Help with se tube amp or kill me!

[stroszek]

Good!

Do you mean the other way around, right? 2.1V for 6N2PB, 1.65V for 6N2PA? If so, yes, this is fine.

No. But if both cathode resistors and both anode resistors are the same, then the two cathode voltages should be nearly the same. In your post #10, you said you had the same 740 ohm cathode resistors for both triodes, but you measured different cathode voltages of 1.9 and 3.2 volts. This showed me that something was wrong with your circuit.


-Gnobuddy

I think I made some mistake there. I measured again and voltages are 1.3 for 6N2PA AND 1.6V for 6N2PB.

In North America, when we say "B+", we mean the supply voltage
-Gnobuddy

Got it! Sorry for confusion.

This type of tone control (Fender Brownface Bandmaster, etc) doesn't cause much signal loss, so by itself, it won't solve your distortion problem.

This is not strange - 10k is much too small, it destroys all the voltage gain from 6N2PB (now that triode does nothing), and as an accidental side-effect, it also destroys the bass response of your amplifier.

I'm attaching a way for you to use the 10k volume pot, and solve your gain problem, without losing all the bass and keeping 6N2PB from working. See the changes in pink in the attached image. You can use any resistor between 100k and 470k, adjust to suit your preference - the bigger the resistor, the more you can turn up your volume control without distortion.

Don't forget: you also have a volume control on your guitar. Turn it down if you want less distortion!

I think you will be happier if you change your whole amp a little - put a Fender Blackface tone control in between 6N2PB and 6N2PA. That will give you the clean tones you want.


-Gnobuddy

I did all of the suggested changes and now the sound is clean, but the volume quite low. I changed R6 back to 320 k, which improved volume a bit. Can I leave it like that?

Well, I know that it's only one output tube rated for 4 watts, I am not hoping to use this amp in gigs, but it was louder before.
I am using now 16 Ohm, 25 watt speaker, no idea about sensitivity. In that old TV there was a small 8 ohm speaker.

The OPT I am using now is very similar to this

Only difference is that p2 is 114 not 91. If I understand correctly it is the number of turns on secondaries? Is feeding 1 Volt to secondaries and measuring primaries the only way to find out the WR?

I read somewhere that 6P14P works best with 6.5 - 8 kohm on primaries.


I wanted to clarify, that the distortion I got before wasn't like rock distortion, but unplasent one - it just sounded wrong. Maybe I am not describing it correctly.

Also since I moved tone controls, I got some constant buzz - not much, but more than before. It's not affected by controls and it increases when I unplug guitar. I looked for cold solders, but everything seems fine. Its grounded in 2 points, first is next to C1 (screen from PT), second next to C4 (tone, volume, preamp, OPT). I don't have chassis yet (all that mess is on piece of plywood right now), maybe that's the problem?


I added schematic how it looks right now.

I will try Fender Blackface tone controls at some point.


Thanks again, I really appreciate all help! I am able to sleep again.

 

[Gnobuddy]

I did all of the suggested changes and now the sound is clean, but the volume quite low. I changed R6 back to 320 k, which improved volume a bit. Can I leave it like that?

Sure! You can also reduce R22, and R21, until you get back the volume you want.

Now that you have a working amp, you can try all sorts of experiments if you like. Just make only one change at a time, and write down what you changed, so that you can reverse the change if you don't like the result.

Is feeding 1 Volt to secondaries and measuring primaries the only way to find out the WR?

I actually prefer to do the opposite, feed an AC voltage to the primary, then measure the secondary voltage from the speaker winding. Because it is a step-down transformer, you get safe voltages to measure by doing it this way.

(If you do it the other way - applying voltage to the speaker winding - you have to be careful not to get an electrical shock from the primary!)

...the distortion I got before wasn't like rock distortion, but unpleasant...

I don't know exactly what you heard, but to get good rock distortion, you have to choose capacitor values to remove most of the bass. Otherwise, you get a muddy, dull-sounding distortion. (You might also get blocking distortion, which sounds very unpleasant.)

I don't have chassis yet (all that mess is on piece of plywood right now), maybe that's the problem?

That's definitely part of the problem. 🙂

Try inverting a big metal pot over the whole piece of plywood (with a ground wire clipped to it.) Make sure there is no chance of 300 volts touching the pot!

I added schematic how it looks right now.

Thank you! That helps a lot.

Thanks again, I really appreciate all help! I am able to sleep again.

You're very welcome! 🙂


-Gnobuddy

 

[Ketje]

Remove C7, C9, R22 and put back a reasonable volume potm of 500k.
The 10k+100k is to much load for the tone control to function properly.
Mona

 

[Gnobuddy]

Remove C7, C9,

This was suggested earlier, but I'm not a fan of the idea. It eliminates one way to implement some "voicing", if the OP decides he does want some overdrive, after all.

The 10k+100k is to much load for the tone control to function properly.

Agreed. When I suggested the 10k+100k, the tone stack was in a different location, and this was a quick way to salvage the 10k pot disaster.


All isn't ideal yet (not that there is such a thing as an ideal amp), but stroszek has a working amp now, a starting point he can use to learn more about amps.


-Gnobuddy