PPMIV In 5f6a Switchable

[Ed_Chambley]

I am attempting install a switchable PPMIV using a dual 500 k pot.  I used shielded cable but when connecting to the 220k resistors, I get a huge changes in bias, therefore it makes it impossible to make it switchable unless I also switch in a bias range resistor.  Can anyone explain a hookup that does not change the bias.  Currently I have the .1 caps lifted and wired to the lug, the wiper returning back to the 220 resistors connected to the underboard wire side.  I am getting about 188k with the master wide open so I assume need to reduce the 220k resistors to 180k.  Is there a simpler way where it will not change the bias where I can make it switchable?

[kagliostro]

Did you remembered to add two cap to decouple bias from PPIMV pot ?

Kagliostro

[stratele52]

Nice sketch Kagliostro but when I look at it it come in "negative "it is black  ???

[kagliostro]

I've no problem with the attached image

however here is the same file in pdf

Kagliostro

[tubenit]

Is your Bassman fixed or cathode biased?   I only used PPIMV with cathode biased amps.

With respect, Tubenit

[sluckey]

The bias voltage should not change at all. I suspect you have something wired wrong. Show us a schematic for your circuit. One of the PPIMV circuits actually connects the shields to the negative bias, not to ground. The circuit posted by Kagliostro connects the shields to ground. Don't get them mixed up.
 

[Ed_Chambley]

Is your Bassman fixed or cathode biased?   I only used PPIMV with cathode biased amps.

With respect, Tubenit

It is fixed, adjustable.

[Ed_Chambley]

Here is a layout.  I have not learned to draw schematics, but I can draw.  I hope this helps.   I used this in a JTM-45 build and it worked beautifully, but the underboard wiring is different.  I assume the connection I have at the 220k resistor cannot work sine the bias connection is in the same location.

[tubenit]

Is this what you are doing?

with respect, Tubenit

[Ed_Chambley]

No, I forgot the .1 caps as I was doing the one Bruce Collins has posted.  Just installed the caps and everything is working great.  Bias is back as should be and the master is out of the circuit at full turn.  I had it on a switch, but after checking it so close to stock wide open that the switch is not necessary.  It is a nice sounding MV as I do not use a lot of overdrive as I mainly chicken pick.  A little hair, but this bassman has a great amount of punch that even at lower volume it is really a gigging amp.  This MV will allow me to get some use out of it at home.

Again, thanks for the help.  Again, I forgot something.

[kagliostro]

I was doing the one Bruce Collins has posted

Unfortunately for me, I'm not Bruce Collins of Mission Amps

Kagliostro

[Ed_Chambley]

I was doing the one Bruce Collins has posted

Unfortunately for me, I'm not Bruce Collins of Mission Amps

Kagliostro

Sorry what I meant to write is I am using the one Kagliostro posted which was drawn by Bruce Collins and can be found in many places on the internet.  I would imagine Bruce did not come up with it either, it seems to be a cross between the Type-2 from the Trainwreck Pages and version 3 of the LAMAR PPIMV.  Nothing wrong with taking credit for providing information.  I do not think anyone invented a lot where amps are concerned, it seems more as a thing that has a life of its own and grows in all different directions.

[kagliostro]

I would imagine Bruce did not come up with it either

English isn't my language so I can't grasp the nuances  and often misunderstand

about the circuit as you say there are many versions of PPIMV

the one I posted is the first one I had under hand with the two cap between pot wipers and grids

other authors have near the same solution

like the attached from Markh and from Weber

the solution given from Bruce adds a 33k/100k resistor in series with wiper as to prevent tone change rotating the knob

the solution from Markh use a couple of very large capacitors to avoid limits in low frequency response (I didn't see this solution implemented fequently)

the Weber circuit is the "Simpliest" but there is shown the difference on applying to both split load and longtail PI

Kagliostro

[Ed_Chambley]

other authors have near the same solution

the solution given from Bruce adds a 33k/100k resistor in series with wiper as to prevent tone change rotating the knob

Kagliostro
[/quote]
Other authors have near same solution is what I was saying, I used lots of words to state it.
I have built the MV with and without the resistors 33-100. I cannot hear any difference, so I don't use them. 

[Colas LeGrippa]

is there any significant improvement in tone using a PPIMV, compare to post tone stack ?

Colas

[kagliostro]

as you say there are many versions of PPIMV

I used lots of words to state it

Yes, I was confirming what you told, didn't want to be in friction with you, sorry if it seemed that I wanted to do

Colas

what do you mean with <post tone stack> ?

Kagliostro

[Colas LeGrippa]

Originally, MV were between the tone stack and the phase inverter, or if you prefer just before the PI. This way, we can use a simple 1 meg or so pot. By putting the MV right after the PI , we have to use a double volume pot, one for each phase. I just want to know the tonal difference between these two different ways. If the tone is really better post PI, it is worth the extra work and parts cost.

Colas.

[Colas LeGrippa]

BTW, Ed, it is PPIMV and not PPMIV  ( that stands for: post phase inverter master volume )

[Ed_Chambley]

as you say there are many versions of PPIMV

I used lots of words to state it

Yes, I was confirming what you told, didn't want to be in friction with you, sorry if it seemed that I wanted to do


Kagliostro

Did not seem that way at all.  Thanks for the help.

[Ed_Chambley]

is there any significant improvement in tone using a PPIMV, compare to post tone stack ?

Colas

I think so, but what someone likes, the other may not.  It seems like the post MV is more of tube saturation, but it does have less distortion.  To me it is closer to the amp turned up rather than an additional gain stage.

[Ed_Chambley]

BTW, Ed, it is PPIMV and not PPMIV  ( that stands for: post phase inverter master volume )

Yes, I know.  I have a little dyslexia.

[Colas LeGrippa]

join the club !

[ac427v]

Hi Ed,
On my 5f6a, I first tried the 1 meg master just before the phase inverter. Later I did the ppimv and prefer the low volume distorted sound with that circuit. However, I connected my dual pot wires to the ends of the 220k bias resistors that are furthest from the bias supply. You might want to recheck that. I love this amp. It makes glorious guitar sounds even when I don't play well!
Craig

[Ed_Chambley]

Hi Ed,
On my 5f6a, I first tried the 1 meg master just before the phase inverter. Later I did the ppimv and prefer the low volume distorted sound with that circuit. However, I connected my dual pot wires to the ends of the 220k bias resistors that are furthest from the bias supply. You might want to recheck that. I love this amp. It makes glorious guitar sounds even when I don't play well!
Craig

Thanks.  That is the way I have it, I just drew it wrong.  Thanks for your input and looking out for me.  I have to say I agree, it is a really nice amp.  I did get my Tung-sol's 5881's yesterday, New in box.  I really like them.  I am trying to decide on rectifiers, my last thing.  It is between GZ-34 and 5R4gy.  They are very different sounding.  I like the sag of the 5r4 as the cleans and chording are better with my telecaster but for the sake of the true bassman sound I will probably stick with the GZ-34.  It is sitting right beside a JTM-45 and I still cannot believe the difference in sound.   My JTM-45 has EL-34's, but that cannot be all the difference.  Sluckey gave me a mod for the JTM-45 to add a plexi voicing to it, but even when I have them set the same, the bassman sounds so much different.  The best way I can describe it is very lively.

I am extremely pleased with the build and really appreciate all those who helped me with it.  I cannot find anything about it that I am not absolutely thrilled with.