6DZ& for ST-35 output?

[Norris Wilson]

Has anyone used the octal based 6DZ7, dual 6BQ5 in one evelope, as the PP output tubes in the ST-35 clone?
I know that they are about half of the cost of other NOS 6BQ5's.
Thanks
Norris Wilson

[erichayes]

Hi All,

Norris, the main problem with using the 6DZ7 as a drop-in substitute for a pair of 6BQ5s in the ST35 or any other ultralinear amp with the same complement is the screens of the DZ are tied together internally. This obviously limits the tube's flexibility regarding anything other than straightforward pentode or triode operation.

But here's a thought . . . Thermion and Shannon have been developing a push-pull-parallel version of the Eiclone. If you have a supply of 6DZ7s (and I envy you if you do), you might consider creating a ST35enstein using Shannon's board and a pair of DZs per channel. The OTs would have to be around 4000Ω P-P and able to handle 150mA at least. Hammond might have something that would fill the bill at a not too outrageous price.

Yet another DIY challenge . . .

[EWBrown]

This was covered in this topic here:

http://www.diytube.com/phpBB2/viewtopic ... light=6dz7

back in mid-2004, still some very good info to be found.

6DZ7s are a bit "tough" to find, but then some bargains at swapmeets and hamfests can be found.

FWIW, 6DY7 is very similar, but even harder to find - as with 6DZ7s, there are no "aftermarket" imports of generic Russian substitutes.
Both 6DZ7 and 6DY7 have the "8JP" pinout.

http://www.nj7p.org/Tube1.php?tube=6dy7

/ed B in NH

[WA4SWJ]

Hi Gang,

If I'm not mistaken the latest issue of Audio Express has an amp design using those tubes (I'm traveling and don't have my copy with me). It also has an interesting zener diode approach to clamping the control grid drive to enable more power output from the amp. Looks like a reasonable approach but the author didn't provide enough power output and distortion analysis data to truly convince me. But he calls it the next great breakthrough in tube amp design. Maybe someday I'll have time to test it; but I always say that and then don't ever have the time. Work gets in the way of one's hobbies!

Regards to all,

[Norris Wilson]

Thanks for the feedback on the 6DZ7, there looks to be some potential in their use.

But, I am not so sure about paralleling any vacuum tube for the outptut.
How does everyone feel about parallel output tubes, especially pentodes?

Also, could someone make a recommendation for some quality vintage 10k output irion for use with the ST-35 board and some 6V6 or 6AQ5/9005 pentodes? I would like to try these 6V6 varients wired in quasi-triode mode for grins. I could live with the possible 3 to 5 watts of output in PP.

Thanks
Norris Wilson

[Thermion]

I am not so sure about paralleling any vacuum tube for the outptut.
How does everyone feel about parallel output tubes, especially pentodes?
Thanks
Norris Wilson

Norris,

I think parallel outputs is a great approach if it can be done correctly. I have been playing around with a paralleled output Ikeclone and the results are remarkable. See the posts for Ikezilla in the Ike forum. However, a parallel output requires additional resistors between the plates and screens. I am not sure how you would implement this using a 6DZ7 since there are internal connections.

JT

[Norris Wilson]

Hi Thermion,
I am hoping to get a copy of the AudioXpress issue that has the 6DZ7 amplfier article in it. Colin Hester was kind enough to offer me his issue, I just have to wait on the USPS.
I see your point about inserting the resistors between the plates and screens as you have stated.
I will report back when I get the magazine and hopefully find out in the article.
Also, I will check out your expereince with the Ikzilla, I love that name.
Thanks
Norris Wilson

[dhuebert]

I am very much in favour of quartets of output tubes. My way of thinking is this: paralleling tubes, whether input or output, doubles their transconductance. This serves to make input tubes quieter by reducing shot noise and doubles the power generated by output tubes. Another benefit for output tubes is a reduction in the series resistance of the output section. This reduction is reflected through the output transformer to the load, increasing damping factor. Increased damping factor provides better control of the speaker during high power transients. My one complaint about the ST-35 is a flabby bottom end, especially evident listening to information such as The Police's "Ghost in the Machine". Doubling up on the power tubes should address this.

Don

PS To be fair, listening to anything acoustic such as jazz, bluegrass, folk, etc. makes the ST-35 absolutely shine and I love it.

[EWBrown]

Ive used paralleled 12AX7 sections in the VA stage of several amps, for just this reason of increased transconductance. I also tried paralleling both halves of 12AU7 as a "ST35 style" cathodyne phase splitter, and it truly did not work or sound right, I would SWAG that some serious re-calculation of the cathode and plate load resistors would be needed here... 33 and 27K were no longer applicable...

In yet another of my not-yet completed (or not yet started) projects, I plan to use PPP EL84s with either Tamura OPTs (3400 ohms CT) or some 1773 OPTs that I got from Eric (appx 4000 ohms CT), and an ST70 driver board. At this point I have about four varying designs for this approach, and have to narrow it down to the final one, and then find the time to build it up.

Original power supply idea was to go with dual mono, using two PA774s and a smaller PT for the driver board, but now I have The Mother Of All Power Trannies that I "rescued" from an all-tube power pulse generator, it has a 360VAC 500 mA secondary, and 6.3VAC @ 14 amps filament winding, and two separate 120VAC primaries This would neccessitate a FW four diode bridge, but that's no biggie... Can't complain for a $5 price for the whole thing, and it also had some real nice Phillips "PQ" 7119 dual triodes inside. Don't overlook that old tube test equipment at swapmeets, there is some hidden gold to be found for cheap, and even for free.

[Norris Wilson]

The thought of lowering the noise and impedance of the transformer seems like a good thing when paralleling tubes.

I would like to build a PPP version of Eric Kingsbury's Musical Machine amplifier using some Sansui 1000A 6.6k output iron and 7C5 output tubes.

http://audiotropic.netfirms.com/Projects/blurb1.html

The 7C5's would have 100k resistors wired from the plates to the screens for quasi-triode. Is this a sufficient value when running (4) 7C5's per channel? Or, would I need to use a different value? If so, would someone help me with the proper formula in calculating this value?
My math stinks!

I would also like to experiment with the ST-35 circuit and try a PP 6V6, or 6AQ5/9005, could someone make a recomendation for a quality 10k output transformer, new or vintage?

Thanks

Norris

[EWBrown]

I recently tried 6DZ7s in a PSET UL amp design. This tube does indeed resemble two 6BQ5s sharing the same envelope (with shared G2s and Cathodes). One observation I made was that the G1 resistors have to be lower than with 6BQ5s, I initially tried 220K for paralleled operation (with individual 220 ohm CC grid stoppers) and found that the G1 voltage was unstable, drifting up as high as 0.2V on the grids) so I added another 220K resistor inparallel, which reduced the grid resistor to 110K ohms, which brought everything back to the "normal" approximately 7 mV or lower on G1. There is a low DC voltage present, due to the "grid leak" phenomenon, but a varying 200 mV is way too high, and instability indicates a low frequency oscillation may be present.

The requirement to use relatively low value grid resistors should be less of a factor in a PP design, as the two G1s are operating separately, and out of phase rather than having them connected in parallel, which was most likely never considered in teh development of the 6DZ7.

One could also add a 10 to 50 ohm "stopper" resistor at each plate, if needed, but I found this to be unnecessary in my particular design.

THe 6DY7 has a somewhat different gm and rP, but are otherwise nearly identical. Probably just a result of Sylvania's way of getting around "infringing" on GE's original 6DZ7 designs and patents.

/ed B