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Ken Morgan

Any RD50 Limiter Side Mods to Reduce Gain?

I'm seeking out a specific tone, one that can only be described as MM RD50 timbre with the ever-so-slight breakup of a Deluxe Reverb or cream faced Bandmaster on about 4...(make sense?) Anyhow, timbre wise, I'm there, of course, with my RD50 head and 2-10 cab stuffed with AlNoCo 10s, but the limiter side is still too distorted, even when gain is almost nil.

Any mods you folks know of/can do that can reduce the distortion but still deliver the 'growl' ? Thanks.

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Tube Gain

Since the Limiter Gain pot is controlling how much of the first tube stage signal is being fed to the second stage, there are three places you could try to reduce the circuit gain or levels. 

1. On the cathode of the first stage of V1 (Limiter), you could reduce the value of the C10 2uF electrolytic bypass cap to a smaller value to lower the tube gain, hence the signal level being fed to the gain control would smaller. 

2.  You lower the value of (or short across) R13 on the bottom end of the Gain control so its output would be "0" or closer to it with the pot set to minimum.

3. On the input grid of the second stage of V1 you could make the value of R14 (470k) smaller or stick a 500K 10-turn trim pot in there instead of the fixed resistor to experiment with controlling the sensitivity of the second stage buffer to the output of the gain pot.

There could be some tonal advantages to changing one over the other or some combination of these.  This is a little like "voicing" an amp and you may have to play with values and balances to achieve what you want without any side affects you don't want (hum, noise, etc.).




Ken Morgan

Thanks - will give those a

Thanks - will give those a try...


Feed the plate

If your RD-50 is an early version with the 4.7meg plate resistor for the second stage of the 12AX7 circuit, I can highly recommend experimenting with the value of this resistor.
Later versions made this resistor 2.2meg and the last version of the RD-50 amp (Rev E) had just 330k.
I just converted my own 'pet' RD-50 to the revision E circuit with 330k plate resistor and the cathode bypass circuit in the second stage added. It makes a ton of difference. The overdrive effect is a lot better and especially the low end is less muddy. The treble bleed cap on the gain pot is also a nice addition - somehow it makes the overdrive channel sound more 'music-man-y' if that makes sense :)

Lars Verholt

*the subject line references the overdrive circuit type used in the early versions of the RD-50 circuit - it's called 'starved plate'.

4-10HD130's picture

Lower Gain "Less Distortion"

Could you just simply replace the 12ax7 tube and put in a 12au7?

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