I've had an RD-112 65 for a couple of years now. The end goal has been to resell it as I got it as a fixer-upper. You've probably seen my posts with the pictures of how I restored the cabinet with my father-inlaw. Anyway, I've since played a few shows with the amp but never thought it sounded quite as good as my pet Sixty-Five 112 (but then again, nothing sounds as good as your pet amp, rite?). A couple of times I'd been disappointed about the lack of headroom with the amp. You could only go so far and it would break up very un-musically. Eventually a friend of mine said his brother might be interested in it, so I let him grab it and take it to his home town for the brother to check it out. I got it back with a note 'he wants to buy it if you can make it stop crapping out at higher levels'.
After more testing and swapping out the clamp diode/capacitor pairs on the 6L6s, to no avail, I had to relent and swap the driver transistors. Until now I've been of the opinion that if they work they work - until they stop working. In spite of this, I put a fresh matched set of 2N6488s in. After setting the bias (I had to turn the pot down quite a bit compared to the original 1692s), I hooked up the speaker and reverb tank and tested with a Stratocaster. Wow - what a difference. The amp somehow seems rejuvenated and has much more punch and clarity (more like my pet Sixty-Five 112...).
Lesson learned - and really, those transistors are very inexpensive. While it's not entirely trivial to replace the transistors it's not rocket science either.
PS - I thougt I'd share that I found a suitable (imo) replacement for the non-obtainable Motorola MR-250-5 diodes used as voltage clamps in the output section. It's a Vishay DO204AL - it's not 5kV as the old one, it's 'only' 4kV, but I'm ok with that. Current rating is .25A like the old Motorola part.
Cheers,
Lars Verholt
Thanks for the info, Lars. I
Thanks for the info, Lars. I used 3KV diodes when I went through my amp, I'll try to get some of the 4KV that you listed.
New Driver Transistors Experience
Restored an early (11/80) RD50 110 about 1.5 years ago, Terry Loose was very generous with his time and I got quite the education. One thing he suggested I do was replace the original 1692 driver transistors.with new 2N6488s (sourced from Mouser). Although they were not contributing to the amp's issues (low/distorted volume, fried resistor) the new drivers definitely provided more volume at a lower setting. However... the new drivers had shorter "legs", so I had to mount them standing up, and not bent over and screwed down to the aluminum rail like the originals. Was concerned about heat dissipation, so I reinstalled the original 1692s with the intention of cutting off the transistor part and soldering the new driver legs to the old longer legs. But once the amp was repaired (new Zener diode) it sounded fine, so I left the original drivers in place.