channel switching with dual op-amp

[961le]

what is needed to use am op-amp for channel switching as marshall did? i have 15v + and -  available already within the amp im wanting to add this to. I can see it used in a marshall schematic but dont know how to implement it.  It appears to be set up as a buffer where the output equals the input. what i dont know about is the voltages.  what do the 15v rails do when used as a buffer?
 i thought maybe a desrciption would suffice for now. the schematic i refer to is the marshall 6100.   

[PRR]

It's not a standard opamp. It is a special, which is out of production, which seems to fail a lot, and you can't get them anymore(?).

Pick another switching technique.

[961le]

WELL I HAVE QUITE A FEW OF THEM.  THE AMP IM WANTING TO DO THIS IN ALREADY HAS OPAMP SWITCHING IN IT ALREADY . SO THE POWER IS THERE.  IM WANTING TO CHANGE MY PCB AND ADD ANOTHER OPAMP TO CONTROL WHAT WILL BE A CLEAN CHANNEL THAT I DO NOT HAVE RIGHT NOW.  WHAT IS IN THERE NOW IS SWITCHABLE MASTERS IN INVERTING FORMAT.  LOOKING AT THE 6100 AMP IT SEEMS IT WOULD WORK PERFECTLY. UNFORTUNATELY, I DONT HAVE THE KNOWLEDGE TO KNOW WHAT TO DO WITH THE POWER RAIL IF ANYTHING....

[961le]

thank you for the response by the way

[shooter]

I DONT HAVE THE KNOWLEDGE TO KNOW WHAT TO DO WITH THE POWER RAIL IF ANYTHING....

look up the data sheet for the IC, that will have voltage ranges and "typical" configurations


if your current rails are +/- 15 you should be just fine, if not steal the rails you want, breadboard it up, test.

[961le]

This is the jist.  what do the rail voltages do when dealing with voltages from tube stages that exceed most examples that i see of 9v and under?

[sluckey]

What is the part number for this IC?

[961le]

sorry about posting in the wrong area.

[shooter]

your data sheet shows +/- 18vdc max, you have +/-15vdc - good to go

what do the rail voltages do when dealing with voltages from tube stages

What do you mean here?  the signal - IN/OUT?
If so, make sure there is a coupling cap (in line with signal) before the IC and after.

post the WHOLE schematic you have, then someone can crayon in the IC for testing

[961le]

yes, the voltage going to pin 2 and 7. all examples ive seen have no more than 9 volts going to those terminals. because we are now talking about plate voltages i dont know the limitations. in looking at the schematic i do see voltage dividers right before the ic. I didnt know if that was for the next tube stage or the ic itself.


the circuit is SLO. the amp is 2500-slx. i want to put the new opamp where the clean and od meet. my plan is connect pin 1 of the new ic to pin 1 of the master volume ic, giving me switchable channels with independent volumes also.
i will try to post the portion of the schematic later.
thank you

[961le]

Marked the opamp insertion at the yellow mark

[961le]

part of circuit that remains in amplifier. Switching master volumes at the top of page. Power rail showing pin 4,8 and 1 at the bottom. I plan to run a trace to 1 to switch the channel on the new ic along with the master volume.

[shooter]

i'm slow and started early so;


you have the amp in the top schematic - reply #10?


If so;
What happens when Vr1 is "engaged"?  VR2?


the .02 caps going "left" on the schematic blocks the DC, AC should be ~~~~1 - 4V


still "unclear" on what you're switching, between clean and OD?

[961le]

Reply 10 is what I’m going to install into an amp I already have (post 11) I showed that schem to show where I was getting voltage from. Vr2 splits the clean (2 gain stages), from the od circuit (3 gain stages). They share the first gain stage and split. The two meet again at the point I highlighted in yellow. This is the point I’m trying to insert the opamp to switch between both clean and od.
I was going to add just the od preamp to my amp but when I thought of adding the clean channel too, well, that’s why I’m here asking questions.
Here is another drawing of the schematic in question. I’m adding this to my existing power section in my amp

[PRR]

Marked the opamp insertion at the yellow mark

That shows LDR switching. While LDRs are going out of production, you can still get them, even in bucket-fulls. They are poor tolerance, but that does not matter in switching. And they are REALLY TOUGH. They can be used for streetlamp control. Many guitar amps use LDRs.

For the chips: ALL input signals have to be attenuated down to about +/-10V max. If you have a plate that swings +100V/-80V from idle, you want 10:1 divider. You show a 2.2Meg:330k divider, 6.66:1, which is not quite enough. (And maybe why I know that chip mainly from burnt Marshalls).

Note that ultimately you don't even need 10V to smack the snot out of any little tube. So this "should work". But Marshall's designers are no fools, yet this chip fails in the field.

[961le]

Then it appears that I have outrun my coverage. The mechanical aspect of this project was not going to be too bad, like remaking the PCB. I assumed that adding the switch would not be too difficult being that the amp is already set up for that. Designing tube amp stages (while I know some basics) is not in my wheelhouse.unless there is another alternative utilizing existing framework I will have to punt. My original plan was to just change to the od circuit and have the two masters that already exist. I can still do that. thank you for the help.and the part about the voltage dividers was the information that I needed. Sometimes it is hard to know how to ask the right questions. this project was also going to utilize the other 1/2 of the 12ax7 that will now be unused.

[961le]

Removed  usable schem

[961le]

And the answer is no it won’t