[Joeztech]

I recently looked at the schematics of a 41-290 to help a friend and in the process noticed the difference between the Williamson type output circuit used in it versus the output schematic of the 41-280. The 41-280 does not use the Williamson design, yet still achieves a push-pull output stage. At first glance I could not figure out how it worked. Thanks to another amateur radio friend the explanation became understandable. You have to look at the phase relationships as waveforms move through the tubes. I took the liberty of adding waveforms to the schematics so people could see the phase relationships. In the 41-280 the screen grid of the top output tube is not bypassed until it gets past resistor 56, a 3900 ohm resistor. This voltage which is in phase with the signal at the plate is coupled to the grid of the bottom tube's grid thereby making its output opposite in phase to the tube above it, thus achieving a push-pull circuit. In effect the output tubes become a phase inversion circuit as well as an output gain stage. A whole tube is eliminated in the process, thus reducing costs.

Joe
   

[codefox1]

Last post must have been lost. This circuit works OK provided output tubes are reasonably well matched, and will not win any awards from instrument based fidelity tests. O course if "upper " output fails, you will get no sound at all. Probably didn't matter much for AM radios and the ultimate limit of loudspeakers of the time. Do a switcheroo of the ouputs to see which sounds better to you.

[Bill Kinghorn]

As far as I know, the Williamson output stage has the screen grids of the pushpull tubes tapped down on the output transformer to give triode-like characteristics while keeping high plate-efficiency. I think its an idea from the 1950's hi-fi era.

The phase inverter of the 41-290 is a much better circuit than that of the 41-280. Is it a Williamson phase inverter?

[Joeztech]

Bill, I think you are confusing the Williamson circuit with the ultra-linear circuitry that came along later. The Williamson circuit is essentially what the 41-290 is. Its screen grids are fed direct from bypassed B+ rather than taps on a center tapped primary. Ultra-linear screen grid tapped transformers are usually much higher priced transformers. The 41-190 was a way of getting more audio power output without the extra phase inverter/driver stage. As Codefox says it would not win accolades as to distortion however and if that top output tube quits the sound goes completely or becomes very distorted if the top tube gets very distorted.

As Codefox mentioned these radios were not high fidelity receivers and the speakers available at that time were not noted for great frequency response or fidelity either. They did have one plus going for them in that many included a hum-bucking winding that minimized hum from the power supply.

Joe

[BrendaAnnD]

http://www.oestex.com/tubes/williamson.htm

[Arran]

The 41-280 uses a power output circuit called "Screen Grid Inversion", the 40-290 just uses a more conventional tube based push pull power output with a phase inverter. The screen grid inversion circuit actually works quite well, but the power output is not as high and the distortion level is higher then a proper push-pull circuit, but less then a single ended output. It did everything that it was supposed to do, the 41-280 and it's 1940 and 42 siblings were extremely popular, and they used the same chassis and circuit in a series of table sets too.
Regards
Arran

[hsusmann]

The Philco 37-650 uses this same output push-pull design (using the
screen of tube #1 to get the phase inverted grid drive for tube #2.

Mine sounds fine to the ear, I have not even checked the matching of the 6F6's yet. Have not measured it for distortion or anything but it does
seem to work good enough.

Herb S.

[Doug Houston]

As far as I know, the 37-650 was the first Philco model to use that goofy phase inversion. I too, couldn't figure out how Philco got inversion. I was in my first year of high school, and a smarter kid than I, explained it.

The "screen inverter" was used by Philco for loads of sets for all of those years. I'm currerntly working on a '39 Packard-Philco set with P-P 41 tubes, and there it is; our favorite phase inverter!

I must say that for all of the years that our family had the 37-650 that father bought new for Chistmas 1936, the set worked perfectly, and sounded very good to all of us. Today, the sound system in my living room is a tad more sophisticated, but what might one expect?

Also, as far as I know, Hazeltine Corporation did Philco's circuit engineering, so I suspect that this inverter is one of their wrinkles. You have to admit...it's tricky, but it works!

[hsusmann]

Doug,

Yes, it is clever and it does seem to work. I now have my 37-650 chassis back together and working. I am waiting for warmer weather and then I will try tackling the horrible cabinet it came in. Someone painted it dark brown.
I think I have a picture of it earlier in this thread.

Herb S.
Ithaca NY

[Randy H.]

Doug,
This is one of the reasons less than great techs of the day HATED Philco. (And there were LOTS of them) They did STRANGE THINGS, and got reasonably good results with them. In their favor, when they DID hit on something that WORKED, they stuck with it.

[Joeztech]

Philco was not the only company that did strange things. I remember RCA televisions that had the audio output tube used as a dropping resistor for the B+ to the IF circuits. When the tube began to loose cathode emission the picture would gradually get less and less contrast until finally video sound and all would go. Of course tube testing would reveal the problem as would tube substitution. Then they also had a weird circuit in some of their radios that used one if the IF tubes as a phonograph amplifier stage when the band switch was placed in the phonograph function position. Most of these strange circuits were done with the goal of reduced cost, while still producing a product with good reproduction of audio and/or video signals. One of the less successful ideas that Philco had was the use of encapsulated RC networks in many of their televisions during the 1950s. It reduced cost, but the technology was not the best and many of them failed more quickly than had been expected. I imagine that a good number of technicians were challenged when they were asked to work one of the 3X-116 or 3X-690 models. The tube count was high in those sets and the motorized station change circuit was not easy for many to understand. They were great performing sets though.

Joe

[Radioroslyn]

Even my 1924 Crosley Trirdyne uses one tube to amplify rf and af signal at the same time(reflex design).Head scratching circuits have been around for along time!!!
Terry

[codefox1]

Muntz!