PI mod.

While troubleshooting my clipping issue, I was reading a lot on PI circuits and was using a simulation program to test various changes in componants.
I ended up making a couple of changes which gives the PI a little more head room before clipping.
Another thing I noticed using signals is that I never could get power tube distortion. The PI will clip long before the power tubes are even close to clipping. What we are hearing when we crank the master to 10 and then bring the channel volume up is the PI clipping.
This was verified in simulation and on the amp with a scope.
Anyway, here is what I did and it does give a bit more headroom in the PI.
Change R48(1.5 k at pin 8 cathode) to 1k. this allows the first stage to drive a little more before clipping.
Change R53 & R54 (cathode and plate 33k on the 2nd stage) to 47k. This also gives this stage more headroom.
Even with this, the PI will still clip before the power tubes.
It was a fun and learning expeirence anyway

sorry guys, I had a little brain fart.
changing R48 increases gain, not what I wanted to do. So I put R48 back to 1.5k.
This mod really only has an effect in low power mode.

Andrew,
I quite like the distortion you get when the master is all the way up and you get up around 7 on the pre. Any idea how to make it so you get more of the power tube clipping after the phase inverter is?
I play with an attenuator so I am up all the way on master and 7-10 on channel volume all the time.
Chuck

Ed Goforth recommends the 'Clapton Mod' for more gain. Do a search on this forum and you'll find it.
All he did was add a the 4.7K resistor across (in parallel) the 4.7K resistor at pin 2 of the final IC in the preamp. I'm getting ready to do Ed's version of the Paul C mod which he also talks about on this board.

with the clapton mod, where would you guess that makes most of the clipping come from?

Hard to tell, Ed Goforth can better answer

Hi guys, The 4.7k paralleled across R-32 going into IC-7 (LM-307) lets more signal through to drive the last opamp stage IC-7 on the schematic I am looking at Chassis is NO. 2275-130 & 2475-130, which pushes the first stage of the 12AX7 and when turning down the master, and turning up the preamp with more gain now with these mods, gives a rich overdrive distortion that is articulate and sweet. So I would say since there are no clipping diodes in this amp, the preamp is a clean drive and when pushed, drives the 1st stage of the 12AX7 with the master turned down, creating the sweet distortion there. With all the right components in the right places, these amps are hard to beat! The modded for MM amps Paul C. PI circuit smooths out the harshness if any, and fattens up the tone and also helps the sweet feedback/sustain we look for in a great amp, where you dont have to work so hard finding the sweet spots. It also is a more stable circuit. A good Mullard 12AX7 works good there. The new GT12AX7M ( Mullard re-issue) is very good in this amp, and I am waiting to try the new re-issue Tung-Sol 12AX7 for the driver.

The new Tung-Sol is a good tube also. Depends on the users taste. The GT 12AX7M seems to have a full all-around tone that reaks fat crunch! The Tung-Sol seems to have a smoother midrange and top end from the samples I tried. Both great tubes for new production. I like some of the Sovtek 12AX7 LPS for their full sound stage tones, if you like full, clear, not so much midrange. The old Mullards have a sweet smooth and fat & warm tone. A 12AT7 gives slightly less drive to the PI, and will not be as fat sounding for those that like a trim, less fat tone.