Paralleling Tubes

[Hollow Mate]

Last Sunday during a lazy afternoon session of tube-related viewing on the Web I came across something called the “Stingray” tube amp built by an outfit called Manley Labs. The description of their amp sounded remarkably like Shannon’s ST-35. It appeared more similar than different. The first gain tube for the amp was a 12AT7 followed by a 6414 tube (military 12AV7) used as the phase splitter. It also uses EL84 power tubes in a push-pull ultra-linear configuration. One big difference is that the amp uses four EL84’s per channel. Each side of the “push-pull” uses a pair of EL84’s instead of a single tube.

BTW, I’m not thinking of an amp modification at my end. Just curious. Firstly, would it be feasible to double the number of output tubes to a total of eight on the ST-35? Would there be a reason for anyone to bother? I was wondering what output watts would be noticed in the ST-35 amp? Component values would need to be changed I imagine. Also is there enough copper in the traces of the circuit board to stand up to the extra power? Would almost all components need to be upgraded? Just wondering.

If the ST-35 will support four 6L6 output tubes in place of the EL84’s (recent previous posting) then it seems to me it might support paralleled EL84’s.

Something in an earlier thread seemed to refer to “running tubes concurrently”. Can a pair of, say, 12AX7’s be run in parallel (or its two sides in parallel) and if so why would you do such a thing?

By the way the Stingray lists for $2250 if any DIYers out there decide that the smell of hot solder is not to their liking.

Donald
(Hollow Fate)

[EWBrown]

Push-Pull Parallel, it's fairly common, I've seen anything from 2X2 EL84s per channel in a Peavey Classic 50/50, to 2X4 6550Cs in a VTL MB450.
(BTW, VTL and Manley Labs are closely related). You would need a power trannie with more current capacity, at least 300 mA, 400 mA would be better (same 300-0-300 to 330-0-330 VAC rating). Also, the OPTs primary impedance should be approximately half of the normal (7-8K) value, aim for 3400 - 4300 ohms primary impedace for a PPP setup.

If cathode bias (as in the DIY35) us used, the cathode resistor should be
the same per individual tube as in the "standard" design, or the pairs of cathodes could be connected together, in which case the cathode resistor will be about half of the "standard" value. Dynaco, in their ST-35 and SCA-35, tied all four cathodes together into one 95 ohm 5 watt resistor. This requres really good tube matching to work properly, indivifual cathode resistors allow for more flexibility.

If you need a matched octet of EL84s, "revolution guitar" on E-bay is a great source, I've bought several from him, the shipping is superfast and the matching is as close as I've yet seen. (I just loaded the aforementioned Peavey 50/50 with a matched octet of JJs, the original Sovteks had a lot of hard mileage on them)

I was thinking of a similar approach with my last yet to be populated DIY35 board, or to use a Dynaco ST-70 replacement driver board (I have several different versions) to drive 8 EL84s or 6L6Gs.

/ed brown in NH

[erichayes]

Hi All,

As mentioned in another post, I have several sets of output transformers originally used on my 1773 amp, until we found out the 8/16Ω secondary was incompatable with the feedback being used. I have breadboarded up a push-pull parallel version of my amp using this transformer and it works perfectly--just twice the power.

I'm going to find out from my winder how much a set of bells would cost ( we used the original bells in the retrofits); my guess is probably five bucks or less for a pair. I'll be selling the trannies for $37.00 each plus shipping and cost of the bells, if wanted. This would make a helluva rear end for a guitar amp.

Specs (measured in circuit):

Primary impedance: 4000ΩCT with UL taps
Secondary Imp.: 4, 8Ω
Power handling capacity: 50 Watts
Frequency response: 10~40,000 cps ± 0.5 dB @ 1 Watt;
20~20,000 cps ± 1 dB @ 35 Watts
Distortion: <0.1% THD @ 1Kc, 1 Watt

Anybody interested give me a holler.

[Hollow Fate]

In the interest of furthering the Art (or perhaps, for the simple base reason of keeping my thread alive) I wish your input on something I’ve been working on lately and I hope to gain guidance from the Tube Meisters as to its viability. Tube Amplifier One is now under construction and the design is locked in. The following concerns T.A. #3 (whenever I get to it).

