C-T OT Needed to Load PP 12BH7's for Max. Output?

[TubeRube]

I want to use (2)12BH7 miniature tubes for a triode-connected,cathode-biased,
PP output stage loaded for max. pwr. output before significant distortion (5%-10% THD?). Not sure what CT pri. impedance needed for a close compromise of goals. For Class A operation, 12BH7A data sheet shows (per triode):

  Max Ratings: Vp=300V...Plate Dissip.=3.5W...Ik=20mA...Neg. Bias Vg=50V
  Typical Oper. Char.'s: Vp=250V...Ip=11.5 mA...Vg= -10.5V

With ~245V on plates of a 5F6A-type 12AX7 LTP P.I. (except .01uf coupler on input & .047uf couplers on plates), I was supposing maybe ~270V-275V on the 12BH7 plates w/~1.8K-2K ohm resistor (per triode) for cathode bias (hopefully,  a moderate amount of clean headroom before dimed distortion)...  or is this a totally whackt guestimation?

In PRR's Reply #4 to tubesornothing's Post #2395: "Cathode resistor for 6L6 PP" (14Feb), there was an "optimum load is near twice the DC load on the B+ supply" estimation. Plugging in my values I get: 3.5W/270V = ~13mA x 4 triodes = ~52mA...  270V/52mA = ~5.2K x 2 = ~10.4K end to end (plate-to-plate load?)...  Does this imply a 10K-CT (or 20K-CT?) OT for unstressed, no red-plate tubes w/40%-60% of 14 watts output? Is this Apples vs. Oranges or is this approach not applicable to triode operation and totally incorrect? I am unaware/ignorant of the many variables that factor into "optimum load" estimation & am clearly in need of some elucidation (or whatever's available) so I can decide what type OT to buy...   HELP!! BRAIN OVERBOARD!! 
   

[PRR]

> 14 watts output?

More like 4 Watts.

> not applicable to triode operation

Correct.

Unless you run very high plate voltage, or enormous cathode (and heater power), or run grid current, triodes rarely reach 33% efficient. You have 2*2*3.5W plate dissipation, 0.3*14 is about 4 Watts. Run 12mA/tube idle current, load with 10KCT-20KCT plate-plate.

> some elucidation

Radiotron Designer's Handbook 3rd edition has analysis of push-pull triode power amps.

The example in there is push-pull 2A3 (6B4) for 10W-14W. The 2A3 is a much fatter triode than a 12BH7.

[TubeRube]

Thanks, PRR, for the info and insight...  your reply posts are always very accurate (as much so as necessary) & most useful. I was hoping you might take time to correct my muddled & partially formed notions on possibilities of using 9-pin mini's in triode operation. I had started considering a Champ-style size & power combo (14"H x 14.5"W x 8.5"D) in PP instead of SE after reading a posted suggestion to use a 12AT7 instead of a 12AX7 as a power tube. I was hoping a 12BH7's 3.5W vs. 12AT7's 2.5W might be more suitable. Also, being able to switch off one of the two 12BH7's for 1/2 power seemed to be a neat idea. Of course, I immediately looked up a 6B4G's 27B outline and, despite design needs of cramped chassis space, (2)6B4G's in lieu of (2)12BH7's might fit (maybe lose the reverb feature)...  for a 10W triode-connected PP output stage instead of a (2)6V6 or (2)EL84 18W-20W pentode-connected PP output. I must find a Third Edition Radiotron Designer's Handbook & learn more about triode-connected output sections. Thanks again...   TubeRube. 

[PRR]

> a posted suggestion to use a 12AT7 instead of a 12AX7 as a power tube.

12AX7, 0.1W if you sweat it. 12AT7, about 0.3W for both sections, and not too fussy about load.

Fine bedroom amps. Not stage amps, not even when you go to the fat 12AU7 then up to the modestly bigger 12BH7 and double-up. You want a shovel, not a bigger spoon.

Size aside, there "are" ways. 6J6 is very cheap. If heater-power were cheap, you could array a dozen 6J6 and make some Power. Or with more than a few 12BH7 or 6J6 it may make sense to use a few as a Transformer-Coupled driver, run the power tubes in AB2 (grid current). Although working triodes above zero-grid does very much the same thing as adding a Screen Grid, except doing the extra push with audio power instead of simple DC power.

Are you sure you want Triode?

Historically, pentodes with overall NFB give much of the effect of triodes with greater efficiency. And, because pentodes work so well, we find a much larger selection.

