Allen Gyrophonic speaker assembly circa 1958-60 with type 90 amplifier

[trobbins]

I am trying to collate any information on the type 90 amplifier that appears to have been in circa 1958-60 Gyrophonic speaker assemblies. My interest is because the amplifier uses a variation of the Williamson amp circuit.

The best photos I have seen of the speaker assembly, but only from the rear, are in the thread:
https://audiokarma.org/forums/index....needed.161084/

I have the July 1954 Dorf write up in Audio magazine, but otherwise I haven't been able to locate any magazine or advertising brochure/catalog type literature on the Gyrophonic range of that time.

If anyone comes across anything, I'd appreciate any information.

Ciao, Tim

[David Anderson]

What information are you looking for? I Googled the Type 90 amp. Looks very rugged. Tube complement shows it has two 0D3 gas regulator tubes, indicating that there's some regulation going on in the power supply. Are you looking for a schematic? (I don't have one.)

I've seen a couple of Allen Gyrophonic cabinets, and they use speakers on a rotating disc baffle to get around Leslie's patents. They are very heavily built and very heavy. They use brush contacts to get the signals to the speakers and very large motors and power supplies to rotate the disc.

[trobbins]

I am hoping to expand on the few details I have come across for the type 90 amplifier (as it is a Williamson variant). The photo in that link is the only photo I know of. I do have the schematic.

Webcor (Webster) appear to have made valve amplifiers for Allen Organ, including the Type 90, with Allen Organ apparently bringing amp production in house from mid 1958 (based on Allen Organ Co schematic date of Jan 1958). I have no earlier Webster details part from two audiokarma forum posts.

It seems Allen used this amplifier only for the Gyrophonic range, however that range extended over a long period, and there doesn't appear to be any model definition and exact timing of models within that range. The Gyrophonic Projector was patented (US2491674) in 1949 by J.Markowitz, the founder of Allen Organ, and advertised that year. Dorf authored an article on the Allen Organ for Audio in 1954 with a photo of the speaker assembly and amplifier. An updated patent (US3288908) was filed in 1963, which is the revision year on the amp schematic. Gyro speaker assemblies used the Type 75 valve amplifier, and also solid-state amplifiers.

[David Anderson]

I'm not sure I've ever seen the Type 90 discussed on here. I'm a tube amp tech, and I don't know much about them.

Organ manufacturers went back and forth on manufacturing amps in-house. Hammond outsourced amps to Webster and Rauland from 1945 to around 1948, at which point they started building them in-house. Then, around 1962, they started outsourcing transformer winding.

As you probably know, the Williamson tube power amplifier circuit was very popular in the 1950s. Lots of companies built versions of it, including Heathkit and various British brands.

Be careful about drawing conclusions from schematic dates. Those dates can be from any point in production -- before, during, or after. For example, the Leslie 22H schematic states that certain changes happened at the same time at a certain serial number, but, in the field, we've found that those changes were actually spread out and didn't all happen at once.

[trobbins]

The schematic in https://audiokarma.org/forums/index....roject.270470/ starts in Jan 1958 as the initial version, then shows revs through to 1959, and then has a 1963 stamp on it (probably some document indexing activity, or related to the patent).

The type 90 amp has some variations on the standard Williamson circuit, so I was also hoping to identify the Webster origins. I note that Miller Organs from UK also used a pentode output stage variation of the Williamson using 300V screen regulation for 807's, and I have one of them made circa 1960, that was known to be used for their mighty Classic IV organ.

[David Anderson]

Engineers did not always follow the original Williamson circuit down to the letter. It was a general topology, not a strict recipe. The original uses triode-strapped output tubes, but Heathkit's versions were ultralinear. I'm not surprised that someone built one with a pentode output stage since that gives the highest power output.

I doubt anyone on here knows more about them than that. I've never seen it discussed. We probably have only a few members who understand triode/ultralinear/pentode output stages.

[trobbins]

Yeh that's fine - queries like this often become a place holder for future viewers/googlers. It took me about 5 years to get in contact with Peter Stinson, who had made contact with Williamson, and had written a comprehensive biographical account of Williamson and how his amp fitted in with the decades before and after.

It also took me a few years to link a Williamson amp I had with the Miller Organs Mark IV - via an amazing organ restoration thread put up by Lucien Nunes (https://www.vintage-radio.net/forum/...d.php?t=121108) - well worth a read through the thread to soak up the enormity and excitement of that restoration :emotion-21::emotion-21:

[kc0yqz]

The term "Williamson" is quite frequently applied to circuits having nothing in common with the DTN Williamson designs published in <i> Wireless World</i> in the UK and shortly after the US version, called "the Musician's Amplifier", by Sarser and Sprinkle published in the <i> Audio Engineering</i> magazine circa 1948-1950. when someone calls any other design a "Williamson" you have to ask, "in what way is this a 'Williamson'?" I am not trying to be contumacious here, I've studied tube amp circuits for 25 years.

At any rate, in general, tube AO Gyro cab amps make very good high fi power amps, I think that they are getting pretty scarce now. A lot were dumpstered over the years. I've parted out a few Gyro cabs but most had the solid state power amp which isn't anything to write home about, probably ok as a utility amp.

[trobbins]

I have just identified the amp chassis in the Gyrophonic Projector photo in Dorf's 1954 article - the chassis is marked as "C-3". That could indicate the Gyro speaker was sold with the C3 organ, as the C3 organ was introduced from 1955 (Jan Giradot's list).

kc0yqz, I am fine with how I have used the Williamson term, and as you indicate, I have purposefully used the term variant:
https://www.dalmura.com.au/projects/Williamson.html