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Old 1st December 2012, 04:26 PM   #1
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Default Matchless Hotbox - troubleshooting

I successfully built a few amps 15 years ago (Champ, Bassman, Bassman with overdrive channel), and I built a Matchless Hotbox that sounded great. A friend took it off my hands and I decided to build another one with a nicer Hammond enclosure that ended up a little cramped. After I built it, I couldn't fix an annoying sawtooth buzz and put it on the shelf.
I recently decided to dismantle it and give it another go.
I suspected the heater circuit, and chose a different Hotbox schematic with 6.3 AC heaters instead of the earlier DC design. I stripped the project down and laid it all out on a temporary board taking cues from a layout posted here by user 'bagudan'.
The sawtooth buzz is still there.
The transformer is a Hammond 269 EX same as I had used in the earlier successful build, but my B+ voltage is 304v instead of ~250v.
Any suggestions? Click the image to open in full size.
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Old 1st December 2012, 04:32 PM   #2
kevinkr is offline kevinkr  United States
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Please post musical instrument amplifier threads to the Instruments And Amps Forum.. See sub-header at top of each forum for explanation.. Thanks..
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Old 1st December 2012, 04:36 PM   #3
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[sorry - failure of due diligence]
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Old 1st December 2012, 05:01 PM   #4
DUG is offline DUG  Canada
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Add a resistor between diode bridge and filter caps...see if there is a change.

20R - 100R
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Old 1st December 2012, 06:34 PM   #5
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A 4.7k inline resistor got me down to ~240v. The humm/buzz persists, but if the gain is turned to about 20% it mostly goes away...
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Old 1st December 2012, 06:52 PM   #6
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There is no shielding. The frames of the pots are not connected to chassis ground.
I'd install a ground wire from the frame of each pot to common ground.
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Old 1st December 2012, 07:20 PM   #7
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Pots are grounded via the front.
My temp enclosure is pretty ghetto, but I thought that laying it out with enough space would help track down the problem. I had used shielded wire for the signal portions in the previous attempt but the buzz is the same.
The tubes are Sovtek, and I've tried a couple different sets.
Click the image to open in full size.
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Old 1st December 2012, 07:37 PM   #8
JMFahey is offline JMFahey  Argentina
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Is that a pressboard board/chassis?
Agree that you will have *terrible * shielding problems.
If possible, get a sheet of aluminum, make proper holes and transfer your build as-is, will improve a lot.
Of course, a proper chassis will be more practical in the real world.
For proof_of_concept, lift your amp from that pressboard chassis, glue a sheet of kitchen paper to it (with contact cement) and remount.
Day and night

EDIT: simulposting!!
OK, you started along the correct way, now cover one side of the full chassis, and remember to ground everything needed to it .

Last edited by JMFahey; 1st December 2012 at 07:40 PM.
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Old 1st December 2012, 11:24 PM   #9
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Default mctube

I recently built a real mctube and ignored the suggested use of shielded cable. What I did was
* twist all AC wires
* use star grounding
* separate signal lines from AC lines
* not applicable to you but I made sure the two transformers used in the real mctube were not aligned.
The result was that the pedal was super quiet. (Despite being in a cramped space just like in your case.

Funny someone else in a forum also mentioned that a friend had borrowed a hotbox never to give it back. I think this will be my next project. I'm trying to work up to building an amp.
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Old 3rd December 2012, 05:45 PM   #10
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Is your sawtooth buzz power line frequency or double? I'd put the power transformer in a separate ferrous steel box, grounded. I'd twist the +- wires out of the transformer to the rectifier, and if that is not enough I'd used shielded cable for the mains feed. Shield Gounded one end, the transformer end.
I find surplus steel file boxes from the charity resale store, or in one case the garbage pickup, useful if not stylish hum eliminating chassis. The transformer could fit in a recipe card file box, the whole amp in an letter file. Cutting a grill for air flow is required. I use a body grinder and a cutoff wheel, or for fine work like connector holes, a stanley carbide blade in a hacksaw frame. Use safety glasses with power tools. I cut fan bezels out of PC cases, or make square grills out of metal mesh. Thin sheet metal doesn't drill well over 3/8", so for bigger holes, even though I own drills up to 5/8", I drill a lot of 1/8" holes and connect the dots with the saw. Punches are for serious pros than can afford $35 a hole size. You look cheap like me. It is essential to have a vise and a dirty shop bench. Watch craigslist, I got my last vise for $10.
dynakit equipment had ferrous steel wrappers around the power transformers, then for the high gain preamp, the signal wiring was in a separate steel enclosure from the aC power wires. All contained in the nice steel box with aluminum faceplate, you never saw the internal steel box unless you took it apart. My hammond organ with 23 tubes has the AC power stuff in a separate chassis, with aluminum foil under the bottom, then the AC wires up to the power switch and back are twisted together and run well away from the signal stuff. I spent winter 2011 getting the hum out of a disco mixer by getting the transformer out of the steel enclosure, and the power switch away from the 50x gain op amps, also.
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Last edited by indianajo; 3rd December 2012 at 05:52 PM.
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