Master Volume 70's Fender - Convert to LARMAR Help

[Yeatzee]

Put a bright cap across the two wires at the mv pot. I'd try 47pf, 100pf, 120, just whatever is handy. Need one cap on each pot. Any better?

All out of caps in those small sizes  Guess I'll order some and circle back in 5 days

[stratomaster]

All out of caps in those small sizes  Guess I'll order some and circle back in 5 days

Scavenge them from the bright switches for proof of concept.

[tdvt]

I have done just 2 LarMar MVs; one in a Bassman, the other in a new Princeton-ish build & as you dial them down, both seem to lose bass quicker than mids or treble.

[Willabe]

Our ears hear less bass as we turn down the amp. There's a 'fullness' to the sound once it's turned up some that we don't perceive as we turn down.

That's why they have a loudness button on stereos. Well, they used to anyway.     

[tdvt]

Our ears hear less bass as we turn down the amp. There's a 'fullness' to the sound once it's turned up some that we don't perceive as we turn down.

That's why they have a loudness button on stereos. Well, they used to anyway. 

Agreed, & I was remembering  those old-school loudness buttons myself.

That's why I thought it odd he was losing top-end.

[sluckey]

Our ears hear less bass as we turn down the amp. There's a 'fullness' to the sound once it's turned up some that we don't perceive as we turn down.

That's why they have a loudness button on stereos. Well, they used to anyway. 

Agreed, & I was remembering  those old-school loudness buttons myself.

That's why I thought it odd he was losing top-end.

You also loose treble end as you turn the volume down.  That's why there is a bright cap on many volume controls.

[Yeatzee]

All out of caps in those small sizes  Guess I'll order some and circle back in 5 days

Scavenge them from the bright switches for proof of concept.

That's a good idea!

[Yeatzee]

Reporting back, happened to have one 100pF cap and then stole the normal channel 120pF for the other one to do the test. Then I also found two 500pF caps in my inventory to try and compare.

100pF to my ears appeared to do very little, certainly wasn't noticeable as the player... yet 500pF was a huge difference and too bright / thin as you turn down. So looks like I'll need to order something inbetween and give that a try

[pdf64]

The most likely root cause of the treble loss issue is the use of 1M pot.
Due to the effective source resistance, that interacts with the output valve's Miller capacitance, rising to 250k at the electrical mid point.
If the goal is to maintain a more inherently linear frequency response over the range of the pot's rotation, then a lower value pot, eg 220k, 100k, may be beneficial.

[sluckey]

You have good ears. Trust them. Get a variety pack of small caps and also get some 100K and 250K pots. Experiment until your ears are happy. Let us know your results.

[stratomaster]

100pF to my ears appeared to do very little, certainly wasn't noticeable as the player... yet 500pF was a huge difference and too bright / thin as you turn down. So looks like I'll need to order something inbetween and give that a try

You can also try a resistor in series with the bright cap.  It'll mess with the pot taper some, but you're in experimentation mode anyway.

If you have a 50k trim pot or so put that in series with the 500pF across the lugs.  As you turn the master down tweak the trim pot to give you the treble response you're looking for.  You'll probably find a suitable value that behaves acceptably through most of the range.

Note: you're more ambitious than I. I would've moved the existing pot to a simple cross-line MV and wired the pull switch to double the negative feedback or disconnect two of the 6L6 cathodes for a "half power" option, buttoned this up and moved into the next repair long ago.

[Yeatzee]

Played with it some more, and ultimately couldn't find a combination I loved. Ended up trying to make the Larmar work and am happy with the results. Instead of jumpering the two bias feeds to the two gain pot inputs, I just ran the two separate feeds from the bias balance to the two separate inputs on the dual pot. Used a 100k pot since the bias leak resistors were 100k.

i=IoA8OJtBk5TxlEMg

[sluckey]

Nice job. Glad you got it sorted out.

[stratomaster]

The 1M safety resistor puts you at the low end of the tolerance band for the grid/bias leaks not including the tolerance of pot itself.  This is likely fine, but the usual 2.2M puts you back in the 5% range.

A safety check you can do since you're messing with bias with this type of master volume is confirming the voltage at the grid pin of each socket.  This will give you a clue as to whether your arrangement is disturbing the bias somehow, and if you remove the tubes you can test that you've fully preserved the bias and balance functions by tracking the voltages at each grid as you rotate each bias pot.

Good work.

[Yeatzee]

The 1M safety resistor puts you at the low end of the tolerance band for the grid/bias leaks not including the tolerance of pot itself.  This is likely fine, but the usual 2.2M puts you back in the 5% range.

A safety check you can do since you're messing with bias with this type of master volume is confirming the voltage at the grid pin of each socket.  This will give you a clue as to whether your arrangement is disturbing the bias somehow, and if you remove the tubes you can test that you've fully preserved the bias and balance functions by tracking the voltages at each grid as you rotate each bias pot.

Good work.

Yeah I confirmed the bias voltage with no tubes in it before putting them in and at the grids it matched the bias voltage straight off the pot. I should have shown that on video, but I was rushing to get it done before the owner came by. He texted me later saying it sounded incredible and he's super happy with it so it all worked out!

[sluckey]

The 1M safety resistor puts you at the low end of the tolerance band for the grid/bias leaks not including the tolerance of pot itself.  This is likely fine, but the usual 2.2M puts you back in the 5% range.

The actual value of the grid leak resistors is not critical. It's more important that they are equal value.

[tdvt]

He texted me later saying it sounded incredible and he's super happy with it so it all worked out!

I recently played a date with the guy whose BF Bassman I converted to a combo & included a LarMar, as stage volumes are now quite low.

He was really happy & from where I was on the small stage, I thought it sounded great. With the MV set about 1/2, it wasn't too loud (we were all worried) & it had a really nice clarity that you wouldn't associate with using a MV. 


Pic added;
It's not a lot bigger than a Princeton but with WAY more under the hood, so a usable MV makes all the difference.