Finished my first build (and one question)

dean

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Finally got the 5F1 kit put together! I got the basic Mojo chassis kit and built the cabinet from scratch - first time for both builds. with a little work on the baffle, I was able to barely fit a Warehouse Veteran 10-incher into the box. It sounds very nice and has some nice grind when set beyond 8. It’s interesting that the Tele single-coils sound better through this thing than the PRS SE ‘buckers. I’m pretty pleased.

I do have one question, though. The amp has a very slight hum when I fire it up, but there’s not enough to hear when playing the guitars through it. I’ve had that before on some of my tube amps, but I’d like to get rid of as much as possible with this one. Would you all take a look at the innards of the amp and tell me if there are wiring or routing issues that I could work on? Up next is the Tweed Vibrolux project.

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andrewRneumann

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Very well done, sir.

Just increasing the value of that first capacitor from 16uF to 24uF or 32uF is usually all it takes to quiet these little Champs down. It has nothing to do with grounding or lead dress. It's just the physics of rectification, capacitance, current, resistance, and single-ended amps. There have been thousands of threads about this. As far as I can tell, the consensus is that the rectifier can handle it.

Once again, nice job. You should be proud of that little charmer.
 

Lowerleftcoast

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tell me if there are wiring or routing issues that I could work on
I agree with @andrewRneumann . That first cap will probably get you there.

It may result in no improvement in hum but here are other areas that could be improved:
Elevating the artificial center tap. Sometimes this will make a difference. Move the 100 Ohm resistors from the pilot lamp and place them on the 6V6. Rob shows this on his sites.
InkedInkedHeater_Artificial_Ground_small1_LI.jpg

Route the signal wires so they rest on the chassis. This provides some shielding and can help keep stray signals from being amplified. On the pic below the signal wires (blue arrow pointed at them), connected to the volume pot, are close to the AC wire (red arrow pointed at AC wires). Leave the AC wires where they are. Move the signal wires to hug the chassis. The green wire (pink arrow) is connected to the ground bus. Keep it away from the AC wires. The signal wires to the tubes might benefit from hugging the chassis as well.
Inked5F1___LI.jpg

All in all, it looks like you have done some well executed work.
 

King Fan

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Nice build. @andrewRneumann and @Lowerleftcoast have great advice here. Let me just add a few ideas; the Champ is an especially cramped space to start fixing things.

1. Single-ended amps like this may always hum a little, cuz we don't have push-pull to cancel output hum. But OK, we can take that as a challenge to reduce 'the rest of the hum' as much as possible.

2. Can you tell if it's 60 or 120 Hz? Below or above low E on the guitar? (This can be confusing, sometimes both, sometimes even some other frequency). The bigger first filter will mostly help with 120, the elevated heaters with 60. Both are really good ideas, but the access here would make it ideal to target our interventions. Or do the ones you can reach with your soldering iron!

3. Another source of 60-cycle hum in the 5F1 is the AC power switch on the volume pot, but drilling a hole, moving the fuse, and adding a separate switch would be crazy hard in a completed amp, so let's take that hum as 'vintage correct.' :)

4. Another source of 120 hum is ripple return from the first filter cap getting into the ground wiring. Rob shows how to minimize this (in theory we can cut it up to 5-fold) by simply anchoring the HT CT (often red/yel) directly on the first filter negative instead of a shared ground anchor -- of course you keep the filters grounded too. This I've done in an existing amp; may need a splice, but access is often adequate.

All that said, from this pic, if you took down the lamp assembly and heater wiring to the 6V6, you might just be able to access the first filter cap, move the HT CT, and elevate the artificial heater CT. I'm no expert; my smarter friends may have better ideas. :)

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dean

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Thank you so much for the advice and the kind words - I appreciate it. I’m going to reread all these comments and suggestions and make a list of things I can do in order of my comfort level. As I mentioned, the hum is minimal, and when I play, I really don’t notice it beyond startup and between songs. I think this amp will continue being my “practice” amp - the one I learn on. Although, if my son really likes it, it will likely end up as his amp (he’s quite good and should be able wring out some nice sounds through it). Again, thanks!
 

King Fan

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Thank you so much for the advice and the kind words - I appreciate it. I’m going to reread all these comments and suggestions and make a list of things I can do in order of my comfort level. As I mentioned, the hum is minimal, and when I play, I really don’t notice it beyond startup and between songs. I think this amp will continue being my “practice” amp - the one I learn on. Although, if my son really likes it, it will likely end up as his amp (he’s quite good and should be able wring out some nice sounds through it). Again, thanks!

Right. I just spent some time modding a big tweed, w-a-a-y less crowded than the Champ, and still came awfully close to burning components and wires. I don't think those factory 5F1s had any of the hum fixes we're discussing here; sometimes it's smart not to fix something if it isn't *painfully* broken. Enjoy the amp!!!
 




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