Method for quickly switching between 2 or 3 output transformers?

[VintageCharlie]

Hi guys,

for a project i am looking for a way to quickly switch between 2 or maybe even 3 output transformers in order to compare the differences in real time (while playing).
I've read people mention they've done something like this, but i haven't seen any technical info.

Has anyone done it and wouldn't mind sharing how this could be done?

Thanks!

[sluckey]

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[VintageCharlie]

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[SoulFetish]

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hahaha

[SoulFetish]

how quickly?
The problem you might run into if you want to switch them in and out in a matter of "seconds" of playing(or seconds of the amp in powered up) is that you are going to be stressing the switching contacts due to the reservoir cap(s) charged to a high DC voltage.
This presents other challenges as well, for instance, you'll probably want to use the same speaker/enclosure if the purpose is judge one from the others. So, you'll want to make sure that you maintain a load on the secondary while switching the OTs.
You don't want to get a nasty pop on the speaker , so you can monetarily short the output during the switching action. If you want to include all of these in a single switching action per OT you plan on demoing, then you're looking at a multi-deck rotary switch that needs to be able to handle the High DC voltages in the primary and the low impedance current load at the secondary. These are available but usually very expensive.
Do do it right, with adequate overvoltage protection & make-before-break switching action, the best solution is to probably use mosfets/SSRs for the job with a miniature rotary switch for the small drive signals.
>>>Here's where I will tap in RG or another more qualified member to articulate how you might go about designing that (provided that they think active switching is a good solution)

[sluckey]

how quickly?

...in order to compare the differences in real time (while playing).

[printer2]

I have been thinking about it. You can keep the power on both OT's all the time. Two poles from the plate to the two primaries. A pole for the output winding to speaker. When you break the plate to primary contact the current stops suddenly and when it makes the other winding it starts suddenly. Certainly not any worse than an amp with a standby.

[pdf64]

...Certainly not any worse than an amp with a standby.

I agree that the scenarios are similar but query your certainty that the proposed application is no worse than standby. Consider that an OT’s primary inductance may be many times greater than that of a typical HT choke, and that with signal, there will be equal but opposite back emf spikes between the switch poles.
Hence I suspect that the proposed application may have the potential to put the switch under greater stress than standby does.
It may be noted that switches used for changing p-p amps between pentode and triode (or UL etc) mode seem to survive ok. But I guess that they’re typically only flipped once in a blue moon, and not whilst playing, ie the amp putting out signal power.

[Stevem]

First off are you looking to do this with a single ended output stage, or push pull?

Anyway I think the way to go about this is to keep all the OTs powered up and just switch the drive signal with a 2 gang rotary switch.

The tuff thing will be trying to keep these wires very short in length so you don't have a oscillating nightmare on your hands!

[SoulFetish]

First off are you looking to do this with a single ended output stage, or push pull?

Anyway I think the way to go about this is to keep all the OTs powered up and just switch the drive signal with a 2 gang rotary switch.

The tuff thing will be trying to keep these wires very short in length so you don't have a oscillating nightmare on your hands!

okay, if I read you correctly, this might be a simpler way to go. I was thinking about he would be using essentially the same power supply, preamp, tubes, etc., and the only component that would get switched out was the OTs. However, you could build 3 simple separate output amps. Each OT with it's own filter and screen supply, and simply switch the drive signals to the output transformer you want to use. Is that what you were suggesting?
You'd have to account for some variations in output tube operation, but it would certainly simplify the complexity of the setup. You would just be trading off power efficiency, and the cost of some more components. But if I were going to do it, I would strongly consider going with this approach. I dig the idea

[VintageCharlie]

Thanks for the replies! It would be a p-p amp.
It would be ideal, if the solution would allow switch OT's in the same poweramp.

It would not necessarily have to be capable to be switched during "active playing", say a power chord. Play - mute strings - switch - play would be sufficient, if that changes the demand towards the solution.

But switching between 2 poweramps is certainly an option to be considrered. What i would not like is that this way there are quite a few variables involved still.

[printer2]

...Certainly not any worse than an amp with a standby.

I agree that the scenarios are similar but query your certainty that the proposed application is no worse than standby. Consider that an OT’s primary inductance may be many times greater than that of a typical HT choke, and that with signal, there will be equal but opposite back emf spikes between the switch poles.
Hence I suspect that the proposed application may have the potential to put the switch under greater stress than standby does.
It may be noted that switches used for changing p-p amps between pentode and triode (or UL etc) mode seem to survive ok. But I guess that they’re typically only flipped once in a blue moon, and not whilst playing, ie the amp putting out signal power.

A standby switch (depending on arrangement) would be between the first node capacitor and the center tap of the OT along with the rest of the amp. One standby switch shutting off the current to the output transformer center tap shuts off current through the both windings where as the above situation has a switch pole controlling half the current in half the primary winding. I doubt the switch would see too much worse service. And it is only a test circuit. How many times do you think you would switch it, 2-300 times? And if it costs you a $10 switch, that is cheap knowledge for something you may not get the answer any other simple way. Heck, have a standby switch, flick that one, the transformer switch, the the standby back again. Lots of ways to skin a cat. I just usually try for the simplest.

[dorrisant]

[pdf64]

A standby switch (depending on arrangement) would be between the first node capacitor and the center tap of the OT along with the rest of the amp. One standby switch shutting off the current to the output transformer center tap shuts off current through the both windings...

Note that OT primary and choke inductances still have a return path to circuit common, ie via the power tubes and the HT cap, that should allow the back emf to dissipate without arcs and sparks.

...where as the above situation has a switch pole controlling half the current in half the primary winding. I doubt the switch would see too much worse service....

The disconnected primary would have no return path for its back emf, which would raising the potential for arcing across the switch contacts as they were being opened.
Hence my previous post, ie the operating conditions for your OT switch idea would seem to be more severe than those applying to a standby switch.
I'm not querying that it may work a few times, just the certainty of the assertion that it was not any worse than an amp with a standby
Apologies for my pedantry

[printer2]

A standby switch (depending on arrangement) would be between the first node capacitor and the center tap of the OT along with the rest of the amp. One standby switch shutting off the current to the output transformer center tap shuts off current through the both windings...

Note that OT primary and choke inductances still have a return path to circuit common, ie via the power tubes and the HT cap, that should allow the back emf to dissipate without arcs and sparks.

...where as the above situation has a switch pole controlling half the current in half the primary winding. I doubt the switch would see too much worse service....

The disconnected primary would have no return path for its back emf, which would raising the potential for arcing across the switch contacts as they were being opened.
Hence my previous post, ie the operating conditions for your OT switch idea would seem to be more severe than those applying to a standby switch.
I'm not querying that it may work a few times, just the certainty of the assertion that it was not any worse than an amp with a standby
Apologies for my pedantry

I agree the switch will see a little worse duty switching the primaries, but in this case it seems to be an acceptable compromise. I also assumed, maybe wrongly, that rather than the magnetic field collapsing and arching across the switch that the secondary would get the energy. But that was when I was envisioning using a dour position switch and the first stop disconnected the primary and the second the secondary to the speaker and the reverse order is made to switch to the other transformer. When going to a two position switch there could be no secondary to take up the energy and greater arcing. I still felt that it is an acceptable solution as the life of the switch will not be exceeded in the time of the testing. I am more concerned with an output transformer than with the life of a switch. I did not see it being harmed in the testing. To be nice to all the pieces a make before break switching arrangement could be rigged up. Even built up by yourself, real old school making a switch out of wood and bits of wire and the possible nail or two.