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Figuring out reverb tank swap on old Alamo amp

skimballc
November 19th, 2010, 07:39 PM
I have an Alamo Futura Reverb that I mentioned in the 'Let's See Your Alamo Amps' thread about a year ago, and I just started playing it again and have been thinking about trying another reverb tank in this thing. The stock tank, as verified by another member with same amp with the same tank, is an O.C. Electronics 'Cascade Reverberating Unit', type 251. That's all the specification it has besides a patent number. I measured the resistance and got 280 ohms at the input and 370 ohms at the output. This is a short tank, but it could easily fit a long one if I so desire, so it's the input/output impedance I need help figuring out.
What can you fellas tell me?

Wally
November 19th, 2010, 08:01 PM
Those ressitances are not in line with any tank that I know of. What is the problem with the tank?

skimballc
November 19th, 2010, 08:13 PM
Nothing much. It works, but I was thinking about experimenting with a long tank and seeing if I can get a better sound out of the reverb. I did a little research on the OC Electronics tanks and didn't find much, but did see a couple mentions of them being weird designs and not having a particularly good sound.
I understand it could just be the amp design, but reverb tanks are cheap enough to try swapping out.

Wally
November 19th, 2010, 08:30 PM
The resistance readings don't correspond to the impedance figures for these reverb tanks. IF you could get someone to give you some resistance readings off of these two tanks, one of them might work.
4FB3A1B---works for Ampegs. I just measured a tank in a vintage Ampeg Gemini II that reads 166 and 190 ohms.
9FB2A1C....which is a 9-spring long tank with higher impedances than the tank above.

I am thinking that there might not be a great deal of difference if you do find a tank with 'proper' impedances, but it could be worth a try.

skimballc
November 19th, 2010, 08:49 PM
Wally-
Thanks for the info. From what I see in the resistances shown at Vibroworld (http://www.vibroworld.com/parts/tech18.html) the closest relative measurements are 200 ohms for input and output.
I should say, I assume the reverb works fine. Its always been pretty weak and you can dime it and it's still not much. This particular amp has a solid state preamp section, so I figure the only thing I could do to play with the reverb is to try another tank. The back is riveted on so I haven't looked at the springs.

EDIT: popped the rivets and springs are fine.

I read a couple people online referring to Dick Dale amounts of reverb from these amps. This amp is not in that ballpark.

Wally
November 20th, 2010, 12:54 PM
FWIW, I have never heard a solid state reverb circuit that would make me spend money on a new tank. YMMV. The only reverb that I have heard that gets any good surf reverb going would be tube driven and recovered springs.

skimballc
November 20th, 2010, 03:08 PM
You're probably right, though this amp does have one 12ax7 whose role I am not too sure off. No schematics exist.
I don't expect surf levels of reverb, just curious if its capable of something more. This thing on ten is like any Fender on 1.5.

Wally
November 20th, 2010, 04:22 PM
simballc, do you get a big crashing sound if you tilt the amp a little bit and let it come back to position suddenly...or kick it real well? IF so, the return is working, and the drive section may have called it quits. In this situation, sometimtes the vibrations in the cab while you are playing can create a bit of return effect.

skimballc
November 22nd, 2010, 03:39 PM
Its not a BIG crashing sound like on other, all tube amps I have had, but it rumbles for sure when kicked. The reverb definitely does work.
I did also find that the hum in the reverb circuit that was greatly affecting the tone is coming from the footswitch, and simply unplugging it improved the reverb immensely.
I did get in touch with the company that used to make the Accutronics tanks here in the USA before they were sold to Belton. Their reply reads "I can tell you that the OC Electronics Cascade type 251 has a 1475 ohm input impedance and a 2250 ohm output impedance. It has a 3 second delay and the connectors are both grounded. This unit can be replaced with an Accutronics model 8FB3A1B or, if you have the space for a 17 long reverb tank you could go to a 9FB2A1B."
There is also a 4FB3A1B tank out there made by Mod. At least now I know what my options are.
My resistance measurements were way off from what they supposedly should be for this tank.

Wally
November 22nd, 2010, 04:37 PM
skim, as I mentioned, measuring the resistance of those inductors yield ohm figures that aremuch different form the impedance measurements.
Ex: typical FEnder tube reverb unit specs for the output are at 2250 ohms impedance. Multimeter resistance measurement will show that resistance at 250 ohms give or take.

skimballc
November 22nd, 2010, 04:42 PM
Yeah, I know they are just ballpark figures, but that's all I had to go on, and with my limited understanding of such things it only adds to my confusion.
Thanks for the help, Wally.

Wally
November 22nd, 2010, 04:55 PM
You are welcome. I only recently got a clue as to the big difference. I have always known that the FEnder tnaks I measured showed different resistance than the spec'd impedance figures, but I never worried about it because I knew what tanks to use. Thsi quesiton of yours made me think about it. I decided to give a rough figiure 10 tens above what I was measuring, and that is how I came up with that suggestion that the 4FB3A1B.
IT appears that the larger impedance specs yield a greater difference than do the smaller specs.

celeste
November 22nd, 2010, 09:12 PM
.......
IT appears that the larger impedance specs yield a greater difference than do the smaller specs.

That is correct Wally. Impedance is a combination of DC resistance, inductance and capacitance. In a coil of wire, which a reverb motor and receiver are, we can pretty much ignore capacitance. If we keep the wire gauge the same, DCR is going to be all about the length of the wire, so if you double the number of turns of wire you roughly double the DCR ( in reality, the DCR will be higher, because when you start wrapping the wire back on itself to keep the coil short, it takes more wire to make a complete turn) but with double the turns, inductance has increased 4x. So you see how an increase in DCR means a much larger increase in impedance.

Wally
November 23rd, 2010, 02:14 PM
Celeste, your knowledge is surpassed perhaps only by your willingness to share with us. I appreciate every bit of knowledge you expose to us here on the forum. Thank you.