SE 6L6 Harp Amp Design

[stingray_65]

I've just got back from repairing a bunch of monitors for a local band and of course the topic of tube amps came up.

It seems the hamonica player is very interested in what he describes as a "Hot tube amp with great distortion"

I found this schematic a while ago thinking it would make a great harp amp.

It was designed for a 6L6G but I will substitute a 6BG6GA.

Other than that I translated his schematic "sketch" verbatim (attached)

From the sound clips he had I REALLY liked the sound, but I have a few questions about his circuit. (he's never answered my querrys)

first is how the volume is applied. I've seen similair designs like that on phono amps but only between the input and first gain stage. Is this a preferential aplication? what would be better?

Second, I'm told the mic he prefers is a Hi Z Xtal mic. Is this input suitable?

Third, he never called out the coupling caps values and I ubiquitiosly chose .1uf as a starting point. What might be better values to start with?

Finally I've got everything in my bins right now to build this with the exception of an OT. I recently went thru all my OT's and hooked them up to a wall wart xformer and wrote thier calculated impedances in sharpie on the frames. I've got a hefty 7K6 primary to 8/4 sec. PP OT from an organ chassis. Could this be used if I do not connect the CT on the primary side?

any help or opinions are as always greatly apreciated.

Ray

****EDIT***
V1 is an octal 6SL7GT

[MWJB]

The 250K pot following the coupling cap of the input triode is your volume control, I'm assuming it's wired this way to give reasonable sweep on the pot, rather than have a 1MA pot that you can't turn past a quarter. If you just used a 250K pot on its own, you would have no series resistance in the circuit & feedback inducing high end would possibly cut your usable volume before feedback. Alternativey, I'd just use a 1MA pot and tack a second resistor accross the ungrounded terminals to tune the pot sweep. You could experiment with caps to ground from the top of the volume pot as a high cut...say start with 0.0033uf or 0.0047uf, these could also be put on a switch?

Input impedances tend to be more critical in larger stage amps, smaller & SE amps an be quiote aggressive sounding anyway, vaue as draw may be fine for a softer tone but you could gou up to 5 or 10-meg. I'd also experiment with larger cathode resistor values at the input triode, you could switch to a couple of different values, look at 2.2K to be the max. Whether it's an Xtal or not itmakes little difference, only the new Shure 520DX doesn't like big input loads (1Meg+).

.1uf are fine for coupling cap values, as a starting point.

OT, I'd just use half the primary winding (3.8K, CT and one end).

[MWJB]

25v for the power tube bypass cap is too low, use a 50v cap minimum, 100v would be better. I'd experiment with running without a bypass cap, or...you guessed it...put it on a switch! :-)

[6267]

I'd mess around with the feedback (4.7k R) a bit, see what works or install a pot. Also, try without the power tube bypass cap entirely, it'll be quieter but you may like the tone. Or not!

Good schematic for excellent star grounding! Make sure the 68k's are right up on pin 4, they can conceivably be increased a touch for harp if the tone is a bit sharp. If the amp has problems with bass harp notes at high volume, reduce the coupling caps to .047uF or lower.

Good design, simple and effective!