More Sparkle in a Plexi build

[jack2911guitarblues]

  Hi to every one!
Super happy, finally finished my Tubedepot JTM-45 build, and also added in  a switchable Hot Switch mod which works great.. So now instead of having a Normal and a Bright input to mix in the Plexi mode, now you only use the 1 input and volume control, which is voiced to give a balanced bright and normal response. Vol. 2 is now used as a Master Vol.

All Good. This thing really roars, only thing I've notice is that I seem to have lost some of the original sparkle and treble in both modes.  Still sounds very good but I have to have the Treble up at max now.

Question :  can I regain that treble by changing the Cathode resistor and by-pass cap to different values?
I am using a 2.2k resistor, and a .68uf bypass cap as recommended at the moment on V1a, (whereas the original V1a used 820 ohms and 330 bypass cap, and V1b used 2.7k and  .68uf bypass cap)

And also I would like to perhaps cut the gain a little on the V1b (cascaded side) if possible.
I did get an earlier response but I was a little confused as to how to do that.
Thank you

[Glennjeff]

Hi Jack,

Would you have a circuit diagram of before and after ?

Glenn

[jack2911guitarblues]

Hi Glenn,

Thanks for your response.

Below is the link to the original schematic and layout of the the Plexi 50  (which is identical to my original TubeDepot JTM-45 plus schematic),  and also shows the Modded schematic and layout of the Plexi 50 which includes the Hotswitch Mod.     (cascade gain mod running input signal  V1 a + b in series )

That's the site where I originally got the schematic for the Mod from.

Cheers,   Jack

http://el34world.com/Hoffman/files/Hoffman_Plexi50.pdf

[ac427v]

Hi Jack,
I prefer the more sparkley sound that comes from keeping C4 (470 or 500pf) in parallel with R12 (470k). Did you remove that when doing the hotswitch mod?
Craig

[Glennjeff]

No worries jack,

You could try try lowering the cathode coupling capacitor from 0.022 to a lower value closer to the original bright channel. value of 0.0022 (C2 off pin 1 of mod schematic). This is the most obvious choice I can see.

You could also reduce the value of C3 similarly, experiment.

Also it might help to put a small value capacitor between the top of the mods volume pot ( R 10 ) and the wiper ( centre tap ) of that pot. Less that 1000 pF probably. ( 500 pF from original circuit )

Also a small value of capacitance across R12a in mod schematic ( less than 1000pF.  500 pF as per original circuit ).
( Look at R12b in original unmodified circuit )

That is the best I can suggest from basic electrical theory.


All the best
Glenn.

[jack2911guitarblues]

Hi to Glenn and Craig,

Had time last night to fire up the iron & do some surgery. This is what I found:  I put the C4 cap back in parallel with R12 as per original schem. That sounded really good, but ended up getting a horrible spattering sound/distortion when hitting the strings hard under full guitar volume. Specially on the high gain (cascade) setting. Back off guitar Vol and was back to normal.
I did take the cap out initially when doing the mod, as per schematic.  Other suggestion I tried was putting the bright cap back on Vol 1 as it was originally,  and then found that the cascade position got quite "hissy" if that makes sense.
Can the bright cap cause the hiss?
I have changed the original .022 cap (C2 off pin 1) to .0022 cap and that seems to have helped somewhat, so I'll get another cap and try that on C3 as well, as you suggested.
Could putting  back the C4 cap in parallel have caused the distortion/spattering?  I would have liked to keep that cap in there.
Anyway, thank you for help so far. Much appreciated.  Jack

[ac427v]

Hi Jack,
The C4a cap allows mids and high frequencies to get past R12a. If you had borderline distortion before...then returning it to the amp could cause excessive treble gain/distortion. In your first post, you had asked about reducing gain in V1b.  Hoffman did that somewhat by using the 2.2k cathode resistor with no cathode cap. Marshall reduces excessive gain and changes distortion quality on some models by using a 10k resistor instead of the 2.2k.  I prefer 4.7k there. Another option to reduce gain is to remove the C4b cap (.68) on R4b (820 ohms) but that may reduce sparkle too. This kind of tweaking is (almost free) perpetual amp-building obsession/entertainment!
I found that using the amp controls also took some relearning. I set bass on 1, treble on 3, and presence on 3. Then used the mid control to dial in my sound. Those settings are more extreme than I used on Fender style amps.
Have fun,
Craig

[jack2911guitarblues]

Hi Craig,

been away for 10 days.  Your comment about lowering "cascaded"  gain somewhat in V1b was interesting. so last night it was back into surgery for my plexi clone. I put a 4.7k as the cathode resistor,  and I put the C4a back in parallel with R12.
Result?  Sparkle is back again, and gain reduced somewhat:  Just what I wanted!   And, and I don't have any of that " spattery" sound in the distortion!  So happy days !  (until the next tweak).   By the way,  i was looking at  Top hat Emplexador schematic, and I noticed that it also used a 4.7k resistor on the cathode of V1b.  There's a ton of gain available anyway, so one can afford to drop the boost a little in that 2nd stage.  I think that a little less aggressive cascade solves quite a few problems.     By the way, would anyone out there have a proper schematic for the 50 watt Emplexador?  I have a hand drawn copy that is a little unclear.

thanks again to both you and Glenn, for the inspirations and help!     


Cheers,    Jack