4 Hole Functionality from 2 Holes (Almost)

[pdf64]

4 hole (input) amps have a lot going for them in terms of flexible input arrangements, however there's a lot of front panel space required; and the need to have a link cable handy. Some amps just use a single input socket to feed both channels, but that loses a lot of functionality.

Having thought about this, I wanted to be able to use one input to feed both channels if required, but also wanted to be able to use the channels independently. Generally with the band I use an A/B box to use one channel as 'rhythm /a bit overdriven' and other channel as 'brighter & more overdriven'; then use guitar vol control and boost pedal to fine tune.

I don't like using a link cable between channels (can't find one when needed) and can't see any use for -6dB padded inputs (just turn the guitar tone down a little if additional loading on the pickup is desired). Plus use of a regular link cable results in the input impedance being lowered, slightly increased noise and treble loss (due to increased input series resistance and Miller capacitence).

It's also handy to have independent access to both channels in case you want to jam with a buddy and that's the only amp knocking around.

I worked out an arrangement so that with 1 regular/mono and 1 stereo switching jack sockets, I could achieve all the above, such that -
Input A (Normal / Link) - plug into this ONLY to use both channels.
Input B (Lead) - plug into this to use 'Lead' channel; doing so disables the 'link' function, allowing the channels to be used independently.



As I see it, this two input socket arrangement does everything that the regular 4 input arrangement allows, other than the padded inputs, which I dislike anyway; and the ability to plug 3 or 4 instruments in.

With existing 4 hole amps, the two spare holes can be used for additional control pots / multiposition rotary switches for selecting various 'bright', cathode bypass or coupling caps (as per Orange FAQ feature) options.

Pete

SITE EDITOR: Missing image link and attachment restored. Spaces added between paragraphs (04/07/2016 Steve A.)

[J M Fahey]

Cool.

[dai h.]

hi, thanks for sharing this. I've been staring at the schemo, and can't quite figure out why the 1M in the "B" channel doesn't go directly to ground. Could you explain this? Thanks.

[pdf64]

My thinking was to retain the regular 1M input impedance whether in the 'link' mode or with the channels used independantly.
The 'B' channel 1M input grid leak resistor only gets grounded when a mono jack plug is inserted into the 'B' socket, thereby grounding the socket 'sleeve' connection.
It would probably be more robust to just leave it grounded, then a mono switched socket could be used, without a 500k input impedance (in 'link' mode) affecting anything significantly.
Pete

[J M Fahey]

Or you could use 2 x 2M2 resistors.
Which although in theory are a little too high for a 12AX7, in practice are not, because either they are in parallel (supplying the proper 1M) or the grid "sees" way less than that anyway, through the guitar circuit itself, so ...

[dai h.]

ok thx, makes more sense. It did appear that the ground connection would be made by the plug but wasn't completely clear (to my own addled brain). I have an old Tascam 488mkII multi-track cassette recoder, and two of the mixer inputs use the plug sleeve to make a connection (grounding out one side of the bal. in when using the mono in IIRC), and sometimes it gets sort of wonky (to be specific, no signal) and usually a bit of contact cleaner restores the sound (never sat down to figure out what exactly that is doing but presumably an oxidation issue?), so was just wondering if reliability-wise the ground connection could be counted on. Also, I (vaguely) recall one time where I didn't have a good solder connection for the 1M input R, and (IIRC) that led to tube damage (positive voltage showing up at grid).

Really like the concept though. I had a similar idea (for my two input JCM800 2204 re-wired as 1987) but guess I was too lazy or not capable enough to figure something out.

[pdf64]

Yes, you're right, if the grid floats off (dc wise) then even a 12AX7 can red plate; something that will ensure a ground reference under any circumstances is needed really. Maybe an additional 10M to ground, or JMs idea of using permanent 2M2 grid leak resistors, or just wiring the 'B' input 1M grid leak in place. 500k is plenty high enough and, as was pointed out in the recent grid stopper thread, almost any and every real setup has a buffered pedal between guitar and amp, even if it's just a tuner.
Pete

[dai h.]

how about this (chime in if I'm seeing this wrong here please) : change both to 2M2, then add a 2M2 to B from hot to ground. This way there is always a solid connection and although there is a reliance on switching to achieve 1M target value, if the switching is non-contacting, dirty (or whatever), the worst case is 2M2.

[dai h.]

ooops. I'm not achieving the 1M input Z when both inputs are used, am I?

[pdf64]

This thread didn't make sense now that the schematic attachment has gone missing, so see Combined Auto-mix / Independent Inputs For 2 Channel Amp Photo by pdf64 | Photobucket
Pete

[deci belle]

Thank you for the photobucket link, Pete!

[deci belle]

Pete— Would you please confirm this?

From your drawing it appears that the switching jack (the one labeled "B"), is equivalent to a Switchcraft 13B.

http://www.switchcraft.com/Drawings/13B_CD.pdf

13B Switchcraft | Mouser

[pdf64]

Yes, that's suitable.
I hope it works for you - bear in mind the amendments suggested by JM, some options are covered in post #7.
I'll have to change the sketch to suit.
Pete

[deci belle]

A circuit with permanently soldered resistors would be preferable after all… is that what Juan had in mind when Juan suggested using 2M2 resistors?

What type of jacks are required in that case and how should I wire the resistors to the jacks to implement your design, Pete?

I could actually understand what you had in mind and could see how that worked in the hand-drawn schematic, but now I'm not understanding yet how to implement the permanent resistors with other jacks/wiring.

[Steve A.]

This thread didn't make sense now that the schematic attachment has gone missing, so see Combined Auto-mix / Independent Inputs For 2 Channel Amp Photo by pdf64 | Photobucket
Pete

Pete, I added Photobucket image link and attachment to your original post here on 04/07/2016.

Steve A.
Moderator