Lar/Mar MV in my Sunn build not quite right

[nateflanigan]

Some time ago I made a mr potato head amp, mostly it's a Sunn 200s, but with the one tube reverb and a 6L6 fender style power amp.  It's cool, but unbelievably loud, so I'm finally getting around to trying a MV circuit in the amp.  I've used the Lar/Mar in my Orange and was really happy with the results so that seemed like a natural fit. 


The bias feed resistors on the Sunn are 100k, so I thought I'd use a dual 100k A pot for the MV.  I've got it wired up, but it's not really working, fully open everything is fine, as I turn it down nothing happens until the very last bit of taper on the pot.  First thought is maybe I should be using a Lin taper, but before I order another pot I want to make sure I have it wired correctly. 


Here's my reference material...


http://el34world.com/charts/Schematics/files/sunn/Sunn_200s_only_schematic.pdf


Thread on MV
http://el34world.com/Forum/index.php?topic=17045.0


Ken Fisher stuff, MV is on pg 42
http://www.blueguitar.org/new/schem/trainwreck/the_trainwreck_pages.pdf




I've removed the original 100k bias feed resistors, I'm going from the coupling caps to lug 1 of the Dual 100k pot, wiper to the 1k grid resistors, -44v bias feed to lug 3.  I'm not using the parallel (2.2m) resistors. 


Smell anything fishy?

[sluckey]

I've removed the original 100k bias feed resistors, I'm going from the coupling caps to lug 1 of the Dual 100k pot, wiper to the 1k grid resistors, -44v bias feed to lug 3.  I'm not using the parallel (2.2m) resistors. 

Sure sounds right. How about your bias supply? Do you have a filter cap directly on the -44v bias feed to lug 3? I don't mean a filter cap upstream somewhere. I mean one that is electrically connected to both lug 3s of the MV. I assume both lug 3s are tied together?

[nateflanigan]

Correct, on the MV pot both lug 3's are tied together.  I have the 50uf filter cap directly between the bias adjust pot and lug 3 of the new MV.


I noticed in the Ken Fisher doc he specifies a LIN taper 100k pot, so that seems like reasonable territory to explore.  I had the dual 100k on hand and figured maybe the taper would be a little weird but not this weird.

[sluckey]

I have the 50uf filter cap directly between the bias adjust pot and lug 3 of the new MV.

Surely I'm misunderstanding this statement. I interpret this to mean one lead of the bias cap is connected to the bias pot and the other lead is connected to lug 3 of the MV. That can't be.

There should be a wire between the bias pot and lug 3 of the MV. The negative lead of a cap should also connect to that wire and the positive lead should connect to ground.

[nateflanigan]

There should be a wire between the bias pot and lug 3 of the MV. The negative lead of a cap should also connect to that wire and the positive lead should connect to ground.

Correct.  Sorry for the confusion.  To be clear, the MV is a new edition to the amp, I built the amp a few years ago and it works perfectly fine (just too loud)

[nateflanigan]

Got it sorted out.  It was my fault, I had it wired backwards.  Meaning the coupling caps were on the wiper and grid stoppers on the 1st lug.  I figured this out by switching to a linear pot, forgetting which color wires went to where, retraced it, realized I was wrong.  For the record audio taper works better than linear.