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Input impedance for cap-coupled "Ampeg" style rev tanks

7/19/2004 2:08 AM
Rob Mercure
Input impedance for cap-coupled "Ampeg" style rev tanks
I know I've got it "somewhere" in my notes but does anyone know "off the top of their head" what the input impedance is/was for the "Ampeg/Gibson" style capacitively coupled reverb tanks. My recollection is around 2K or so but I'll be damned if I can find my notes. My 1976 Ampeg service manual lists all of these units as "4C."  
7/19/2004 7:04 AM

a type 4C (Hammond organ style) has a 1.5K (~200 ohms DC) impedance. These are the 4 spring (looks like two) long tanks similar to Fender's save the impedance.
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7/19/2004 4:52 PM
Ray Ivers

The highest input Z I can find for an Accutronics tank is 1925 ohms, corresponding to an 'F' as the second designator (e.g. 4FB3A??, which would be a 4-spring pan with 1925 ohms input Z, 500 ohms output Z, long delay, and grounded input & output. Some Ampegs have a wire-form lock (2nd-to-last digit = 3) or no lock (1), and there are six pan orientations so I can't really help you with the last digit without more info.  
7/19/2004 5:02 PM
Mark Lavelle
Re: Input impedance for cap-coupled "Ampeg" style rev ta
"The highest input Z I can find for an Accutronics tank is 1925 ohms, corresponding to an 'F' as the second designator"
That's what I used in my Gemini I...
7/19/2004 6:19 PM
Rob Mercure

Ray, Mark, Dr.P, thanks for the responses. What I'm dong is "back-assward" engineering (wouldn't honor it with "reverse" engineering). A customer has an old Music Man 2275-130 and has asked me to see what it would take to convert it into an all tube amp. The existing pan is a 4FB3A3B Accutronics and I was trying to determine if I could adequately drive it with an Ampeg cap coupled type circuit - from Mark's reply it seems that this should work. A few years ago when I was creating my various "reverbs from hell" designs I crafted several cap coupled circuits that I really loved - by far my favorite reverb circuits.  
Loud clean amps aren't in much demand so I was thinking about eliminating the voltage doubler B+ circuit which would give me a 255 VAC secondary to rectify - switching to 6V6s from the existing 6CA7s should give me lotsa extra heater current so I was going to propose a single channel 40W-ish amp with reverb and lotsa transformer reserve - but you could still get almost 100W out of 6L6GCs at these voltages so I may consider switching options. He's already spent a few dollars with me on a 1964 Vibroverb "rebuild" and a YSB-1A and Magnatone "overhauls" so I can't "tap him out" too much :).  
7/20/2004 7:33 PM
Jack Koochin

Hi Rob!  
I'm still scratching my head about a JC-120 reverb problem, have no schematic, and was wondering if you could help. The DC resistance of the reverb tank (O.C. Electronics tank) output transducer is about 200 ohms, and the input transducer is about 1 ohm. I thought Ahaa!! ... burnt out transducer, but then I checked this against my Bandmaster tank, and it's about the same reading on both transducers... 1 ohm DC??  
Anyhow, as you might've read in my other thread, the JC-120 reverb works, but puts out a distorted signal. The Bandmaster reverb works fine.  
7/21/2004 7:10 AM
Carl Gigun

Here's a chart for normal resistance of reverb tanks relative to their AC impedance.  
-Carl Gigun
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