P.I.

[Deisel401]

Hi all. Could someone please tell me which is the P.I. in my RM20 ? From all the threads I've read , I'm thinking it's the preamp tube closest to the power tubes. So, looking at the back of my amp , power tubes to the left and 2 preamp tubes to the right. The 3rd preamp tube is a bit farther back ( closer to the front ). I'm sure this has been discussed dozens of times but I went thru a lot of threads and had no luck. Thanks and all have a great day. :roll:

 

[JKMV12]

the PI is the phase inverter. it is the final preamp stage. in the amp it is closest to the power tubes. As its name suggests it inverts the input signal and sends two versions of the signal to the power amp, each one being 180 degrees out of phase with each other (they are opposite of each other). This is only seen in push-pull power amps as far as i know. I haven't studied power amps and phase inverters too extensively so i have said all i can so far. It is a very interesting subject.

The next preamp tube is the effects loop buffer. Again a subject i haven't studied yet so i can't say much. One thing i will note is that each preamp tube is comprised of two triodes (so it is essentially two tubes in one glass). The basic signal path for the effects loop is INPUT < Triode Buffer 1 < Effects Loop < Triode Buffer 2 < OUTPUT. The input would be the signal coming from the preamp stages and the output will go to the phase inverter. (someone please correct me if i am wrong about anything).

Now the preamp tube farthest back, near the input is the first gain stage of the amp..typically called V1. It is the first thing that the guitar signal hits (after the necessary resistor/capacitor network of course). The resistor/capacitor network is comprised of the basic components needed for a gain stage and initial tone shaping as well as components to block high radio frequencies. The following gain stages are in your preamp modules with the final preamp tube (V2) driving the tone stack (your tone controls).

Sorry i kinda described the tubes in reverse order but hopefully i have cleared up the confusion w/o causing more. Let me know if you have any questions.

Take Care,
John

 

[crankyrayhanky]

is the V1 the spot where people are enjoying the 5751? A little less gain, lots more clarity?

 

[JKMV12]

is the V1 the spot where people are enjoying the 5751? A little less gain, lots more clarity?

never tried it there but i use it in the PI and what i noticed was a significant reduction in noise. The thing with 5751's is that they have similar specs as a 12AX7 but have less gain. I will have to try it in the V1...might be interesting

Again i haven't studied phase inverters yet so i could be wrong with what i am about to say. The way i see it is that the PI doesn't need as much gain as the other three gain stages (technically there are six gain stages). The PI's job is to send the original signal as well as a 180 degree inversion of it to the power amp. It should retain as much of the original quality as possible. I think more gain would be unnecessary and would cause a loss of clarity and increase in noise. This is just informed speculation...i would have to know more about PI's and see the actual circuit to be sure. Though my argument seems logical.

 

[Mattfig]

is the V1 the spot where people are enjoying the 5751? A little less gain, lots more clarity?

I think there's some confusion at times here 'cause we all call V1 something different...

JMV12 is absolutely correct that V1 is on the amp or RM4 and V2 and V3 are in the modules...

However, many discussions here reference V1 as in the module...I am guilty of that myself, I try to say "V1 in the module." Anyway, V1 in the module is a great place for a 5751 to tame a mod without losing its tonal characteristics...If you want every ounce of gain available it's better to stick with a 12AX7....There has been some debate as to how much gain is lost in this case...If you are playing Slayer, skip the 5751...If you are playing classic rock - go 5751 and get some nice punchy clarity...

As far as V1 on the amp or RM4, I personally don't like a 5751 there...It's like cutting your signal down right off the bat IMHO...There isn't anything to "clear up" yet anyway...

I also agree with JMV12 that a 5751 in the PI is a good move...I don't do it in all my amps, but a few that need some quieting down...

Hope that makes sense....

 

[HughJasol]

I try to put 5751's in all my PI sockets, but at $30 a pop it get freakin' expensive as I get more amps.

