803 tube for audio

[Opcom]

I concede it is done in the traditional way with a large modulator and a resistive divider feeding from the plates to the cold ends of a split grid winding.

I also maintain that it is important to take feedback from the secondary of the transformer and if there was a way to add about 100 turns to the mod transformer and center tap it, and give the modulator the cathode (filament center taps) feedback it wants as in diagram #6, the result would be better and reproduce more accurately than the resistive divider method.

I believe that an UL circuit diagram #2 would be an improvement upon the resistive divider scheme from an output voltage regulation standpont, but I do not believe it works better than the cathode feedback circuit in reducing the output impedance and giving good regulation.

[Bill, KD0HG]

It's just conjecture as to why the UL design was never adopted by AM transmitter manufacturers. I'd guess the cost of a special modulation tranny with tertiary screen windings was outrageous. You also take somewhat of a hit on power output capability when using semi-triode UL as opposed to a straight pentode design. Overall, it probably wasn't worth it for a commercial rig.

The Williamson UL design has a lot of merit. It has a lot of immunity to distortion caused by load changes. I use a Williamson with a pair of 6L6GCs to drive my class B modulators where the grids of the 304s really 'swing' into conduction.

My home stereo is also a UL design using P-P 6550s. Unbelievably low THD and IMD.Built 15 some years ago. The output trannies are a product of a company called Magnequest. He also makes dupes of the old Peerless and Acrosound transformers.
http://www.magnequest.com/products.htm

I think the experimentation by Dave is in the best tradition of ham radio.

[KE6DF]

It's just conjecture as to why the UL design was never adopted by AM transmitter manufacturers. I'd guess the cost of a special modulation tranny with tertiary screen windings was outrageous.

I use a Williamson with a pair of 6L6GCs to drive my class B modulators where the grids of the 304s really 'swing' into conduction.

In my junk box I have a driver transformer from a BC-5P transmitter that used 4 x 6550 in push pull parallel in UL mode. The transformer has screen and plate taps on the primary.

The driver output drove a couple big triodes (3x2500's?) in class B.

It's interesting that you had success using UL in a driver.

I figure my transformer would work well with 6L6's or 6bg6's instead of the expensive 6550s.

So UL was used in the broadcast industry at the speech amp/driver level.

[Steve - WB3HUZ]

Very interesting. UL in the driver stage makes sense, since good regulation is vital when driving Class B tubes.

There was a short article in QST many years ago (might have been the Hint & Kinks section) on using small triodes in the screen leads of tetrode modulators. The triodes were set up as pass tubes (IIRC) and you could set the operation of the tetrode modulators from full tetrode to full triode, or anywhere in between. I'll see if I can dig up the article.

[Opcom]

I figure my transformer would work well with 6L6's or 6bg6's instead of the expensive 6550s.

psssst.. use 6CD6's. $6.00 each. Mucho Ib capacity, low G2 voltage

[WBear2GCR]

found a set of 803s at Nearfest yesterday, so went looking to see about them...

...since the post with the schematic from Orr's 1959 handbook was deleted, I've included it here for completeness
and historical record. And in case anyone else runs across 803s... The main issue with these appears to be the high Plate Z,
but run at lower plate voltage, the Z would be lower but the power still pretty good, although not 500watts...

[W2PFY]

Yup dem dar are nice old tubes and the price is good most of the time. The navy loved them big time. Other than the navy, who else used them? BC? Nope! SW? Maybe? Hams? Limited! Audio Fools? Yuk!!

[W3RSW]

Reg Williamson may haved begged to differ with questioning the worth of "ultra linear" vacuum tube circuits.  All games are inexorably improved or proven inflexible given an impetus.  

Distortion control of commercial BC AM transmitters probably peak when mandated limit requirements are met.

Similar thought for modern times. Is pre-distortion correction in the digital audio domain being utilized somewhere in the audio chain as in the RF domain? Have to dig up my Broadcast World Wide catalogues.