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

[SteadyEddie]

Update, after cranking the amp it does sound somewhat different. A bit fuller with just twisted wires. A tad bit "fatter." I like it.

[Trevor]

Hey guys, I did this mod on my 50 watt a while back and just love it. I have a Vox AC30 TBX and I'm wondering if anyone knows if it can be done in this amp and how to do it. I've searched this thread for info on it and have found a few posts that make me think its possible, does any know how to do it? Any help would be greatly appreciated!

[CoffeeTones]

Cathode bias with Lar-Mar - It all installs the same way except the wire that normally goes to the bias supply, now goes to ground.

[Twistingcrow]

ok, work is done.

Seems everything works fine. I just could do a short test on minimal bedroom level, but no noise or malfunction detected so far.

maybe the amp drives a little bit less into overdrive, but I have to check this tomorrow. And my god, this amp develops a lot of heat...

Here the picture:


Hope this helps for other 1974x owners.

Best
Markus

It will help me a lot thanks! I'm about to install a Lar/Mar in an 18w TMB. This pic is perfect!!! Why did you choose a dual 500K pot instead of 250K by the way? Does 500K make more sense in an 18w or...?
Thanks!

[CoffeeTones]

Many 18w amps are built with 470k resistors that the 500k / 4.7M Lar-Mar components replace. It gives more drive to the output and changes response. Also see the .01uF output caps. Those correct the time constant along with the 470k. http://www.ceriatone.com/images/layoutP ... iatone.jpg" onclick="window.open(this.href);return false;

[Twistingcrow]

Many 18w amps are built with 470k resistors that the 500k / 4.7M Lar-Mar components replace. It gives more drive to the output and changes response. Also see the .01uF output caps. Those correct the time constant along with the 470k. http://www.ceriatone.com/images/layoutP ... iatone.jpg" onclick="window.open(this.href);return false;

Thank you.
I will install it in the second output jack though, unlike the Ceriatone option. I've never been a fan of master volumes installed on the front panel...
So basically I need a Lar/Mar MV with a dual 500K instead of the 250K and 4.7M resistors instead of the usual 2.2M? And follow the pic pictured above for install?
Thanks!

[CoffeeTones]

Yes, and also .01uF PI output caps unless your amp has them already. .022uF may sound just fine, but bass response may not be what you like.

[Twistingcrow]

Awesome! Thanks!

[Trevor]

So the bias feed wire, is the blue wire in Marks diagram that goes from the board to the ground lug on the pots? So basically that wire is omitted?

[CoffeeTones]

Yes, that wire is shown in blue and can be extend or omitted and replaced by a wire to ground, in a cathode bias amp. http://metroamp.com/wiki/images/thumb/9 ... PPI-MV.jpg" onclick="window.open(this.href);return false;

In a cathode bias amp such as Vox. The wire that normally leads from ground to the junction of the PI output grid resistors / bias splitters which are replaced by the Lar-Mar components, can be extended to the left lugs, if viewing from the rear of the dual pot. If desired, you can simply run a new ground wire and not use / extend the original ground wire.

In a fixed bias amp such as Marshall, that wire normally leads from the bias supply to the junction of the PI output grid resistors / bias splitters, not from ground. That's the difference concerning the Lar-Mar and these two bias types.

[julkke]

Hey, my ppimv is a little scratchy on the first 1/4th of the pot. If I don't play anything you cant notice the scratchiness but if you hit a chord and turn the knob it scratches a bit when turning. Is this acceptable or should I replace the pot? My other amp doesn't have this scratchiness, but it is using a 100k dual pot. I also get a quite big volume jump fron pretty quiet to loud room volume, my other amp doesn't do this either. I haven't measured the pot and might just swap it, just asking if this stuff is normal for peace of mind. It is alpha pot. Thanks

[CoffeeTones]

Spray the pot with cleaner and it is possible that with some break in from turning, that it could smooth out once the track and wiper settle in. I never use the PPIMV that way but it is not surprising that you get some noise and thinking deeper, I doubt there is a problem with the pot since it makes no noise other than the circumstance you are creating.

[Roe]

I'm experiencing a strange problem in a twin with larmar: the bias on two of the powertubes (6l6s) start to drift when the master is at 10, from 30mA to 50+ mA (I'm turning off at that point). when the master is at 0 the bias is stable around 30mA. The negative voltage at pin 5s is stable, except that it drops gradually on one side after the master has been on 10 for some time. I tried changing the pot and a PI signal cap but it didn't help.

any suggestions?

similar case here: http://forum.metroamp.com/viewtopic.php?f=28&t=35220

[CoffeeTones]

Are you getting current across the PI output caps? What is the pot value and resistor values? Any pictures of the install?

[Roe]

its a 250k pot measuring 250k/240k, using 560k and 680k safety resistors for a total around 170k