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Re: Odd reverb tank, is it usable?

12/30/2004 4:28 PM
Mark Lavelle
Re: Odd reverb tank, is it usable?
"The power for this modules seems to need B+, Screens and preamp, ground and two wires for the heaters. There are five wires on a Molex like plug that I have only one end of. [...] The tube complement is a 12AX7 and 2 ECL82's."
Two of the pins will be 6.3VAC for the heaters (no duh, eh?), which should be easy enough to figure out. The "B+1" would have been very near 300VDC, straight to the CT of the OT, and ground shouldn't be hard to find, either.  
That leaves the supplies for the screens (B+2) and the 12AX7 (B+3): You're going to have to figure out if B+2 is derived from B+1 via an RC filter or if the B+3 is derived from the B+2, making the last wire B+3 or B+2, respectively.  
In the closest Hammond schematics I've got ("AO35", which uses 2 12AX7s and 2 EL84s), the screens are at 290V and the preamp plates get 240V.  
Hope that helps...
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12/30/2004 8:31 PM
Dr. Photon

That's a hammond amplifier that I'm no aware of, although I do own two hammond amplifiers that I have not seen anywhere else.  
I have two hammond tanks, one with a 1.6K input (200ohms DCR) and annother with a 8 ohm input (lesss than an ohm DCR). The DC resistance of these things is muh less than the impedance....pretty inductive I'd guess. the output impedance of all hammond/accutronics (they're related, along with Gibbs, some offshoot company or somthing, I forgot) is the same, at around 1.5K (160-something DCR). I'm not sure what your input impedance is, but you should be able to kinda-sorta match it up with the DC-resistance on the table of values of accutronics tanks. either a solid state low power amp (like an LM386, or several parallel op-amps with some isolation R's like in some marshall amps) or a fender transformer-driven tube diver (you ain't gonna drive that low impedance tank with a capacitor coupled tube stage).  
It's not a stereo tank per se, it's wired up more like a dual voice coil subwoofer would be connected to a stereo system - two coils for left and right in the same system that sum together into a mono signal in the transducer. It's not center tapped, it's essentially two windings that have their common connected.
12/30/2004 10:51 PM
clint b
Thanks for all the info.  
It is a motorola console stereo and it is missing the main amp. It uses B+1 to the center of the output transformer primary and B+2 for the screens then an extra filter section for the preamp plates. There is only one, two terminal, terminal strip in it. It is very simple and elegant inside without any extra bull crap. 3 sockets and tubes, 16 resistors, 6 capacitors, 1 transformer, 2 jacks, and screw terminals for the speakers. Oh, and the power plug. I'm thinking... get a power supply and speaker on this thing and see what happens.  
And the original Jensen P15RJ speaker that came with this thing sounds great in its new cabinet! (It wasn't connected to this small amp...)  
1/1/2005 8:06 PM
Dr. Photon

The amp should work fine as a guitar amp with just a little reworking. the output level from a reverb tank is a little weaker than the average guitar output. I built an amp with a Hammond AO-35 reverb amp (2 6BQ5, 2 12AX7, and a 5Y3). Actually the first tube stage is spec'd as being a ECC83/7027 and the second is "just" a 12AX7 - even had two different brands of tubes in it (remarked GE and Mullards? factory orignal, the first one was lower in microiphonics than usual). Even had some special precision low noise resistors instead of carbon comps in the first gain stage. doesn't have a gain control. After cutting out the 10K resistor and cap in th input, it sounds awsome on guitar and bass, although it'll easily ovedrive into a nice crunch with the instrument's volume on half. I thought about adding a gain controll, but I was much happier just sticking some lower gain 12AU7 and 12BH7's in there and leaving the damn thing alone (and added an L-pad to the output for crunch at sane volume levels!).  
Your ECL86/6GW8 tubes have a triode stage in them (that's vaguely 12AX7-ish) and a power pentode in them (kinda like a weaker EL84/6BQ5). they prolly used the triodes in these tubes as the PI and the 12AX7 as the preamp/voltage gain.  
For you power supplies, the main B+ to the output transformer is probabally 250 or 300 volts, and you can probabally just use a 1K resistor and a cap to produce the screen/everything else voltage
1/4/2005 12:31 PM
clint b Reverb tank update
Well I decided to hook the reverb tank up to an amp and see what happened. The results are mixed but promissing. The output side seems to be working just fine. A tap on the tank and the normal clanging noise fills the room! The input side needs a little work... I only hooked one side of the input to the extra output jack on my amp. It is trying to work. I get some reverb and the light bulb glows with the sound when the guitar is played. The main problem is with feed back. When I turn up the channel the reverb is on it goes into feedback. It is a high frequency feedback, much above the normal guitar frequencies. I think if I could filter off those high frequencies it would work just fine. Maybe I could put a volume pot and tone stack on the input to control this thing. I run the output into the second channel on the amp so it has a volume control.  
I'm thinking maybe a 500k pot as a variable resistor in series with a 1 nf capacitor to ground after a volume pot wired as a variable resistor.  
Does this sound doable? Are my values way off?  
1/4/2005 1:04 PM
Mark Lavelle

"The main problem is with feed back."
Very likely a "mechanical" problem. Where was the tank in relation to the speaker, amp, etc.? Have you tried moving it away from the amp & providing some sort of shock absorbers?
1/4/2005 5:42 PM
Dr. Photon

Did you plug the input of the reverb assembly into the amplifier's reverb tank output, or sis you plug it up in parallel with the speaker output. Since if you wire it up in parallel with the speaker. the later is known as "foldback reverb", and it only works for a small amount of reverb (otherwise it will feedback, since it reverbs the reverbs and the reverbs). I would avoid this connection and instead wire it up like a regular reverb tank (and possibly ditch the resistor/light bulb stuff, but that shouldn't cause your feedback).  
If you are using it in the way hammond intended by powering the tank from annother amp's speaker output through the lightbulb/resistor circuit and then using the SEPERATE amplifier that it came with along with it's own speaker cabinet, you should not be getting any feedback unless it is a mechanical issue.  
Either use the tank as hammond intended with a seperate amp/speakers for the reverb, or connect it as a standard reverb tank. plugging it into a single amp's speaker output and then feeding it's reverb output back into the same amp is asking for feedback at any reverb levels other than "very subtle"

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