I want to build an amp selector (two amps a/b, one cab)

[Old Tele man]

Absolutely! "Opening" the OT output while signal is applied is akin to "automotive breaker points 'opening' on older ignition systems," i.e.: reflecting a VERY high voltage...back into the OT rather than to a spark plug.

A 10-times-Z, non-inductive, "swamping" resistor in parallel with each amp's OTs winding input to switching circuit (not the speaker output) should mitigate this problem.

[Gtr0]

Well it all sounds dangerous for the amps. I'm not sure if I have enough faith in components to feel confident that this would work in a long term.

I know George Metropoulos has/used to have one, which triggered my initial thoughts to have one... this was a while ago, I will go see if I can find a video. He doesn't show the switch but he seems to have full faith in the item he is using. Assuming that he is using the same cab. I will go seek that out in the meantime.

[Gtr0]

Found it... He's demoing A/B'ing one of his 12,000 series heads

https://www.youtube.com/watch?v=Fcabc00kZ7w

He seems to have faith in the switcher, especially switching with an original 1968. His switcher is built into a little metro style head.

I assume he is using the two switch amp input/output to ground.

Okay, faith renewed and reading over the last several posts with great care.

Thanks everyone!

[mhuss]

I built a two-relay box similar to the one shown above, but used full-time 200 ohm 5w resistors across the amp jacks (to keep a minimum load). 200 ohms because... that's what I had laying around! I used it for a bit with no problems, but haven't used it in a while. (I was using it for two different amps/sounds, not for A/B testing).

[Chuck H]

I built a two-relay box similar to the one shown above, but used full-time 200 ohm 5w resistors across the amp jacks (to keep a minimum load). 200 ohms because... that's what I had laying around! I used it for a bit with no problems, but haven't used it in a while. (I was using it for two different amps/sounds, not for A/B testing).

Any details about pop noise? How you mitigated it? etc...

[SoulFetish]

The main thing to be careful about when switching speakers is to avoid intermittent load, which can cause voltage spikes and destroy the output transformer.

Even if the unused amp is switched to a dummy load, the relays will bounce a bit when switching. If there is no other protection, this would be very risky.

A simple protection is to put a resistor across the amp speaker out signal (e.g. 270 ohms/5W) that is always in the circuit. This is enough load to protect the amp, without affecting the tone.

The safety resistor can be added to the amp (I use it on all my builds), or part of the switch box (on the speaker signal input from each amp, before the relay).

I would also recommend adding a circuit to mute the input of the unused amp.

I like this idea ^^

For switching tube amp outputs, wouldn't MBB power relays be the way to go as well?

[loudthud]

What are MBB power relays ?

I tried running two 5E3 type amps into a stereo wired Marshall 4x12. Caused some kind of oscillation problem, but only when a note or cord was being played above a certain volume. I even isolated the grounds on the jack plate and that didn't help.

[Chuck H]

What are MBB power relays ?

I tried running two 5E3 type amps into a stereo wired Marshall 4x12. Caused some kind of oscillation problem, but only when a note or cord was being played above a certain volume. I even isolated the grounds on the jack plate and that didn't help.

Interesting.

[SoulFetish]

What are MBB power relays ?

I tried running two 5E3 type amps into a stereo wired Marshall 4x12. Caused some kind of oscillation problem, but only when a note or cord was being played above a certain volume. I even isolated the grounds on the jack plate and that didn't help.

Yeah that’s kind of wild. ‘Gotta wonder if there was some acoustic feedback getting picked up by the 2 unused speakers, and getting reflected back to the output stage and causing instability. Of course, that presumes you were switching between them and not running them both at the same time.

MBB stands for “Make-before-break” referring to the switching action. The benefit of that kind of power relay, is you remove a lot of the risk of opening up the OT secondary in the act of switching.

For those who might not know what that means, it means that, in the act of switching, the switching contact makes the connection to the following terminal, before breaking the connection with the terminal it is being switch from.

[Chuck H]

MBB stands for “Make-before-break”

I'm sure, like me, no one else can believe they didn't put that together right off.

[Enzo]

I never heard it before. NC and NO are my letters for relays. MAke before break switch contacts are called "shorting" contacts.

In my work in amusements we had pinballs FULL of relays, as well as jukebox control circuits. We used the Form A, B, or C terminology for make or break. A form C switch could be shorting or non-shorting.

https://forum.digikey.com/t/understa...figuration/811

[nosaj]

What are MBB power relays ?

I tried running two 5E3 type amps into a stereo wired Marshall 4x12. Caused some kind of oscillation problem, but only when a note or cord was being played above a certain volume. I even isolated the grounds on the jack plate and that didn't help.

Both 5E3 in phase to a stereo cabinet, Would it have help to reverse the phase on one 5e3? Seems a similar concept like a PI, no?

nosaj

[mhuss]

I didn't really have any pop problems because the amp input grid resistors keeps the input DCV at zero, and I used small-signal relays to switch the inputs. The biggest noise IIRC was the 'clack' of the two power relays on the speaker side switching, but nothing from the speaker.

[d95err]

I like this idea ^^

The safety resistor idea comes from Kevin O’Connor’s books. He wrote somewhere that he had never seen an amp damaged by a mismatched load (wrong speaker impedance), but he had seen dozens of amps with broken output transformers due to intermittent load (e.g. unplugging speaker while playing or glitchy speaker jack connection).

[Gtr0]

The main thing to be careful about when switching speakers is to avoid intermittent load, which can cause voltage spikes and destroy the output transformer.

Even if the unused amp is switched to a dummy load, the relays will bounce a bit when switching. If there is no other protection, this would be very risky.

A simple protection is to put a resistor across the amp speaker out signal (e.g. 270 ohms/5W) that is always in the circuit. This is enough load to protect the amp, without affecting the tone.

The safety resistor can be added to the amp (I use it on all my builds), or part of the switch box (on the speaker signal input from each amp, before the relay).

I would also recommend adding a circuit to mute the input of the unused amp.

I built a two-relay box similar to the one shown above, but used full-time 200 ohm 5w resistors across the amp jacks (to keep a minimum load). 200 ohms because... that's what I had laying around! I used it for a bit with no problems, but haven't used it in a while. (I was using it for two different amps/sounds, not for A/B testing).

Sorry, Fellas - I am not experienced enough to be comfortable with "experienced amp guy vernacular" - of course I say that in a nice way - so when you say "to put a resistor across the amp speaker out signal" can I take that to mean from the output jack's tip to ground? And is it enough to do it once for parallel outputs or should I do that per jack?

Thanks!!!!