp drops to the point where the amp is far quieter. So any suggestions how to get that lost gain back later or how to get a similar result w/o a drastic volume loss? Funny, but this is one area of the amp i've never experimented with, and i think it may be that key to what i'm after because i feel i've tried everything else and have the rest pretty much as good as it's gonna get.

[iknowjohnny]

Ok, i now think i need this tweak ! Amp sounds rounder and fuller than ever and i am really digging that, as it's always been one of my main goals. I could never even get that out of any of my marshalls.

that said, the main question again, and i really need to figure this out....how do i regain the volume? the amp is loud enough to gig with, but it's so much lower that at the master would have to be up about 1/2 or more, and at that setting the noise is too much. Should i start looking at gain mods at the PI? If so, how would i best up the gain w/o issues?

[Andy Le Blanc]

Its partly impedance matching, and getting the gain of the CF stage nearer to 1.
You really do need to mind the heater to cathode voltage.

Put a pot in the plate circuit of the inverter, while your in there.
AC/DC balance, cant do both at the same time, explore the range in between.

[Jana]

Seriously, look at the tube data for a 12AX7 and see what the max heater to cathode DC voltage is. Then measure what you have with the 200K cathode on the cathode follower. 35 or 40 volts more tells me nothing--more than what?

For more gain, try changing the .022/68k before the CF to .01/220k.

You're not going to get any more appreciable gain out of the PI.

You could also change that 68K on the 1M gain to 220K to 470K. That should give you all the gain you need in the pre-amp.