The Autumn of 6T9

[TomMcNally]

I built an amp with a pair of 6T9 Compactrons tonight, pretty much
following the Nuts and Volts schematic, as long as I had the parts on hand.

The OPT's are Edcors, and I used a Hammond 269EX PT -
the voltages were within a few volts of the schematic, even
though I used a lower voltage transformer.

There is a little low level hum I'll deal with later.

It sounds good - solid bass and plenty of sizzle on the highs,
and it definitely packs some punch for small tubes !

Here are some quick pics ...



The strange rings around the tubes are just reflections.



The five turret terminal boards mounted on the tube sockets are
made by James, and available from Euphonia, very handy.
They have the forked turrets like the bottom of the James transformers.

The only hassle with Compactrons is squeezing everything into
a small space, and the fact that the sockets are cheezy quality.

More pix of my amps at http://tmamps.com

I built an ST-35 the other day with the new stainless steel
diytube chassis from Triode. VERY nice ! As soon as the
sun shines I'll take it out and get some nice pics.

[wiredbecker]

Wow Tom, that's a really elegant, understated design. Well done!

[TerrySmith]

Excellent work Tom! I think there are some big discoverys to be made with compactrons.

Here's a link for new ceramic sockets:

http://www.vacuumtubesinc.com/socketsnew.html

[Sal Brisindi]

Nice job Tom, as usual.

Its funny how the Edcor output transformers are larger then the Hammond power transformer. We have to find some nice round or square can to mount on top with the audio transformers hidden inside since the Edcor transformers don't have leads. I was told by someone you can ask to have leads on them for about $2.00 extra per transformer. They use a different core.

BTW, which Edcor transformer did you use?

Sal

[SDS-PAGE]

It seems to me like anything can be built on those hammond chasses and look neat. Nice work, Tom! So, how much output you get out of these tubes? -Min

[EWBrown]

I built up one of the 6T9 amp kit boards last year, but never got around to finisnhing everyhting into a chassis. (I just bought the boards and not the complete kit, their OPTs were real cheapos)... I did give it a quick "smoke" test just to make sure that it actually worked :o Yellow_Light_Colorz_PDT_09

Power output is around 4 to 4.5 watts per channel, into a 5K primary OPT.

The original 5K OPTs (from Trafomatic) that I had intended to use with this one got "stolen" for a 6N1P / 2X 6P14P-EV UL / SEP amp.

The PC board is kinda funky design, long and narrow, one channel at each end and the power supply components mostly in the middle, though some mods / additions they did looked like afterthoughts.


I suppose it wouldn't be a long streatch to do up a simple PP version.

/ed B in NH

[Kyle K]

There was a P-P 6T9 amp in AudioXpress last month:

http://www.audioxpress.com/magsdirx/ax/ ... er2834.pdf


Kyle

[SDS-PAGE]

That's a cool magazine. Think I will subscribe to it. Thanks! -Min

[TomMcNally]

Thanks for the comments and links.
I am getting a little static from the tube pins
being dirty on one side -- I'll hit them with
some ScotchBrite. The Edcor OPT's are the
XSE15-8-5K's I use in the Darlings.

Does anyone know of a link to the original
GE Tube Manual schematic for the 6T9 amp ?

The tubes I have had one of the cheapo brands
stamped on them, but I am thinking only GE made them,
the top etched number looks like GE.

[EWBrown]

The 6T9 is also in the RCA tube manual. They don't have the curves, just the basic OC info. I have both RCA and GE 6T9s in the "stash".

The PP circuit has a couple of "irregulartities"...

The 8.2V zeners for cathode biasing are an interesting idea, though I dunno just how good of an idea, I'd go with 270 ohm 2W resistors and 100-330 uF bypass caps. The Zeners don't allow for tube mis-matching, I'd SWAG this could invite some mis-matched plate current and the resulting imbalances. FOr 35 mA, just lower the cathode resistors to 220-240 ohms, the tubes have 12W plate dissipation, so there shouldn't be a meltdown or red-plate situation.

Also the "New Sensor" choke specified appears to be a 4H, 40 mA unit , similar (or perhaps identical) to the Fender Deluxe and Vibrolux choke offered by Triode Electronics. Since this feeds only the driver/LTPI portion of the circuitry, the low curent handlong capability is OK (I had to look at the diagram again, I'd originally thought that the full B+ and power ran through it, which would create a definite short MTBF situation).


My 6T9PP ideas was more or less along the lines of the K-502 design, one triode as VA, the second as a cathodyne phase splitter, then the two pen-toads in the usual PP arrangement, OPT around 10K P-P impedance. Good use for those leftover UTK K-502 OPTs...

