Two-Channel SE Channel Switching Question

[jhaas]

My current build is a two-channel SE amp (EL34) with a single TMB tone stack that is shared between the two channels.

Schematic:
http://i44.tinypic.com/347y0wj.jpg

The schematic shows two switches, one DPDT, the other SPDT. In the amp I'm using relays for these switches, both triggered together for the clean/crunch channel.

The issues I'm having, I believe, is with the first switch (DPDT). When the amp is switched to the clean channel:
a) The gain knob is still "active", increasing it to max causes an audible increase in hiss, and it slightly effects the tone of the clean channel. I would expect it to be out of the circuit at this point.
b) I'm getting a popping noise when switching channels. The pop is louder as the gain pot is increased, and the pop goes away when the clean volume pot is set to 0.

Any suggestions on what might be happening and how to troubleshoot, fix, etc.

Regards,

-John

[Cliff Schecht]

It's never a good idea to leave an amplifier gain stage, especially two with that much gain, floating at the input. I'll bet if you throw a scope on the output with the gain cranked and the amp on clean, you'll see some pretty wicked oscillations at the output of V2A. You need to figure out how to ground V1B's grid in your switching scheme, perhaps with relays. Also, a time delay relay would get rid of the popping I believe, many hard switches are notorious for this.

[ChrisM]

You could try wiring your DPDT relay so the input of the Crunch channel is ground when it is bypassed. This is common practice when wiring bypass switches in high gain pedals.

Do you have a diode across your relays coils?
Are you using a regulated supply?
Do you have a NP cap across the switch that makes the coils connection to ground?

How it is drawn will work, so something went wrong with the execution.

You may have a faulty relay. I'd try another one, they are fairly cheap.
What kind of PSU do you have to power the relays? I might be inclined to use one relay to do everything, a 4PDT (I don't think 3PDT relays are common). This is just a guess but maybe you don't have the current to fully turn off one of the relays, that's why you are getting the mixing.


Link to bypass switching with grounded input.
http://www.generalguitargadgets.com/dia ... ut_grd.gif
You can use your existing DPDT.

[jhaas]

Thanks guys,

I need to go back and digest that wiring diagram, but I agree that grounding V1B's grid is what I'm going to need to do. In the meantime I'll try to address some of the questions:

This is the relay supply that I am using, which has worked fine for me in the past. This is the first time I've tried to run two relays of of it though:
http://i34.tinypic.com/2djc3gg.jpg

The PT is an Edcor XPWR106 which has a 6.3V secondary rated at 4A.
Two 12AX7's, an EL34 and a couple LED's should be using less than 2A of it.

I'm using two pf the following DPDT Omron Low Signal 5A relays:
Mouser P/N: 653-G5A-234P-DC5
Coil Voltage: 5 VDC
Coil Current: 40 mA
Coil Type: Non-Latching
Power Consumption: 200 mW

Looks to me like I should be fine with that but let me know if think otherwise.

Thanks again,

-John

[ChrisM]

Personally I would add a little transformer and have a dedicated regulated relay supply.

[Structo]

You should check out the Dumble relay switching with relays.

When in clean mode it grounds the grid of the OD section, killing the output.

On a typical DPDT relay you have the normally closed and normally open contacts, on both sides of the relay.

Also attached is how Dumble ODS amps do their channel switching.
Of course the ODS amps are not true two channel amps since they cascade the stages for overdrive but you can see how that lower right relay contact goes to the ground lug.
That grounds the OD tube so it is quiet when in clean mode.

[jhaas]

Hey Chris,

Why do you feel that my existing 6.3V/4A supply will be insufficient?

For those interested, here is now the pedal wiring diagram that Chris posted transfers to the schematic of my amp. It's simple and brilliant, thanks for posting that!

http://i41.tinypic.com/6r1yt5.jpg

-John

[jhaas]

Thanks for the Dumble pointer Tom. I'm not very familiar with them, so I wasn't aware of their switching setup.

I'm aware of the normally open, normally closed contacts on relays, but my question is, is there a convention to the way the relay diagrams are drawn? Do they always show the switch in the "normal" position?

Seems like I always end up guessing, and >50% of the time guess wrong.

[Structo]

Yes when they say normally closed and normally open, that is the off state of the relay so in my diagram, lugs 13 and 11 are closed when the relay is not energised and 13 and 9 are open.

When the relay coil is energised, the opposite is true.

13 and 11 open and 13 and 9 close.

Then having that same setup on the other side of the relay makes it very useful for switching channels and other things in an amp.

I use a 16 pin IC socket for my relays which works great.
If you are interested here are the Mouser part numbers for what I use.
Since I use a 8v regulated supply and 5v relays it is a little different but you just need to choose the right relays for your voltage.
The diode across the relay coil is to quench the transient voltage caused by when the coil gets hit with voltage.

Relay 653-G5V-2-H1-DC5

Socket 575-199316

Diode 621-1N4007

Hope this helps.

[jhaas]

Thanks Tom, I've been using the same 16-pin socket, but your Omron is almost half the price of the one I've been using, so I'm glad to hear it's working for you. I'll order that one the next time I "restock".

Cheers!

-John