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Old 30th October 2008, 10:01 PM   #1
diyAudio Member
Join Date: Jun 2008
Default Need help with a Gilmore Jr. build

I finished assembling it a few days ago, but it is very noisy, and seems to be oscillating at a low frequency (I've never built an amp before, so I'm not sure this is what's happening, honestly). I have tried rerouting the filament power supply wires, the OT wires and the input wire, have reslodered a couple of leads on the turret board, and have generally poked and prodded all over the thing, but nothing I've done has had any effect at all.

I am emailing with Rich Guy at Guytronix about it, but we haven't really made any progress at all so far. Any advice anyone can give would be really helpful. I have some stuff here that should help:

-photos of my current layout
-readings taken at the tube sockets with the amp off and unplugged, nothing present at the input, and the tubes removed (html and excel files)
-an mp3 demonstrating the problem

The mp3 has a few different things:
1 - amp on, nothing plugged in to the input. Volume starts at lowest setting and sweeps to highest setting. tone is at highest setting.
2 - Same as 1 but with a guitar plugged in.
3 - Volume set low, guitar plays low E, high E and a cluster
4 - same, but with the volume set about halfway up.
5 - same, but with volume on full

The lower notes sound like they are being doubled an octave down, the higher notes sound kind of chimey, almos t like they're being ring modulated. The amp breaks up very early and the distortion is harsh and nonmusical.

I've noticed that the volume control crackles a little bit, a lot like the crackle that happens in a Rangemaster circuit because of the voltage across the pot. I haven't measured this yet.

I really have no experience troubleshooting circuits, and the most complex things I've built up to now are fuzz face and Rangemaster clones from scratch (which have worked perfectly, so I've never had to actually troubleshoot them). Help me please!

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Old 1st November 2008, 05:01 AM   #2
diyAudio Member
Join Date: Oct 2008
Default filter caps

Hello Everyone

The sound I'm hearing is classic lack of filtering. I would take a good long look at the filter caps following the rectifiers. I looked at the photos you posted. Is it possible that the positive of the main filter cap (47uf 500vdc) which seems to be running to turret 18 ,should go to turret 19 which seems to be the output of the rectifiers? Clearly from the sound clip, The output is being modulated by ripple in the B+.

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Old 3rd November 2008, 03:22 AM   #3
diyAudio Member
Join Date: Jun 2008
Thanks, that was the problem, I missed it three times in a row when I checked the layout. Amp's working great now.
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Old 3rd November 2008, 02:21 PM   #4
diyAudio Member
Join Date: Oct 2008

Glad to hear it. I'm also glad I could be of help. Score one more for the good guys
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Old 3rd November 2008, 02:35 PM   #5
diyAudio Member
Join Date: Jun 2008
It's definitely an excellent sounding low wattage amp now. Those Mercury Magnetics transformers are living up to their reputation so far.
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