Standby Switch as a Speaker Kill Switch?

[Alexo]

Hello hello,

So I'm building a bass amp for a client. He wants a line out so he can use it as a recording preamp, which is fine. He also wants a "speaker kill" switch so he won't have to hook it into a cab under these circumstances.

Soooo... I've been pondering this for a while, and I've been thinking of just using a DPDT switch for the standby: when it's on, all will be normal, when it's turned off, it will interrupt the connection between the power supply and the output tube plates and screens, inserting a 240K or so resistor between these tubes and their power supply nodes to avoid cathode poisoning. The switch would leave the connections between the power supply and the preamp tubes unaltered.

I'm using Zeners for the 300 volt drop between the plate and screen/preamp supplies (6550's at 600 volts) so the preamp voltages shouldn't go haywire when the current draw on the power supply is changed like this. SS-rectified, btw.

So my question is concerning the don't-turn-an-amp-on-without-a-load rule:

Can you disconnect the load if you still have a couple volts on the plates via the 240K resistor between the OT and the plate supply? ...and of course a 240K between the screens and their supply.

I'm thinking this will be fine, but it never hurts to have a reality check ...unless it blows up your OT.

[Cameron]

Hello hello,

So I'm building a bass amp for a client. He wants a line out so he can use it as a recording preamp, which is fine. He also wants a "speaker kill" switch so he won't have to hook it into a cab under these circumstances.

Soooo... I've been pondering this for a while, and I've been thinking of just using a DPDT switch for the standby: when it's on, all will be normal, when it's turned off, it will interrupt the connection between the power supply and the output tube plates and screens, inserting a 240K or so resistor between these tubes and their power supply nodes to avoid cathode poisoning. The switch would leave the connections between the power supply and the preamp tubes unaltered.

I'm using Zeners for the 300 volt drop between the plate and screen/preamp supplies (6550's at 600 volts) so the preamp voltages shouldn't go haywire when the current draw on the power supply is changed like this. SS-rectified, btw.

So my question is concerning the don't-turn-an-amp-on-without-a-load rule:

Can you disconnect the load if you still have a couple volts on the plates via the 240K resistor between the OT and the plate supply? ...and of course a 240K between the screens and their supply.

I'm thinking this will be fine, but it never hurts to have a reality check ...unless it blows up your OT.

Why not just lift the cathodes of the output tubes ?

[Cameron]

...

[Alexo]

I thought about that, but I think it'd be cool to have the standby switch double up this way and eliminate the need for an extra switch in an already-drilled chassis. Although I could just have the standby switch lift the cathodes too.

But this poses the same question... if I lifted the cathodes, I'd want to keep them connected via a 10K or so resistor, to keep some current flowing through those tubes so they don't suffer from cathode poisoning (NOS 6550's are pricey). Can I safely disconnect the OT under those conditions, when there's still a couple volts/ma through the power tubes?

My gut says yes, especially saince this is a really hefty 100watt OT that feels pretty indestructible, but it never hurts to discuss.

Thanks for the suggestion.

[lastwinj]

or better yet, run a dual supply. one supply for the 6550, one for the preamp. simplest solution, IMO.

germ

[Alexo]

OK, regardless of how I implement this design, I will end up with a little current flowing through the power tubes to keep them happy when they're turned off.

Under those conditions, either via a 10K r between the cathodes and ground, or a 390K R interrupting the power between the A/B nodes on the PSU and the plates and screens, anybody want to venture a guess if my OT will be OK if it's disconnected from a load?

[benoit]

Wouldn't it be way easier just to put in a dummy load on a switch or even just have one that could be plugged into the speaker output jack? Or just have a switch that interrupts the signal after the recording out but before the PI?

[Alexo]

I was originally going to do a dummy load, but this is a 100 watt bass amp, and the power resistors would cramp the layout and generate a lot of heat. Altering the standby design to power down the power tubes seemed like a more elegant solution to me.

You may be on to something with disconnecting the signal before the power tubes. The amp still needs a load under these conditions to keep it from kicking back some extreme voltages in the OT as the idle current tries to bridge a near-infinite impedance in the primary, but if there's no signal present, it's not critical what that load is, I would think.

EDIT: Scratch that, impedance only impedes AC, the DC idle current would just be faced with the DC resistance of the OT primary, barring any PSU hum (which there shouldn't be with 400 mf on the first PSU node).

Since the signal won't be swinging across that load line, we could leave it wired up so that there's a short across the secondary when the cab's disconnected. There'd be a zero ohm impedance primary, but that shouldn't matter if there's no signal ...should it? The tubes would still just be sitting there with their typical idle current, no matter what the load is, right?

[Alexo]

I think I'll leave the output jacks to short when disconnected and use a DPDT for the standby switch - one pole inserting 10K between the 6550 cathodes and ground, the other pole disconnecting the signal at the input to the pi.

Thanks for the help, gents!

[benoit]

Keep in mind that your dummy load resistors wouldn't necessarily need to be in your chassis. The Dr. Z Mini-Z, for example, puts the resistor for it's built in attenuator outside the chassis. Just sayin'. Good luck, in any case. Another thing you could try is to make the preamp and poweramp operate independently of each other and have a preamp out and power amp in. That way if you were just recording you could fire up the preamp without turning on the power amp. I guess the only issue there would be making the power supply work right, so that's probably more trouble than it's worth.

[FUCHSAUDIO]

I'd worry that the preamp tube voltages would skyrocket if the output tubes weren't drawing current anymore.....just a thought.

[benoit]

I'd worry that the preamp tube voltages would skyrocket if the output tubes weren't drawing current anymore.....just a thought.

Right, the PS design would be a major pain, probably take up too much space.

[Alexo]

Howdy guys,

Well I realized/remembered that I already built an amp with switchable pairs of output tubes - when you flip the switch and one pair activates, the other pair is switched off via a 10K R between the cathodes and ground. So I pulled one pair, and did some tests - with that pair on, into a dummy load, there was roughly 80VAC RMS on one of the plates under heavy signal. With no load and that pair shut off via the 10K Rk, there was about 1 volt AC on the plate under heavy signal.

So I guess it's safe!

But then of course I lose the opportunity of letting the tubes warm up before flipping the standby switch, because the standby switch will only insert/bypass the Rk and maybe disconnect the input to the pi.

I suppose this is probably fine, as any cathode stripping that occurs should have to pull those electrons through the cathode's ground connection, which will be blocked by R10K. Anyone care to comment on this assumption?

I'd worry that the preamp tube voltages would skyrocket if the output tubes weren't drawing current anymore.....just a thought.

Good point, but as I said, the PSU is being brought down for the screens and preamp via zeners (it's a bass amp, so this sort of thing is OK), and the rectifier is solid state, so the preamp voltages should stay pretty much the same, regardless of plate current.