From Audio Karma:
As I answered in the active Allen 75 thread, the Allen 90 did precede the 75. In fact, the Allen 90 was the oldest (to my knowledge) high power Allen tube amp. It started life as a direct buy PA amp manufactured by Webster (Webcor). The earliest Model 90 amps found in Allen Organs didn't carry an Allen nameplate. By the mid-1950's the nameplate was Allen (although the Webster/Webcor name was sometimes still referenced) and the OPT was upgraded from the original. This was the major, and maybe the only, difference between the PA amp and the musical organ amp. By the late 50's there was no reference to Webster at all.
As I answered in the active Allen 75 thread, the Allen 90 did precede the 75. In fact, the Allen 90 was the oldest (to my knowledge) high power Allen tube amp. It started life as a direct buy PA amp manufactured by Webster (Webcor). The earliest Model 90 amps found in Allen Organs didn't carry an Allen nameplate. By the mid-1950's the nameplate was Allen (although the Webster/Webcor name was sometimes still referenced) and the OPT was upgraded from the original. This was the major, and maybe the only, difference between the PA amp and the musical organ amp. By the late 50's there was no reference to Webster at all.
All good points. A lot of good amps do start from the PA amps. Take Quicksilver example which seems to be a direst copy of a Bogan PA amp. Granted the Quicksilver has much better transformers than the Bogan did. As I pointed out most generally the tube PA amp were frequency limited. The Altec 1570 for example did a lot of duty in drive in theaters where frequency the lack of lows and highs wouldn't be missed. They had the ability to generate a massive amount of power for their time. Tube in PA systems in my opinion worked well in its ability to run grossly overloaded 25volt or 70 volt lines.
AS I mentioned I had a pair of them for home audio for a while. They made a great midrange amp but had nothing in the low end.
The 75's are good amps. This is the third pair I have owned. Generally when someone comes over to listen to my stereo they end up offering me something that I can't turn down for the pair of 75's
Still trying to find out more on the 90's that I haven't already read on Audio Karma or other audio forums.
AS I mentioned I had a pair of them for home audio for a while. They made a great midrange amp but had nothing in the low end.
The 75's are good amps. This is the third pair I have owned. Generally when someone comes over to listen to my stereo they end up offering me something that I can't turn down for the pair of 75's
Still trying to find out more on the 90's that I haven't already read on Audio Karma or other audio forums.
Agree in all counts.
I have rebuild several pairs of AO75 and published bench results on AK. They are truly good amps with extended bandwidth. Those Acrosound OTs, and AO/Saratoga OTs are all good. Difficult to get better.
At some point I was offered a pair of AO90 and after studying the schematic, I decided to pass.
It would be interesting to test the bandwidth response on those big mama's OTs. They seem bigger than the 75.
I have rebuild several pairs of AO75 and published bench results on AK. They are truly good amps with extended bandwidth. Those Acrosound OTs, and AO/Saratoga OTs are all good. Difficult to get better.
At some point I was offered a pair of AO90 and after studying the schematic, I decided to pass.
It would be interesting to test the bandwidth response on those big mama's OTs. They seem bigger than the 75.
Well it is interesting that the Allen 90 is a slightly tweaked Williamson circuit, with fixed bias and pentode mode output stage, and shunt RC roll-off of high frequency for HF stability. Does anyone know the history of this amp and its use in organs?
I note that the English organ firm Miller Organs also used a similar pentode mode Williamson for their power amp.
I'd be guessing that the bass response and low distortion of that amp circuit choice were an influence.
I note that the English organ firm Miller Organs also used a similar pentode mode Williamson for their power amp.
I'd be guessing that the bass response and low distortion of that amp circuit choice were an influence.
And to add to the confusion, I discovered the Allen Organ C-3, aka Allen Organ 60.
This is a parallel 6L6 design, with 2 tubes on each leg, so 4 power tubes. Each tube has it's own dedicated primary winding. It is too a regulated screen voltage design, with fixed bias with it's own dedicated supply PT. The interesting piece is the feedback. It has a separate winding acting as the cathode resistor on the driver stage. No classic RC GNFB. It has 2 tube rectifiers with a 1000 VCT supply.
That thing was design for robustness as it can run with only 2 tubes, or more. I have to try if it can run with only 1 tube. No bias adjustment, no balance, no AC balance, nothing. Plug the tubes and play. But it does play well 🙂. Sometimes there is to say for simplicity... All the transformers are made by AirDesign, which was big during after WWII for high quality professional equipment, located is PA. It closed down early 50's.
The OT is even bigger than the AO90, with dual 4 ohm winding secondary only. So you can parallel the 4 ohm or serial for 16 ohm speaker. No 8 ohm unfortunately.
There is very little data on that amp. I bench tested one and was getting 40 watts out, with quite some distorsion but this was with original components, most of then out of specs.
I am listening to it as we speak, just for fun.
This is a parallel 6L6 design, with 2 tubes on each leg, so 4 power tubes. Each tube has it's own dedicated primary winding. It is too a regulated screen voltage design, with fixed bias with it's own dedicated supply PT. The interesting piece is the feedback. It has a separate winding acting as the cathode resistor on the driver stage. No classic RC GNFB. It has 2 tube rectifiers with a 1000 VCT supply.
That thing was design for robustness as it can run with only 2 tubes, or more. I have to try if it can run with only 1 tube. No bias adjustment, no balance, no AC balance, nothing. Plug the tubes and play. But it does play well 🙂. Sometimes there is to say for simplicity... All the transformers are made by AirDesign, which was big during after WWII for high quality professional equipment, located is PA. It closed down early 50's.
The OT is even bigger than the AO90, with dual 4 ohm winding secondary only. So you can parallel the 4 ohm or serial for 16 ohm speaker. No 8 ohm unfortunately.
There is very little data on that amp. I bench tested one and was getting 40 watts out, with quite some distorsion but this was with original components, most of then out of specs.
I am listening to it as we speak, just for fun.
With limited searching, it appears that the type 90 amp (with schematic showing production dating from 1958) was only used in the Gyro speaker product, but the Gyro speaker product seems to have had a few variations and only a limited time-span of production had the type 90 amp in it.
The best photos are in:
Allen C-4 organ pics, and help needed! | Audiokarma Home Audio Stereo Discussion Forums
There is a good write up by Dorf in min 1954.
https://www.americanradiohistory.com/Archive-Audio/50s/Audio-1954-July.pdf
The best photos are in:
Allen C-4 organ pics, and help needed! | Audiokarma Home Audio Stereo Discussion Forums
There is a good write up by Dorf in min 1954.
https://www.americanradiohistory.com/Archive-Audio/50s/Audio-1954-July.pdf
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