600 Ohm C.T. Secondary Driver XFMR???

[W2XR]

Guys,

I have an tube audio amplifier that has a 600 ohm center-tapped secondary. It can deliver 30 watts of clean push-pull class AB1 audio to a 600 ohm load. This translates into about 132 volts RMS across 600 ohms.

I am wondering if this may be even remotely usable as a driver to my push-pull class B 833A modulator. The driver transformer in my modulator is a high-fidelity Dynaco Stereo 70 output transformer with the normal plate-to-plate primary winding connected as the center-tapped secondary, with the grid bias for the modulators connected to the center-tap. The low impendance side of the XFMR is connected to the 8 ohm output of a high-fidelity tube audio amplifier. Really just a pair of audio output transformers connected back-to-back. Nothing new here, of course.

Before I put my P-P class A 845 audio driver amplifier into service (currently under construction), I 'd like to eliminate this back-to-back transformer topology.

My thinking here is that the 600 ohm C.T secondary of this amplifier/transformer probably would not provide enough voltage swing to really drive the 833A grids into deep class B operation (i.e drive the grids sufficiently positive into grid current) before the driver amplifier runs out of steam. The 833As are biased at about -70 volts with 2600 VDC on the plates; this results in a quiescent current of 50 Ma per tube. But I remember hearing a long time ago (don't recall from what source) that a driver XFMR with a 1000 ohm secondary was closer to the ideal for driving class B triodes of the 833A or 810 class. It is interesting to note that many broadcast rigs in the 500 to 1000W class use driver XFMRs with a 5000 ohm C.T secondary. This would, of course, provide much more voltage swing to the push-pull modulator grids than a driver XFMR with a lesser secondary impedance. I've got to assume that the engineers that designed those old rigs knew what they were doing!

Any thoughts on this??? I could try it by experimentation, but maybe one of the team has been there, done that!

Thanks & 73,

Bruce, W2XR

[AB2EZ]

Bruce

Looking up the specification sheets for 833A's operating in class B, push pull...

It would appear that you need a few hundred volts (300 volts peak or more)... from the center tap of the output transfromer to each side of the output transformer... to drive a pair of 833As in push pull, even if you don't want to reach the full output power that the tubes are capable of.

The output impedance of the driver isn't that important... except, indirectly: in that a step up transformer (from 600 ohms to a higher impedance) would produce a higher output voltage.

Another issue is that a driver with a 600 ohm output impedance, ***if*** it were properly terminated in a 600 ohm resistive load, would be better able to deliver grid current to the 833A's when their grids are driven positive... but that would come at the expense of the much higher drive power that would be consumed by the 600 ohm load on the driver. I.e., a 600 ohm driver, terminated in a 600 ohm resistive load would have to deliver about 300 watts of average drive power to produce a 300 volt peak output voltage from center tap to one side of the output transformer.

Best regards
Stu

[W2XR]

Hi Stu,

Thanks for the reply, and confirming exactly what I had suspected.

I think I'll stick with the 5000 ohm C.T. secondary driver transformer I pulled from an old Gates BC-1F, and use this with the pair of class A p-p 845s I currrently have under construction as a driver to the p-p 833As. This will give me way more headroom in the driver section than I'll ever need, and linearly swing all the grid voltage I'll need for the p-p 833A modulator.

Best 73,

Bruce, W2XR