Amp input with relays for channel switching?

[Balticnoiseforge]

Noticed this pcb board/kit on a amp component site.

Iam not familiar at all when it comes to channel switching through relays at the input - i have heard it may not be a good idea to power relays from heater wires? But if using another vac source would it be fine? Or fine either way? ( If you got mA for it ofcourse. )

But my main question is about wiring and possibilities, could two dpdt relays and a input socket like this do 3 things? Switch between two triode sides through resistor and also bridge them? Without having some weird crossover.

So basically like a classic JTM with two high inputs you can swith between, but also third option to bridge them?

Sorry for my ignorance but never played around with these really and struggled to find info about it when it comes to a tube amps input.

Thanks for reading.

[tubeswell]

a couple of methods to think about


1) opto isolators (look at the SLO100 schematic)


2) relays (e.g. Peavey amps)

[sluckey]

You have the coil circuit figured out. Here's the contacts circuit...

[Balticnoiseforge]

To sluckey:

Thanks! That i did not think about it like that :/ now it seems like a ridiculously easy way to switch it that i should have thought about. Thanks!

Do you have any comments about powering relays with heater current at the input? A dedicated vac must be better for noise? Or?

[sluckey]

Using the existing 6.3v filament supply works just as good as a separate 6.3v transformer. ***BUT***, you should not connect any part of the relay power supplY or coil control circuit to chassis ground. IOW, let the circuit ground float. This is easily done by using a Cliff style stereo FS jack and connecting the diodes/filter cap ground to the ground lug on the Cliff jack. NONE OF THIS CONNECTS TO CHASSIS!

[Balticnoiseforge]

On the same principle that you cant have a AC ground and a DC ground on a dc rectified tube heater then?

Got it, thanks 👍

[sluckey]

On the same principle that you cant have a AC ground and a DC ground on a dc rectified tube heater then?

Yeah, sorta. The idea is that if you connect the negative side of the bridge rectifier to ground ***AND*** the filament winding also has a center tap connected to ground, you will have a diode connected directly across the upper half of the filament winding. There will also be a diode connected directly across the lower half of the filament winding. Best case scenario is the two diodes will immediately blow apart, creating an open circuit. However, now you are left with a conventional rectifier that only puts out half the voltage as the FWB rectifier, and it will not be enough voltage to operate the relays. Worst case scenario is the diodes fail shorted, overheat the PT, and burn down the gambling house. I guess you could write a song about it.   

I've had a bit more time to draw out an improved switching circuit. This circuit will put a ground on the unselected channel and prevent any noise on that channel. It's a bit more wiring effort but I think it's worth it.

[d95err]

Doing the switching at the input means any switching noise is amplified by the full gain of the circuit.

I suggest bridging the inputs and then switching at the channel mixer stage. One relay to select either of the channel mixing resistors, one relay to bridge the first.

[shaun]

FWIW, I recently used the Hoffman design from this website (EL34world) to take power off the 6.3v, and it was the best system I've tried so far. Previously, I had tried converting the 120v to 5v, and it worked, but I found the 6.3v system more convenient because you already have the 6.3 power distributed throughout the amp, so you can simply add the relay anywhere along the stages you want. I get no noise from it at all when switching. See attached pics of the Hoffman offerings - the one titled "screenshot" converts the 6.3v to 5v. The other is for converting the 120v to 5v.
Of course, you can also find more info on this site in the parts section, where you can also purchase the relay etc. Use the relay socket - recommended.

Just on a side note, I seem to hear a slight signal loss when running through a relay (obviously, the clean channel runs through the relay no matter which channel you're using). Am I imagining it? Not to hijack the thread, but has anyone else had that experience, or does anyone have a fix? Maybe a high-end relay suggestion?

Thanks.