Push Pull rk71 transformer phase split

[Paul Barker]

I wanted to employ my last two Black Gates in a PP amplifier, because the Sowter PP transformers have been collecting dust in one of the many men in sheds spaces in and around my house.

I dreamed of transformer phase splitting, but in the event the two pairs sailing over from the USA one pair electraprint one pair schumacher, appear about the same. Primary current tolerance 15 to possibly 20 mA. Hoping 20 is ok, because any driver needs lifting off the bottom line on the chart. If the laminations seem likely I could interleave them then I would parafeed for sure and lift any driver up to the parts of the load line benefit from current. But lets wait and see. Anyway end game is wind my own Nickel mix lams and parafeed. Baby steps apply first.

Just quickly on the purpose of the chosen output valves. Conditions, voltage limit of Black Gates. Not many transmitting tubes do any worth except the GK71. Thats why. In UL it gives quite some power certainly over 20 watts. But in triode Id expect better sound, though lower power. 14.4 watts. So I’ll try that first.

Well the first obsticle was I spent ages tied in notts trying to find a valve to sit well with the phase splitter primary. Eventually the 6em7 power section is the closest to the required voltage swing, but falls just short.

So like Williamson I shall have to put balanced driver, not requiring much more gain, once again 6em7 sprung to mind. So that fixes the lack of drive swing after the transformer, which isn’t as it seems a one part fix everything package.

Not only that but the 6em 7 also requires voltage swing so nothing drastic; a 6j5 gives a gain of 17 making the 6em7 operate at full potential from a source of 1.5v peak.

After the phase splitter transformer the rc coupled pair of 6em7’s can boss the rk71 grids.

The bias for the rk71 is required as fixed so max anode volts available. No detail of grid resistance so I shall use grid chokes to feed the grids with -50v. The benefit of this is smaller coupling cap in case of any unplanned minor grid current.

And thats about it. At least with an additional after stage the it splitter can do the job with a little help from a pair of 6em7’s.

The bill for the four transformers had vat added so that was a shocker. But still economical compared to Hashimoto and Tango.

The test image of both sets of transformers show linear from 20hz to the latter element closing in on 20khz but only 1 db lacking. Id never hear that anyway. In my ear spectrum its all flatline.

[Paul Barker]

I admire Morgan’s perfect PP diagrams. Mine looks nothing like his.

[Paul Barker]

This pair converted to £ UK and VAT and shipping eventually cost £245 the pair. I am happy with that.

[Paul Barker]

I found enough gain with parallel 5687 as the driver.

[Paul Barker]