New JCM800 Mini Presence Issues.

[boostedsuby]

Hi all. This is my first tube amp build and my first post. I have built a JCM800 Mini based on Rob Robinette's drawings and I am using two EL34 power tubes. Everything has gone surprisingly well and it sounds amazing where I have it set, but the range is narrow. I realizing some voicing may need done. Still there are a few issues that I don't understand. Possibly the issues are related? The presence, between 0 and 5 is very screechy. The further I go past 5, the better it sounds. I leave it at 10 and it sounds great, but I have to roll the treble off to 2 or and the mid to 3 or so to get good tone. Beyond those points, it is very trebly and harsh. Does anyone have any thoughts? I was going to switch the plate leads on the OT, but I'm not sure that is my issue.

[thetragichero]

screechy would lead me to trying to swap the ot leads

[boostedsuby]

Thanks for the reply. Being a new builder, I am filled with questions and uncertainty. I swapped the leads this morning and the amp it much more stable. If I back the presence off though, I still get these harsh spikes. They seem to occur during the attack if I am really digging in. I am going to start making adjustments to the voicing though. It is terribly bright with everything on 5 and this is with the bright cap removed. The low input has been removed from this circuit as well as the treble peaker on the input, which I replaced with a 68K resistor.

[thetragichero]

what is the bias set at? have you confirmed that everything is the proper value? it's easy to pick up a 2.2nf (222) cap thinking it's a 22nf (223). some brands of resistors the orange and yellow (or red and orange) third band are nearly identical

[xtian]

Post a link to the schematic you used. I loved my JCM 800, but I took out the bright cap AND the treble peaker. These amps were made to BITE through the mix, but I prefer a rounder sound.

[boostedsuby]

Sure, and thanks for your comments. https://robrobinette.com/RR2104_Master_ ... JCM800_6V6. Somehow, I mistakenly used 180K plate load resistors. I replaced them with the appropriate 100K. This cleared up a lot of the fizz and other issues that I was hearing. It took at lot of gain away though With all knobs set at 5, I get a usable tone, but the get it really to my liking, mids have to be cut back to 3 and no more than five. It starts to sound harsh and brittle with mids or highs above this point. I think maybe now that I am really at a point where I just need to work with tone stack values to get where I want to be? Bass is slightly farty past 6 or 7. I would like to tighten in up slightly. So now I am an addict. I really dont even care to build guitars anymore. Amps are far more fun.

[boostedsuby]

xtain,
As noted above, I removed the peaker and brite cap as well. Also remove the low input. I have never used them. I put a 68k resistor in place of the peaker. Is there a better option there?

[xtian]

There are a few smart folks here who have all kinds of "mods" for Marshalls. I will just point out that it's common for folks to run these 800s with Treble and Presence controls set to zero. So, twist the knobs and trust your ears.

[Littlewyan]

What are you after with this amp? Did you specifically want the JCM800 tone? Or do you simply want a Marshall tone at lower volumes? As I can give you some mods that will give you a nice Marshall tone at lower volumes, but you'll be moving away from the JCM800 sound (Which I've tried and same as you're describing now, I wasn't happy with).

[boostedsuby]

I guess that what I am trying to accomplish is the get the the tone stack set to a point where from zero to ten, it is at least musical. That said, I would welcome any mod suggestions, even if they move away from the JCM800 sound. Ultimately, this amp is a keeper for me, both because it is my first and because it nails the Billy Duffy sound on Electric. But, as I have already started on another one I would like to explore mods, especially darker and gainier ones. Thanks for all the input.

[Littlewyan]

Do you crank the master volume? Or do you keep it below 5? I’m asking as this kinda dicates where we add gain etc

[boostedsuby]

I hover around 5 at home, but I do spend some considerable time below that when wifey is doing her work form home.

[Littlewyan]

Ah we’re definitely talking just preamp distortion here then. Going by the original schematic, try removing the second 470pf treble peaker cap from the 470k/470k voltage divider and put back the 470K/470pf config on the first stage.

To add gain, bypass the final stage 820Ohm cathode resistor with a 0.68uf cap.

These are just starting points, but generally I think it’s better to cut bass early and cut treble later when increasing gain. Let me know how you get on as there’s plenty more we can do after this to add gain and keep the treble/bass under control.

[boostedsuby]

Been away for a few days, but I will try these and let you know. Incidentally, is there is there a theoretical reason for the bass early-treble later approach or is it something you have learned from experience?

[Littlewyan]

Learnt from experience. If you look at various designs you'll see they do the same thing. Usually you have to cut treble because so many harmonics get created that the tone becomes fizzy. If you let too much bass through then again, so many harmonics that the tone becomes boomy or farty.