90's Blues Deluxe switchable reverb?

[Dingleberry]

Hi.

My friend asked me if it is possible to add a reverb footswitch to his 90's Blues Deluxe. I told him of course it COULD be done but HOW, that's a different story. But I thought to give a try. Before I enter his amp I was thinking that maybe some of you have experimented with that circuit, or other op amp driven reverb circuits before? I'll have to admit I haven't...

Could it be done by just using a regular "fender type" reverb switch arrangement or is it better to be done by relays? There is schematic attached so you can maybe point out what could be the best place to short the reverb signal to the ground.

Fender used to ground it before the reverb recovery gain stage in tube amps. Could same principle be used with op amp driven reverb?
Will it introduce noise (the long cable run to the switch) if the switching is done after the recovery stage? (shorting reverb pot wiper to ground)

Will it introduce noise in any case if the switching is done passively?
Even with using good quality shielded cable?

Shorting R33 would kill the signal before recovery gain stage and shorting R35 would do that also. Any no-no's for shorting R35 i.e grounding pin 5 of the op amp?

With relay you could probably just short the reverb pot wiper to the ground and not to worry about exess noise because long cable runs are not needed then. That would be simple and effective way.

If going for relays, could it be better use the existing relay supply or tap it off of heater winding? Will adding one extra relay stress too much the relay supply? Would it make sense to build a switch box with indicator led using pedal board power supply for powering the relay and led?

If some of you can point me which way to go I will gladly appreciate.
Thank you very much.

-T

[DonMoose]

The traditional Fender approach would definitely add noise.

You can route the signal out to a footswitch and back using a shielded twisted pair cable - a microphone cable, for instance. Out on one wire, back on the other, all shielded by the, uh, shield.

If you go for a relay, I'd be inclined to replace C15 with a series assembly of that cap + a 100k or 220k resistor, and the use the relay to short U2-2 to U2-3. In that way, you can slam a 'verbed note and hit the switch to cut off drive, but still get the decay.

[Dingleberry]

I think I'll go for relay.

I was thinking this kind of arrangement as seen in the schem attached. Reverb footswitch would be part of his pedal board so I thought of using the pedal power to power the relay and the status indicator led.

Relay switching box on his pedal board and the relay inside the amp.
Should probably be low noise solution.

If I can't find 9Vdc pcb signal relay from local electronics store I could probably make it with 5V or 6V relay and use LM317 regulator for creating correct voltage. They usually carry 5V and 6V relays.

I added 1000uf/100nf reservoir/smoothing cap combo to the schematic just for safe and the 15nf anti-pop-cap is there between the switch contacts for the same reason. And also wired the two sections of the dpdt switch in parallel just for the worst case scenario, if one of the switch sections fails.

That "cut the drive & save the decay -mod" is actually a very clever idea!
Some things of your text I don't probably get because I haven't been playing with op amps lately... and sorry if my english is sometimes hard to read... my native language is not english...
What are the benefits for adding series resistance with C15?
Before or after the cap?
And if I got it right, you mean that I should short those pins 2 & 3
(+in and -in) of the U2A with that relay?
Will that mute the driver stage?

Well, just take a look at the schem I drew. Every comment welcome.
Does that sound worth doing?

P.S. Windows Paint is your friend;)

-T

[Dingleberry]

Ok. Tried that relay switching arrangement and it works like a charm. Used 12Vdc relay because that's what I had on hand at the moment. Now just have to decide how to wire it. Could you DonMoose explain it a little more?
Thank's already for input and that clever idea.
-T

[DonMoose]

That "cut the drive & save the decay -mod" is actually a very clever idea! Some things of your text I don't probably get because I haven't been playing with op amps lately... and sorry if my english is sometimes hard to read... my native language is not english...
What are the benefits for adding series resistance with C15?

Without the series resistance, when the relay shorts the drive signal, it will mute the whole amp. Even with the resistor, it may reduce the amp volume to cut the reverb - a larger resistor will reduce that effect, but also affect the reverb.

Before or after the cap?

Doesn't matter - whatever is physically easier to implement.

And if I got it right, you mean that I should short those pins 2 & 3 (+in and -in) of the U2A with that relay? Will that mute the driver stage?

It's supposed to. An op-amp works by driving its output to try to force its inputs to the same voltage. Shorting the inputs together should effectively shut it down.

Well, just take a look at the schem I drew. Every comment welcome. Does that sound worth doing?

That would work - and be simple. When they say insulated connectors, they mean isolated from the chassis. For simplicity - at least in the US - I'd go for a 3.5mm stereo jack and only use the TIP and RING contacts (SLEEVE usually connects to chassis). That's a pretty small hole or could be tucked into an existing vent slot.

Hope this helps!