I've been reading the same PI balancing thread as you Hywell.
I had originally fitted a 'balanced' tube from Watford Valves, think it was one of their standard Harma (JJ) type.
I changed it last week to a Harma Retro ECC83, not specified as balanced and I have to say that I noticed the difference and prefer this new valve without doing any more PI adjustment - my pot is pretty much at the midpoint.
I have read that some rave about the Long plate Sovtek in the PI position so I've bought one of those to try as well, again the non balanced version.
I found a 'dumble/two-rock' schematic somewhere that was hand drawn, and was apparently the innards of a two-rock custom reverb.
The preamp section, from what I could work out (terrible diagram) was similar to the OTS, but had 100k plate resistors...
What does increasing the voltage on the plates do for the tone?
as a side note, are there any resources for learning what mods do what? I've got experience in solid state amplification, more of a hifi background, so I do kinda know what I'm doing, but these terms like 'snubber' 'bleed' etc are all new to me.
Thanks in advance.
Changing the plate resistors will change the gain and tonal characteristics of the pre-amp. Lower plates = lower gain and more harmonic distortion. I can recommend that you read up at http://www.aikenamps.com/ or http://www.freewebs.com/valvewizard/
Sounds like you've been reading up at Ampgarage? That's where the two-rock schematic was posted and yes it was basically a Dumble #124 with built in reverb.
The term 'snubber' is not really used correctly but as installed in many Dumble derived amps it is used to describe the 2 capacitors that are attached to the base of V2. They filter out high frequencies to tame treble / smooth the overdrive sound. Increasing their value will give a less harsh overdrive (only impacts when overdrive is in use). The Two-Rock schematic in question used 470pF.
Treble bleed usually describes a capacitor that is placed across a Pot (volume control). It allows high frequencies to bypass the Pot when the POt is turned down (lower volumes) at higher volumes it has no impact. This makes for a brighter sound.
Sorry to revive this old thread but I too had the feeling that my OTS 50 was a tad on the dark side.
At home it sounds perfect but with the band at rehearshal or gig it lacks a bit of sparkle and the treble and presence are already at around 8. By experience I know that my Boogie F50 sounds bright and harsh at home so I turn the treble down but at practice or gig I'm lost in the mix and I bring those treble back up and lower the bass.
So today I bought some 68pf and 47pf caps and tried them, with aligator clips to be able to swap them easily.
68 pf was too much especially on the OD that got to fuzzy.
47pf was much better. With the treble and presence back to 6, the clean channel is chimier and by turning down the tone on the guitar I can get close to the tone without the cap. The OD channel gets a little edgier, somewhat more aggressive, the picking more so than the sustain. Overall the amp sounds a bit less mid heavy.
So I soldered the 47pf in the amp and closed it back. Let's see what it does at band practice. I might have a push pull MV pot put in there for more flexibility if I like the tone.
I use a 2x12 cab with Emi Swamp Thangs and those speakers aren't very bright but well balanced and open, I like those very much. As a comparison, playing on the G12-65 I used to bring the treble from 8 back to 6, and even lower with the Emi Tonker. The amp might be too bright for those now. Haven't tried yet.
Cheers Fabien
The higher you turn the master volume, the less impact the capacitor will have.
Personally I've never had any issues about switching between Clean / OD on my OTS. There again perhaps I'm not that demanding!!
I don't have a problem with balancing the relative levels and generally set the EQ for my OD sound, the clean takes care of itself. Occasionally I might turn the Treble up a bit higher to get a bit more sparkle on clean chords but all I do then is back the tone control off a bit on the guitar for OD sound.
If you use a Klein / C-Lator then it is quite feasible to put an EQ pedal in the loop, I have read that some other OTS amp users do this.
Remember also that the clean and OD sections are not separate. They cascade into each other. Therefore, the tone on your clean section affects the OD tone.
On my HRM BM this is true until I engage the PAB - then the tone stack is bypassed. This is one of those things that I'm not sure is true for all models or not - maybe someone knows? It's not mentioned in the manual. I was getting a bit frustrated with my drive channel until I discovered this - without the PAB engaged, I just couldn't find a really clear sounding overdrive - now I never use the drive channel without the PAB engaged.
All of the HRM type amps have 2 tonestacks, with the OD engaged your signal will pass through both unless you engage the PAB which bypasses the 1st tonestack giving you more gain but then allowing the 2nd stack to shape the sound.
