Would THIS Be A Bad Idea For Tube Amps?

[MattHooper]

Hey folks,

I use Conrad Johnson Premier 12 monoblock tube amps and recently got hold of another pair to try out. (Long story).

I've been switching back and forth testing them out, different tubes 'n such, but it is a bit laborious. I power down everything, switch over the speaker wires and RCA signal cables, power up...

However my preamp(s) do have two sets of main outputs. If I bought a few more, longer interconnects I could have both sets of amps permanently hooked to the outputs of my preamp, and then IF I KEPT THE AMPS POWERED UP I'd just have to switch the speaker cables between each set of amps.

So the question is: Can I do this while both sets of amps are powered up? What if anything does that risk to the amps?

It would obviously mean:

1. At any one time, one set of tube amps will be powered on but without being attached to speaker cables.

2. Plugging and unplugging speaker cables in to amps while they are on.

3. Possible risk to the speakers themselves of having the signal plugging in/unplugging from the amp while the amps are on(?).

My foggy memory is telling me you generally don't want to do this, that tube amps especially don't "like" being on while not hooked up to a load, and that maybe plugging in and unplugging speaker cables with the amp on is a bit risky. Basically, you'd want to power down, as I have been, to do switching like this.

I sure as hell don't want to risk anything happening to the amps or speakers, so I'm looking for some insight here from more electrically knowledgeable folks.

Thanks!

 

[DonH56]

The output transformer when left open can produce high-voltage spikes that can destroy output tubes and the transformer. Basically the current still flows but with the secondary open has nowhere to go so the voltage spikes very high. You should never leave the output of a tube amplifier open whilst it is powered on. Some may have protection against the open circuit but most IME do not.

One thing I did in the past was to place a power resistor across the output terminals when switching amps so I did not have to power down. Make sure the resistor can take the power, natch, and make sure it is connected first. I had some with banana plugs wired to them.

FWIWFM - Don

 

[Blumlein 88]

https://dalmura.com.au/static/Output%20transformer%20protection.pdf

This article covers the issues though maybe in more detail than you wish. Using a resistor on the output transformer is mentioned on page 8. In short I've done what DonH56 suggests, in putting a resistor on the output to load the amp. Now you don't need full power or 8 ohm resistors. A 5 or 10 watt 220 ohm resistor will prevent issues. I've also at times used 470 ohm resistors of 5 watts. Just make sure it is solidly connected and you can't accidentally lose connection.

 

[DonH56]

https://dalmura.com.au/static/Output%20transformer%20protection.pdf

This article covers the issues though maybe in more detail than you wish. Using a resistor on the output transformer is mentioned on page 8. In short I've done what DonH56 suggests, in putting a resistor on the output to load the amp. Now you don't need full power or 8 ohm resistors. A 5 or 10 watt 220 ohm resistor will prevent issues. I've also at times used 470 ohm resistors of 5 watts. Just make sure it is solidly connected and you can't accidentally lose connection.

Yes, good catch! I should have mentioned that I used a fairly high-value resistor, but also used an oscilloscope to check that the high resistance was enough to load the amp and not cause problems (varies by amp). By using a 220-ohm (or higher) resistor you can leave it in place without significantly affecting the loading.

 

[MattHooper]

Thanks very much for the reply! Much appreciated!

I did have some foggy reminiscence of why I shouldn't do this. Problem is I'm as far from a techie/EE/DIY guy as you'll find, so when we get in to wiring in resistors to save the equipment...this is why I've never got around to doing this stuff, and just power up and down.

But just out of curiosity, if one is checking for possible differences in how a tube amp (when changing tubes) sounds with a pair of speakers, do I presume correctly that adding a resistor wouldn't be a variable in there? In other words, it wouldn't change the character of how the amp might be working with the speakers?

 

[Blumlein 88]

Thanks very much for the reply! Much appreciated!

I did have some foggy reminiscence of why I shouldn't do this. Problem is I'm as far from a techie/EE/DIY guy as you'll find, so when we get in to wiring in resistors to save the equipment...this is why I've never got around to doing this stuff, and just power up and down.

