The model 75's big brother the 90 seems to be quite different. I've never seen one up close only in pictures. None of the pictures I've seen have showed a bias control. I believe they do show two test points. Are the test points to balance the output tubes like the model 75? How do you set the bias on the 90?
I saw a pair for sale but I'm not going to pull the trigger until I have more information and possibly get to listen to a pair from someone in my area.
Are they that better than the model 75? I did see it looks like they use a pair of 0D3's for screen voltage a nice touch.
I saw a pair for sale but I'm not going to pull the trigger until I have more information and possibly get to listen to a pair from someone in my area.
Are they that better than the model 75? I did see it looks like they use a pair of 0D3's for screen voltage a nice touch.
The 90’s came before the 75’s, and many early ones were simply sourced from Webcor - those were just high power PA amps, while the later ones had better OPT’s, the design remained the same. Do you really think you can benefit from the small increase of 15W from the 75’s?
Can I benefit going from the 75's to the 90's? Slight increase in power transformers the same quality. The 75 is very full and robust sounding in my opinion. I do like the regulator tubes in the 90 and its somewhat different schematic. The differences in the topology have me wondering how it might sound. After all isn't that what audio is all about? The differences in sound as a result of different circuits. Sure, I realize I have to double the power to gain 3db increase in level. Its not all about power in my opinion its about the composition of the circuit and its refinement. Then again I could be totally wrong.
If you have the budget, sure go ahead and try them. But the 90 was a comparatively dated design, I’d stick with the 75... btw, there is no regulator tube in the 90 afaik.
You are right, I missed them completely... they were no doubt used to maximize the output power for PA applications.
From your answer it would seem that you consider the Allen model 75's and 90's to be nothing but a PA amp? It may have started out in its early years that way but I believe Allen spared no expense in decent transformers and components. After all an organ needs to function correctly and last a long time without failure. Unlike PA amps...organ amps need to be able to output full range and a lot of it. PA amps the tube amps were for the most part frequency limited. The 1570 Altec tube amps which I had a pair of were limited on the low end and not so great on the high end. But the 811's were pretty.
To be blunt here....I'll put my 75's against a lot higher priced amps. They may be heavy and somewhat ugly but they have quality transformers to go along with the somewhat dated circuitry that sounds great by the way.
Like I may have mentioned the 90's are supposed to sound even better.
Interesting thread. Since the Allen organ experts are here, how does the model 20 (6l6gc) compare to the 75 and 9?
I'm no expert. There are several on this forum that have commented on one of the other audio forums about the 90. Maybe they will comment.
The 20 is a good sounding amp with good iron in my opinion. I've seen them posted for sale at a decent price. I think it ll boils down to your speakers, room, accoustics, and your other gear. Everything in my opinion contributes to the sound. The 20 in my opinion again doesn't have what the 75 does but it will beat a lot of other amps with twice the power.
Not at all, 75's are considered by most to be hi-fi and a great effort from Allen, but the same could not be said of the 90's. 🙂
Low C on the pedals, a commonly used organ note, is going to consist largely of a 32Hz fundamental component for a 16' principal and diapason stops; it is often used close to full power. The engineers that designed this equipment, if it is purpose designed, would have known this as organ applications tend to be more demanding (and less ambiguous) as to the requirements for an intermodulation free bottom end. IM is very easy to detect and quite objectionable in organ music due to the duration and sustained nature bass notes. The screen regulation certainly speaks to this. I would expect core saturation to be high on the list of concerns. Many organs of the era were bi-amped, I don't know if that applies to this unit as well. What we don't know is how the top end was treated or will perform (e.g., transformer coupling and leakage inductance specs, etc.). It is a pentode amp and I would expect decent high end performance--even if it's not perfect. I would think these units are going to be capable of good performance, perhaps with some adaptation. The circuit looks decent and reasonably robust; a good starting point for an interesting project IMO.
It looks like the last schematic that I found has some modifications. The input tube and cathode resistors have been changed to a lower gain tube.
In 1958 they would call the repair man who'd pull the service literature, come out and ask you why you want to set the bias. Then after the discusion and he says, OK, you're payin' the bill, he flips her over and puts the meter on the point where the -35v is designated and then he turns the pot that's only accessable to a tech or the brave and curious, 'till he sees -35v.
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Thanks for the reply.
The underside picture of a 90 that I was looking at doesn't seem the have the bias pot shown in the schematic. I found what must be a newer version and it does have a control mounted underside. I was somewhat amazed that it doesn't have the balance circuit that the 75 does.
The underside picture of a 90 that I was looking at doesn't seem the have the bias pot shown in the schematic. I found what must be a newer version and it does have a control mounted underside. I was somewhat amazed that it doesn't have the balance circuit that the 75 does.
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