Champ 12 hum

[Leevi]

I'm trying to eliminate ground hum of Champ 12


https://el34world.com/charts/Schematics/files/Fender/Fender_champ_12_schem.pdf


I have changed the filter caps, tried choke, cleaned ground contacts, changed power tube without success.


Question: Could the half wave rectifier cause the problem?


/Leevi

[Leevi]

By changing the 3 filter caps to 200uF caps solves the problem. That's unfortunately a workaround.


/Leevi

[shooter]

an SE running 500v and 1/2wave with all the extra stuff probably shoulda been designed with 200's

[jjasilli]

Don't know if this is half wave.      Anyway it seems D102 forms a voltage doubler circuit    Champ 12's are known to have a hum issue, complete with a Fender Service Bulletin.  See:  http://www.fenderforum.com/forum.html/searchthreadgateway.php?topic_number=783751


Short of a major mod to the PS, increased filtering is one solution.

[PRR]

It's a full-wave voltage doubler. Hum is 120Hz, but only 23uFd total capacitance.

Without utterly re-engineering an old plan, here's a more-parts less-uFd way to reduce B+ ripple a LOT, at the cost of slightly lower power output:

[PRR]

Here's a sim of the hum, stock and with added R-C filter. Ripple is down about 10:1, and the higher harmonics (buzz) are down more than that.

However when I clean-sheeted a similar plan, I think I used three pairs of 220uFd with 220r series resistors. On 'scope the hum was low but not zero. I didn't get as far as connecting a speaker before my life changed and the project abandoned.

[Leevi]

Thanks PRR for your clarification and information. I see now that it's a full-wave rectifier. I'm going to order 220uF or 270uF caps for the power supply. It's a bit hard to operate with the PCB and I think the caps must be placed outside PCB.


/Leevi

[Pietro]

What makes you think the hum originates from the Ht line?

Are the Heaters centertapped? If not...i.d start with that.

Never had an amp the could not be silenced with 50 + 16 + 16 uf.
220 uf is overkill.

[sluckey]

What makes you think the hum originates from the Ht line?

The fact that the big filter caps fixed the problem is a big clue.

Are the Heaters centertapped? If not...i.d start with that.

Did you look at the schematic?

Never had an amp the could not be silenced with 50 + 16 + 16 uf.
220 uf is overkill.

220µF is probably bigger than needed, but it may just be what he had on hand. These cheap single ended amps are more susceptible to B+ hum than push/pull amps. More filtering is always an improvement. Bigger filter caps is one way to add more filtering. Adding a C/R stage before the plate node is a more efficient way to add nore filtering. Adding a C/L stage before the plate node would be the best (but pricey) solution, because the choke must be able to handle the entire B+ current load.

[Leevi]

Are the Heaters centertapped? If not...i.d start with that.

I think the 100Hz (Europe) hum is not caused by the heaters (50Hz).


The whole amp is a nightmare what comes to maintenance. You can not even see the pots.
Removal of the PCB is a huge risk.


/Leevi

[PRR]

Here is what I *think* did for a slightly larger non-profit "big Champ". The main B+ ripple is 14mV, which in a good guitar speaker (I had a hunk) computes to around 20dB SPL @ 4', which is below audibility (might be audible within 2 feet). Idle hum was very very low. Hum came up with Volume, because the preamp was open on the bench, but was comparable to hiss. I believed it would be heard as "no hum at all" even in a small teaching studio. But it didn't get that far. And that was a HEAP of snap-caps.

[ac427v]

My Champ12 hummed badly. I changed all three of the 47/350volt power supply caps for 100/350volt caps. Physically they were about the same size as the originals so that part was easy. Sorry I can't remember the brand so you would have to measure yours and find some that fit the printed circuit board. They fixed the hum. I was disappointed that it still had minimal bass response so I sold it.

[Leevi]

100uF were not enough in my case. 150uF improved a bit but 200uF made it quiet.
/Leevi

[Leevi]

It is dead quiet now, see the pic.
/Leevi