Triode power-amps are mellow. Electric guitar needs more flavor.

> 6B4G... might fit

Oh, bah, that's not the way to go "tight". Take fer instance 6EM7. 'Bout the size of 6V6. Has a fat triode and a high-gain triode in one. A pair of bottles at ~~270V in 10K load is a complete 8 Watt push-pull (albeit at 50% more heater power than say 2*6V6 and a 12AT7).

I mentioned AB2 operation. '53/6A6/6N7 twin triode has specs for 10 Watts output per bottle. The drawback is that this needs 20mA grid current on positive peaks with no current on negative peaks: R-C coupling can't be used. (6J6 is kind a small '53 in new bottle.)

[jojokeo]

> 14 watts output?

More like 4 Watts.

> not applicable to triode operation

Correct.

Unless you run very high plate voltage, or enormous cathode (and heater power), or run grid current, triodes rarely reach 33% efficient. You have 2*2*3.5W plate dissipation, 0.3*14 is about 4 Watts. Run 12mA/tube idle current, load with 10KCT-20KCT plate-plate.

> some elucidation

Radiotron Designer's Handbook 3rd edition has analysis of push-pull triode power amps.

The example in there is push-pull 2A3 (6B4) for 10W-14W. The 2A3 is a much fatter triode than a 12BH7.

Ol' Gerald Weber has an amp advertised as 7 watts w/ a single 12BH7 & 14 watts w/ two!?!

The all new Kendrick So-Lo 7 recording/practice/gigging combo amp delivers the unmistakable tone and the unique feel of a big amp, yet weighs only 22 lbs! The secret comes from the true push-pull design - like a Marshall or a Twin, but using a twin triode output tube (12BH7), which puts out 3.5 watts per side for a total of a full 7 watts. ***Note: There is also an extra tube socket that can accept an extra 12BH7 which will put the amp at a full 14 watts for small club gigs, should one desire more power. This optional extra tube is entirely plug and play and no adjustment is needed.


http://kendrick-amplifiers.com/Merchant2/merchant.mvc?Screen=PROD&Store_Code=KOS&Product_Code=KEN0104-so-lo7&Category_Code=0104

[HotBluePlates]

I am grateful that Gerald Weber wrote the books and articles he did, as they were the best source of information at the time.

But there's a lot I've had to unlearn or relearn, because of things that were incomplete or exaggerated. Obviously, the output claim for this amp is exaggerated, unless he's using some kind of hybrid tube/SS output stage, using the solid-state to boost the output power of the 12BH7.

[TubeRube]

Oh Wow, PRR!!! A 6EM7 is a way cool tube!! I coulda looked thru tube manuals forever & not seen this one! Like you say, it eliminates need for an entire 12AT7 P.I. 9-pin socket and is more compact than a 2A3! This opens new design speculation for a small pkg. amp that, I agree, shouldn't be jam-pack crammed. My original notion was toward a "bedroom" amp that, with unknown losses & inefficiency, would put out considerably less than 14W peak.(before info re: ~30% efficiency of triode oper., going for 8W...  4W 1/2 pwr.)
Anything close to 8W in triode-connected PP output in a similarly compact combo pkg. would be perfect!!
 
Unless, as noted, I don't like how "bland" a triode-connected output might sound at low power. I guess I'll have to stop procrastinating & put together some sort of bread-boarded circuit testing arrangement (seen pics of some on this forum) so I can actually sound-check design ideas that I can't dismiss until eventually brought to fruition in some form. Somehow, I've drifted away from a safe zone of initially wiring conversions & add-on circuits off to the sides of existing amp chasses for audio sampling tests. How did this happen???

[PRR]

> puts out 3.5 watts per side

The ad copy-writer seems to use the Plate Dissipation as the Power Output. "3.5" is the Pd number in the 12BH7 data-sheet.

That's like calling the maximum energy (heat) output of a car engine (via radiator and exhaust) the "Horsepower".

There's some relationship. Small tubes won't make big power; it is silly to use a big tube for small output. (Tho you can find headphone amps with a 13W 6080 making 0.05W into a load.)

> some sort of bread-boarded circuit testing arrangement

Ikea or the Dollar Store has breadboards, or find scrap plywood behind your garage. Nail/screw terminal strips, sockets, transformers. Just watch your fingers, and remove the power cord when kids or cats might get into the room.