Fixed bias Class AB Push-Pull to Single Ended, Power reduction.

[varaso]

Hello,

I have a Fender Blackface clone, an upgraded princeton, that I'd like to use in lower volumes and make it break up earlier for home use.. After building myself an L-Pad attenuator from guitar magazine, changing the GZ34 to a 5U4GB, and still not being very excited with the changes, I thought about building a 6W single ended tweed amp and get over with it..

But how about shutting one power tube down ?

Could a simple mod with a switchable electolytic cap from the grid of the one tube to ground do the trick? I guess the tube will then operate at idle as all the signal from the one end of the PI will flow to ground and the negative voltage will still be applied to the grid. The only info I could find was this website:

https://www.ampbooks.com/mobile/amp-tec ... t-7173488/

Is there anything else to consider? Impedance, B+, biasing? I'm no expert, I just have a basic understanding of how the sections of an amp work..

Thanks in advance!

[tubeswell]

Sure - lots of guys here have done that. Ground one of the output tube grids - either with a switch or a pot.

The 'off' tube will not be conducting, so the Pr:Sec winding ratio 'seen' through the OT halves (which changes the impedance ratio - lowering the reactive load to 1/4 of what the plate-to-plate impedance is, but since you're now only running 1 tube, this amounts 1/2 the 'nominal' impedance seen by that tube). This reduces output power, but that sounds like its what you want anyway. (Note that the OT is still a PP-OT, and the laminations are not stacked in a manner which permits an air gap, so there will be added inefficiency from running it in 'Class A' as the signal amplitude increases, or as frequencies get lower, but this is part of the mojo. However, saturation is not a severe problem one the whole, because the amount of iron in a PP OT is usually more than what it would be for an SE OT designed for one tube of the same type).

Or, if the amp is fixed bias, or has separate cathode bias resistors for each tube, you can simply pull one of the output tubes out.

[pompeiisneaks]

Sure - lots of guys here have done that. Ground one of the output tube grids - either with a switch or a pot.

The 'off' tube will not be conducting, so the Pr:Sec winding ratio 'seen' through the OT halves (which changes the impedance ratio - lowering the reactive load to 1/4 of what the plate-to-plate impedance is, but since you're now only running 1 tube, this amounts 1/2 the 'nominal' impedance seen by that tube). This reduces output power, but that sounds like its what you want anyway. (Note that the OT is still a PP-OT, and the laminations are not stacked in a manner which permits an air gap, so there will be added inefficiency from running it in 'Class A' as the signal amplitude increases, or as frequencies get lower, but this is part of the mojo. However, saturation is not a severe problem one the whole, because the amount of iron in a PP OT is usually more than what it would be for an SE OT designed for one tube of the same type).

Or, if the amp is fixed bias, or has separate cathode bias resistors for each tube, you can simply pull one of the output tubes out.

wait I thought pulling one tube out on half of a P-P output transformer was 'really bad' is it not?

I see how grounding the grid would remove all signal, but the tube is still there conducting, completing the circuit for that half of the p-p side no?

~Phil

[varaso]

From what I've read I think it's really bad to pull out a tube because then there will be high voltage across the winding of the OT and may cause arcing etc..

[martin manning]

Sure - lots of guys here have done that. Ground one of the output tube grids - either with a switch or a pot.

That would be ok for a cathode biased amp, but grounding the grid on a fixed bias set up wouldn't end well. Here you want to take the grid to the bias voltage, at the junction of the 220k grid resistors, and leave the idle current flowing. To be clear, you'd disconnect it from the PI coupling cap, and connect it at the junction of the 220k's, same as the description in the Ampbooks link.

The 'off' tube will not be conducting, so the Pr:Sec winding ratio 'seen' through the OT halves (which changes the impedance ratio - lowering the reactive load to 1/4 of what the plate-to-plate impedance is, but since you're now only running 1 tube, this amounts 1/2 the 'nominal' impedance seen by that tube)...

The off tube will be conducting, just sitting there idling and not responding to the signal coming from the phase inverter. That's the same as being in cut-off as far as the other tube is concerned, so that one will run on its normal Class B load line at 1/4 Ra-a. It will clip early, though, because it is not center biased like a single-ended amp would be. That may be ok for what you are looking for.

Or, if the amp is fixed bias, or has separate cathode bias resistors for each tube, you can simply pull one of the output tubes out.

I wouldn't recommend that. It's better to have both tubes conducting to balance the idle current in the OT.

Disclaimer: I don't think you'll break anything trying this, but I have not done it myself.

[varaso]

To be clear, you'd disconnect it from the PI coupling cap, and connect it at the junction of the 220k's, same as the description in the Ampbooks link.

Wouldn't it be the same if you just used a switch to disconnect the PI coupling cap (or send it to ground?) from the grid resistor of the output tube?

image just for reference

[martin manning]

That would work too, but it would also change (unbalance) the load on the PI.

[varaso]

That would work too, but it would also change (unbalance) the load on the PI.

Then what do you suggest? Could you please explain me this again, I didn't get it

To be clear, you'd disconnect it from the PI coupling cap, and connect it at the junction of the 220k's, same as the description in the Ampbooks link.

Connect the grid at the junction of the 220k resistors and let the grid resistor floating? Wouldn't that change the bias of the "disconnected" tube, because of the missing resistors (220k+1k5)?

[martin manning]

Like this:

[varaso]

Like this:

Thanks!! Wouldn't that change the bias of the "disconnected" tube, because of the missing 220k resistor? That way it'll run cooler right?

[martin manning]

There isn'y any current to speak of flowing through the 220k so the bias will stay the same.

[varaso]

What if I connect a 20uF cap from grid to ground? Wouldn't that sent the signal from the PI to ground and keep the tube at idle? I'm asking because it recures no modification in the wiring at all, so I can test it and see if I like it.. Then I could keep it or do what you said!

[tubeswell]

Silly me, I was thinking ‘put the signal to ground’, and I said ‘Ground the grid’, which of course would be bad in fixed bias. Happens when I’m in a rush.

Killing the signal on one side of the power amp means that output tube isn’t effectively conducting AC, so the OT primary on that side will not be in the equation of load impedance. (Transformers only transform changes in voltage/current.)

I’ve run push-pull fixed bias amps with only one output tube plugged in and the amp still works. Tube idle current for the remaining tube changes a bit, and the amp is bit hummier, but it’s not the end of the world.

Peavey uses a ‘Class A’ control, which kills the signal one one side of the power amp, in their Valve King series.

[varaso]

Killing the signal on one side of the power amp means that output tube isn’t effectively conducting AC, so the OT primary on that side will not be in the equation of load impedance. (Transformers only transform changes in voltage/current.)

So it's like changing the ratio of the transformer ? The current will be only flowing on the one half ot the OT primary, right? Would it help if I used a different tap on the OT when using just one tube, and if so it should be the tap with half the impedance of the speaker, I guess?

[tubeswell]

Yes, the Pr:Sec winding ratio is halved, and for reasons I explained earlier, the load to the ‘on’ tube is halved ( because only 1 tube is delivering AC- through half of the primary winding). So it’s the same as running a tube ‘1-knotch’ away from the optimal load. Output power will be reduced. The load line will be steepened, so you are less likely to risk overdissipating the screen grid. However, the plate will be running into a lower load (which means it won’t be as efficient).

As long as the ‘off’ tube is still biased, there wont be any emergency.

The Peavey Valve King ‘Class A’ control works this way (by cutting the signal to one side of the power amp)

(Or you can pull out the off tube as long as it isn’t running off a shared cathode bias resistor).