Master Volume ideas...

[bren-hur]

So... I've been mucking around with my amp (again)...

I have a '69 Bassman modded to be a blackface/tweed/marshall... I used to have a MV on just the tweed channel, using a 1M pot before the 220K mixing resistor, pre PI section. (Type 4 from the trainwreck pages....)

Now... I wanted a MV on both channels AND I wanted to make it so that when it was fully cranked it was essentially bypassed, making my amp stock again... I went with the simplest..Type 3 trainwreck... Post PI, A single
1MA pot tapped off the 2 .1 caps. (At 0 the signals are run into each other at 10 they are separate, as normal)

Now this works sorta fine..its the most un-intrusive to the circuit, the amp sounds better at 10, the distortion sounds gooder with the PI tube cranking as well...but the problem is... when I dial it back, it doesn't really turn the amp down much, till about 3...then its off a 2...hmmm. I get a good cranked sound with the pre's turned up, but the MV don't work till 2...and its way loud already...

Would switching to a linear pot make a difference? smaller than a 1M?

I dunno..I was thinking bout trying out the Type 2 trainwreck, (Post PI, Replacing the 2 220K resistors with a dual gang 250K Linear pot) Anyone got any comments on this one?

I actually just thought of another possible MV (Someones done it already I'm sure...) and that would be to switch out the 2 220K mixing resistors for a dual gang 250k pot.. PRE phase inverter....

I dunno.... ramblings...I'd just like to get a cranked sound, without buggin the wife to much...

[txstrat]

Could you post a sketch or schematic the way it looks right now?

[Amp Kat]

Also take an ohm reading of where that cut off point is on the pot. If it's less than 500k than no it won't do any good.

[cbarrow7625]

FWIW, I'm a big fan of swapping out the 220k resistors post PI for 250k pots. I do it all the time in my amps. If it is just for you, this should not be a problem but if I was building it for somebody else I would likely opt for a more conservative circuit (If the 250K pots go bad, etc. it could lead to power tube failure).

I agree that the cranked PI makes a huge difference in sound. If you have a PI before AND after the PI you can dial in just the right amount of PI overdrive (too much can be too much).

[Jag]

I think the type 1 MV from the Trainwreck pages looks like a good option, although I haven't tried it. If the pots go bad it won't bugger the bias.

FWIW, I find that the Cross Line MV (TW type 3) pretty much always works the way you describe and haven't found a way to even out the control. The Cross Line is my favorite sounding though.

I agree that a cranked PI sounds great.

[cbarrow7625]

The problem with the cross-line MV control is that in order to reduce the volume the pot has to be in a position where it is creating a decent voltage divider with the output impedance of the tube. If the tube output impedance is around 68k (just for example, it is of course circuit dependent) then you won't even really start to hear a reduction in volume until the pot is also around the 68k resistance point (or whatever equals about half of the impedance of two tubes working on a single resistance - one of you math whizzes can figure that out...but I digress). No matter how you cut it, that means the pot has to be on a pretty low setting to start making noticeable reductions in volume. From that point down it works great as a voltage divider and reduces volume.

I don't care for this type of MV because it loads down the PI too much. I don't find that sound too appealing (just my personal taste).

To solve both problems (and create a different one) you could try putting 100k or 200k in series after the caps & before the pot. That way the volume would begin to reduce earlier in the rotation and not load down the PI nearly as much. Of course, it would compromise the performance at full output. To compensate for that somewhat the power tube grid resistors could be increased from 220k to 470k, thereby creating less of a voltage divider with the added series resistance.

A third option would be to add a "nighttime" switch that switches these series resistors in and out. When you want full cranked tone it is there, when you need reduced volume with better control over it you can switch that in as well.

Might be worth a try.

[bren-hur]

hmmm...sounds interesting! So those 100k resistors would be before the pot? ie: could attach to the pot lugs? (I attached a real crap picture of what I mean...)

[bren-hur]

then you won't even really start to hear a reduction in volume until the pot is also around the 68k resistance point (or whatever equals about half of the impedance of two tubes working on a single resistance - one of you math whizzes can figure that out...

So... If I had say a 100K pot, It should kick in earlier?

[cbarrow7625]

Not quite. The 220k power tube grid resistors need to be attached at the POT side, on the other side of the 100k resistors (as you have it drawn).

i.e., the pot still hast to be attached directly to the power tube grids

[cbarrow7625]

Yeah, but your cranked tone will not be nearly as good. That's why a 1M pot is usually used. Most people feel that 1M is sufficiently "out of the way" for their full cranked tone.

[bren-hur]

Thanks for all the help!.... I guess my reasoning for going with a smaller K pot was that I thought when the pot was cranked, it was totally bypassed with this type of MV circuit, like the 2 wires are basically free floating out of the circuit when the pot is all the way up... like the left 2 lugs are disconnected, and the one wire going to the right lug goes nowhere....

i dunnno... crap. I'll porbably stick in a dual gang 250.. ha!

[Alex R]

If you get a pot with a push-pull switch you can wire it to open the post-PI MV circuit and switch it right out. Then you can fit the pot that suits you best, sounds like a 220K log might do the job.

[Amp Kat]

Or you could add the resistor to the MV wiper going out and achieve the same thing but maybe easier to mod.

[tubecrunch]

Hi Bren,
I want to do the tweed/marshall mod to my bassman. I found plenty of people talking about it, but no specs. Could you point me in the right direction?
Thanks

[bren-hur]

Hey man... heres a schem of my blackface/tweed/marshall... no master volume on it and I added a mid pot later on to the tweed channel....