18 watt with JCM800 preamp Questions

[jdbenge]

Sorry for the long post. Summary: High input sounds out of phase on my build.

Hi all. I have been lurking this excellent forum for quite a while. I haven't posted yet because I've found the answers to all my previous questions up till now.

I built an 18 watt power and PI with a VVR and a JCM800 preamp. Diode rectifier. 33k grid resistor on the first stage of V1.

It sounds pretty close to what I expected. Sounds really good after I've done some tweaking (feedback loop, bright cap values, etc.).

When I plug in the high input (the cascade V1) with my San Dimas with a Duncan JB in the bridge it sounds like it's out of phase and thin. If I put a buffer (boss pedal) it brings back all the frequencies. I have played several guitars through it and this one seems to not like the input impedance or something. The neck pickup is nice and full sounding. The 57 classic pickups in my LP don't do this. P90s don't, either.

Has anyone experienced this kind of thing? I'm going to try raising the 33k grid resistor to 68k and see if that helps, but it's weird that it's happening.

I can post my layout if that will help.

Thanks!

[JMPGuitars]

Perhaps I read your description wrong, but it sounds like only 1 pickup on 1 guitar doesn't sound right? I would check the guitar, not the amp.

If you want to check the amp, post your voltages. I wouldn't change the grid stoppers.

Thanks,
Josh

[jdbenge]

Lol...when you put it like that I sound crazy...I probably am.

I played the guitar in question through my VHT Standard 18 (18 watt TMB) and it sounds full and rich. I think my build might have some kind of weird blocking distortion or something cancelling out frequencies. Can an impedance mismatch do that?

The issue seems to be in the first stage. When I use the low input, bypassing the first stage, I don't have the out of phase effect. Using a buffer also cures the out of phase effect.
Here's my voltages for V1
B+ 301.4V
I wired the 2nd triode as the first stage:
V1 pin 6 191.1v
V1 pin 7 ---
V1 pin 8 1.325v

1st triode is the second stage. This normally has the 10k cathode resistor but I'm working with voltages less than in a normal JCM800. I wired a 3 way 2pdt switch to vary the value of cathode resistance at this stage.

Voltages at all three cathode resistances:
V1 Rk @ 10k, pin 1 = 271.5v, pin 3 = 2.746v
V1 Rk @ 5k, pin 1 = 251.6v, pin 3 = 2.312v
V1 Rk @ 1.5k, pin 1 = 199.2v, pin 3 = 1.445v

Thanks for humoring me.

[JMPGuitars]

Now you have me thinking that JB is possibly your hottest pickup, and you're driving the tubes harder with it. If you're running EL84s, you might want to try the Paul Ruby Mod.

Just for fun, try that guitar again, and see if you still have the same problem with the volume lowered a little on the guitar.

Grab one of my voltage docs, and use it to show us what your power amp looks like.

[jdbenge]

I think you're onto something here.

I turned the volume down on the bridge a little and there was no change. I turned down a lot and the lows and highs came back.

For giggles, I measured the DC resistance of the pickup through the guitar cable. 538K! Obviously there is something messed up with the wiring and I never noticed it until I plugged into my latest build. The neck 59 measures normal at around 15k.

I bought the guitar used so I've got to trace down what might have been modded.

Thanks for humoring a crazy man.

[JMPGuitars]

After you get the guitar sorted, I would still consider the Paul Ruby mod. It's really simple to implement. Check out the docs for the EF86 Xtra, it shows how to do it.

Thanks,
Josh