Bridging the inputs on Marshall

[greg]

Hello

I have built a little Lead\Bass 20 head and I'm interested in bridging the inputs (as I feel I have to be losing some signal using a Y cord). Can anyone tell me how this is done?

Thanks in advance,
Greg

[Matt T.]

I doubt you're losing any appreciable signal by using a Y-cord...however many/most of the Y-cords I've bought over the years were junk and would fail with any use...it's amazing how many Y-cords I've had fail. So I wired my smallbox jacks like this:


So the low-gain Normal input is actually the 'Y-cord input' and you can still 'jump' the channels with a jumper cable the way most people do (i.e. plug into high-gain Bright jack and then jumper the low-gain Bright jack and the high-gain Normal jack). But whether I used a Y-cord or this arrangement...the 1M resistors are still in parallel either way for a total reistance to ground of only 500K. That might not apply to your amp (not familiar with a Lead/Bass 20) but it might give you some ideas. HTH.

[Chris / CMW amps]

You're probably having some problems with lower input-Z. If you only do use the amp jumpered change the 1M Rs to 2M2 and see what the effect of the 68k Rs does.

[stokes]

V1a and V1b are out of phase with each other,no?If you bridge them you will get phase cacellation,or am I missing something here?

[Matt T.]

stokes,
If you're refering to my little (or maybe not so little ) schematic above...that's for a 4-input Marshall like a JTM45/100 or a Super Lead or whatever. So V1a is the input triode for one channel and V1b is the input triode for the other...so they're in phase.

That may not be the case with a Lead/Bass 20.

[stokes]

The two halves of a dual triode are out of phase with each other,that is how a phase inverter that uses each half of a 12AX7 to feed each side of a PP output works.If they werent out of phase with each other the phase inverter would drive both sides of the output to swing positive at the same time.

[Matt T.]

That's not the case stokes. The two triodes in a dual triode are just...triodes...it's the external circuitry that makes them in or out of phase.

The plate is out of phase with the grid and the cathode is in phase with the grid. So with a typical common-cathode gain stage (input signal to grid...output signal from plate), the output is out of phase with the input...because the plate is out of phase with the grid. But with a cathode follower (input to grid...output from cathode) the output is in phase with the input...because the cathode and grid are in-phase.

[greg]

To everyone who responded to my post - Thanks for your "input".

Greg

[Regis]

If you want to bridge your amp internally, solder a wire between pins 2 and 7 on your 12ax7 preamp tube. Simple.