Bogen RP40A transformers....what to build with them?

[deicide67]

OK,

10k looks like the value to go with.

I started at 22K because it was on the high side of what I have seen:

Results:

Load: 8.75 ohms

Pulled NFB off 8 ohm tap

39.7mV no NFB for 3.07V
57.0mV with 20K NFB resistor to 3.07V
74.8mV with 10K NFB resistor to 3.05V

It's a pretty sensitive amp.

Blair

[bigaudioscotto]

Sounds like you got her up and running great job.



Scott

[rongon]

6H30P is a horse of a different color. You could use 10k resistors in the plates, no problem. Its rp is around 2k or lower.

IMO, that's overkill. 6H30P is a very expensive tube, so you might want to save those for a more ambitious project, like an all-triode amp (big output triodes need a beefy driver to overcome their Miller capacitance).

I was thinking the 6CG7's you have in there now should work pretty well, and since this amp will have to have NFB around it, you might wind up liking the sound just fine. If you don't, then you can experiment with 12BH7, or maybe 6N6P (a Russian near-equivalent to ECC99 that is available for not too much money and can sound excellent). 5687 is another tube that could work.

Bear in mind that 6N6P or 5687 will need about 12 to 16mA plate current per triode. Does your power transformer have enough capacity to deliver 48mA to 64mA just for the LTPs?

The 12BH7A can get you there with about half the current draw. That's what I used in my Stereo 70 re-do.

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[rongon]

I just saw your post about the NFB resistor. 10k it is. Looks good. How does it sound?

Yes, it's bound to be a very sensitive amp. You certainly won't need to use a preamp with it. Just a volume control on the front of it and you'll be fine.

Next step -- What is the voltage out at the onset of clipping?

If it's about 12V peak, then that should be about 12.5W rms.

With 12V peak output, is the input voltage about 300mV?

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[deicide67]

Hi Ron,

I did not install it yet. Just gator clipped in.

With the open loop performance, I'm not too sure looking for a better tube than the 6CG7 is the right direction. If the NFB get any better results, it should be very nice!

I'll get it installed and let you know what happens.

Since I got around 12.75W into 10.25 ohms, you are correct that the 6L6 are biased at around 13.5W which is probably what I'd get into 8 ohms.

Blair