6SN7 output question

[AmberB]

This question has probably been answered several times in the past, but I couldn't find it.

I want to use a 6SN7 as an output tube for a low power amp. I found a thread that talks about using a 20K ohm primary for a push-pull output, but I didn't find any info for an OT rating for using the tube with the plates in parallel single ended operation.
Any info on this would be appreciated.

 I also don't know if the primary impedance would be the same as for the 12AU7 with parallel single ended, but that's a different story.

[Stuff4bikes]

I have seen quite a few with that $20 fender reverb transformer as the output.

[PRR]

Figure a hot section of 6SN7 as 7k plate resistance. So 3.5K for two in parallel.

For good power without much regard for THD numbers, load a triode with 2*Rp. So find a 7K SE OT.

Oddly the Fender Champ is right in that zone. It is at least 4X larger than the 6SN7 needs, but you won't find much cheaper.

A 10+ Watt 6.6K or 10K push-pull (CT) OT may be another bet. Way more iron than you need, but two-6V6 amps are everywhere including scrap piles.

[valvetones]

For a guitar Amp or what?

I have the perfect transformer for you.

[valvetones]

Also, assuming it is for a guitar amp, I’d suggest a very low power speaker.
A typical 25-50 watt speaker is not what you want.

I’m pretty sure the Merit transformers I have are 3.5k primary with a 4 ohm secondary, but give me a few days to verify that.
They are in storage.
Should be perfect if you can find a low power speaker.
I think the transformers are rated at 3watts but again, I’ll need to check.

[valvetones]

Check out this item number on EBay.
115239411774

If you use a 8ohm speaker, it’ll turn that 2k primary into a 4K primary.
It’s single ended , and rated for 3watts.

Ding ding ding 🛎🛎🛎

[AmberB]

Yes, this would be for a small guitar amp.  I want to try it just to see what I can get from it.
I want to run the tube with 400 volts on the plates just to see what happens.  The RCA manual says that they're rated at 450 volts max on the plates, so I should be ok as long as I have a 6SN7GTA tube.  The older type isn't rated that high.
I've read that you can build the same kind of amp using a 6CG7/6FQ7 or the 12BH7 as long as you stay within the max plate voltage of the tube, 300 volts.  I suspect I can get a touch more power from the parallel 6SN7 at a bit higher voltage, but I could be wrong.
The max plate dissipation for both plates of the 6SN7 is rated at 7.5 watts, and the 6FQ7 is 5.7 watts.
I have a bag full of 6FQ7s, so I may have to experiment with that tube also...

I'm assuming that the plate resistance of the 6SN7 will go up with the voltage.  The RCA book doesn't show a plate resistance for a voltage above 250 volts, but it shows 6700 ohms at 90 volts and 7700 ohms at 250 volts.  I would guess that it would be at above 8000 ohms above 300 volts, so I'll work with that.

Thanks for the calculations, PRR, I can use that to figure this out.  :-)

[AmberB]

I have a small output transformer that I took out of an old tube powered reel to reel deck.  I think the output tube was originally a 6V6.  I tested the OT with a variac and came up with a 49 to 1 turns ratio.  That gives me a 2400 to 1 impedance ratio.  So, I have 9600 ohm to 4 ohm ratio, or about 19000 to 8 ohms.  I'm guessing the 6SN7 in parallel would work with that either way.  I don't have much in the way of a 4 ohm speaker, but I found a couple of 4 ohm low watt guitar speakers that won't break the budget, so I think I can go from there.

[valvetones]

Cool deal, let us know how it works out.