Cross-line master volume problem

[Fred G.]

I'm experimenting with a cross-line master volume (Type 3 of the Train Wreck pages) on a Fender Bandmaster, and something sure ain't right!

Here's what I did:

I used a 1M audio double pot (it's all I have handy) and just used the back 3 lugs.

From the FRONT of the amp, numbering the pot logs from LEFT to RIGHT, 1-2-3: Center lug (wiper) to the right-most 220k resistor (closest to the bias pot) and lug #3 to the leftmost 220k resistor. Lug #1 is not connected.

The master seems to jump in volume and become VERY LOUD almost immediately, say on a setting of about 2. Also quite noisy.

What have I done wrong here? It's sure not acting like a master volume! HELP!

Thanks,

Fred G.

[Satamax]

Sounds like your pot is linear or reversed. Try pin 1 and 2 I use the other side of the dual pot for increasing NFB loop resistivity, which increases the volume at first, and then lowers it. HTH.

Max.

[Fred G.]

Hmmmm...looking at a Matchless schematic, it looks like pins 1 and 3 are connected to the resistors, with the WIPER connected back to either 1 or 3. Is this where I made my mistake?

[Chris / CMW amps]

Fred,

with that style mv the volume-rise is pretty large and fast, most seem to happen below 3 (from 10).

[Fred G.]

Thanks, Chris.

That's pretty much the impression I get. Seems like a pretty useless master volume, to me, anyway. It doesn't seem to do much at all. I tried wiring it the way the Matchless schematic looked, and it made no difference. I guess I'll stick with the 4-cap PPIMV, unless someone comes along and sheds some new light on this. What a disappointment.

I had tried out a couple Matchless amps in a local music store several years ago, and was totally floored by them! I thought their master worked great - or at least it seemed to at the time. Oh, well.

[HTH]

I have this type of MV in my 6V6 amp and I've got the same problem - once you're above 1/4 on the volume it's pretty much maxed out.

[drewl]

Try another value pot.
I know the schem calls for a 1M, but I've seen 500k in these amps.

[Fred G.]

What about trying a linear taper pot as well?

[Bruce / Mission Amps]

I've never liked the way the X-line MV sounded in guitar amps or the abrupt/poor control feel either.
Maybe just me but the dual gang MV always sounds and works better.

[Fred G.]

Bruce,

Does the Cut control have the same problem (abrupt control behavior)?

[Bruce / Mission Amps]

Bruce,

Does the Cut control have the same problem (abrupt control behavior)?

Well I'm not sure what you are asking... but the MV that I like is a dual gang 250K to 500K log or audio pot with large value, high quality polyprop coupling caps sending signal into the pots and coming out of the pots to block DC plate and bias voltage from appearing on the pot.
If your amp is using a .022uF cap to feed a power tube grid then use at least a .047uF to .100uF set of caps in the dual gang MV circuit.

Bruce

[Fred G.]

Bruce,

Actually, I was curious if the "treble Cut" control had the same abrupt sweep behavior as the Crossline master volume, since the two are similar (the only difference being a cap on the Cut control, right?).

In this particular amp, the MV setup I'm using is .1 uF into a 500k ganged pot coming out into 1.0uF caps. I've got the negative feedback completely disconnected.

Thanks,

Fred G.

[Bruce / Mission Amps]

Yes a X-cut master volume is high cut control with a capacitor big enough that all frequencies are cut not just the highs!
Typically you'd see a 250K to 1M pot with a .0047uF cap for a high cut and in the X-cut MV the cap would be at least 10 to 20 times larger so all freqs are cut.

[cgiff]

Post PI MV

Fred,
I built 3 amps with Post PI MV Matchless style . I used 500K audio taper (alpha's) I believe. On the el34 OP amps the MV is range is fairly smooth & the brillance control is smooth. In the 6v6 amp the MV is on or off It's is definitely not right compared to the other 2 amps ( I need to investigate) I think it might be a 1M linear, the brillance control on this amp is smooth acting though. But in all cases this is not as precise as the typical gain control in the preamp.
Could be that I'm used to it. I usually rock on with MV set about 4 -5, 2-3 when the wife's at home and 10 :-) when I'm having a concert definite difference in power level at 10.
Cgiff

[HTH]

fwiw, I just removed the 500k crossline MV in my 6V6 amp and put a typical pre-PI MV (JMP Marshall style) using a 500k pot.

The amp sounds 200% better now AND the volume is controlable (yay!!!)