• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Single Ended Stereo Amp

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I want to build and HIFI tube amp for my home. One of the main objectives would be reuse all the components that I can.

I've recover one UPS transformer.

- 8V-0-8V secondary that can handle 2,7A more or less.
- 16V-0-16V with 1A more or less
- 0-252V without CT

This transformer can handle two 6L6GC and two 12AX7 without any problem ( I think ).

I've a bunch of 6L6GC and 12AX7 tubes

I've a 10H 150mA choke.

But the schematic that I have uses two 6SN7GT tubes and a pair of 6V6GT output tubes in a single ended configuration.

My question is:

I can swap the 6V6GT with the 6L6GC output tubes?

In single ended config, with 6L6GC I win 2 watts per channel.

I can swap the preamp tubes with a 12AX7?

What changes I will need to make?


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PSU part

Here's a diagram for how to use that funky transformer.



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Okey… I'm a newbie, a big newbie. This transformer hasn't mains windings… what a mistake. But if I use the low side for the HT there is no danger of shock or someting like that? this winding are not isolate… sorry about that…

Don't jump to that conclusion, yet. Do all of us a favor, and measure the resistance of the white-pair of wires, compared to the 0–252 pins. It'll give us a 'big hint' whether you have a separate primary or not.

There's every likelihood that the white-pair is the so-called 220 V primary winding. IF SO, then the diagram I drew up wil serve you well.

Just Saying,
-= GoatGuy ✓ =-

Then the diagram is for naught. … or not! You could always purchase a quite-inexpensive isolation transformer (way cheaper typically than a 'proper' multiple-secondary tube-amplifier grade transformer.)

Example: https://www.newark.com/block/st-250-23-23/transformer-iso-230vac-250va/dp/55X2525

Forty four bucks. Not bad for 250 VA. Still, now that I'm looking around a bit, there are a lot of very useful transformers with the 'right voltages' out there that aren't overly expensive. Not pretty, not US or EU made, (cough, ahem, err…) but you get the picture.

-= GoatGuy ✓ =-
They work. And they're pretty cheap. And, if you adopt the “let's understand Goat's schematic diagram”, you'll find that it will address the needs of the 6V6 SE amplifier (diagram) that you posted early in this thread.

So… perhaps in the Season's Spirit, you'll give it a look-see, and perhaps have a few questions?

-= GoatGuy ✓ =-
Once I solved the problem of power transformer. My second question was:

2a - I can change the 6V6 with 6L6 tubes?
2b - I can change the preamp tube with a 12AX7?

If this two changes needs a lot of component substitutions i/o modifications, I will search another schematic.

6V6 and 6L6 are fairly similar, although they will need different bias-points. Something easy to adjust by changing out the ZENER diode.

12AX7 and 6SN7 are substantially different, needing quite different biasing schemes. But in truth, mostly just resistor changes throughout the first circuit you posted.

If you have access to the 12AU7 tube, it would be a somewhat similar tube to substitute for the 6SN7. There are better valves though. What I like about the 12AU7 is that it has an unmistakeable 'valve-sound' that it imparts to music. Biased right, it can be quite nice. But … there are others. Plenty of them. Some pretty aggressively inexpensive.

I don't have time today to set aside the 2+ hours to transcribe the circuit for new target valves, and compute the resistance / zener substitutions, but if you want I can look at this early tomorrow morning (my time).


-= GoatGuy ✓ =-
But in truth, mostly just resistor changes throughout the first circuit you posted. -

I suspect it is not just rebiasing.
First: 12AX7 has way too much gain for a 2 stage pre.
Second: The AX7 will have massive problems to cope with the 100k plate-to-plate feedback causing heavy distortion. Of course one could increase it to 1meg or so, but that would increase gain even further ...
12AU7 might work or 6n3p or 6n1p or even 6DJ8 ecc88 but 12AX7 ... ?
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