SE OT Question

[burt_toast]

1st regular post for me. thank you EL34 for your website and this forum; man, what a reference goldmine!

ok, got an old Conn organ amp (probably early '60s cuz the tranny leads aren't that crunchy). it was originally stereo, with one and a  half 12au7s, and 2 x 6v6s on one side (pedal), and the same except 2 x 6L6s on the other (manual - that means "keyboard" to organ folk). i can't find a schematic of it but it's sorta like a stereo version of the attached jpg.

anyway, i already have my old plexified ab763 bandmaster and a '72 Traynor YBA-1 (coming back from the dead), so i don't need another 50w PP amp. i'm making the Conn into a el34 se, and so far it's enormous! the conn trannys are super huge, so will the OT be ok in this se-monster?

thanks,
scott

ps, please factor into all my posts, that i'm broke, and can't fix problem by throwing $ at 'em; i have to do a lot of substitutions and make-do fixes. cheers

[sluckey]

the conn trannys are super huge, so will the OT be ok in this se-monster?

The Conn OTs are made for a push/pull circuit and will perform poorly in a single ended amp.

[Iannone]

When I built my EL34 SE amp, I used a Hammond T-125ESE output transformer, and was very happy with the result.  At 390V on the plate of the output tube, I get 10W of output.

[burt_toast]

how much for the Hammond and from where? in the meantime, if i can run this amp wide open (and crazy loud) for a while and my (incorrect) OT doesn't even feel warm, is it at least safe til i can afford the Hammond?
*sigh*

ps, diggin the gentle giant head,  Iannone :)

[Iannone]

About $60 from Antique Electronics, www.tubesandmore.com.

P.S.  Nice to know that some of us still remember Gentle Giant.

[eleventeen]

Appropos of not much, I would estimate your Conn to be a solid decade older than your estimate because of the tube designations (eg; 6L6*G* [woulda been 6L6GA, GB in the 60's ...6SN7*GT* woulda been GTA, and 5U4G woulda been GB in the 60's)

I wouldn't worry too much about the use of a PP transformer, certainly if you already have one...but I'd bet it would work better using only HALF the tube winding...meaning, don't use (and tape off & insulate!) the unused end of the primary.

[burt_toast]

Appropos of not much, I would estimate your Conn to be a solid decade older than your estimate because of the tube designations (eg; 6L6*G* [woulda been 6L6GA, GB in the 60's ...6SN7*GT* woulda been GTA, and 5U4G woulda been GB in the 60's)

I wouldn't worry too much about the use of a PP transformer, certainly if you already have one...but I'd bet it would work better using only HALF the tube winding...meaning, don't use (and tape off & insulate!) the unused end of the primary.

yeah, i think that schematic is older than the amp - ie: it has 6sn7s instead of 12au7s, and the ink on my chassis says that at least the 6v6s have a GT after 'em.
wait a sec - half the windings? you mean i only send the el34 plate/output to one side of OT primary, and just chassis ground for the speaker negative? huh?? that's a new one on me (like a great many things)

hang on, i can't use only one OT pri lead. did you mean use center tap as the new negative?
my brain hurts. . .
thank you, clarify if possible
scott

[eleventeen]

To use a PP output tranny as SE, you would feed B+ to the center tap, connect one of the ends to your power-tube plate, and do nothing with the other end of the winding that would go to a plate under normal PP config. Uhhh, no, do not connect that to ground, LOL. Just tape it off and/or insulate it VERY WELL since it will essentially have full B+ on it. Maybe solder it to an otherwise unused terminal. The speaker side of the tranny you would connect normally.

I'm not saying this is anything near an ideal situation, but it would be free, since you already own the tranny. It *may* also occur that using the entire primary of the OT would work better. I would make that experiment before securing the primary wires to the OT in final fashion. That arrangement would probably be a lot louder, but louder may not better in this case. Technically, you're going to have a good-sized impedance mismatch, but I seriously doubt you'll blow anything up. Hey, maybe you could incorporate a switch that used all/half the OT primary.

[eleventeen]

By the way...should you construct such a thing as an SE amp using your PP output transformer and then add a switch so that you can flip from using half the primary to using the full primary....I imagine you would be well advised NEVER to switch that switch under power. Because you will be throwing (or un-throwing..even more radical) ~~350 volts across half the primary and I have little doubt that would generate a massive voltage spike across the speaker, quite possibly blowing it. It would also create quite a spike on the plate of your output tube. In short, I think you could do significant damage should you ever flip that switch under power. Do not do!

[burt_toast]

thanks, Eleventeen (and everyone)

boy, it only took me a day on this forum to feel like a retard - oh well.  no, i didn't mean to suggest i would connect my OT to both b+ AND ground, but given the lunacy of my whole SE project, you'd have every right to assume that. i wish my initial post had been about some harmless tone stack question or something, so as to reveal my ineptitude gradually, and not all at once, ha ha...

i may try your 1/2 primary trick, but no switch - thanks
 

[sluckey]

I would build a push/pull power stage using the conn iron. This will keep the static current equal in each half of the primary of that OT. Then just simply connect drive signal to only one of the power tube grids. You'll have a single ended amp and the OT will probably perform much better.

