Fender Pro Jr Hum + more

[phelar]

I´ve got this Pro Jr that staring to give me headache.
It got a nasty hum that goes up with the volume. It allso have ghost notes...or maybe a springlike sound that follows the guitar tone.
When i got the volume pot all down it leaks in to the speaker.

Ive tried with other preamp tubes in V1, same same.
Checked the pot wiper againts ground....it dosent short....6 ohm. When i turn up the volume pot it will rise to 60k then down to 20k when its maxd.

Please check the link, recorded some of the issues

https://www.youtube.com/watch?v=0WKt2jYuARs&feature=youtu.be

Help!!

 

[phelar]

Update:

The springish, rattling sound was the speaker. Tried it today with my new 10" cab and the sound was gone.

Still this nasty hum after i pass 3 on the volume, is this "normal hum" for this amp?

 

[abbey road d enfer]

Update:

The springish, rattling sound was the speaker. Tried it today with my new 10" cab and the sound was gone.

Still this nasty hum after i pass 3 on the volume, is this "normal hum" for this amp?

Difficult to say without hearing, or having hard measurement figures, but does it hum with the input shorted?
You may want to check the smoothing caps and resistors R24 & 25 (screen grid).

 

[phelar]

Update:

The springish, rattling sound was the speaker. Tried it today with my new 10" cab and the sound was gone.

Still this nasty hum after i pass 3 on the volume, is this "normal hum" for this amp?

Difficult to say without hearing, or having hard measurement figures, but does it hum with the input shorted?
You may want to check the smoothing caps and resistors R24 & 25 (screen grid).

I´ve post a youtube link with the behavior and issues in the first post. The hum goes up with the volume so i think the problem is around V1, the preamp tube.

I,ve done a bias mod and have changed the powertubes , the amp hade a crackeling noice when i got it. This is all fixed up.

When the jack shorts the amp is silent.

 

[CJ]

single coil pickups are noisy, sometimes there are issues inside the guitar,

try a different cord also,

try a different guitar if you can,

pc mount input jack?  could be a bad jack, re-solder the jack or replace with a metal one, this requires tedious wiring and the circuit can be hard to figure out as to how to wire a 3 terminal jack into a pc board which has 5 holes in it, there is a practice nowadays of not only shorting out the input jack but shorting opamp inputs as well, why they do this is beyond the scope of this post, 

 

[abbey road d enfer]

I´ve post a youtube link with the behavior and issues in the first post. The hum goes up with the volume so i think the problem is around V1, the preamp tube.

When the jack shorts the amp is silent.

Looks to me like normal behaviour, but just a bad case of single-coil hum. Hum should disappear when orientating the guitar.

 

[RuudNL]

Strange that in 2016 the definition of an instrument amplifier is still:

"A device that amplifies the sound coming from an electronic instrument and adds hum and noise"...  ;D

 

[abbey road d enfer]

Strange that in 2016 the definition of an instrument amplifier is still:

"A device that amplifies the sound coming from an electronic instrument and adds hum and noise"...  ;D

You forgot distortion, which is the signature sound of "clean" electric guitar for more than 60 years...

 

[phelar]

The guitar is fine, have tried another cord. The hum is the same with just the cord, the cord and the guitar and no hands on the strings....and with the hand on the strings. Its there when i plug in my function generator to.

This is how the plate looks like on V1, the guitar is plugd in and hand is on the strings.

 

[abbey road d enfer]

The guitar is fine, have tried another cord. The hum is the same with just the cord, the cord and the guitar and no hands on the strings....and with the hand on the strings. Its there when i plug in my function generator to.

This is how the plate looks like on V1, the guitar is plugd in and hand is on the strings.

Does hum change when you put the guitar away from the amp?

 

[CJ]

let's see how clean the pwr supply is for V1,

scope it out, this will tell you if the filter cap is working, low chance of this being the problem as you have filters up stream already,

also pwr down and measure ground resistance from guitar jack ground to chassis ground, to do this plug in the guitar cable and put an ohm meter clip on the chassis and the other meter lead to the guitar cord sleeve,

you can also try swapping V1 and V2, there are the same tubes, 12AX7a, then scope the plate again so we can eliminate the tube as the problem,

measure the B+ volts on V1, s/b 232 VDC

looks like a nice sounding amp, you need a fuxx box in front to get it to crunch?

 

[musika]

also pwr down and measure ground resistance from guitar jack ground to chassis ground, to do this plug in the guitar cable and put an ohm meter clip on the chassis and the other meter lead to the guitar cord sleeve,

Loaded question.  What is acceptable resistance for ground resistance from guitar jack to chassis?

 

[CJ]

minus 5 ohms


s/b zero ohms + resistance of leads

 

[phelar]

looks like a nice sounding amp, you need a fuxx box in front to get it to crunch?

;D ;D

 

[phelar]

let's see how clean the pwr supply is for V1,

scope it out, this will tell you if the filter cap is working, low chance of this being the problem as you have filters up stream already,

also pwr down and measure ground resistance from guitar jack ground to chassis ground, to do this plug in the guitar cable and put an ohm meter clip on the chassis and the other meter lead to the guitar cord sleeve,

you can also try swapping V1 and V2, there are the same tubes, 12AX7a, then scope the plate again so we can eliminate the tube as the problem,

measure the B+ volts on V1, s/b 232 VDC

looks like a nice sounding amp, you need a fuxx box in front to get it to crunch?

I,ve tried 3 difrent tubes...among them new and known working, have also swapped V1 and V2

Resistance guitar cord- chassis ground 0.3 ohm

The B+ voltage seems steady as a rock

I forgot to tell you its a US-version so i run it with a AC AC convertor(Sweden got 230v 50Hz). The  c24 is missing in this unit because of this. They was only fitted in the export units. I´m not 100% shure what they are good for?

 

[abbey road d enfer]

I forgot to tell you its a US-version so i run it with a AC AC convertor(Sweden got 230v 50Hz).

!!!!!!!!!!!!!! I guess it's a transformer. Where is it physically located? Stray field from the magnetic core can generate hum like crazy. Also, are you sure you have correct grounding of teh chassis?

The  c24 is missing in this unit because of this. They was only fitted in the export units. I´m not 100% shure what they are good for?

CE marking requires RFI/EMI protection. It does not make any difference to hum.

 

[phelar]

!!!!!!!!!!!!!! I guess it's a transformer. Where is it physically located? Stray field from the magnetic core can generate hum like crazy. Also, are you sure you have correct grounding of teh chassis?

Its outside the amp, picture below. I´m not shure but i think ive checked most or all of the ground points.

CE marking requires RFI/EMI protection. It does not make any difference to hum.

Ok, thanks!

 

[CJ]

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,

 

[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.

 

[abbey road d enfer]

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?