But just out of curiosity, if one is checking for possible differences in how a tube amp (when changing tubes) sounds with a pair of speakers, do I presume correctly that adding a resistor wouldn't be a variable in there? In other words, it wouldn't change the character of how the amp might be working with the speakers?

The change is too small to matter. It consumes a small fraction of power and otherwise won't matter unless you had one very strange speaker load.

 

[MattHooper]

The change is too small to matter. It consumes a small fraction of power and otherwise won't matter unless you had one very strange speaker load.

Thanks again.

Ok, so I have speaker terminal outputs on the back of my amps which take banana jacks. Would these resistors be in the form of adapters
that would fit in between the speaker jacks and the speaker cable? So I'd need resistors that are essentially in the form of a banana jack adapter with a male and female end? E.g.

4mm Banana Jack with 250 Ohm Resistor (R-LOOP-BP)

4mm Banana Jack with 250 Ohm Resistor (Replaces Emerson 00275-0096-0001 & Meriam ZA36821)

www.transtek.com.au

And then with these resistors, plugging and unplugging the speaker cables from the amps while on wouldn't provide any threat to either the amps or the speakers?

 

[kchap]

Sensible answers! What's going on?

 

[Blumlein 88]

Thanks again.

Ok, so I have speaker terminal outputs on the back of my amps which take banana jacks. Would these resistors be in the form of adapters
that would fit in between the speaker jacks and the speaker cable? So I'd need resistors that are essentially in the form of a banana jack adapter with a male and female end? E.g.

4mm Banana Jack with 250 Ohm Resistor (R-LOOP-BP)

4mm Banana Jack with 250 Ohm Resistor (Replaces Emerson 00275-0096-0001 & Meriam ZA36821)

www.transtek.com.au

And then with these resistors, plugging and unplugging the speaker cables from the amps while on wouldn't provide any threat to either the amps or the speakers?

The resistor you show is rated at 1 watt. I would want 5 watts for this. You could mount them for use with banana plugs. Or simply add a short wire with a banana plug end.

Should be safe for speakers and amp. Maybe think about an amp switcher too.

 

[egellings]

I have what is called a 'tweeter saver' in my homebrew mono block circlotron style tube amps. It's a series RC placed across the OPT secondary. At audio frequencies, it has very little loading effect due to the cap's high impedance at audio, but at high frequencies where the oscillations, if any, are likely to occur, the cap's impedance drops and places the R across the speaker terminals in order to add damping effect on any oscillation. I can take my amps and play them at volume with no external load, and a scope just shows a large audio signal with no misbehavior at all.

 

[MattHooper]

The resistor you show is rated at 1 watt. I would want 5 watts for this. You could mount them for use with banana plugs. Or simply add a short wire with a banana plug end.

Should be safe for speakers and amp. Maybe think about an amp switcher too.

Ok thanks Blumlein. I think I've got enough to go on now if I want to pursue that. Much obliged!

And thanks to DonH56 !

 

[kchap]

I have what is called a 'tweeter saver' in my homebrew mono block circlotron style tube amps. It's a series RC placed across the OPT secondary. At audio frequencies, it has very little loading effect due to the cap's high impedance at audio, but at high frequencies where the oscillations, if any, are likely to occur, the cap's impedance drops and places the R across the speaker terminals in order to add damping effect on any oscillation. I can take my amps and play them at volume with no external load, and a scope just shows a large audio signal with no misbehavior at all.

The classic "snubber" circuit. What value of R & C did you use?

 

[egellings]

I used 0.047uF film cap in series with a 22-ohm resistor.

 

[kchap]

I used 0.047uF film cap in series with a 22-ohm resistor.

Thanks for the info. It works so no need to change anything but even a 0.1μF would be OK.

 

[egellings]

That would work, too. I tend to prefer just the minimum needed to assure stability, since although small, the 'tweeter saver' circuit is an additional load on the tube amp. If the amp is a low power one, I'd cringe at wasting even that tiny little bit. But it is also likely to be an unnoticeable loss.