12BH7A PPP Amp project

[EWBrown]

I got this one from Gary Kaufman's webpage, and, for now, the photos are from his page.

12BH7A PPP amp design by Nobu Shihido, presented in his book "Transmitting Tube AMplifiers".

Gary gave me a couple of the boards that he designed for it, plus the working completed prototype, with which to experiment.

Schematic, copied from Shishido's book:



I have not been able to find any data on the U-15-8 OPTs or the PH-120 Power Trannie, but these can be readily substituted.

Prototype board undergoing test:

B+ voltage 250VDC, current consumption: 40 mA. OPT is from a Fisher 500C.
This is not going to be a mega-powered subwoofer earthquake-making machine



Gary's completed amplifier, OPTs are from a Fisher 500C, power trannie was salvaged from a console carcass.



I've been running simulations for possibly changing the output tubes to 6GU7s, or 6350s (which would require a minor
bit of etch surgery to accomodate the 6350s somewhat different pinout). I'll go with the 12BH7As for now, I have
a decent quantity of them.

/ed B

[rmyauck]

Nice amp Ed!

Thanks for showing it!

Tell us how it sounds when completed!

Must be <2W/CH!

Floating Paraphase right?

Randy

[nyazzip]

hm i may have just the PT for a little amp like that...whats the ideal primary impedance of the OPT...?

[mesherm]

Nice idea running the input cables over the power tranny to inject 60 hz 180 degrees out of phase to lower the hum.

[EWBrown]

I haven't yet built the completed amp, just the boards (now I have three of them), so far. The amp in the pictures was built by Gary Kaufman several years ago.

I suspect that the Fisher 500C OPT would betoo low an impedance, I have some 5W, 23K A-A: 4, 8, 16 PP OPTs to try for this.
Or use Hammond 1609s with 8 ohm load on the 4 ohms secondary setup. Fool it into behaving as a 20K A-A OPT ;-)

It is not a floating paraphase, nothing so exotic. The bottom 12AU7 grid takes its signal voltage from the tap between the 220K and 18K resistors.
At best, this is an approximation to get the two phases of the driver stage to roughly equal voltages. With this design, "close enough"
is probably good enough

It'll be a while before I get around to finishing it up...

For the PT, I'll use an Antek 05T200 which I've had laying around for a couple years. I've load-tested it, and it would be perfect for this application.

/ed B

[soundbrigade]

Maybe this schematic can be of some help.

[Geek]

Ham terminology in audio, a rare and pleasing sight

[soundbrigade]

????

[tomlang]

Magnus, I see you've done a good job trimming the fat but there's still too much pork!!

[Gingertube]

The one thing in those schematics from Hiroshi San which stood out when I looked at them is the series power diodes in the B+ feed.

Anyone used these diodes? Do they make any difference at all?

Cheers,
Ian

[battradio]

I have be looking the altec amp as a line stage which appears very similar to the above amp , only it uses 12AY7 instead of a 12AU7 for the front end , and was wanting to use a UTC A-18 for the input transformers and UTC A-26 as outputs
http://www.bunkerofdoom.com/xfm/UTC_1955_WEB/028.jpg
http://www.waltzingbear.com/Schematics/ ... c_459A.htm