Improve RD-50 Reverb?

inertian

Recently acquired a very clean RD-50 head, dates from 12/83.  Has the Mid Shift toggle in place of the Clean/Limiter of the earlier models.  All original caps, no signs of leaking, and the amp is super quiet at idle.  Sounds great with no extraneous noises.

Reverb was dead on it, tank was shot.  Correct replacment (MOD brand) connected, Reverb now works, sounds good but not strong. 

Cable between chassis and tank checks out fine, as do the components (resistors and caps) and grounds around IC-3.  Swapped in a different IC-3 opamp, helped quite a bit. However, set to 10 (full up) Reverb is where I'd expect it to be at about a setting of 5.  Below 5 Reverb is barely audible, at 3 it is gone.

Plan to do a full recap, until then is there anything else to check/replace to make the Reverb stronger?  

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Music Man Equipment
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mm210 Fri, 04/10/2026 - 04:48

Just curious if the transformers on tank measure the same as the old one or was it just SPEC'd the same? Also, I assume you looked at the little transistors driving that end of IC3 (ztx109's). Not much else there other than pot and caps. 

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Ian Azrikan

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12 years

inertian Fri, 04/10/2026 - 07:03

I've used the MOD tanks in a number of RD50s with good results.  Tried two of them I had on hand, both sounded the same.

Maybe test/replace the ZTX109s?

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Music Man Equipment
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mm210 Fri, 04/10/2026 - 15:45

That's where I'd look first. There's really nothing else around to screw with it. C20 could be shorting to ground early, I suppose as could c28. C27 is tiny so I doubt it there. You did check to make sure the + and - 16v is good didn't you? Also, unplugged the footswitch? 

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Ian Azrikan

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inertian Sat, 04/11/2026 - 13:34

Gonna go double check everything.

What's a recommended replacement for the (largely extinct) ZTX109?  I have searched online, found lots of opinions  Have some 2N3391s on hand, would they be sufficient for testing purposes?

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Ian Azrikan

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12 years

inertian Sun, 04/12/2026 - 11:28

Thanks for that.  Only place I found any ZTX branded items was on eBay for way too much $.

Ran a ZTX109 substitution search on AllTransistors.com and got this:

https://alltransistors.com/crsearch.php?mat=Si&struct=NPN&pc=0.3&ucb=45&uce=30&ueb=5&ic=0.1&tj=125&ft=150&cc=5&hfe=240&caps=TO92&cf=on

Also, going through some saved notes from the forum found others had used NTE 199 and BC550C with good success.

Going to track down some of these and if the full recap doesn't fix the problem, will install them.

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