Just cleaned and recapped (all 12 electrolytics) an early '82 RD50 110 (black tolex) and a late '83 RD50 112 (blonde tolex/Mid Shift switch). Played through the same speakers, the 112's clean channel sounds bigger, cleaner, warmer and generally more pleasing. The 110 sounds a bit harsher and nasal. Assuming I didn't miss anything (always a possibility) were there any circuit changes made to the clean channel in the iterations of this amp? Or could tubes account for the difference (haven't tried swapping them yet)? Thanks.
Thu, 09/04/2014 - 13:37
#1
Tube swap can't hurt...
If they both use the same tubes - try a tube swap, it can't hurt. If the tone follows the tubes you'll know where the problem is.
Do the tubes need to be biased or are these models cathode biased? Probably something Lars or Mike can help you in determining.
Maybe the 110 is tired beyond the filter caps...I'm not sure what IC's these amps use, but I just did an IC swap (5 of 7) on my '79 and it brought the amp back to life.
Does the 110 sound bad, or just different? Different flavors are always nice :) I have a '79 212 Sixty-Five, and an '81 212 Seventy-Five that are essentially the same amp with a few component changes - including the power tubes. The '81 has a blackface fender sound (bell like - scooped mids - big lows) while the '79 has a clean sound with more mids/honk and less low end. Both sound great - just different.
Update...
Both use 6L6 power tubes, I swapped the used Svetlanas in the 110 for the fresh JJs in the 112. Amp is self-biasing.
Used the 110 at band rehearsal and...it sounded great! The old JBL E-110 speaker that came with it had a surprisingly smooth top end and plenty of bass, made the tone controls very usable for all kinds of voicings. Didn't need to get the Volume past 2 and it was fine in the mix (band is one guitar, bass, drums, keys and two vocalists).
My conclusion is that either this amp just prefers JJ tubes (or they are in better shape than the Svetlanas) or that the cap job I recently did needed to "burn in" if you believe in such things.
I also swapped out the Fender labeled Eminence 12" speaker (a Blues Jr standard 8 ohm/50 watt) in the 112 for an older red label Celestion Seventy 80, and it is a night-and-day improvement. I know these low end Celestions get a bad rap sometimes, but this one seems to work very well in this amp.
But back to my original question...
Were there circuit changes made to the clean channel over the production lifespan?
Not really
The 1650-RD circuit did change quite a bit over the years. However, these changes were almost exclusively implemented in the overdrive/limiter channel. The main output amp was changed once: The initial (before revision A) version has 27 ohm cathode(emitter) resistors (later versions has 6.8 ohms) and bias is provided from the +30V grid supply as opposed to a bootstrap resistor on the later ones.
Cheers,
Lars Verholt
Thanks...
...that was what I needed to know. I'll probably update the earlier RD50 to match the later one's Distortion channel.