The PPIMV (Post-PI-Master-Volume) thread.

[syscokid]

syscokid - would you want a different value for 6L6Gs?

I would just keep it to the EL34 specs, and check and or re-adjust the bias. You might have to alter the bias-range resistor, that leads to the bias pot, to get it in the proper range.

[hotguit]

Sorry if it's already been discussed but I didn't read the 100+ pages.

Is it possible to replace the 220k resistors on the board with the 2m2 instead of soldering them on the pot ?

The recommended pot is 250k of resistance. To bring the Grid/Leak Bias resistors to a value of 220K for EL34's, you must install the 2.2M R's parallel to the 250K pot to achieve the desired results.

For 6550's, you would want Grid R's value to be about 150K. Therefore, using a pair of 470K's parallel with the 250K pot, would give a value of 160K...close enough!

so if my 1975 SL has stock two 82 K instead of the two 220 K, for installing the LarMar do i have to use a couple of 130K right?
1/(1/130 + 1/250) = 85K
Is it right?
thank you
hot

[tommi]

Sorry if it's already been discussed but I didn't read the 100+ pages.

Is it possible to replace the 220k resistors on the board with the 2m2 instead of soldering them on the pot ?

The recommended pot is 250k of resistance. To bring the Grid/Leak Bias resistors to a value of 220K for EL34's, you must install the 2.2M R's parallel to the 250K pot to achieve the desired results.

For 6550's, you would want Grid R's value to be about 150K. Therefore, using a pair of 470K's parallel with the 250K pot, would give a value of 160K...close enough!

I thought the 2M2 resistors were there to provide bias voltage in case the pot fails and goes open circuit. Which is kind of strange though as it's way over the EL34 spec (which is something like max 700k grid leak resistance). I would probably use a 680k resistor if the pot is 250k or less so that the low resistor value won't affect the behaviour of the volume control.

I don't think 250k vs 220k will make much of a difference in the tone as the corner frequency of the high pass filter formed by the decoupling cap and the resistance the cap sees to ground is approximately 1/(2*pi*R*C) and the corner frequency will either way be very low... And if you're actually using the PPIMV to attenuate signal you can pretty much neglect the parallel resistor. Even with 6dB attenuation the high pass filter will see a resistance of 242k to ground...

I would be more concerned about the bias if the pot fails. 2M2 resistors might bias the tube quite hot as the grid current coming from the power tube will decrease the bias voltage and therefore increase the plate current. This might damage the tubes and the output transformer.

But that's just my opinion

[themidnighter]

Hi everybody,

I am the proud owner of a Bassman 5F6A clone as a head, not a combo, (here is the layout : http://www.ceriatone.com/images/layoutP ... 100508.jpg" onclick="window.open(this.href);return false;) but the 6L6 have been replaced with 6V6.

I recently installed a LAR/MAR PPIMV, and it turned out really great. The bias as been controled, and there's no problem with it.

But I have one single trouble with my amp. When I turn the PPIMV at more than 75%, and the presence controle is less than 50%, the amp is howling and buzzing loud like a whistle, and it goes louder as I turn down the presence control.

I have installed the PPIMV in the exact way as it is described on the metroamp.com/wiki.
(1) Dual 250k audio pot Log taper, although Linear will work fine. At least ¼W and High quality (so as to ensure 220K matched sides)
(2) 2.2M resistor 1/2W min (or any value between 1.5M and 2.2M).
(2') 2 conductor shielded coax (Shielded 2-conductor cable (RG174/U). Rated for 300v, .22 gauge)

Everything listed is correct and th ePPIMV as been installed exactly as the shematic on the wiki page.

Anybody got any advices to fix this whistle, or maybe have any recommandation on this ? Hope you guys can help.

[rooster]

Sure, you crossed your wires leaving the board and going to the pot. You can switch them at the board, the pot, or the tube sockets.

[SteadyEddie]

A friend and I both built plexi clones with PPIMVs.
Then he came across a 2204.
We marveled at how "active" the tone stack was compared to our plexis.
Then, we installed a PPIMV in his 2204.
Whoa! The tone stack sounded just like our plexis.
The revelation is that the PPIMV somehow makes the tone stack less relevant in the circuit.
Not sure how, maybe saturation of the PI reduces the dynamics.
I know a PPIMV makes your presence control less active due to the decrease in relative negative feedback. But that doesn't affect the tone stack, does it?

[CoffeeTones]

Which settings of the PPIMV are doing this?

[SteadyEddie]

Which settings of the PPIMV are doing this?

All settings. Except when you run in almost "bypassed," that is, turn the PPIMV all the way up and turn the volume (plexi) or master (2204) down.

I think it's just when the PI gets overdriven (which normally happens at ear-bleed levels) that there's less dynamic range...

[sickman82]

This thread is such an interesting read. I managed about 30+ pages so far. I have a '96 1959SLP reissue that was modified with the Lar/Mar PPIMV when I bought it. I've just been inside the amp this week because I needed to clean the pots, jacks and switches etc due to the amp cutting out intermittently at high volumes. The master volume has been done correctly by the previous owner, apart from twisting the wires going to the output valves. Which I've taken care of.

I'm also going to remove the presence control and put in the cut control that is discussed early on because the presence is pretty pointless as it is. I just have a quick question regarding the presence control.. if I'm right, I can add a 4k7 resistor in place of the C22 jumper on the pcb (the jumper linked the 10k from the PI to an offboard wire going to the presence control).. then put the 100n cap in parallel with the 4k7 and wire them to the ground bus on the pots? Does that sound right?

The link is in the amp from factory, I'm not at all sure why. With regard to the cut control, as I understand it.. I'm just putting a 250k log taper pot in series with a 2n2 cap across the plates of the PI?

One more question as a side note, I noticed the B+ at the output valves is sitting at 501v.. the amp has LCR filter caps rated at 500v, is this cause for concern? It's knocking on for 20 years old, should I change the caps for some decent ones with a higher V rating?

[CoffeeTones]

http://forum.metroamp.com/viewtopic.php ... 40#p287143

I'd lower the plate voltage and test the caps ESR before replacing the caps. Although replacing the caps could be cheaper than an ESR meter. I don't know that you will find the needed caps with a higher voltage rating, but you could use caps in series, with balance resistors for higher voltage handling.

[SteadyEddie]

One more question as a side note, I noticed the B+ at the output valves is sitting at 501v.. the amp has LCR filter caps rated at 500v, is this cause for concern? It's knocking on for 20 years old, should I change the caps for some decent ones with a higher V rating?

I don't see the downside to doing this. At 20 years old, the caps are due for a change anyway. Get some caps rated at 600+V, and enjoy a little peace of mind. Make sure you form your new caps as per the sticky.

[sickman82]

Thanks for the replies guys.

If possible I would rather leave the B+ as it is and put some 600v caps in there instead. I have the amp sounding great at the moment, so I'd rather not fiddle with voltages unless I have to..

However, I've been searching and haven't come up with any dual value caps rated higher than 500v though.

[sickman82]

By the way Coffee Tones, I love your avatar! So true!

[neikeel]

If possible I would rather leave the B+ as it is and put some 600v caps in there instead. I have the amp sounding great at the moment, so I'd rather not fiddle with voltages unless I have to..:

if it ain't broke don't fix it

[sickman82]

Good point! I need a new project to work on!

Can anyone confirm what I said about the presence and cut control? Haven't actually seen a diagram for either, but going by the description that's what I got.