Matching up the Princeton PI

[mbratch]

I've read that that Phase Inverter on a BF Princeton has an inherent mismatch, and that it's a nice mod to "match it up". But I haven't found any information on how to do that. Is there a link to a description? Thanks.

[Old Tele man]

...you want the "Paul C" mod, look here: http://people.cornell.edu/pages/mt24/Amp/mm/mm_main.html

[mbratch]

Thanks OTM. I had a hunch that's what the Paul C mod was, but I wasn't sure.

Is the added 2M2 a 1 watt resistor?

[Old Tele man]

...naw, that's over-kill, because the current is SO LOW that wattage-level is actually VERY LOW...here's why:

...assuming B+ is 350V and R1 = 2.2Meg (resistor from grid to B+) and R2 = 1.0Meg (resistor from grid to ground), first figure the current (I):

I = (V / R) = 350V/(2.2M+1.0M) = 0.000109A or 0.109 microAmps (uA)!

..then, figure the wattage (W):

W = (R*I^2) = (2.2M * 0.109uA^2) = 0.0263W, or less than 0.03W!

..hence, a 1/8W would be fine, but a 1W will work too, just WAY bigger than necessary.

[willhutch]

Hey Tele Man (or similarly knowledgeable forumite):

Could you please explain the theory of the Paul C Mod? I see that you are applying DC voltage to the grid. What does that do? Does it "match up the PI"? By "match up", does that mean it matches the waveforms that are being sent to the dual output tubes in a push/pull arrangement?

[donnyjaguar]

Ideally you want to have 1/3 of the supply rail's voltage across the tube for maximum dynamic range. Perhaps this Paul C modification brings that closer to reality?

Most split-load phase invertor amplifiers I've seen have the grid directly coupled to the plate of the previous tube. In this case the operating points are set up in the previous stage.

[Old Tele man]

...you're "tweaking" the grid-bias so that the signal off the PLATE-resistor and the signal off the CATHODE-resistor signal go into "clipping" at exactly the same point...hence, both maximum headroom and output.

...the easiest way to do this on a "split-load" phase invertor (PI) is to "tweak" the bias point...that's what the 2.2Meg and 1.0Meg "voltage-divider" resistor (approximately) simulate.

...to be SURE, I'd use a pot and adjust until equal/simultaneous clipping occured, then measure the R1, R2 values and replace with closest BIG-value (so as NOT to "load" the grid!) resistors.

[KLB]

...you're "tweaking" the grid-bias so that the signal off the PLATE-resistor and the signal off the CATHODE-resistor signal go into "clipping" at exactly the point...hence, both maximum headroom and output.

...the easiest way to do this on a "split-load" phase invertor (PI) is to "tweak" the bias point...that's what the 2.2Meg and 1.0Meg "voltage-divider" resistor (approximately) simulate.

...to be SURE, I'd use a pot and adjust until equal/simultaneous clipping occured, then measure the R1, R2 values and replace with closest BIG-value (so as NOT to "load" the grid!) resistors.

I'm getting a case of Oscilloscope Envy! :)

When I modded my PR a few years ago, I installed the values Paul Cochrane recommended, but I don't objectively know how close to balanced it is. As I recall, the PI plate is 205v and the cathode 100v. For sure, it sounds better with this mod, along with using a higher voltage from the power supply.

I spoke to Paul about this topic a couple of months ago, wondering whether I should convert to a long-tailed PI and give up the tremolo. He said the PR power section usually distorts before the preamp. He likes to have the preamp break up just before the power section -- another thing that can be easily confirmed with a scope.

Since the Victory head arrived (!!), I don't use the PR very much, but I can't bring myself to part with it because of the beautiful reverb and tremolo. It is a great small amp for pedals, too.

[Old Tele man]

...Yes! I'm a "Missouri Engineer" -- ya' gotta "SHOW ME" what those electrons are doing before I believe anything...HA, ha!

[PaulC]

That mod isn't really a mod. It's the text book way of setting up this type of inverter. I just stuck it into a princeton and talked about it.

But this is the way to balance the inverter. Matching up the resistors really wont do a thing. You do want equal loads, but it's really all about bias points.

Some people have a problem with the idea of putting DC on the grids, but this is common practice, and in fact it's already going on with the self biased setup - it's just being produced by the tube instead of being tapped off of the B+. The actual number is closer to 25% of the anode supply instead of the normal thought of 1/3. I always do it with a pot myself. That 2M2/1M thing came from just using what's already there plus a common value 2M2 resister. It does seem to get it "better" put you can nail it down with a scope and a pot.

I've done this mod to many amps for over 10 years here in Nashville without any of them having a problem. I have heard stories of doing this along with what's known as the "stokes" mod burning up some pwr trannies, but I've never actually seen anybody say it happened to them. It's always a friends amp. But you never know...

PaulC

[willhutch]

Hey guys, writing in to report that I just completed the so-called "Paul C" mod.

I'm happy with the results. I don't seem to get significantly increased clean headroom with this mod. However, the nature of the distortion when the amp is pushed is much nicer. Overall, the amp is more useful across a broader sweep of the volume dial.

I did the Stokes mod last week. Being new to modifying my amps, I learned some things in the process of completing these mods. During both mods, I made stupid mistakes that had me stumped for a while, but I eventually figured out. LOL! Last week, I put the rectifier in wrong and was wondering why I was blowing fuses. This week I put a 2.2 Ohm resistor wher a 2M2 resistor was supposed to go. This set me back a long time as I went over and over my work before discovering the problem. I'm lucky nothing was damaged in all my buffoonery.

I'm sure you pros would be shaking your heads at the foolish neophyte, but y'all have probably probably done more ridiculous things.

Anyway. I'm enjoying a sense of accomplishment and a great sounding Princeton Reverb!:BEER

[Old Tele man]

Yoda wisdom: "Wise is the man who learns from HIS mistakes, but even WISER is the man who learns from OTHERS mistakes!"