1950's Allen S12 - Poor sound quality

[matthewb]

I spent some time with it today, mostly cleaning the outside - a damp rag worked great to get the dirt off the keys. I played it a bit too. I didn't see your post beforehand, JQA , but I will definitely check the fuses next time. Unfortunately I have another repair to make first - the audio amplifier appears to be a drop-in commercial product (see attached nameplate photo), and was plugged into a standard outlet in the side of the power supply. The lamp-cord style wire was as brittle as glass, and a bunch of insulation broke off of it when I unplugged it. I'll wire in a new cord end next time I'm over there, before I power it on.

It is a 25-note pedalboard. Unfortunately I forgot to get a good picture of it.

The gyrophonic projector has a bearing with a oil port on it, so I gave it a few drops of sewing machine oil. I noticed something very odd with it - it has two medium-size speakers on it (maybe six-inch drivers or so), but only one of them is wired. The other one is not connected to anything, and by all appearances never was. Would they really use a permanently disconnected speaker just to balance the spinning wheel?

I called Allen's number yesterday morning, but I think they were closed for Good Friday. I hope to try them again sometime this week.

Would the hum leakage be coming from the "main" power supply in the center of the console, or from somewhere in the amplifier chassis?

[JQA]

Ah yes, gotta love crumbling lamp cord. Thankfully, that kind of rubber insulation is almost never found on the electronic wiring within the instruments – I've only ever seen crumbling insulation within 1930s–40s radios so far. Anyway, a cord like that is easily replaced. Note that the strain relief bushing can be removed either with pliers (with some difficulty) or with a strain relief bushing tool such as this one.

Aha, it's a Webster amp – classic! I was mistaken about the nearby shoulder-style tube being the rectifier; it is one of the pair of 6L6G tubes used in the push-pull output stage. The large cylindrical tube to the left of the 6L6Gs is the rectifier, probably 5U4GB if I had to guess. The metal-can tube on the right is probably the preamp (maybe a 6SJ7 pentode? I can't help but to guess, please indulge me!), and the small glass tube beside it is probably the phase splitter (6SN7, final answer!) Note the additional fuse holder on the amp chassis.

As for the gyrophonic projector's unpowered speaker, I would not be surprised if it was added purely for balance – Allen was known never to spare a dollar where quality was concerned, so if they decided it was best to balance the wheel with a real yet unpowered speaker physically identical to the powered one, they would not think twice about the cost. As compared to the penny-pinching race-to-the-bottom products of today, it does seem extremely odd!

The source of the hum is likely in the amplifier chassis. There is a simple test you can perform to be sure: remove the organ signal plug from the amp and insert a shorted-out RCA plug (which could be made e.g. with a normal RCA cable and an alligator clip lead shorting out one end.) If the amp still hums badly, the problem is completely unrelated to the organ section. If the hum is eliminated, the problem is unrelated to the amplifier.

[myorgan]

With the unpowered speaker in the gyro, at least you have a spare, should anything happen to the one which is presently wired.;-) Fortunately, with those speakers it's uncommon for them to fail in any way–they just grow old and work forever until they die.

Michael

[matthewb]

I'm no expert, but I would be surprised if a speaker that's never been powered in its nearly 70 year life isn't dry-rotted or something. The functional one could well be better than the "spare" in that regard.

[matthewb]

Regarding the speaker, that is something else! Proof of the value of this instrument. I have a replacement cord (that I'll need to rip the ground pin out of :) ) in my car now. None of the other wiring seemed that fragile and I'm very glad you don't think it will be anywhere else!

That's a great idea to test the amplifier with. It will make my life much easier if the most crucial repairs for sound quality could be limited to subassemblies such as the amplifier. I was also able to borrow a small oscilloscope I hope to poke around with as well.

Allen Organ called me back on Monday, and sadly they no longer have records for this instrument. If anyone out there knows where I could find service manuals, circuit diagrams, etc. for this instrument or closely related ones please let me know.

[tucsondave]

I have the schematic for the S15 but the amplifier is different. If you list the tubes in your amp perhaps I can find another model organ in my service manual that used it.
I will then scan them and post them.

td

[matthewb]

Thank you! It will be a few days before I make it over there to examine them, but I will mention you when I get that info.

[myorgan]

I may also have the schematic for the S-12B. Let me look and see what I find.

Michael

[tucsondave]

Here's the S12 schematic and based on JQA's guess is a schematic for an ampliier.

[myorgan]

Yours is better than mine, Dave. Which amplifier model is the one you pictured?

According to the scan, the S-12B (Matthew's model according to the plate pictured) uses external speakers rather than internal.

Michael

[JQA]

Thanks for those schematics, Dave. They even confirm that the unpowered 8" speaker is an intentional counterbalance. By the way, my reading of Matthew's nameplate is "S12 S", so neither A nor B suffix. I'm guessing it predates both of those revisions, given that all the tubes look like octal types rather than miniature, yet the above schematics show the generator section using all miniature tubes (12AU7 instead of 6SN7/6SL7). The amp in the S12A schematic looks probably close to Matthew's, except with a 12AX7 miniature preamp tube instead of whatever metal-can octal tube is used. Honestly, the amp looks simple and conventional enough that both amplifier schematics provided should be "good enough". I'm not impressed by Allen's overly simplified generator and stop selection schematics, though; unless further info comes forth, it may be better to rely on Richard Dorf's diagrams.

[tucsondave]

I'm guessing the "S" suffix is for the sustain circuitry. Not mentioned in the S series schematics.
Allen's analog service manual mentions that the early Webster amps used what they called plate expression control instead of audio signal. I can find no schematic in the manual for such an amp. They say the difference is a shielded cable for the audio and three wires for the plate. Hence the three pin connector for expression.

The separate amplifier schematic is from the Organette model 44. There is no model number listed for the amp. Since the 6J5 can be a metal tube I'm thinking that might be the amp he has.

td

[matthewb]

Thank you so much, everyone! It is an S-12S, and it has sustain functionality, so that would make sense.

I'm hoping to get some time early next week to replace that cord, which will require taking the amplifier out. I'll take some pictures, and hopefully I'll be able to correlate them with some of these schematics.