Amp into open load at idle?

[casey73]

I've been asked to build a speaker switch box for someone to replace a custom built switch box built by someone else. It takes the output from three amps going to a rotary selector that directs the output of one amp to one of several speaker cabs via another rotary switch. So, when amp 1 output is selected, amps 2 and 3 are going into an open load. Only amp 1 is being played through and the customer claims it has been used this way with no apparent harm being done. I tested it out and it runs quietly and didn't do any harm to the idling amps during the short time I played with it but I'm still not crazy about the idea.

I suggested I'd add a load so the amps not being played still see a resistor load instead of being open as a fail safe. I'm thinking an amp at idle with an open load might be okay, but it still makes me nervous. Then it occurred to me that if I added a load I'd have two amps passing at least a little current through a resistor to ground in the same box that is passing a third amp's full output to a cabinet, and all sharing the same ground.

Am I overthinking this? If I did add a resistor load for the idling amps, could it cause some issues with the amp that's going to a cabinet? Or is it okay to leave an idling amp run into an open load?

[Leo_Gnardo]

Transistor or tube amps? Transistor amps rarely care whether they're driving a load or not. Tube amps, some react badly if driving no load. For instance some Mesa amps go into a low frequency oscillation so hard you can see output tubes flashing blue a couple times a second, that's definitely not good. It's a good safety measure to have a load resistor instead of a speaker in any case.

Also, some amps have speaker outputs where neither one of the leads is at ground. Some transistor amps have an equal amount of signal on both leads, one inverted compared to the other. It would be a fatal mistake to ground one of those speaker leads. Fatal to the amp anyway.

With some tube amps you can get away with not loading the output as long as they're not sending thru a signal. Apparently the switch owner has been lucky to have these in use so far, BUT it would be a big misteak to assume that you can get away with that in all cases. Load resistors = cheap insurance, compared to blown output tubes & transformers.

Then it occurred to me that if I added a load I'd have two amps passing at least a little current through a resistor to ground in the same box that is passing a third amp's full output to a cabinet, and all sharing the same ground.

If that's a worry you could switch both sides of the speaker leads, and return the ground side only to the amp that's driving the speaker or load. More more switches... but it would be safer that way. Consider what would happen if one amp's speaker output became unplugged or ground side somehow disconnected or compromised, not a good scene.

[casey73]

Forgot to mention, all tube amps. The setup had a Marshall JCM800 and a SF bassman head when I checked it out. I've built quite a few standard tube amps over the years, but this is my first venture into a speaker switch such as this. I'm duplicating what he already has but adding a few speaker outs. He's using it for demo purposes. I'm thinking of putting in a load resistor for each amp input (it's a 3 pole rotary) and if it causes issues, I'll just remove the load since he's suffered no consequences up to this point but making sure he understands my reservations.
I've done other things for him so I'm not worried about being sued.

[eschertron]

...and you haven't specified a switch. Industrial-strength rotary switches can be had* that allow you to add as many levels as it takes. 2 poles for each amp (to allow separate grounds) also allow loads to be switched in for the idle amps.

*well, back in the old days at least. I've been through my share of plant closings where (think 50's sci-fi) equipment became free for the looting. Depending on where you look, you may find used old stock, or maybe there are current supplies?

[Leo_Gnardo]

...and you haven't specified a switch. Industrial-strength rotary switches can be had* that allow you to add as many levels as it takes. 2 poles for each amp (to allow separate grounds) also allow loads to be switched in for the idle amps.

*well, back in the old days at least. I've been through my share of plant closings where (think 50's sci-fi) equipment became free for the looting. Depending on where you look, you may find used old stock, or maybe there are current supplies?

Given enough budget, a lite duty switch could be used to run several contactor relays with way plenty enough contacts and current handling to never arc or break down with guitar amp amp/speaker combinations. I've used plug-in Potter & Brumfield parts for this sort of thing, not cheap but also not $$$ to break the bank.

All my decades old looted switches and relays - from college dumpsters & abandoned factories - are pretty well corroded now & good for a Frankenstein lab only. Use new good brand name parts & enjoy reliability & not having to make excuses. "It worked OK in 1968 .... "

[casey73]

Thought I'd follow up on this since I was the OP. I built a switching box to feed the output of three tube amps to one of 15 separate speaker in 6 different cabinets based on a design built by someone else. I'll cut to the chase. The initial test didn't go well and confirmed my doubts about the existing switch. I proceeded to build a new one, isolating all in and out jacks and providing an individual load resistor for each amp so when not being played through they didn't go into an open load. It worked well and no amps were lost in the process. Thanks for all the input.

[Enzo]

If it works, that is the test.

When I worked at a music store years ago, they sold Allen Organs to churches. They came to me to build a speaker selector I used plug-in relays. That way all the switch had to do was turn on a relay - no speaker current. Mounted in a wall box, the six position switch made a satisfying CHUNK with each new position.

They had a couple sub-choices, so I added a small board of diodes to create AND logic.

[soundmasterg]

Weber used to build a FourHead switching unit that could switch up to 4 amps to a speaker, and when not going to the speaker, each amp had its own resistor. I've got the last one that he made. It works well for the one amp project I needed it for, which was a Conn organ amp that I modified and added to. It has 3 complete amps in one head that share a power transformer. The Weber unit sounds similar to what you came up with. Glad you got something working well!

Greg

[Pedro Vecino]

Mesa uses in the Roadking to assign different speaker outputs to different channels 10A relays. The model is NEC MR301-6HSL.