[burt_toast]

Oooh! Thanks Sluckey,
any schematic examples of that kind of power section laying around?
scott

ps, uggh. gotta tune a huge pipe organ in Cleveland today - my ears hurt already...

[DummyLoad]

Oooh! Thanks Sluckey,
any schematic examples of that kind of power section laying around?
scott

ps, uggh. gotta tune a huge pipe organ in Cleveland today - my ears hurt already...

... just begging for your conn iron and power supply, except loose two big power resistor divider ckt. for the screen supply, add a choke or 2500ohm R for screens - everything else should remains the same. the 5E5 looks intriguing.

http://www.el34world.com/charts/Schematics/files/fender/PRO_5E5.pdf
http://www.el34world.com/charts/Schematics/files/fender/SUPER_5D4.pdf

peace.

--DL

[sluckey]

Maybe not an exact schematic but plenty of schematics that can be easily adapted. I would probably choose a known good SE schematic and add the 'dummy' tube. Since you mentioned EL34, you may be interested in Iannone's "Critter" amp. You'll need the free program Jschem to view the schematic.

This is a modified PA for Iannone's Critter that shows what I'm suggesting. You could simplify even more by connecting the cathodes together and using a single 250 ohm and 25uF (or 50uF) cap for both tubes.

I still don't recommend using PP iron for a SE amp, but doing it this way should work better. I think Dummyload's idea makes a lot of sense.

[eleventeen]

I would build a push/pull power stage using the conn iron. This will keep the static current equal in each half of the primary of that OT. Then just simply connect drive signal to only one of the power tube grids. You'll have a single ended amp and the OT will probably perform much better.

Ehhh...don't you think that the quiescent output tube would act as a "snubber" and tend to damp response through the active output tube? As long as we are talking about weirdness....

[sluckey]

no

[burt_toast]

wow, thanks for all the great suggestions, guys!  back to the ol' (push-pull) drawing board - i especially like the pseudo-SE tricks and the attenuation stuff i saw the favorites archive, http://www.el34world.com/Forum/index.php?topic=4401.0 . that fender pro looks really intriguing, too!
i can see i'm gonna do 2 things hangin' around this forum: occasionally make a fool of myself, and learn.
thanks a lot,
scott

[burt_toast]

tried the critter-trick this morning - it f***ing worked! could only try it super quiet with my family still asleep, but look forward to dimin' that sucker tonight! Yes!!!
MANY thanks,
scott

[PRR]

> tend to damp response through the active output tube?

With pentodes, only 10%. 5% if the unnecessary cathode cap is omitted on the dummy-side.

[burt_toast]

UPDATE:
this amp as it sits, has got a champ-style front end (w/ vol-tone between v1a & b - and Marshallier values for bias and the cap after v1a plate - what's the term for that one?) then right into Iannone's critter backend, except a shared 820r 15w and 20 uf 500v for bias, cuz that's all i had lying around. Also a 5u4 rectifier.

so - a lot less SE hum (is it technically still SE?), still loud as hell thru a 2x12", a little cleaner w/lots of headroom (big iron?), and chime for days! it's nice, but sounds more like a blues amp now, so i'm thinking of adding a second 12ax7; my question is: should i go with a regular PI 2nd tube, or right to a drive tube (i have the parts to do a PI, or a fender pro-style drive tube)? which might work/sound better with the critter backend?

thanks again fellas,
scott

[sluckey]

It's a single ended amp. It doesn't need a PI circuit unless you're gonna go back to push/pull output.

[burt_toast]

cool, thanks Sluckey - sorry for the dumb questions. i'll just look at some other 2 - 12ax7 SE amps.
thanks again,
scott

[DummyLoad]

cool, thanks Sluckey - sorry for the dumb questions. i'll just look at some other 2 - 12ax7 SE amps.
thanks again,
scott

the ax84 hi-octane looks interesting. if you're into insane gain amps. IIRC it's a marshall-like 2204 pre-amp joined to a SE output stage. i'd lose the MV. IMO, using the wiper of a pot for a grid-leak of the output tube isn't a good idea. if you must have MV there, use a quality sealed 1W-2Watt part.

check it out here.

--DL

[sluckey]

so - a lot less SE hum (is it technically still SE?),

I 'think' it's technically a single ended amp due to only having one drive input to the power tube. But you still get the power supply hum cancellation due to having the dummy tube connected. This allows idle currents to flow thru each half of the OT primary just like a push/pull amp. Since the equal (if matched tubes) idle currents flow thru the halves of the primary in opposite directions, the mag fields caused by the 120Hz ripple on the B+ will be 180 degrees out of phase and will cancel, reducing hum just like a push/pull amp.

[burt_toast]

D'oh!
i was using the term, "phase inverter" when i meant "cathode follower" - my bad.
ax84-hi octane and a few other SEs look like they're using a cathode follower for v2, so i'll try that next.
and yeah DummyLoad, no MV on this one, i'm kinda 'over' jcm800-style preamp distortion, just wanna push the el34 a little harder.

thanks,
scott