Question about possible B+ switch...

[Chuck H]

Here's my modification on the LARMAR type circuit. Since most pot failures involve the wiper this is potentially safer because it keeps the path from the bias supply to the grids on the outer lugs.

http://music-electronics-forum.com/t42008/

Besides, you probably wouldn't have a problem with the typical LARMAR circuit anyway. If putting a pot wiper between the bias and the grids were THAT problematic we'd be hearing about it all the time because of all the vintage designs that use bias wiggle tremolo.

I wouldn't attempt altering the HV much without analogous bias adjustment. To me EL34's sound "right" at only one bias point in any given amp. Change the HV and you change the amp. If you don't change the bias you change the tone more dramatically. I don't think switching between your available taps will make a noteworthy volume difference anyhoo. But you could...

Switch between 635(ish) volt winding and the 690V winding, but add a fat resistor to the 690V circuit to emulate rectifier tube sag. That you keep the Vp the same (and therefor the bias) but still get an instant gratification switch that alters the tone. If not the actual volume much.

[mozz]

This one works well.

[pdf64]

Chuck, I was thinking about the 'wipers first' version of the type 2 master, and it struck me that its mode of operation, ie loading the LTP plates rather than a potential divider on their output signal, was the same as a type 3 / crossline master.
I haven't got around to comparing them yet, but wondered if you had?
A type 3 master being very simple to implement, doesn't require a dual gang pot.

[Chuck H]

I only use the cross line master circuit when it aids in bench testing. I've never scrutinized it for tone. I've also never noticed anything objectionable. I don't use PPIMV's. I use attenuators I only came up with the alternate circuit because a poster wanted to use a LARMAR "type" circuit with a bias wiggle trem. That it removes the pot wipers from the bias circuit was just a fringe benefit.

[Justin Thomas]

I think the Crossline is just fine. <IF> you play loud! Crossline MVs are for saying "a little off the top, please..." When used for radically lowering the amp's volume, you're just learning exactly how unbalanced your PI is.

Look at the amps that made them "famous" - Matchless. Not exactly a "high gain" design, and highly favored by professionals who were playing on stages and needed to cut through a mix. IOW, LOUD.

This makes me think that the original intent of the Crossline design was similar to the way Fender's 70s design ended up working in practice - great for reducing overall volume levels, but if you're using it to try to get a Marshall/Mesa sound, or as a bedroom amp, forget it - Fenders and Matchless were designed with a different goal in mind. Maybe the MV in a Matchless was added as a selling point, not as an actual useful feature... "Well, this is the easiest and most cost-effective way to give them what they want, but I don't necessarily agree with it..." Like the effects loop in the Prosonic...

Justin

Edit: what the hell, I'm plugging in a 12DW7 in the PI spot just for shots & grins...