Here's another 6T9'er I found on the 'net:

http://www.bright.net/~geary/projects/

So far just a photo, no schematics or other info. Nice lookng build!

BTW the "Net Nazis" here have the original Nuts 'n' Volts site blocked for "inappropriate content". Probably their dirty little peurile minds have rather sordid ideas of just what a "6T9 amplifier" just might be Yellow_Light_Colorz_PDT_21 :o Yellow_Light_Colorz_PDT_09 Yellow_Light_Colorz_PDT_21

Update 11/16/07: we have just been assimilated by the Borg, er, acquired by a new company, and the "Net Nazis" won't be an issue much longer, they stay behind with the former parent company Yellow_Light_Colorz_PDT_06 Yellow_Light_Colorz_PDT_06 Yellow_Light_Colorz_PDT_06


/ed B in NH

[Sal Brisindi]

Does anyone know of a link to the original
GE Tube Manual schematic for the 6T9 amp ?

You ask and you shall receive. I just scanned this from my GE tube manual.
http://www.tuberadios.com/temp/6T9amp.jpg

Regards,
Sal

[EWBrown]

Looks quite similar to this version

http://www.duntemann.com/12vtubes/6t9stereo.swf

He added a balance pot in the input.



The whole site starts here:

http://www.duntemann.com/radiogallery.htm

/ed B in NH

[EWBrown]

This thread lit a little fire under my butt , to git 'r' done Yellow_Light_Colorz_PDT_03

Last night I dragged out an old aluminum 6X10X3.5 Bud box, similar to the one in the photo above, of which origins I have long ago forgotten, and started measuring, drilling, punching (and only some moderate cussing, as I managed to draw somewhat less than a full pint of blood, as I always do while attempting metal-mangling). I didn't bother with any painting, just want to keep this one "quick 'n' dirty". Yellow_Light_Colorz_PDT_06

As it stands, the completed board, power tannie and the two HME 734 OPTs are mounted. The OPTs are hung under the chassis, and only the two tubes and the power trannie appear on top. Because of the dimensions of the board, my layout is rotated from that shown in the image above, and there is no choke.

The I/Os will be mounted on the bottom half of the box, and I will even get a little bit "fancy" and use one of the cheapo Hong Kong 100K step attenuators as a volume control..

/ed B in NH

[EWBrown]

I finished the 6T9er last night and got it working after a couple of minor tweaks, the main one being that I had to swap the OPTs' output leads in order not to have a positive feedback power oscillator, as usual Hammond trannies have a reversed phase relationship between primary and secondary, and the HME 734s are a Hammond product.
The other fix was an unintentional solder bridge on the Hong Kong step attenuator that tied the two channels together in a fairly bizarre acting way - there is very little space around the I/O connections.

The first hour or so, it sounded kinda "rough", but started smoothing out with continued use.

It now sounds pretty darn good, though I still have to take one more step to eliminate a weird little hum / buzz problem - currently there is no electrical connection between the circuitry and the enclosure, so I just have to add in a single ground point by better means other than a clip leads jumper Yellow_Light_Colorz_PDT_02 Yellow_Light_Colorz_PDT_08

I used the originally spec'ed 269JX power trannie, 250-0-250 @ 60 mA, and it seem to run a tad hot, as it is being run at 100% capacity, at least.

(Update 11/19/07) I grounded the PCB ground plane to one of the PT mounting screws, hum and buzz are now totally gone, and the sound seems clearer, and the power trannie doesn't run as hot.

I suspect there was some spurious / parasitic oscillation occuring, and grounding the PCB to the box eliminated it, as well as the hum / buzz.

I may replace the power trannie with a 270DAX 260-0-260 @ 90 mA trannie (I already have one from another unfinished (OK, never started) 6BM8 guitar amp project).... Or I' may go ahead and build up another unit with some added mods 'n' tweeks :o I still have an unstuffed "gizmos" board, for future experimentation, and al ll of the necessary parts.

It'll never be a contender against the G*S*G, or the "SEX" amp but for a cheap-amp, it preforms admirably! I think using somewhat better OPTs helps, as well, the original spec'ed cheapies were just a tad TOO cheap... :o Yellow_Light_Colorz_PDT_06

Photos coming soon, it has my standard "Soviet Military Surplus" look to it Yellow_Light_Colorz_PDT_08

/ed B in NH

[TomMcNally]

I used a 269EX, skipped the first resistor off the diodes,
and used a 510 ohm 5 watt dropping resistor between
the caps. The transformer runs warmer than the Darlings,
and the voltages are almost right on with the schematic.

I like the sound of it. A hand wired version isn't really
a beginners project, too much going on in a small space,
but for $ 16.95 for a PCB - it's a good deal.