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RI Bluesbreaker mods...

5/28/2000 1:55 PM
Ian Anderson
RI Bluesbreaker mods...
I’ve just got a used RI bluesbreaker (finally) and am well impressed with it. I know there have been posts about all this, but I want to know specifics of why/what these mods do.  
I’ve already got some Mullard ECC83’s lying about that should do nicely in there.  
* I’m ordering some ‘square-top’ Tesla EL34s (the guy said these have more gain than the regular JJ ones – do they?). I know that the bias circuit needs some resistor values changing and pins 1 & 8 ties together – anyone have the specifics? (I’m going with EL34’s because I’m looking for angus’ tone more than clapton’s). How should I bias these – around 35ma okay?  
* The rectifier – my tech measured 410V on it and said replacing it will have no effect on tone at all and all that happens with rectifiers is that they get more saggy with age. The one in there is quite small with no markings or lettering.  
* what’s the improvement (or otherwise) of the cap mods? My tech said it would just introduce more hum.  
* My tech did say that upgrading the caps on the board and some of the ¼ watt resistors would give a ‘slight’ (debatable) improvement in tone – anyone have the specifics of what to change and to what type? The caps my tech had were much too big physically to fit neatly on the board.  
* Anyone tried changing the cap values on the volume pots? – from the schem they both appear to be 100pF. I’m going for more gain and aggressive sound, so anything in that direction would be great.  
Finally, I am really happy with the stock sound but just want to experiment a bit with getting it 100% there.  
5/28/2000 5:47 PM
Ray Ivers
RI Bluesbreaker mods
Here are a few mods for more gain and less 'mud' from your reissue. You did say you want a more aggressive sound, right? The schematic is available at I'm not going to change the non-master nature of the amp, or (hopefully) the operation of the tremolo channel. I would hold off on the rectifier, power supply caps, complete PCB resistor/cap replacements, and changing the 100pF volume-control cap. This cap should have little effect above 4 or 5 on the volume control when you're cranking the amp for overdrive, and can add a nice hi-treble boost with the control turned down for a clean sound - even 50 pF will work here, and I personally prefer no cap at all to avoid getting 'locked in' to using only a limited range of the control.  
Power amp: change the 470 ohm, 5W screen resistors to 1K, 5W; change R4 (15k resistor) to get about -40 volts bias at pin 5 of the output tubes at midpoint of the bias control rotation - I'm going to guess this value will turn out to be somewhere between 18K and 27K; change C12 / C13 driver tube coupling caps to .022 uF or even .01 to reduce possible blocking distortion and muddiness; change R25 feedback resistor to 47K (a little more power-amp gain and bottom end); change R33 to 91K (possible better phase-inverter balance). Why not start at 25 mA bias current per tube and go up from there? You might also experiment with connecting the feedback wire to different OT secondary impedance taps - stock is 16 ohms for the 6L6's with a 27K feedback resistor, but with the tube change, try them all and see which you like best (presence control operation will be affected quite a bit).  
Preamp/tone stack: change C3 to a 2.2 uF cap (less mud and pre-overdrive bass); change R32 to 150K (more gain); change C5 to .0033 uF (less muddiness); change C6 to 680 pF or .001 uF (slightly more mid boost - 100 pF here will accentuate pick attack); change C7 to 470 pF (shift treble control range downward in frequency), change C8 to .005 uF (shift bass control range up); bypass R15 with a .47 uF cap (more gain); change R15 to 2.7K and change the plate resistor of this stage (no schematic designation) to 270K (!), which will keep the bias conditions of the cathode follower the same as stock while increasing gain.  
Except for the power-tube substitution mods, which of course should be done as a set, I would suggest trying these modifications one at a time - all of them at once will greatly change the tone/noise level/stability of your amp, and you said you were pretty much happy with it as is. The second gain stage plate resistor/cathode resistor swaps should also be done together. I would start with the power amp mods, and then the preamp mods in the order presented. Let me know if you have any questions about getting started.  
Ray Ivers
5/28/2000 6:36 PM
Peter S

The mods suggested here by Ray are excellent ones if you are looking for more gain/aggressive sound. I have a few amps that have most of these mods done to them and let me tell ya' these beasts are fire breathing monsters. There is only one thing that I would do differently(keep in mind thsi all a matter of personal preference). I would remove c-6 altogether, instead of increasinng it's value.  
as far as the filter cap changes you referred to, I think what you are talking about is some posts where the point was to make these amps as close to the original as possible. To do this, in part, you have to lower the filter caps values to 32+32 and 16+16......this will give the amp a bit more sag at high volume and make it sing a bit more. This would be going more towards the original tone of the amp(closer to the so called"vintage" sound of the originals), and in the opposite direction from the more aggressive, and IMHO, more modern sounding mods Ray suggested. As far as lower the filter cap value contributing to more hum, you wont hear any more hum.....the caps that are in there now are a slight bit of overkill. The original circuit that this amp was copied from back in '62, the '59 bassman used an 8uf cap in the preamp B+ and even these amps dont hum too much. That said, you should leave the 50uf caps in the power supply if you are going for the "more aggressive" mods.  
5/28/2000 9:19 PM
Re: RI Bluesbreaker mods...
"anyone have the specifics? (I’m going with EL34’s because I’m looking for angus’ tone more than clapton’s). How should I bias these – around 35ma okay?"
You would need to go with KT66's for those tones you mentioned. They sound amazing and are very cool power tubes!!  
5/29/2000 1:42 PM
Ian Anderson

Thanks for everyone's respones - very informative. I'm thinking of modding the normal channel to the bright channel specs and modding the bright channel for more dirt and agressive tones (think Iggy Pop's 'Raw Power' LP and Angus at his most dirty)  
In regard to the KT66's v EL34's debate, for what I want, everyone (almost) suggests EL34's for that classic AC/DC sound with a more aggressive crunch and KT66's for a smoother overdrive and fat midrange.  
As far as this amp sounding muddy - mine doesn't, maybe I got a good one!  
Cheers all... Ian
5/31/2000 10:34 PM

"In regard to the KT66's v EL34's debate, for what I want, everyone (almost) suggests EL34's for that classic AC/DC sound"
That's funny... all the interview I've read with Angus's amp tech say he uses KT66's.
5/31/2000 11:02 PM

"That's funny... all the interview I've read with Angus's amp tech say he uses KT66's. "
KT66's are tubes he used for the "Back In Black" album.  

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