 

[Deisel401]

Wow . . .lol . . . that's a lot of info and I really appreciate the responses. You folks cleared that up for me. I was confused cuz in my RM20 , the 2 preamp tubes together closest to the back , have a small label next to the tube socket that says "12AX7". The 1 by itself in the middle has no label so I thought THAT 1 was the P.I. I'm talking about actual paper labels stuck to the wood in front of the tubes. Weird, huh ? Anyway, last night I changed out all my tubes again. I put in a pair of G.T. EL84's ( h. rating = 4 ), and loaded all preamp sockets with E.H. 12AX7's. Then set my bias to midway, about 17.5 ma.'s. After all these years I have THE PERFECT Marshall tone !!!! Ya' know that glassy , crunchy , sustaining , shiny , colorful wall of syrupy tone. And this beautiful tone is WITHOUT my Keeley Comp or Screamer. Just my stock SL+, Les Paul, a good cable, and a pick. Oh, and my 1x12 closed back cab with V30 in it. I've had about 10 or 11 different Marshalls since I started playing in '82 and this is IT. And I really enjoy reading all the threads on this forum. I've learned a lot from you ppl. So I guess I'm gonna try a TunG-Sol in the P.I. soon as I can afford it. Thanks everyone.

 

[Deisel401]

I just read the responses again and I can now "see" the signal path in my head . . . . . .This is frikkin' great !! I've spent my years practicing my *** off and not really worrying about the technical aspect , other than making sure someone competent was taking care of my amps. Input , V1 in amp , V2 and V3 on module , V2 in amp , then P.I. , then power tubes . Is this right ?

 

[JKMV12]

Wow . . .lol . . . that's a lot of info and I really appreciate the responses. You folks cleared that up for me. I was confused cuz in my RM20 , the 2 preamp tubes together closest to the back , have a small label next to the tube socket that says "12AX7". The 1 by itself in the middle has no label so I thought THAT 1 was the P.I. I'm talking about actual paper labels stuck to the wood in front of the tubes. Weird, huh ? Anyway, last night I changed out all my tubes again. I put in a pair of G.T. EL84's ( h. rating = 4 ), and loaded all preamp sockets with E.H. 12AX7's. Then set my bias to midway, about 17.5 ma.'s. After all these years I have THE PERFECT Marshall tone !!!! Ya' know that glassy , crunchy , sustaining , shiny , colorful wall of syrupy tone. And this beautiful tone is WITHOUT my Keeley Comp or Screamer. Just my stock SL+, Les Paul, a good cable, and a pick. Oh, and my 1x12 closed back cab with V30 in it. I've had about 10 or 11 different Marshalls since I started playing in '82 and this is IT. And I really enjoy reading all the threads on this forum. I've learned a lot from you ppl. So I guess I'm gonna try a TunG-Sol in the P.I. soon as I can afford it. Thanks everyone.

if you can afford it then i recommend the JAN 5751 for the PI. As i mentioned it is like a 12AX7 with a lower amplification factor. Don't worry...it won't detrimentally affect the gain of your amp...but what it will do is reduce the noise levels and possibly add more clarity. Your gain comes from the amp V1 and the module V1 and V2.

http://thetubestore.com/sylvania5751.html

That is the basic signal path. The V2 in the amp is the effect loop buffer and should only come into play if you use it. I don't think it really adds anything to the tone. Then after the power tubes come the output transformer followed by the speaker load. And if you want to be even more detailed you can say that after the V3 in the module comes the tone stack. It is really cool stuff. I recommend this link for more technical information about stuff like preamps, phase inverters, power amps, and power supplies:

http://www.freewebs.com/valvewizard/index.html
Here is another good one:
http://www.aikenamps.com/

 

[Deisel401]

Thanks JKMV12. That's great info. And thanks for the links. I'm quite decent with a soldering gun so I'll be doing some really simple mods soon. Then again . . .I have the best tone ever so after I get the tube you are talking about for the P.I. , I may just leave well enough alone. . . . . . . . .Ha ! No I won't !! LOL. I'm like a lot of you guys. If the possibility is there for experimenting with tone , I won't be able to stop myself. Thanks again JKMV12. Have a great day. Rik J. Smith