23 posts / 0 new
Last post
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.

admin's picture

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'.


Revision E !!!

Just came here to say a BIG Thank you for the info in this thread. I updated my RD-50 head [Revision A] to Revision E specs and the difference is amazing. The limiter channel sounds better now! Thanks Lars, that 4.7 meg to 330k swap was the ticket.
I would still love to have a brighter limiter channel. If any body has an idea how to accomplish this I'm all ears.


Your take on the RD-50 mod

Hi Lars,

I know your post is ~ 7 years old but I'd like to ask you some questions about it if I may; I'm hoping to modify my RD-50. Specifically, I'd like to gain some more understanding of your RD-50 mod before trying it.

It appears that you made two (2) modifications to the limiter circuit (Is this Correct?).

The 1st modification was to change the plate resistor for the 2nd stage of the 12AX7 to a 330k ohm value (Is this correct?). I have the Revision D schematic in front of me and it is 2.2 Megs. On the schematic it is R11. You changed this 2.2 Meg R to a 330k R?

The 2nd modification was to add a cathode bypass circuit to the 2nd stage of the 12AX7 (Is this correct?).

Typical bypass caps are placed in parallel with resistors where both the C & R are connected in parallel between the cathode and ground; like on the 1st stage of the 12AX7. On the schematic there is no C or R attached to the cathode on the 2nd stage of the 12AX7; here, the cathode is grounded. What exactly did you do here? More specifically, did you add both a C and R here? If so, what values of C and R did you employ?

If there is something here I failed to mention or an additional important point you wish to make please inform me so I can learn.

I'm a big fan of MM amps having owned a 212 HD-130 since 1976. These amps are still, by my ears, great sounding amps!





Hey Cincy, yes thats correct, find the revision E schematic and you'll see those values.


In my previous post I stated that I still wanted the limiter channel brighter after changing to Rev E. However, the one change I failed to make was the 75pf bright cap on the gain pot. I've got my chassis out today as I'm putiing in Belton sockets and raised heater wires, so I will add that cap and report back!

4-10HD130's picture

Lower Gain "Less Distortion"

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


doesn't sound right

Tried that, and it didn't work for me. Its not a typical preamp tube but more like a tube overdrive pedal. To me, it sounded bland and lacking harmonic content with a lower gain tube in there.


gain pot bright cap

Ok,i put a 100pf cap across the gain pot and that was the missing ingredient.
Of course, its out of the circuit when the gain is full up, but for lower gain its just perfect, much closer to the clean channel, just a beefier version! I might even go higher, say 200/500pf next time I get her out, which won't be any time soon!




Thank you! After reading your posts I searched online and found a Rev E Schematic. When I get a break in the action from life (we all have been there) I will make those changes. It may be a while as things stand presently!

From what I have absorbed over the years, it appears that Bob W(? - I guess he disappeared???) made changes largely in the R and other values years ago, then Inertian modified/improved on them, then Lars took it in a different scaled-back direction by embracing the REV E MM modification that appeared to be in a similar but different direction as Bob and Inertian. And, as I can gather, all have expressed satisfaction with these various modifications. If I am missing something here please tell me. Regardless, all should be applauded for their efforts!

Thank you again!



That about covers it!

I tried the Bob W mods at one time, but prefered just the stock Rev E version. They got it right that time IMHO. [just don't forget the bright cap on the gain pot like I did]
Enjoy your LOUD, easy to carry gig amp!


PDF of RD-50 Revision E Schematic

Someone may wish to use this and/or add it to the great database of schematics that already exists on this forum! See attachment.


some #s and decimals are difficult to see


Another question regarding component values; hopefully this is an easy one.

I was looking closely at the Rev E schematic I secured online and I can't definitively make out the values of the elements including the 1st or 2nd stage cathode bypass circuit values of R or C. The margins are a bit fuzzy.

As best as I can tell the caps are polarized electrolytics and the resistors, including the 330k ohm plate resistor, are all 1/4 watt (Is this correct?):

The 1st 12AX7 stage R12 = 5.1k ohms (1/4 watt) and C10 = 2 microfarads, 25 volts. (Is this correct?)

The 2nd 12AX7 stage R (#?) = 3.3k ohms (1/4 watt)??? and C(#?) = 2 microfarads, 25 volts. (Is this correct?)

Just from looking at other plate and cathode resistor pairs in Fenders, Marshalls, Vox, etc, it appears that many designers have Rplate/Rcathode ratios in the neighborhood of 10:1 (with values chosen for both gain & high-pass filtering purposes - avoiding flubby bass responses). That is why I am guessing the 2nd stage R = 3.3k ohms??? Again, I can't make out the number very well so I am seeking some clarity.

So are these values correct according to your copy of the Rev E schematic?

Thank you very much, again, in advance.



Clearer Schematic

Hi Cincy, This site has a much clearer Rev E schematic in the Downloads and Resources page. Scroll to the bottom of the 1650-rd.pdf file to see. http://www.pacair.com/mmamps3/sites/default/files/docs/1650-rd.pdf Just sayin' and good luck. -mgriffin



your values are right, 1/4w resistors are fine. I used 2.2uf non polarized axial electrolytic caps.



C12 & MG155 --- Thanks Again! This site is a wonderful resource and the folks are incredibly helpful!


Nice to see it worked!

Hello all, sorry for the late reply. For some reason my email provider decided to send all forum posts to spam so I didn't know I was being pinged. Looks like you got your answers even so. As per my previous post, years ago I changed my Rev A limiter channel to Rev E, mostly because the original design was not very useful to me. I don't think Rev E is perfect, and that opinion is shared by many others judging by the sheer number of alternate designs and mods created over the years. Personally I think Rev E should be better balanced for volume against the clean channel (limiter channel is super loud) and I could use a little more overall gain for single coil guitar use.

Still, such a nice little amp that will cut through pretty much anything.

Lars Verholt


late reply

No problems Lars. I understand completely.

And thank you for your comment; maybe a different tube like a 5751 or 12ay7 may help with the volume difference? Or maybe add a boost pedal up front for the single coils?

FWIW, I have found that adding a compressor pedal improved the tone of the limiter channel's overdrive significantly.

When I get the time, I will mod it to Rev E and report back to this forum!

Again, thank you!

Polytone's picture

Hi all,

Hi all,
I uppgraded my rev D today (to E specs) with the 2 uF catode bypass cap in parallell with a 3.3 K resistor on the second gain stage of the limiter channel and also put a 100 pF cap on the gain pot. I am very pleased! Though, I can imagine that the swap of the 2.2 meg resistor (R11) to 330 K resistor would be even better. Is there a good way to change the resistor without unscrewing the board?

mm210's picture

Just clip the existing

Just clip the existing resistor right AT the resistor, stand up the ends and wrap the new resistor leads around it and solder. Or you can twist them a little and then solder for a little more friction. Done it thousands of time unless somebody was anal about what they LOOKED like. Mike.

Polytone's picture

Thanks Mike! Good to know it

Thanks Mike! Good to know it has been done before and worked out well.

Polytone's picture

I changed the 2.2 meg

I changed the 2.2 meg resistor to 330 K today (R11). It made the limiter side a bit stiffer then with the 2.2 meg resistor. I have to higher the gain knob to receive more overdrive now and the limiter side is now even louder then before. The one thing I have not changed to rev E yet is R9 (470 K to 560 K). Its the plate voltage to the first gain stage so I can imagine that the amp will be a little softer with that change, right? Do you think it makes a big difference? Is it worth pulling the chassi out one more time for that?

Log in or register to post comments