Is there switchable push-pull <-> single-ended mod?

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psychepool
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Is there switchable push-pull <-> single-ended mod?

Post by psychepool »

I am collecting information to make my own AC30 style amplifier.
I am going to put the quad <-> pair switch with reference to the Matchless DC30.
I am trying to find out if there is a way to switch to a pair <-> single-ended with a little more greed.

Of course I think it's not very useful mod.
Because I will put Larmar ppimv, and even if the EL84 is single-ended, it's hard to crank up at home without an attenuator anyway.
But if it is not so difficult to apply, I'd like to try it.

Please let me know if there is a way to switch from quad to paired, and push-pull pairs to single-ended.
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pompeiisneaks
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Re: Is there switchable push-pull <-> single-ended mod?

Post by pompeiisneaks »

My general understanding is that this would be very hard. Not impossible, but you'd need two output transformers, you'd need to potentially adjust the max B+ voltage between the PP vs SE, as well as change the tubes from parallel operation for SE to split sided for the Push Pull part, On top of that, for SE you don't need a phase inverter, but for PP you do.

lots of complexities to manage with a simple switch.

IS it worth it for the extra cost of two output transformers? Not sure I'd think so personally.

~Phil
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psychepool
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Re: Is there switchable push-pull <-> single-ended mod?

Post by psychepool »

As expected, this is a very complex work.
I expected there would be a way to do it without adding a single-ended transformer, but that does not seem to be the case.
I did not even consider the phase inverter. The work is really getting bigger.

I've used the Bad Cat Minicat2 clone before (the preamplifier circuit is almost identical to the top boost circuit)
I adjusted the master volume so that the sound is not too loud and the clean tone is also beautiful,
so I was wondering if I should switch the AC30 to the EL84 single ended.

Thank you for answer.
tictac
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Re: Is there switchable push-pull <-> single-ended mod?

Post by tictac »

Well... It's actually pretty easy and not very complicated.... Peavey calls it a Texture control. It's basically a post PI volume control for one of the power tubes. You turn the volume off for one of the power tubes, the remaining tube becomes your SE amp output. If you play with the value of the 82k resistor on the lower part of the control you can partially or completely eliminate any signal going to the power tube....

TT
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frankdrebin
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Re: Is there switchable push-pull <-> single-ended mod?

Post by frankdrebin »

Exactly, lots of people did it, it was also in Koc books, Mesa, Brunetti, Blackstar, and others for sure.

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ChopSauce
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Re: Is there switchable push-pull <-> single-ended mod?

Post by ChopSauce »

pompeiisneaks wrote: Wed Feb 14, 2018 3:37 am... but you'd need two output transformers[...]
Having the phase inverter act as an extra low gain stage might be interesting - soundwise, as you raised that point - at least now that I have taken that option out of my small SE build I am missing it.

However, I also am curious about how they manage the single-ended vs. push-pull transformer question :?:
psychepool
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Re: Is there switchable push-pull <-> single-ended mod?

Post by psychepool »

tictac wrote: Wed Feb 14, 2018 6:17 am Well... It's actually pretty easy and not very complicated.... Peavey calls it a Texture control. It's basically a post PI volume control for one of the power tubes. You turn the volume off for one of the power tubes, the remaining tube becomes your SE amp output. If you play with the value of the 82k resistor on the lower part of the control you can partially or completely eliminate any signal going to the power tube....

TT
Oh, it's interesting. But it's cathodyne phase inverter. The AC30 has long-tail phase inverter.
Can you show me the example of the "Texture Control" in long-tail PI?


frankdrebin wrote: Wed Feb 14, 2018 7:03 am Exactly, lots of people did it, it was also in Koc books, Mesa, Brunetti, Blackstar, and others for sure.

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Do you have a example of 15W/5W switching poweramp?
I want the example of EL84 Cathode Biased poweramp because I will make Vox AC30(or 15)
pdf64
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Re: Is there switchable push-pull <-> single-ended mod?

Post by pdf64 »

An LTP phase splitter wouldn't require any changes to the texture control circuit.
But cathode biased power tubes would obviate the need for the 3rd 100nF cap.
tictac
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Re: Is there switchable push-pull <-> single-ended mod?

Post by tictac »

Oh, it's interesting. But it's cathodyne phase inverter. The AC30 has long-tail phase inverter.
Can you show me the example of the "Texture Control" in long-tail PI?
There's a Post PI Master Volume that's popular with the Trainwreck crowd among others. (Allen Amps uses it too) It uses a dual pot as a volume control for each half of the PI output. If you you use half of that MV control it will do the same thing as the texture circuit I posted earlier. In fact if you replace the 82k resistor with a piece of wire it would look something like that...
Maybe it's called the Lars MV? The Trainwreck pages show this MV too if I remember correctly. It's a volume control isolated by a cap at the input and output, that's all...

TT
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Kagliostro
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Re: Is there switchable push-pull <-> single-ended mod?

Post by Kagliostro »

Well... It's actually pretty easy and not very complicated.... Peavey calls it a Texture control.
Which are the Peavey models that use that solution ? (at least one medel)

Thanks

K
sluckey
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Re: Is there switchable push-pull <-> single-ended mod?

Post by sluckey »

Valve King 100 uses a texture control. Looks like the exact same schematic as shown above...

http://el34world.com/charts/Schematics/ ... %20212.pdf
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roberto
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Re: Is there switchable push-pull <-> single-ended mod?

Post by roberto »

I suggest you to use this solution I've tested years ago.

All you need is a dual log pot and an on-on-on switch.
What you can obtain is:
  1. Single ended amp with PPIMV
  2. Push pull amp with transition to SE
  3. Push pull amp with PPIMV
Follow the attached schematic.
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psychepool
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Re: Is there switchable push-pull <-> single-ended mod?

Post by psychepool »

roberto wrote: Fri Feb 16, 2018 10:23 am I suggest you to use this solution I've tested years ago.

All you need is a dual log pot and an on-on-on switch.
What you can obtain is:
  1. Single ended amp with PPIMV
  2. Push pull amp with transition to SE
  3. Push pull amp with PPIMV
Follow the attached schematic.
Thanks!
Would it be equally applicable to a cathode-biased amplifier? (with connect to ground instead of connect to a bias supply)

And can the switch use the low voltage specification?
psychepool
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Re: Is there switchable push-pull <-> single-ended mod?

Post by psychepool »

rocket power.jpg
I tried to draw again for confirmation.

I plan to make an AC30 / Trainwreck Rocket / Matchless DC30 type of amplifier.
The schematic you provide is shown as a fixed bias on the pair output tube.
The amps I want to make is cathode bias on the quad output tube.
And I'm going to put a quad / pair switch.

Is it okay to make it like this?
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pdf64
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Re: Is there switchable push-pull <-> single-ended mod?

Post by pdf64 »

Yes, that looks good, apart from the 2M2 'fail-safe' resistor should go between the switch output and circuit common.
As the switch may fail open circuit, which would cause the power tube control grids to lose their dc reference, and result in them red-plating and burning out in short order.
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