[sdiy] Arp 2600 replacement op amps

[Dave Manley]

Another reverb rework suggestion from the archives to increase the drive 
and cut the receive gain:

"...replace R276 (220k) w/ a 1M metal film, and replace R288
(2.2M) w/ a 560k metal film."

The full post is here (along with more op-amp discussion): 
http://search.retrosynth.com/synth-diy/search/lookit.cgi?-v9701.188

You can also change out the reverb tank for a longer 6-spring model.  
The reverb in my 2600 is model Accutronics "1AB2A1B"
This translates to: Type 1 (2 spring, 9.25") , 8 ohm input, 2250 ohm 
output, medium delay, input grounded, no lock, horizontal mount down.  I 
see some mounting studs on the back of my 2600 case that makes it look 
like Arp sometimes used vertical mount reverbs so you may have a 
different unit.  See:  http://www.accutronicsreverb.com/prodspec.htm for 
how the part numbers are decoded.  There is also other useful info on 
the Accutronics web site - schematics for drive/receive circuits, etc.

You could upgrade to a "9AB2A1B" which is a 6-spring 16.75", otherwise 
identical unit for about $25 at http://www.tubesandmore.com/.  Search on 
their webpage for "Accutronics". 

Accutronics has three different delay ranges available: short 1.2 to 2 
seconds, medium 1.75 to 3 seconds, and long 2.75 to 4 seconds.  I'd be 
interested to hear if anyone has a 'long' delay model and what they 
think of it.    The similar long delay unit is: 9AB3C1B.  Note the input 
on this is insulated.  If you plug this in you will get no output since 
the input has no ground reference - you'll need to add a ground wire ( I 
ran into this with an old surplus reverb tank I bought 20 years ago and 
never used).

-Dave


Michael Bacich wrote:
>
> BTW, you can greatly improve the signal to noise of the 2600's  reverb 
> section by simply increasing the gain of the reverb send amplifier.  
> This will increase the volume of the reverb, which will allow you to 
> lower the reverb noise level by lowering the reverb return level.  
> Here's how:
>
> Try decreasing the value of R273 on the 4-2 board.  It (in conjunction 
> with various other parts) determines the gain of the reverb spring 
> driver.  Right now, it's 100K -- you could roughly double the gain by 
> replacing with a 47K (or by simply paralelling another 100K resistor 
> across it).  I would probably not go much lower than 47K, however, as 
> doing so may cause audible distortion or unwanted artifacts in the 
> reverb.