el84/6bq5 PSE

[IsaacTeller]

Shalom, y'all!! First post but not first tube amp build. I've made about half a dozen low-powered, "traditional" single-ended and PP amps using 6V6, el84 and ecl82 tubes and I'm pleased to say that I've become addicted. I now have a nice chassis with two octal sockets and a 9-pin socket that I acquired from that auction site with the blue, red, yellow and green logo. Instead of making a single-ended, stereo amp with two 6v6's and a ecc83 I want to attempt a PSE with two 6dz7's and a ecc8x. After searching the forums I've come up with the following assumtions:
1)the opt primary must be half the impedance of single-ended(2500 vs. 5000)
2)the circuitry for the the output section must be duplicated for each pentode(i.e.- 2 SE circuits coming off each triode section)
3)due to the shared screen grids there is no possibility of UL operation
4)obviously the power supply must be strengthened to supply the extra current.
This all seems fairly straightforward and easy to implement, however, as with just about everything in life I'm sure that there's more to it than that. Can anyone fill in the blanks for me before I do any damage?

 

[Eli Duttman]

Isaac,

A look at the 6DZ7 data sheet provides some clues. Not only is the g2 connection common, the cathode connection is also common. You don't have access to some techniques that correct imbalances between sections. You have to rely on the tubes being made to very tight tolerances. Use either cathode or combination bias. Pure "fixed" bias will exacerbate the enevitable differences between sections and result in current hogging.

6DZ7 drive voltage requirements seem modest enough. Have you given thought to the O/P trafos that will be used? If you have stuff made to order, a tertiary cathode feedback winding is a very good idea. OTOH, if "off the shelf" O/P "iron" is employed, the "plate to plate" NFB technique, along with an outer global loop, is in order. Pentode mode "finals" require substantial amounts of NFB in order to obtain a satisfactory damping factor.

Regulate g2 B+, to maximize open loop linearity.

 

[kevinkr]

Unless you've got 'em already the 6DZ7 is pretty rare. Would not be my choice ever regardless of how good they sound. And they are decent sounding in PP amplifiers - their linearity might not be the best for SE applications however.

 

[Tom Bavis]

6DZ7 was a GE design, and they characterized it as a dual 7189.

 

[Jaime]

This is an 6DZ7 amplifier abouth te Gary Pymm 47PP.

http://www.audioasylum.com/forums/tubediy/messages/12/124726.html

 

[kevinkr]

I had what I think was a Stromberg Carlson 20wpc stereo power amplifier that used them for a short time (20yrs ago) and locating a reliable supply in those days was difficult, and I'm not sure how much better the www has made that situation. They weren't widely used in that earlier heyday of tube audio and I don't know of other vintage amplifiers that used them.

 

[Tom Bavis]

The Stromberg used the Sylvania 6DY7 - closer to a dual 6L6, though it's the same pinout as the 6DZ7. These are pretty much unobtanium - I've yet to see one for sale except for a few used ones on eBay.

Heathkit had a SE stereo amp that used one 6DZ7 for both channels.

 

[kevinkr]

The Stromberg used the Sylvania 6DY7 - closer to a dual 6L6, though it's the same pinout as the 6DZ7. These are pretty much unobtanium - I've yet to see one for sale except for a few used ones on eBay.

Heathkit had a SE stereo amp that used one 6DZ7 for both channels.

Hi Tom,
Thanks for setting me straight - in fact that is true and the 6DY7 was unobtainium and 6DZ7 were substituted by the previous owner with an adjustment to the bias supply to compensate. The 6DZ7 were very difficult to get as well. They did not sound good compared to the 6DY7 even with much tweaking probably because the loading was not close to optimum for this type.