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+--- Thread: Power Amp using preamp tube (/Thread-Power-Amp-using-preamp-tube)
Power Amp using preamp tube - Strelok - 01-17-2025
Hi all,
I would like to build a Power Amp using a preamp tube.
Also because power tubes are getting too expensive for me.
And I am very curious about the sound of course.
I looked at the AX-84 site but like Kevin said there is only one design, the Moonlight.
But it uses a custom transformer.
In TUT5 there is a chapter about AX-84.
There a 125A is used.
As far as I can see, the primaries use colors, like in the schematic.
But the secondaries use numbers.
I don't see them in the schematic.
Wiring should be done in a way that reflects the highest Z.
With an 8 Ohm speaker, it would be 2 & 4 on the secondaries, the highest Z would be 22,500 Ohms then.
Is this correct?
For the Herzog secondaries 1 & 4 are used, giving 11,600 back.
And another question: just for testing, can I use that replacment Reverb TX for our friends over at NewSensor?
I have a few around here.
I will be using an 12AT7.
Kind regards,
Strelok
RE: Power Amp using preamp tube - K O'Connor - 01-17-2025
Hi Guys
Ideally, you use the plate curve of the tube you wish to use and go from there to decide on OT primary impedance.
Yes, you can use a reverb driver transformer and have a low-watt PA. It may be limited to 100mW or so, which is enough to make some sound through an efficient speaker.
The 125-series OTs come with a sheet giving 90 matches of load to reflected primary impedance. Start high and see what works.
Have fun
Kevin
RE: Power Amp using preamp tube - Strelok - 01-21-2025
Hi Kevin!
Thanks for your reply!
I have no idea how to do use plate curves. I read some about it and it was understandable so that was nice to learn.
There were two shcematics of the Firefly that I could use.
Then I downloaded some video's that showed the Firefly in use and I did not like the sound.
Too flat.
So that's settled then.
Strelok
RE: Power Amp using preamp tube - K O'Connor - 01-21-2025
Hi Guys
Keep in mind that the firefly is self-split and class-A - part of a minimum-tube-cunt approach. What you do in the preamp makes a lot of difference. This ouptut stage will not colour it too much.
However, you can add a tube and do the splitting separate from the output stage, which gives more degrees of flexibility, just as with a larger conventional power amp.
You asked about using a standard SE reverb transformer, and that will yield a dirtier tone than the push-pull circuit, so maybe more what you were hoping for?
Have fun
RE: Power Amp using preamp tube - Strelok - 01-23-2025
Hi Kevin!
True, perhaps I am judging too fast.
I only wabt to use it as a Power Amp anyway.
My dream would be a Herzog with a 12AU7 as power tube.
I can do that with a Firefly schematic.
And then a package I can carry around.
Thanks again!
I will try the Reverb Transformer, very cusious about that.