Star Grounding My Gregory Mark X Tube Guitar Amp
By Paul Marossy
Last updated 5/26/05
Still not satisified that this amp had the
lowest possible amount of hum, I decided to
star ground it in the same manner described by RG Keen at his
most excellent website. His example happened to be the Vox Pacemaker, which is very similar to the Mark X,
so I used that as a guide in star grounding my own amp. The results were nothing less than astounding! After
I was done, there was absolutely no hum whatsoever and at least as quiet as any equivalent solid state amp
||Here is a view of the chassis with the star grounding completed. There were only
two wires that had to be routed from the terminal strips to the star ground point, which is
towards the left side of the chassis. The heater ground is also bundled with these wires.
The rest of the wires had a very short distance to go to the star ground.
||I used some plastic input jacks to
isolate the input grounds from the chassis. Before I did this, I had done all the star grounding
minus the input jacks, speaker jack and tremolo bypass jack and the hum was still significant with
the volume turned all the way up. After I replaced the input jacks, the hum had literally disappeared!
The jack to the left of the two plastic jacks is a phony, it's only there for cosmetic reasons
since I don't foresee the need for three input jacks. I left the speaker jack and tremolo bypass
jack grounds connected directly to the chassis, and this seems to work fine - they do not
contribute any hum, so I left well enough alone.
||Just for kicks, I added a 10uF bypass cap
to the cathode of the driver tube but I found out that it caused hum to reappear, so I
removed it. Some people have questioned the 1M resistors in the amp. You can't really
see the colors well in this picture, but they are there. I think the designer of the amp avoided
blocking distortion by using 0.005uF coupling caps to the grids of the power tubes. It seems to
work just fine as the amp is loud and clean.
added some 1K/1W screen resistors to the power tubes since this circuit didn't have any screen
resistors at all. I then checked to see what the power tube plate dissipation was, and I
found that plates were dissipating over 14 watts! After
adding the screen resistors and going to individual 220 ohm cathode resistors instead of one common
160 ohm cathode resistor, I got the plate dissipation down to 11.5 watts, just under the
recommended maximum of 12 watts. I had to add a little terminal strip and rewire the B+ supply
a little bit, but the operation didn't take long at all.
||Here is a view of the chassis in the
enclosure. I moved all the
interconnective wiring around to avoid parallel runs and made everything cross at right angles
where I could.
||This is a shot looking at the same area from another angle.
||Here is a view of the internal speaker jack wiring. Just to the left of the
speaker jack, I installed a switch that can remove the negative feedback for more gain.
||Here is a picture of the speaker jack with speaker connected.
||This is the chassis back cover that I made, showing the internal shielding and the method I used
to ensure that the shielding was in contact with the amp chassis - a spring. The spring
pushes against the chassis vertical support and provides continuity to ground.
||The jack on the far right is the phony. I painted the threaded portion black
to match the plastic ones. You can't tell unless you look real close that they aren't quite the same.
||I liked the original control knobs, but it bothered me that three of the four knobs
were missing the chrome insert, so I improvised and made them all look the same. It's not
perfect, but I like it a lot better. It's not in this picture, but I added a standby switch
where the fuse holder is shown and moved the fuse holder to another part of the chassis.
||This is how the knobs look when the chassis is in the amp. I think it looks a
lot better now that all the knobs match.
A few more hours of effort really paid off! The amp sounds a little stronger and clearer, which I think
has to do with rerouting some of the wiring, scrutinizing the lead dress, and the star grounding. The
tremolo also does not throb with no input signal anymore, nor does the hum increase when the strength
control is turned up. This amp was a great test bed for star grounding versus just grounding stuff to
the chassis wherever convenient and not using isolated input jacks. Now I'm a believer! Not that I wasn't
before, I just hadn't exerienced such a dramatic change like this before. Anyhow, I also learned some things about
various tube amp circuits that I didn't know before, so I think all of this was a worthwhile exercise. I think
this officially completes this project, finally.
I am really surprised by the sound of this amp, it has a very nice tone when used in conjunction
with my Shaka Tube overdrive pedal and my BSIAB. The tremolo has a nice feel to it as well. It's pretty loud, and
if you use a guitar with an onboard preamp, it's louder still. This is a case
of a "diamond in the rough" if I ever saw one!
Star Grounded Chassis Layout
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