Plexi 50W with MV and PPMIV question

[Auke Jolman]

I've build a 50W plexi according to Doug's layout and added an MV in front of the PI and one after the PI. The first is done with a 1M pot and the latter with a dual 250K pot,I think iit's called Larmar PPIMV.

I really like this amp a lot and due to both MV's I get all kinds of different tones out of it.

Last weekend I noticed however that turning the PPIMV past 4, I don't hear an increase in volume. The question I have is wether this is normal behavior of the PPIMV or is there something wrong and if so, what can be done about it.

[Ed_Chambley]

If you are using an audio taper it is possible.  Measure both pots and see when the reach 220k.  If it is close to "4" then that is all there is.  I prefer linear PEC pots for this, but most use alpha.

[Auke Jolman]

Thank for the reply Ed.

I did use a dual Alpha pot 250K, I don't remember beeing an audio or linear pot. When I measured the the resistnce between the left lug and the middle lug, it ranges from zero to about 690K, at about 4.5 it reads 220k. So that explains why I don't hear a volume increase, right?

Is it normal to get the readings I get?

With regards,
Auke

[Ed_Chambley]

Sounds normal. I have a the same master using an audio taper pots do it acts the same.

[Ed_Chambley]

Thank for the reply Ed.

I did use a dual Alpha pot 250K, I don't remember beeing an audio or linear pot. When I measured the the resistnce between the left lug and the middle lug, it ranges from zero to about 690K, at about 4.5 it reads 220k. So that explains why I don't hear a volume increase, right?

Is it normal to get the readings I get?

With regards,
Auke

Not sure how you are getting the 690k reading but if you are using a dual 250 audio what you have is common.

[hesamadman]

That doesn't sound right. If you have a 250k, the most resistance you should get is 250k when turned up. Since its a dual pot, check the reading from left terminal and middle terminal while knob is on about half. Write down your measurement. Don't turn knob. Check the same two terminals in the other part of the dual knob. You should have the same thing. Or close anyway. Do this exact same thing with pot all way down. Then all way up. Make sure readings are the same on both the front terminals and the ones on rear. If your getting 690k on a 250k pot, you may want to switch it for a new one.

[Ed_Chambley]

That doesn't sound right. If you have a 250k, the most resistance you should get is 250k when turned up. Since its a dual pot, check the reading from left terminal and middle terminal while knob is on about half. Write down your measurement. Don't turn knob. Check the same two terminals in the other part of the dual knob. You should have the same thing. Or close anyway. Do this exact same thing with pot all way down. Then all way up. Make sure readings are the same on both the front terminals and the ones on rear. If your getting 690k on a 250k pot, you may want to switch it for a new one.

It may be because he has it connected to a series resistor network.  That is why I mentioned I did not know where he was getting the 690K from, but you are correct that he should have only 250k between 1 and the wiper or 3 and wiper max with nothing connected on both pots.  Being that they are not precision pots the position of where you check resistance can vary which is common.  The front pot could measure 200K at 1/2 turn and the rear pot could be 150k.  Not uncommon.

[Auke Jolman]

Have you ever done something and you're sure you know you've done it right with the proper stuff......

Well, this is my moment. Turned out the dual pot was a 1M instead of 250K. Changed it out and now everything is fine. Thanks for putting me on the right track.

With kind regards,

Auke

[Ed_Chambley]

Have you ever done something and you're sure you know you've done it right with the proper stuff......

Well, this is my moment. Turned out the dual pot was a 1M instead of 250K. Changed it out and now everything is fine. Thanks for putting me on the right track.

With kind regards,

Auke

Sure have, all the time.

Every time I flip the switch on I believe I have it correct.  The fact is I have only ever built 2 amps that I did not have to retrace more than once.  An AC15 and a AB763 Super Reverb.  Both of those I took my time.

Glad you got it.  Does she sound great?  I love the Marshall Plexi and I am getting ready to mod a build I have.

You running el34?  I am thinking of trying KT66's.  I have never heard them in a Plexi circuit.

[Auke Jolman]

Hi Ed,

She sure sounds great. I build it stock per Dougs layout, except that I used a rectifier tube GZ34. And I run EL34's.

For the trannies I used some white label iron what I could get locally. Always wonder if it would sound even better with some other iron, like MM or Classic tone. The are quit expenive here.

With regards,

Auke