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I have a homebrew JCM800 2204 (no fx loop), that I built out of an old Fender Bassman head. I am attempting to run it into my Weber Mass Lite 100 at full attenuation with no speaker connected. I then run the line-out of the Mass into my time based fx (just a carbon copy for now). The fx then go to my Fender Bandmaster and into a speaker cab.

The line-out signal is too strong for the pedals, so I swapped the 10k/1k voltage divider with a 100k/1k. The signal is still line level. It's like there is a line level signal in parallel with my fx signal. If I put my tuner in the fx chain and turn it on there is still a loud signal coming through.

So my assumption is that there is still a strong signal coming from the shared ground between the amp in/speaker out and the line-out on the attenuator. We are talking AC so there is still signal at ground. Is this correct? Anyway, when i lift the ground at the line-out I get the expected guitar level signal with the 100k/1k divider (50k/1k seems to work better).

So the question is, does this harm the amp, attenuator or anything else in the chain? I have emailed Weber and am still waiting on a response. Just thought I would get as many opinions as possible. Maybe this post could help someone else.

This happens on my home-made power resistor dummy load as well.

Anyone? Soulsonic?


Here is the Mass Lite schematic. So my calculations show 14V for 50W at the output of my JCM800. It uses a 4ohm Bassman output transformer. Square root of (50W x 4ohm)=14.14V.

To drop 14V to guitar level I used a voltage divider.
14.14V x [1000ohm / (1000ohm + 50000ohm)]=0.277V

There is still over a volt at the line-out. It also acts as if the line-level signal is in parallel with the instrument level signal. If I isolate the ground the signal becomes the value that it should be. So the line-out signal is not grounded in the attenuator, but is grounded at the pedal.

Yes I've had grounding issues with similar attenuation/load box systems and it can be quite confusing.
A wire with alligator clips might be needed, start bridging other ground paths that might not be obvious and see if it makes a difference.
I'm not the expert on this subject but in my experience circut ideas like these might only work with a ground lift.
As long as the Amplifier Speaker output looks into low Ohms,, then what happens after that is of little concern for the amplifier. So if ground lift works then use it as is.

If yo u want I can up load my DIY Loadbox with line output schematic it has a ground lift switch built in for the exact problem you mention.

HTH, Phil.

I know this is an old thread now, sorry, I haven't visited here regularly in awhile.

I would say the reason why you're getting higher voltages on the line out than what you calculated starts with the fact if you're running the amp full blast into the dummy load, it's probably putting out more than 50w, especially at peaks. The stated RMS power of an amp is an average over time, and peaks can be several times that value, and they don't often measure its RMS at full meltdown heavy distortion level.
So... probably need to calculate much more conservatively. Assume 100w and see what you get. Or howabout after the fixed attenuator, add a 100k pot to act as a variable volume control?

If you're having ground issues, you can isolate the output with a transformer. I've had good luck with the Edcor PC10K10K in some splitter boxes I've made.