Quick question about making sure tube amp has load while on

[rotobadger]

Hope this isn’t a silly question but I’ve always been super careful about never running my tube amps without a load connected. I have a amp switcher with an an amp on each of the four connections.

Here’s my question.

If my tube amp is wired to the switcher but the channel for that amp is NOT engaged, does the amp still see a load and, thereby, still protects it? Or does the switch need to be engaged and actively routed to the speakers?

Thanks!

 

[gadget73]

depends on the switch, but a typical multi amp to single speaker sort of thing will not have anything connected to an amp that is not selected.

 

[rotobadger]

depends on the switch, but a typical multi amp to single speaker sort of thing will not have anything connected to an amp that is not selected.

Ok, so if I’m understanding you correctly, you’re saying that (assuming my switcher is as you describe) when the amp is not selected it is NOT seeing a load?

Thanks for the quick answer.

 

[anchorman]

It seems like having a load disconnected suddenly while the amp is powered and has a signal through it would be worse than no load.

 

[rotobadger]

It seems like having a load disconnected suddenly while the amp is powered and has a signal through it would be worse than no load.

Agreed. I’m just trying to determine if the switcher provides a load/resistance even if the channel is not active. I never take any chances for obvious reasons but, mistakes can happen. Especially after a few vanilla Crown Royals.

 

[gadget73]

Got an ohm meter? If you do, connect it to one of the amp inputs with it not selected. If it reads open circuit you have your answer.

not sure it matters so much no load vs suddenly disconnected load. Its a no load situation. The voltages that can develop in an unloaded output transformer are quite large and can damage it. Thats the real danger. Also some amps could become unstable and oscillate with no load, which leads to those high voltages. In theory a stable amp with no input can be run with no load but unless you know its absolutely stable that isn't a good bet to make.

 

[Mr Boochie]

Got an ohm meter? If you do, connect it to one of the amp inputs with it not selected. If it reads open circuit you have your answer.

not sure it matters so much no load vs suddenly disconnected load. Its a no load situation. The voltages that can develop in an unloaded output transformer are quite large and can damage it. Thats the real danger. Also some amps could become unstable and oscillate with no load, which leads to those high voltages. In theory a stable amp with no input can be run with no load but unless you know its absolutely stable that isn't a good bet to make.

Prolly best to have the secondary outputs short-out, when there is no load. Than stay open (near infinite impedance). Shorted out --- built up EMF field can be somewhat shorted to ground? You'd need a connection (like a 1/4-inch jack) that has a short-out built-in when the speakers are unplugged.


Two 1/4-inch jacks, for stereo.

 

[tonyg88]

I read else where if you can't have a load on it then something about putting the selector on the headphones port gives it enough load. Someone correct me if i am wrong.

 

[Mr Boochie]

I read else where if you can't have a load on it then something about putting the selector on the headphones port gives it enough load. Someone correct me if i am wrong.

I suppose if the headphone jack works off the output transformer. Many receivers have pre-amp section circuit for the headphone. So those wouldn't load the secondary of the OPT.

 

[maxhifi]

Prolly best to have the secondary outputs short-out, when there is no load. Than stay open (near infinite impedance). Shorted out --- built up EMF field can be somewhat shorted to ground? You'd need a connection (like a 1/4-inch jack) that has a short-out built-in when the speakers are unplugged.

View attachment 2435102
Two 1/4-inch jacks, for stereo.

Unless any solid state amps are connected to the same switcher, in which case say goodbye to the output transistors!

 

[maxhifi]

Agreed. I’m just trying to determine if the switcher provides a load/resistance even if the channel is not active. I never take any chances for obvious reasons but, mistakes can happen. Especially after a few vanilla Crown Royals.

I would be very surprised if it provides a load. If you want a switcher which will do that, you will need to make it yourself. I have a similar setup for going between movies and stereo, and I've had the switch in the wrong position a few times without any crown royal... it is a setup which invites mistakes.

 

[tonyg88]

I suppose if the headphone jack works off the output transformer. Many receivers have pre-amp section circuit for the headphone. So those wouldn't load the secondary of the OPT.

My Fisher doesn't have a special preamp section for headphones... Instead it is powered by the 7591 power tubes and has 2 big ceramic resistors to drop it to the headphone jack audio level. Which I am guessing those resistors would cause transformers to have a load even if nothing is connected to headphone jack but im not 100%

I think scott or someone might have used a special transformer for the headphone jacks but how fisher did it was more common practice.

 

[Tinkerbelle]

There are switchers for tube amps that specifically address this for tube amp use but most units probably don't have this protection built in. So check the manual or test as suggested above.

I suppose if the headphone jack works off the output transformer. Many receivers have pre-amp section circuit for the headphone. So those wouldn't load the secondary of the OPT.

Looking online, this write-up suggests 150 Ohm, 5W added in parallel across each of the speaker connections for the Niles product it discusses. So check your switcher user manual if it offers any such guidance. I'd be inclined to try a higher power rated resistor, maybe 10W+, depending on the amp and switcher.

 

[rotobadger]

Thanks for the input guys. I’ve read about adding a resistor to show a load for the amp. I may look further into this just as a precaution.

 

[gadget73]

I suppose if the headphone jack works off the output transformer. Many receivers have pre-amp section circuit for the headphone. So those wouldn't load the secondary of the OPT.

lots of tube stuff just uses resistors across the speaker outputs to provide a load and knock down the signal so it won't melt your ears. Its low impedance output and a couple of resistors is cheaper to implement than a dedicated buffer stage.

 

[toddalin]

What if the switch box has a relay that switches the amp's output to a 100 watt, 16 ohm resistor when the other amp is selected?

Obviously the set-up is viable, but what of the "moment" that the relay is between poles? How hard is that on the amp? I ask because I have such a set-up and a Melton KT-88 P/P amp and would hate to do damage.

 

[kward]

If it’s a make before break then no problem the amp will always have a load. The problem occurs if the tube amp isn’t stable into no-load, or if switching while the amp is playing. Switching while playing, especially if playing at high wattage output, could likely damage the amp.

 

[OBMG74]

My simple and effective (for me) solution.

My rules for switching my Valencia speakers to another amp:
Turn off amp.
Remove speaker plugs and IMMEDIATELY insert 8 ohm 5W dummy load.
Insert speaker plugs to other amp.

Not taking any chances with switches.

 

[gadget73]

my even easier solution, I don't have multiple amps connected. If I switch amps it requires unplugging input and output, and to do that it means reaching across the tubes to get behind the thing. I shut it off if for no better reason than big power tubes are bloody hot when you lay your arm across them.

 

[Mr Boochie]

I had a setup a long time ago with two amps. I ran 10-ohm, 25-watt wire-wound resistors across the "B" speaker leads. I'd switch to "B," lower volume to zero, while messing around with the "A" channel.