modify vh4S to add dual standby switch

[vincentvdlocht]

Hi Guys,

I've bought a vh4s (second hand) and for practice at home I would like to use only 1 power amp instead of the two.
In several topics it has been discussed to have a load box connected to the channel not in use, or by turning off the volume.

The make sure nothing ca go wrong, I would like to modify the amp by adding a standby switch per power amp.
I'll probably keep the original standby switch and add a switch per poweramp. (probably on the backside).

Does anyone known if this is done previously.
The new switches could switch the high-side of the power amp, thus the high voltage connected to the output transformer.
A second option is connecting it to the low side, by switching the ground connection on the power tubes.

What is the best solution in your opinion?

 

[Ventura]

Best solution is keep 1 side zero'd on the volume with load, use the other.

Messing around with an integrated standby switch mod is a bit over-the-top.

Uncle Mo

 

[MississippiMetal]

I wouldn't add any extra switching for the HT voltage side of the amp, honestly. The easiest and least obtrusive way to integrate a power amp standby, or in this case a "kill switch" would be to connect the cathode-ground connection from the power tubes you want to cut off to a SPST switch. You kill the cathode-to-ground connection, and you still have voltage at the tube plates, but no current, and the power section is basically shut off. This should be fairly straightforward, if yours has the valve sockets mounted directly to the chassis like the other VH4's I've seen.

 

[vincentvdlocht]

Thanks for your replies!!

@Ventura: Besides the volume, I would like to also limit the weight and stuff I have to carry with me all the time (approx. 2 gigs a week and at least 1 rehearsal) That's why I'm looking for such solution.

@ MississippiMetal: So low-side swtiching is the easiest choice. This trick i've also seen for switching 50W vs. 100W. Are there any risks with this solution?

I'll make two toggle switches on the back, above the speaker connections. Each switch will turn off the power amp. So when connecting the speakers, I can select which power amps are running.

It would be great if someone can confirm the "low-side switching" is safe to use (just to be sure !!!)

 

[MississippiMetal]

@ MississippiMetal: So low-side swtiching is the easiest choice. This trick i've also seen for switching 50W vs. 100W. Are there any risks with this solution?

I'll make two toggle switches on the back, above the speaker connections. Each switch will turn off the power amp. So when connecting the speakers, I can select which power amps are running.

It would be great if someone can confirm the "low-side switching" is safe to use (just to be sure !!!)

Think about it. All you're doing is breaking the cathode's chassis-ground reference. Without that 0 dcv reference, the tube shuts off, unable to pass current. It's simple law of electron flow.

If the switch is wired right, worst case scenario is the switch fails, in which case the tubes remain off. No damage incurred. And if the switch shorts (which I've never heard of internally) big deal, it's doing the same thing it did in the ON position. In either case it's safe.

I would like to point out that I bear no responsibility for any injury or damage incurred due to accident or lack of knowledge when working in a high voltage circuit. I'm telling you all this assuming you know how to safely work in an amplifier, and how to identify the right tubes and connections.