Channel switching in tube amp with solidstate relays

[Michael Allen]

Started out as a 5E3, but changed the tone control to the Spitfire control.

Had some initial trouble with squealing, put a snubber across the second stage plate resistor, and now it's quiet as a mouse. This is the most quiet amp i've put together, and the wiring is far from ideal. No cross talk between channels at all

[J M Fahey]

Hi Michael.
That's GOOD and very useful
Thanks for sharing your successful experiment.
K1-1 isn't switching *that* strong signal, but K2-1 gets hit real hard !!! Easily a 250Vpp squarewave or worse.
And yet you report no feedthrough, not even through wiring capacitance or ground loops.
I would have thought that grounding V2A's grid would have been necessary.
Congratulations.
We need more of this high quality work posted here.
Again.

[Michael Allen]

About the V2a grid,

My first scheme had V1a being switched out of the circuitry, with v1b into v2a. Input went to v1a all the time, and also to the SSR. Input to the second stage was taken from either v1a plate or the common input. This howled, and i thought it was becaue I didn't ground the plate of v1a when second stage input was switched. So i redid the circuit and switching so that V1a either went to V1b and then v2a, or straight from v1a to the PI. This scheme grounded v1b grid when not in use. Still howled so i figured out it was oscillation and put a snubber on v1b plate resistor. that cured it, and the switching was fine. But there wasn't enough gain in one stage before the PI so i switched the circuit back to having either two stages or three, as in the current schem. I was worried about having the third stage ungrounded but it works great!

There should be a lot of voltage swing going into the SSR after the second stage but it doesn't bleed at all.

[Steve Conner]

Very nice! Can you tell us the part number of the SSRs you used? I'd like to get some and check them out.

[defaced]

For my own education, what is the appeal of solid state channel switching elements?

[Michael Allen]

These were Avago ASSR-4128. I've also used NEC PS-7141 series in my Trex, no problems there either.

To defaced: The big attraction is that you can switch silently as sometimes mechanical relays will pop. also FETs can't handle as much voltage as SSRs. and these thins are DIP8 packaged, and also come in SMT so they can be real small. I know there are some relays like 7mm square but i think they're a lot bigger than these $2.50 chips...

[Lostfollicles]

Started out as a 5E3, but changed the tone control to the Spitfire control.

Had some initial trouble with squealing, put a snubber across the second stage plate resistor, and now it's quiet as a mouse. This is the most quiet amp i've put together, and the wiring is far from ideal. No cross talk between channels at all

Hey Guy's! As a newbe to this forum I thought I would let you know that Crate Guitar Amplifiers have been using the Vishay VTL5C3 opto couplers for ages and my own Crate VC-3112 Combo sounds fantastic with no problems with switching! The only problem with these amps is the solder on the valve bases becoming bad! But Hey!!
That's an easy fix and you can pick these beautiful babys up real cheap because of this!

[Steve Conner]

Those aren't solid-state relays though, just the old-fashioned Vactrols.

[Lostfollicles]

Those aren't solid-state relays though, just the old-fashioned Vactrols.

Hi Steve! Yeah! I get what your saying but if it works why fix it!
Do you see any advantage with using the SS type relay for the switching of valve amp circuits?
The VTL5C3's do a very nice smooth transition from channel to channel and very reliabliy so job done as far as most musicians are concerned!
Am I missing something in this thread? Apart from experimenting of course! Nothing wrong with that!!

[Michael Allen]

SSR are much smaller than vactrols, and aren't a resistor so you don't need to worry about passing signal/bleed through. Maybe in some cases, but in my experimenting you don't.

And they're getting cheaper while I bet vactrols aren't being made in large quantities anymore

[hewo]

vtl5c3 degradation

Hi Steve! Yeah! I get what your saying but if it works why fix it!
Do you see any advantage with using the SS type relay for the switching of valve amp circuits?
The VTL5C3's do a very nice smooth transition from channel to channel and very reliabliy so job done as far as most musicians are concerned!
Am I missing something in this thread? Apart from experimenting of course! Nothing wrong with that!!

The resistive properties degrade by guitar square wave abuse.
This attenuates total passage when on.

[Jazz P Bass]

Not to argue but where did that little nugget come from?

[eschertron]

Not to argue but where did that little nugget come from?

Not sure, but I'm glad the thread was bumped. I missed finding it when I posted my own concerns about the LH15xx SSRs here http://music-electronics-forum.com/t38020/. I've got some LH1500s and LH1502s on order from Mouser. Hope to find out for myself if they are OK for switching and audio control applications.

[Jazz P Bass]

I would much rather have a 1.5 ohm resistor in the signal path than a 50 ohm mosfet.

But that's just me.

Does the element type & resistance have an effect on the signal?
I would imagine that it depends on the application.

Edit: taking note of the mosfet datasheet, there is a flat .12 db insertion loss.
No such data is available for the Vactrol.

[eschertron]

Regarding 'silicooties' I'm willing to unlearn what I haven't learned yet

but - as it has been argued at length (and above) - if its possible to put the SSRs into a shunt circuit avoiding a series insertion, then any nonlinearities or hysteresis become a non-issue. We'll see soon enough, I hope