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?
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?
An externally hosted image should be here but it was not working when we last tested it.
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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.
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.
An externally hosted image should be here but it was not working when we last tested it.
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 .
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 .
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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.
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.
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.
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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I had exactly the same problem on my last project, using a similar Hammond PT, with diode rectifier. I tried all sorts of capacitor/resistor suggestions, different diodes, etc, to try to kill the noise. Couldn't get rid of the noise. I clearly identified the source of the noise as the filament supply (receiving noise from the B+ winding) coupling noise into the grids.
My solution was to use a small filament transformer to supply my preamp tubes. Using the noisy filament supply on the push-pull power adds no noise.
My solution was to use a small filament transformer to supply my preamp tubes. Using the noisy filament supply on the push-pull power adds no noise.
Better, but still not perfect:
Not dead quiet yet, but the original problem is gone. Need to double check all my joints and see about a little shielding on the the power supply side.
An externally hosted image should be here but it was not working when we last tested it.
Not dead quiet yet, but the original problem is gone. Need to double check all my joints and see about a little shielding on the the power supply side.
Perfect now:
An externally hosted image should be here but it was not working when we last tested it.
Make sure that the transformer's B supply winding's center tap is grounded AND that the transformer's filament supply winding's center tap is grounded. If so at least you got that as designed!
Put small ceramic disc or mica capacitors across the diodes to eliminate any diode switching noise at its source.
Put some small-value poly caps across the main filter cap (the only one connected directly to the diodes) to do a better job of filtering out any diode switching noise.
Keep in mind that a single-ended amp is unforgiving of power noise.
A common fix easily tried is to make a 10:1 voltage divider between B+ and ground and reference the filament to that instead of to ground. Put a 1meg resistor from the filament center tap to the B+ (right after the diodes) and a 100K resistor from the filament center tap to ground. Oh, and bypass the 100K with a small cap to ground out any high frequency noise on the filament supply. This really works! It made a huge difference with my Sound City amp.
I would also mention that my Alembic preamp had a lot more stages of filter cap & resistor, filter cap and resistor, filter cap and resistor...
And as Indiana Joe asked, is it 60-hz noise or 120-hz noise?
If you inject a very very low-level input with the amp's gain way up (but input so weak it's still not loud) if you pull the power plug the music will (hopefully) fade away slowly as the amp continues to run on the caps and filaments take a few seconds to cool. BUT: did the hum disappear as soon as you pulled the plug?
Did you try putting the heaters on a battery with one side grounded or lifted to some additional + with respect to ground (one side conneted to that voltage divider I described)?
Since you're using shielded wire to the input jack, sometimes it helps to use an insulated jack that doesn't connect to chassis ground, and let the shielded cable supply the ground to your guitar. Other times the grounded jack isn't a problem.
If you decide to put the transformers in cans, get some "loaded" heat-conductive potting epoxy and fill the can with the trans inside, instead of making any cooling holes. Wrap the trans in mu-metal?
Does the hum disappear if you turn the volume off?
Good luck.
If you inject a very very low-level input with the amp's gain way up (but input so weak it's still not loud) if you pull the power plug the music will (hopefully) fade away slowly as the amp continues to run on the caps and filaments take a few seconds to cool. BUT: did the hum disappear as soon as you pulled the plug?
Did you try putting the heaters on a battery with one side grounded or lifted to some additional + with respect to ground (one side conneted to that voltage divider I described)?
Since you're using shielded wire to the input jack, sometimes it helps to use an insulated jack that doesn't connect to chassis ground, and let the shielded cable supply the ground to your guitar. Other times the grounded jack isn't a problem.
If you decide to put the transformers in cans, get some "loaded" heat-conductive potting epoxy and fill the can with the trans inside, instead of making any cooling holes. Wrap the trans in mu-metal?
Does the hum disappear if you turn the volume off?
Good luck.
You have the speaker jack is grounded to the chassis too. Sometimes speaker current thru the chassis can make a better high-current path than the wire, but induce currents in the chassis that can couple to other things like the input jack. So, you might want to try using an insulated speaker jack.
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