Thanks, really appreciate the input and suggestions. I'm not trying to keep the amp close to original. Just want it to be working and useful.

Some comments/questions on your suggestions:

1. Why so important to keep the power resistors so close to PCB/power transistors? Will moving them 10" away, with robust conductors, really cause a problem?

2. The 4 output transistors mounted on the chassis (2N3055) don't seem to get very hot, so not sure if I need to move them. (It's the 40410 transistor and the 2 power resistors that seem to be generating most of the heat.)

3. What's the advantage of putting a speed control on vent fan? Maybe just a 120v disc thermostat?

Yessir you are correct, the 134 has a cabinet with 4 x 10" Eminence speakers.

If you know Acoustics well, you'll def see some mods here (pic attached) – vents and a small fan installed, also the 10w resistors on the power PC board have been changed to 25w chassis-mount. All to try and dissipate some heat. Still need to find a new place for the filter cap, but should be easy. The newer ones aren't as big as a farm silo   

From what I've read these amps tend to run pretty hot, and a real design flaw (in my opinion) is there's almost no way for them to cool - there's no venting or means to breathe AT ALL once the head is installed in the cabinet.

From what I can tell (might be wrong) it appears the OEM reverb tank was a small one mounted inside the head. I suppose a 17" tank could be shoehorned in here (the inside width is about 23"), but it would probably be touching the main transformer and be very close to Q302 (40410), both of which seem to run hot (especially Q302). So I'd rather defer to a smaller tank to allow as much breathing room as possible. Having said that, I like your suggestion of mounting the tank outside the head in the cabinet; might consider it. I have a modest background in electronics but am not real familiar with guitar amps & reverb tanks.

This unit has been neglected and abused over the years, and clearly others have worked on it. It hasn't worked in 15-20 years; the drive transformer was blown and after searching for one unsuccessfully for years (as it sat in my attic), I decided to just rewind the bad one. I've already had it up and running so I think that was a [surprising] success. It def needs some TLC, and I really hope to get it working for good. These amps apparently competed with the Marshalls back in the day.

Just finishing up a recap; we'll see how it goes after that. Still need to see if I can get some solid hours of running time without something burning up before I invest in a reverb tank. If it seems to run really hot I may be back here looking for some additional ideas and help ...    

(The data sheet on the Q40410 says it can operate up to 200 C. It's running well below that at around 110 C. Still, that's pretty hot. Not sure if that's normal for this amp or if there's another issue somewhere. I'm concerned that kind of heat can damage nearby components on the PCB.)

My next challenge is the physical size. The 4 series tanks are 17" long, which will not fit in this head. I probably need an 8 series, but there's nothing offered in that series which exactly matches the impedances of the 4FB (1475 ohms in, 2250 out)

8E series is 800 ohms in, 2575 out
8F series is 1925 ohms in, 2575 out

Do you think either of these will work? 

Thanks everyone for the replies!

The Acoustic 134 is a solid state amp so I'm not sure how much benefit there would be comparing its circuitry to a tube amp.

The reverb tank is horizontal mount, open side up, so according to the current numbering system wouldn't a compatible replacement need to end (7th character) with the letter "A"?

I have the schematic for the 134 and the reverb driver circuit is exactly the same as what's shown for the 150. (I've read that the 134 and 150 were the same head with different cabinets.)

Sooo ... the 4EB2C1B is an interesting suggestion. Can this be mounted either way? – Open side up or open side down?

I'm repairing / refurbishing a 1970s Acoustic 134 guitar amp. It's missing the spring reverb tank (someone obviously removed it some time in the past) and I'd like to install a new one but I'm at a loss as to what the proper specs are or how to find them.

The service manual/schematic does not provide any specs; the only information listed there is Acoustic part # 315006 (which has proved useless so far). I have contacted several online companies that deal with vintage equipment and/or reverb tanks (Studio Sound Electronics, Accutronics, Model Electronics, Tubesandmore, Ruby Reverb, etc.) and no one has been able to find specs or suggest a proper replacement.

There are 4 mounting holes in the head chassis for the tank - long dimension between holes is 222mm (8-3/4"), short dimension between holes is 99 mm (3-7/8"). I'm told this makes it an "8 series tank", but that just indicates the physical size. Need to know input and output impedance and grounding arrangements.

Anyone have a suggestion? I guess ideally I would need someone with a 134 to take a look at the reverb tank and see if there's any numbers on it. Or, tell me (1) what is the DC resistance reading across the input and output jacks? and (2) is the outside ring of the RCA jack tied to the reverb case on both input and output jacks? With that info I could probably identify a compatible replacement. Thanks for any help.