I De Tweeded my 5F6A Bassman! !

[Greg Smith]

The MV pot halves are grounded correctly in the second picture.

However, the bias supply is still connected in the wrong place. The bias supply is connected to the two 100K resistors, which connect the bias supply to the two sides of the signal path, at the junction between the second coupling cap and the 1K5 resistor (on each side).

Take a look at this schematic...
http://www.lynx.bc.ca/~jc/73_JoePiazza_YGM34.gif
....notice how the bias supply is connected to the 6BQ5 output tube grids through the two 100K resistors. That should get you in the right ballpark.

[Casey4s]

Sooner or later I am going to figure this out...

[Casey4s]

As soon as I posted the last schematic I could see the problem but I am not allowed to edit so here is the correct rendition. (I hope)

[Geek]

Why are you sticking with that double volume pot again... ?

If you insist on it, use a heckuva good one, like this:
http://search.digikey.com/scripts/DkSea ... 2541S28-ND
Cheers!

[Greg Smith]

I am a big proponent of the post-PI ganged MV. You can get some great overdrive tone from an overdriven PI. However, an overdriven PI can sound fizzy - you need to filter some of that out. An AC30-style cut control would be a good idea.

Why is a high-quality pot such a necessity here, as opposed to a plain-jane Alpha pot?

Greg

[Geek]

Why is a high-quality pot such a necessity here, as opposed to a plain-jane Alpha pot?

Because their tracking is horrible.

Actually, those PEC's are the exact same ones Alessandro rebrands as their own and doubles the price

Cheers!

[Greg Smith]

Is it just the audio taper pots that track horribly, or the linear ones as well? I don't know how much the tracking errors would matter in the lo-fi world of guitar amplifiers. What I WOULD be concerned about is one of the wiper contacts becoming intermittent and losing drive to one half of the output stage.

I always thought those Allesandro pots looked eerily similar to the PECs....

[Geek]

Hi,

It's a lot of them. I've tested Alpha, Bourns, CTS, PEC and Black Velvet... the PEC and Black Velvet were the only ones to track beyond my meters ability to differentiate on the cobbled bridge.

As for will it matter? Not likely below a certain point (like 5% diff).

Cheers!

[Casey4s]

Most of the moderately priced pots are not true audio taper but rather several straight lines overlaid into a lumpy arch in that taper. That's why so many track badly.

[Geek]

Yes, 4-6 steps of "logarithm".

The Precision and Alps (Blue and Black Velvet) pots are true log.

Cheers!

[dhuebert]

Orange Amps uses the post PI master volume with great success in their "Tiny Terror".

Don

[Casey4s]

I prefer the post PI MV over the pre PI version. I like controling the output power the way a post PI does.

I originally had a double Bournes pot in the MV and I hated the feel of the pot so I opted for the Alpha because I was in a hurry but want to get a Clarostat or PEC if I can find one. I like the cheap Alpha better than the Bournes.

In my Active tone stack I had Bournes pots and didn't like the feel of those either so I used 3 linear Clarostat pots there.

My Gain and Presence pots are CTS which I really like the feel of.

I haven't tried any PEC yet but want to try some on a project.

[Greg Smith]

I actually used a 500k dual linear pot for my PPIMV. It was all I had on hand at the time. Surprisingly, the rate of volume increase as I adjust the volume does not sound abnormal, as I thought it might with a linear pot.

[Casey4s]

In the original Weber Schematic, it calls for 3, 500K Audio pots, but they wouldn't center and made the controls act funny so I replaced the three Bournes audio pots with the Clarostat Linear pots, and it's much better.

[Casey4s]

Why are you sticking with that double volume pot again... ?

If you insist on it, use a heckuva good one, like this:
http://search.digikey.com/scripts/DkSea ... 2541S28-ND
Cheers!

Thanks for this link. I see they (PEC) will make custom combinations for you, that's pretty cool.

I wonder if they make a dual concentric pot? A good quality 1M & 100K in a concentric configuration would be a great find for my next amp MOD.