6V6 / 6SL7 / 5Y3GT SE Amp

[TomMcNally]

I'm building a single ended 6V6 amp, single 6SL7, 5Y3GT rectifier.
It's the "RH" schematic. I did the mechanical work tonight.
Hammond 272DX power transformer, James 6113HS OPT's
and a Radio Daze RDC-100 choke.
I'll wire it up tomorrow night.

[Quad]

Is the 7F7 a better alternative to the 6SL7 in this circuit?
Apparently, it has lower Miller capacitance compared to
the 6SL7. Of course, the downside is loctal base, and no
new production, but they are cheap.

[azazello]

I thing, IMO, Rfb-470 kohm will press the sound, without it sound will be "wide" - You can try....

[TomMcNally]

I've never messed with Loctals, but they do seem to have their advantages.
Mike Sherman uses them. Gotta tell you though, the eBay sellers think
they are gold, they are charging a lot for them.

I'll experiment with that "feedback resistor" it's not a common practice
to do it that way.

... tom

[Ty_Bower]

Tom, do you prefer those James outputs over the Edcors? I'm pretty sure you've used a fair number of both.

I have some Edcor GXSE15 (2.5K:6) which I like, but not as much as I like the Transcendar 10 watt 5K:8. I'm thinking of replacing my Edcors with something a little nicer and can't quite decide which way to go.

Nice looking amp. I like the all black profile, right down to the power tubes themselves.

[TomMcNally]

I think the Edcors are pretty close to the James. I'm building this
for someone who liked the James, and they are available faster
from TCTubes, who seems to have a good stock.

BTW - Edcor is back to just about 4 weeks from order to delivery.
even quicker to the West Coast.

[Geek]

I love anode followers
(not invented by RH Audio, contrary to popular myth)

They have a sound very clean and unique... if difficult to drive with heavier FB

Cheers!

[EWBrown]

That resistor feedback scheme goes back to at least the 1940s, I've seen it used in several old circuits. Typically, the value of Rfb should be about 4 to 5 times the resistance of the 6SL7 plate resistor. I've had good results using this with 6BM8 / 6F3P and EL84s, never tried it with 6V6s. As it is effectively in parallel (in DC sense) with the 120K plate resistor, you will have to calculate the 6SL7 plate current accordingly.
This combination of 120K and 470K should behave as if the 6SL7's plate resistor is close to 100K.

You'll probably have to "massage" the optimum resistance value 270 RK for the 6V6, more likely will be closer to 470 ohms, just for a quick SWAG. I've generally used 240-270 ohjms for a PP or parallel pair.

It might be good to also try UL operation, in addition to the standard pentode operation.

If you want to "economize" it, just use 12SL7 and 12V6GT, those are usually considered as being "plinkers", and are accordingly priced. These also have Loctal equivalents, both in 6V and 12V filament versions, though they use 7 or 14 as the prefix.

/ed B

[TomMcNally]

I wired the amp up last night, but got finished too late for a LOUD
listening test ... but so far so good. I did not install the Rfb resistors
yet, I'll evaluate it without, and with one, thanks for the ideas.

I bought some predrilled board and forked turrets and tool to make
tie points. They are mounted on the tube socket hardware with
4/40 3/4" spacers. I cut the board with my Dremel, hence the burned
edges (and fiberglas in my lungs) They work pretty well when there
are lots of tie points. The turrets are made out of really hard metal,
and I tapped them in with a hammer. Maybe I'll get a little 1/2 ton
press, or try using my drill press to squeeze them in.



Bigger pix and lots more amps on black Hammond chassis
at my site: http://tmamps.com

[azazello]

Congratulations!
I see You use resistors....maybe from Radioshack . If yes, these are from
"China town". When You adjust all modes, I advice You to change to Allen Bradley or tantalum. I did the same with my 6V6 DIY in Chicago with tantalum....The difference was very sensitive....

[TomMcNally]

No, not from Radio Shack ... they are Stackpole Carbon Film, 2 watt.
I'm not convinced that anyone can actually hear resistors, except maybe
in a high gain circuit like a phono preamp.

[azazello]

IMO...I can....

[EWBrown]

Since you haven't yet installed the 470K Rfb resistors, it might be a good experiment to use a couple of 1 meg trimpots, set them initially to 500K, and then tweak them up / down to determine the optimum NFB setting. Just a variation on the DIY ST35 theme, with its variable NFB resistors

My small 6BM8 SET used a combination of the plate feedback, plus "global" NFB from the 8 ohm secondary back to the VA cathode (all documented elsewhere) , and I also used a UL connection for the screen grid, and this was an excellent combination.

The global feedback had the least effect, by itself, it had only a subtle difference, but teamed with the plate feedback, it definitely made a difference. I haven't tried 6V6GTs with UL yet, it should work well with these tubes. The only pentodes I have encountered which do not play well with UL are 11MS8s and 10GV8s.

/ed B

[TomMcNally]

I think the 1.2K isolation resistor from B+ to the 6SL7 was a typo,
the voltage at the plate was too high, I changed them to 10K and
it sounds better. I'm still at 200 Volts and 1.5 at the cathodes.

I did put 470K feedback resistors in already. I can tweak a little
more though.

[EWBrown]

I was wondering if 1.2K for the VA plate supply decoupling resistor was correct, it was probably meant to be 12K, but 10K will work well, too.

The actual value isn't really critical...

/ed B