Bluesmaster's have limited headroom because of the PI they use (Marshall/Fender PI),especially the 50w BM.
I don't see the BM breaking up @ around 2 o'clock. On my 100w amp the only thing that's bluesmaster spec is the phase inverter (The Clean tonestack is a Skyliner and the HRM is just standard HRM) and it breaks up (with the Master) @ 12 o'clock and the (Pre amp Vol) @ 6 o'clock. And the amp is pretty friggin loud @ that point. With those settings the amp feedbacks into itself (of course I use a D-lator as the Final Master too).
I recommend finding someone to mod your PI to Skyliner specs and keeping the BM tonestack if you don't dig the sound now and if your not getting enough headroom.
Some folks don't have headroom problems with the 50w OTS although...?
my BM50 starts to break up at 2 (as in 2/10), not 2 o'clock (and 3 o'clock with a quieter strat SC pickup). I run the master vol at 8 (3 o'clock) both OD at noon (i.e. 5/10) tbh, most of my amps do break up at 9 o'clock, I'm talking about breakup rather than overdrive. I run my Fuchs ODS30 at 10 o'clock, and that's very fendery clean there. I'm just wondering if anyone's tried different tubes in the BM50. I do like its sound (although it's a little dark, even with a Celestion Gold I'm setting the presence and treble very high, which is very unusual for me), so I'm not looking to mod it (other than to reverse the deep switch and lower the PAB gain)
Cheers Tone
That sort of breakup would be consistent with the BM Phase Inverter configuration. The 'standard' OTS PI has a bit more headroom....too much for many. Combine that with the fact that the Master Volume is Pre Phase Inverter and I was initially confused as to why I couldn't get any dirt out of my clean channel (standard OTS).
Cheers Pickmaster - I'm going to do something similar myself as my Head Cab / Speaker cab combination is far from stable when one is placed on top of other....I should have made my speaker cab a bit deeper.
Overtone Special is Skyliner hi-plate nonHRM don't know why this amp is considered as #124. I don't see it this way, like I said OTS is hi-plate amp and #124 has fender style values in there. Very smooth and warm sounding OD, lots of OD on tap but not as much as S&M, mid heavy cleans (in a good way). Amp that likes to sustain and compress. Easy amp to play... All knobs at noon and you have a sound to work with. I think this one is the warmest amp compared to others.
Overtone S&M (Sami and Marin) Special Skyliner nonHRM Overall brighter amp compared to standard OTS... clean tone is more or less the same, on the other hand OD is brighter, more aggressive sounding. Like OTS this amp likes to sustain. In my experience this amp is not designed to switch from clean to OD on the fly. You have to set the clean and stick with it... than go to OD, set the tone again and play that for a while. BTW this was the first D-style amp from Nik and it was his take on D-stayle amp... some said, it's to harsh sounding but some others liked. Sami and Marin didn't want to mod it (like OTS "jzucker" mods) and that's way it's called S&M model. I don't blame them they sound very good with this amp.
Now I don't have any experience with FM or 183 models... but looking at layouts, I can say that FM is as well as OTS hi-plate amp but has some things in there which will make the amp sound brighter (master cap, presence cap, smaller snubbers)... so I guess this amp is designed to be used with D-lator... without it it might be just to bright. 183 is again hi-plate amp but with some interesting turns... like very small subbers, no LNFB, different style PS. I really don't know what to say about this amp... but you heard TAGs Politician, what's more to say.
Like I said this is my take on these amp... I played the OTS and S&M and I hope I'll try the other two as well some day. ...but you all know this, tone is a personal thing and what is smooth and warm for me, it might be dark and dull for somebody else.
#183 features what is known as a "precision power supply" this is not a Dumble invention but a well established alternative way of delivering stable and filtered voltages to the various stages in the amp. I know where the #183 information came from and I have to say that I'm sceptical about the very small snubbers, in my opinion these amps need to have some kind of Low Pass filtering 'post overdrive' in order to tame the brightness / overtones. Either the Low Pass filtering is happening elsewhere or the snubbers should be larger. I know that there were debates on this subject over on TAG, what was written on the snubber caps, did it mean nF and not pF, were the snubbers actually measured 'out of circuit'......the only true test is to play one I guess.
I don't think that Locktite was the way to go on the voltage regulator. Heat sink compound (sticky white paste) would be more appropriate for something that gets hot - it helps to transfer heat from the component to the chassis. Other than that, nice looking build.
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