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Blue Sky thoughts - 6 x 6v6?

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5/3/2000 3:37 AM

Blue Sky thoughts - 6 x 6v6?

Tested an old OT the other day - heavy enough to run between 50-100 watts but only about 2100 ohms primary, with 4-8-16 secondary. I immediately thought of running 4 EL34's but I already built an amp with those tubes and want to try a different sound.
I had built a sorta Deluxe - pair of 6V6's with just the one normal channel preamp with a bit more gain at the top of the vol control - cathode biased. Sounded fabulous! Lots of mids!
So I thought about using 6 of the 6V6's to match to a 2k load! After all, I don't have to buy the OT so I could put the money into the tubes. Besides, it would be fun!
Has anybody got any tips about layout/design with 3 tubes on a side? Can I just strap the grids and plates or are there other factors? Strap the screens or separate dropping resistors? Ditto the cathodes?
Since their voltage driven, will the 12AT7 PI have to be modified or not?
Most important, any predictions on the sound? Will I likely have that DR sound, only LOUDER?
Any thoughts before I start would be appreciated...

---Wild Bill

5/3/2000 11:43 AM
anonymous	I can't answer some of the technical questions regarding 3 tubes to a side, but I built a 4x6v6 Bassman last year, and the sound is a lot like a Deluxe but louder, and more in your face when cranked. It sill, in my opinion, retains a lot of the Deluxe 6V6, vibe & MoJo. Casey4s

5/3/2000 2:39 PM
Don Symes	Separate Rk/Ck for sure. How else are you gonna balance the bias? Strap the plates, but I'd do separate Rg. As to driving the pile, consider a CF stage running from the PI - maybe an/3/6 IRF540 instead of a tube. Should be cool.

5/3/2000 10:52 PM

Just got the latest issue of Glass Audio, the feature article is by Joseph Norwood Still about a hifi amp using 6 of the Svetlana 6550's that has some ideas that might apply to sextets of smaller tubes as well.

After the coupling caps from the PI, bias is injected from a supply having less than about 100K of impedance per grid. There are also a pair of 470K, one to ground on either side of the balance pot, resulting in about 250K effectively to ground from each grid. The stabilizing effect of both of these *sinks* is in parallel so the result is equal to (probably better than) using less than 100K bias resistors to each grid, in place of the usual 220K in many guitar amps. Then the signal goes through 2K7ohm 1W resistors to each grid of the first pair of power tubes, then has 330ohm 1W isolating them from the grids of the second pair. The third pair is connected only to the second pair through more 330R 1W's as well. The final pair has an additional 1M added to ground from each grid.

On the plates of the power tubes he used a 50R 4W between each one & the OPT (isolates them from each other, this will waste power to an extent), he was also running in triode mode with no separate screen supply, instead there is just a 220R 2W between the plate & screen of each power tube.

If you are going to use regular pentode mode, I would put at least 220ohms on the screen tab of each socket to isolate them from each other, before joining them and adding another screen resistor (1K? Wattage?) between them and your screen supply.

That's probably all the applicable highlights, but it still is an interesting article.

Hope this helps,
Mike

5/4/2000 3:27 AM
Wild Bill	All good thoughts and much appreciated! I always like to get as much input as I can before I start a project. Any more would be just as helpful! ---Bill

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