Single Tube Modulator

[K9ACT]

Is it a matter of convenience or laws of physics that modulators always have two tubes in push-pull.

I have an extra 811 and socket and wonder if there is some way to make a modulator for my 811 CQ rig.

js

[KA8WTK]

Why not try Heising plate modulation? Some of the older Radio Handbooks show how this was done. Requires only one modulator tube and a choke. You don't even need a modulation transformer.

[Bacon, WA3WDR]

A few thoughts:

An analog series modulator; not too efficient, but relatively simple if (filamentary) cathode modulation.  Rough guess about 40 watts carrier out.

A PDM modulator.  A bit complex, would need unusual drive on an 811 switcher tube.  Pretty efficient.  Needs unusually high voltage supply, maybe 2 or 3 KV.  Rough guess about 140 watts carrier out.

A Heising modulator.  Needs a modulation inductor, a version could be done with a center-tapped power transformer winding if DC is balanced, but not too much power possible.  Not too efficient.  Rough guess about 40W carrier out.

Might set up two 811 in parallel, and grid modulate or (better) grounded-grid linear.  Not too efficient.  Rough guess about 60 watts carrier out.

[kf6pqt]

It wouldn't work for the 811a, but there is Screen Grid mudulation!

[kf6pqt]

page 2

[K9ACT]

Thanks for all the thoughts. 

The amp is done so screen mod is not an option.

I had considered cathode but it seems more complicated than plate mod and I have an SS22 mod iron collecting dust along with a 700 v power trans, choke and bunch of diodes. Also have a 10w ss audio amp left over from the 8000 project.

I just happen to have another 811 and it would be neat to use it to modulate the other one.

I was also wondering if a pair of 807's could be used as zero bias triodes as I did with the 813's.

That would be pretty simple and probably enough power.

js

[AB2EZ]

JS

A few comments to address your original question:

The reason that push-pull modulators are typically used is that a) A pair of tubes in push-pull operation, each biased for Class AB1 or AB2, will be much more efficient than a "single-ended" amplifier biased for Class A operation; and b) In single-ended operation, the primary of the modulation transformer will usually be used to carry the average plate current for the modulator tube... leading to an unbalanced average current in the primary... leading to additional magnetic saturation problems in the modulation transformer.

Of note, the highest-priced audiophile, vacuum tube amplifiers employ single-ended, Class A operation ...and feature this as being associated with the best audio quality (whether true or not). As such, they operate with very low efficiency (lots of heat, huge tubes for modest output power, ...) and require a large/heavy output transformer that is designed to accommodate the large unbalanced average current in the primary (or, alternatively, a large/heavy, supplementary Heising choke).

Best regards
Stu

[John K5PRO]

Another reason for push pull using 2 tubes: If they are operating in class B (as higher powered modulators using triodes often are), for efficiency, then each tube is cut off for half the audio cycles, conduction is 180 degrees. The second tube is inverted and only operates on the other half cycle. When both are coupled to the mod transformer primary, the resultant is full audio waveform again. Thanks to Loy Barton of RCA for this class B audio scheme.