Here we go…. Since it is possible to parallel 6BQ5’s, as per earlier postings, would it not be a simple affair (and cheaper) to parallel an octal 6DZ7? My first 6DZ7 was free. The next seven NOS (just received) cost me $5.00 each. But inside each 6DZ7 is two good ol’ American NOS 6BQ5’s. I’ve forwarded a drawing with specs to Shannon to post here.

I think it is possible to jumper their octal socket to take both the 6DZ7’s as well as 6L6’s (see drawing). No need for any switches to toggle between the two. The two 6BQ5’s within the single bottle should (maybe) act like one 6L6. It’s hard for me to compare numbers when different reference points are used on the spec sheets but I feel that the numbers are close enough to put the question to the experts.

If it seems to work electrically then some neat things are possible. Firstly you can put in the equivalent of 8 6BQ5’s into 4 octal sockets and connected them to Shannon’s ST-35 amp board with a concurrent boost in output power. As well, at a whim, you can pull out the 6DZ7’s and pop into the same sockets those Russian 6L6’s you bought for cheap in the hopes you’d have a place for them one day. Also, and within the same amp design, you could directly compare sonic differences between 6BQ5’s and 6L6’s.

OK guys, (girls??…. please… just one….) What are the other issues? There’s got to be a way that this will work. I don’t know myself. I’m going by both feel and intellect here because “Tubes” is new territory for me.

BTW, 6DZ7’s are rare birds (as are 6DY7’s which also might work). Rare maybe but a NOS pair was had off Ebay last week for $9.95. To me that sounds like four American 6BQ5 NOS for $2.50 each. Most people ignore this tube. My current acquisition of seven was from a stock of ten. If what I propose here will not work, please don’t tell me to stick to the white wine and avoid the red. I prefer red.

Donald.
(Hollow Fate)

[EWBrown]

The 6DZ7 and 6DY7s look like interesting tubes. It should work by paralleling both halves within each tube. Connect 3 to 6 for the plates and 1 to 5 for G1. It may be a good idea to use a 1K "grid stopper" to each G1 (and perhaps a 100 ohm resistor in the plates, if the halves aren't well matched). A single 6DZ7 would be OK for PP pentode mode, but wouldn't fly for UL or triode as the G2s are tied together internally, as are the cathodes.

I just snagged one off e-bay, and AES has 'em for $13.00 FWIW
I'll have to triage my stash of "mystery tubes" to see if I have any.

PDF data sheet here:

http://www.mif.pg.gda.pl/homepages/fran ... 6/6DZ7.pdf

/ed B in NH

[Hollow Fate]

The only photo on the Web I've seen of the 6DZ7 tube is at:
www.trideel.com/ham1.gif
and
www.trideel.com/ham2.gif which also gives a small mention about the tube and its construction.

Donald
(Hollow Fate)

[Hollow Fate]

Ooops... Try Again....

The only photo on the Web I've seen of the 6DZ7 tube is at:
www.triodeel.com/ham1.gif
and
www.triodeel.com/ham2.gif which also gives a small mention about the tube and its construction.

Donald.

[erichayes]

Hi All,

The 6DZ7 is a pair of 6BQ5s with the screens and cathodes tied together internally. This means UL operation with a single tube is out, but I've played with a pair of them in push-pull UL and they sound great.

The main problem with them is availability. They weren't all that ubiquitous to begin with (production only ran for a couple of years), and a lot of them got scooped up by Fisher 500C owners, who rewired the 7591A sockets to take the 6DZ7 with its sections paralleled. Maybe now that New Sensor has reintroduced the 7591 at a reasonable price some of the 6DZ7 hoarders will allow some to return to the open market.

[Guest]

Hi all;

I am still wondering if the ST-35 amp can be set up to take both 6DZ7's and 6L6's. Once the two pentodes in the 6DZ7 are connected together they have exactly the same pinout as the 6L6. See drawing at:
www.diytube.com/6dz7.gif
The pinout is exactly the same. Once both pentodes of the 6DZ7are connected together it is electrically close to the 6L6. The idea being that either tube could be used in the four octal output tube sockets on the amp.
Are they close enough to use them interchangeably? Any takers on this one?

Donald
(Hollow Fate)