hello, yes, I KNOW this is an old thread, I'm replying to it to set the record straight on a few things for future gents to be able to sort out their amp easier.
I used to be "Amperex" on the OFG and the plexi palace eons ago, and Im the one who "suggested" to remove the 220K resistor to ground before V1.
It does NOT belong there, period, remove it. Its not on any other oranges pre-73, and not on any mattamp/fender/marshall/vox/hiwatt etc etc etc guitar amps, period.
you do NOT need a grid load here first of all, and secondly the gent who said the gain will decrease, NO, its just NOT true, Im sorry. No offense.
A resistor does not belong there on any amp and just bleeds precious input signal to ground.
This is why they did it.
The reason whoever at orange was, who decided to screw with the original design, really messed things up here. They added it to offset their other bungle, taking a perfectly good balanced cathodyne PI and making it unbalanced by directly coupling the two triodes together, but with NO coupling cap to keep DC from entering the next stage from the previous one! You cannot hear this cap in the audio path.
thats the very much needed .068 cap thats on all pre-72 oranges and mattamps/fender tweeds/ampegs/gibsons, and basically ANY amp that has a cathodyne PI...along with the 1 meg and 1.5K resistors that also belong there.
if your wondering why your post-72 orange with this circuit has two of the EL34's burning out before the other two, now you know why. Unbalanced PI.
That being said, even if you dont rewire the PI (but please do it), removing the 220K resistor from the input, is like taking a blanket off the front of your cab; its just better fidelity ALL around, and not necessarily brighter, just fuller and doing what that stage is supposed to at this point. It sounds ALOT less compressed/mushy and tighter like the orig amps did.
The reason they added it was because directly coupling the PI, and also changing the nominal 100K plate resistor to 390K (!) was rediculous...it completely overloaded the PI and made the gain mushy and unlcear and spongy/untight...so they realized they needed to remove some of that mush, and decided to do so at the input, after the fact...WHY, lord only knows! This is backwards engineering in the classic sense, lol.
these amps dont sound very much like a classic pics-only or mattamps b/c of this, so if you want your post-72 amp to sound like the early ones, you must rewire the PI and remove the dreaded 220K...its the best thing you'll ever do. You'll be 99% there, especially if you use vintage glass, which is absolutely necessary IMO.
It can be done without pulling the board, and you only need a 1 meg resistor, the .068 cap, a 1.5K resistor, and a 100K resistor for the plate as additional components, thats it. A good tech can do it in about 15 minutes.
Lastly, removing the 1000Pf cap across the 100K plate resistor in the PI on early amps (if they have it) (and on the changed value 390K on post-72 amps) is also something you should DEFINITELY do. It was put there for possible parastitic oscillation b/c of the gain changes, but Ive never encountered an orange amp that needed it.
For the record, Ive seen several pic-only amps that didnt have it, (even though its in the scheme that everyone has) and Ive NEVER seen one in a mattamp. Its also like taking a blanket off the front of the cab, clearer and better fidelity...we DONT want any capacitance there on the plate!
You wont be able to hear any difference when the amp is played clean with this cap removed; only when driven, and the more you crank it up, the more you'll hear how much this cap was squelching highs away.
There you go. This all being said, I just scored a 73 OR120 a week ago that I couldnt even play b/c it sounded so buzzy/squishy...not anymore
Rock on!
Angelo