Fender Pro Jr Hum + more

[phelar]

you might try a small cap from pri to ground of the pwr transformer,

or try it across the wht and blk wires, after the fuse just in case the cap blows up,

usually there is already capacitance in the pri wind but sometimes not enuff ,

ferrite beads can help also, but where would you get those so skip that,

Ok, thanks! I will try.
Didnt have anything lyin around, order ceramic 1KV .01 caps so ill have to wait until they arrive.

I would not recommend this mod; that has been original in earlier amps, when grounded plugs just didn't exist. Any leakage in this capacitor is a potential killer, since it brings the chassis to mains voltage in case the ground is not adequate;
that's the reason it's been discontinued in the 70's.
If you want to go this route, you'd better go the whole way with a set of Y2 capacitors, but please read that first, from top to bottom:
http://www.justradios.com/safetytips.html
Anyway I seriously doubt this would cure your hum problem. These caps are intended at preventing EMI/RFI from entering the amp; it may be helpful if your problem was with receiving the next TV transmitter or the ambulances.
You haven't answered my question: does hum change when you move the guitar? Does hum change when you turn down the volume on the guitar?

I´m sorry....i guess i´ve made this thread longer then it it had to be :-[

The hum does change, gets louder when i turn the pups against th amp, when i walk away a few meters the hum decrease. When i turn the guitar volume down it dissapear (iddle hum).

Ive played through maybe 20 amps with this guitar (Ibanez pf350) and never experienced this.

 

[abbey road d enfer]

The hum does change, gets louder when i turn the pups against th amp, when i walk away a few meters the hum decrease. When i turn the guitar volume down it dissapear (iddle hum).

Then it's definitely magnetic pick-up; nothing you can do on the amp, except maybe replace the power transformer with one specifically made for 50Hz mains.
Another possibility is to put the step-down xfmr close to teh power xfmr and find a position where the fluxes would more or less cancel (humbucking); this may or may not work.
Anyway, I've had the opportunity to play with the exact same amplifier in New Orleans recently and I noticed I had a lot of hum with my P90 equipped Gibson, so maybe there is not much to do...

 

[CJ]

seems like the hum is coming from that converter, techno geeks say fix hum by stopping it at the source,
so internal amp mods seem to be going down the wrong stree, plus the technical police don't like the ,01 cap thing even though there are millions of amps with that cap in there,

one last internal mod you might try is a piece of sheet metal with a 90 degree bend in it, you drill holes for the input jacks on one side, and the other side shound hang over the jacks about 1.5 inches,  Roland uses this on the Jazz Chorus 70 and it really helps keep the hum down,  i have seen it on other modern amps also,

does that converter have a ground connection for the amp pwr cord?

 

[pH]

I had one of these years ago, it was a good sounding amp but mine also had a lot of hum. The tube sockets were mounted on a separate pcb and the connection to the main board had wiring that was long and messy with non-twisted heaters. I don't think cleaning it up helped very much though. Back when I had mine they were biased really hot, you might check on this and bring the bias down. The heat eventually cooked my tube socket pcb, so I ended up gutting it and built a point to point circuit in its place, it's dead quiet now!

 

[phelar]

seems like the hum is coming from that converter, techno geeks say fix hum by stopping it at the source,
so internal amp mods seem to be going down the wrong stree, plus the technical police don't like the ,01 cap thing even though there are millions of amps with that cap in there,

one last internal mod you might try is a piece of sheet metal with a 90 degree bend in it, you drill holes for the input jacks on one side, and the other side shound hang over the jacks about 1.5 inches,  Roland uses this on the Jazz Chorus 70 and it really helps keep the hum down,  i have seen it on other modern amps also,

does that converter have a ground connection for the amp pwr cord?

Yes, there is a ground for the plug.

I´ll may be try the sheet metal thing when i got more time.....ill think it will push the pcb away from its mounting holes

 

[peterc]

I had a similar situation regarding a step-up converter chain.

The converter was rated big enough to handle the power required, but gave a lot of hum. It was just on the edge of not working properly.

Tried the amp with a bigger converter and no hum.

Looking at the size of your converter and knowing the size of an amplifiers' mains transformer, it seems physically to be quite small? Hard to tell from the pics, though.
If you can, try it the set up with a larger converter transformer.

Hope this helps....
Peter

 

[phelar]

I had a similar situation regarding a step-up converter chain.

The converter was rated big enough to handle the power required, but gave a lot of hum. It was just on the edge of not working properly.

Tried the amp with a bigger converter and no hum.

Looking at the size of your converter and knowing the size of an amplifiers' mains transformer, it seems physically to be quite small? Hard to tell from the pics, though.
If you can, try it the set up with a larger converter transformer.

Hope this helps....
Peter

The ac ac is 100w max. The amp require 70w.

 

[phelar]

I had one of these years ago, it was a good sounding amp but mine also had a lot of hum. The tube sockets were mounted on a separate pcb and the connection to the main board had wiring that was long and messy with non-twisted heaters. I don't think cleaning it up helped very much though. Back when I had mine they were biased really hot, you might check on this and bring the bias down. The heat eventually cooked my tube socket pcb, so I ended up gutting it and built a point to point circuit in its place, it's dead quiet now!

Yes, the wiring is not so "pro" in those amps....not the bias either. There was allready mods done to this amp, messy though. Have reworked the bias mod and removed some kind of bright switch. Everything in the amp is in order, it has some iddle hum...acceptable though. This one got new tubes with right bias voltage.