AB763 hum and fix

[Leevi]

I have problems with a small hum in AB763. The amp is a modified version
of AB763 with no tremolo and reverb.

There is  a PPIMV master volume. The basic ground hum exists
only when the master is fully opened. If I decrease the master little bit
the hum disappears.

I solved the problem by adding 82K serial resistors to the power amp before the PPIMV dual pot.
I didn't hear any big difference in the sound, the volume decreased little bit.
What do you think about these resistors there, do they ruin the original sound?

/Leevi

[stratele52]

There is no reason to have the "hum" in a well-built circuit. Added resistance will affect the sound.

Show us the schematic of your Master Volume and pictures of your installation. Sure that there is a problem there.

Which circuit of MV you use ?

Or the hum was there before MV mod ? And amp need to be fix first .

Yesterday I have just installed a master volume in a similar circuit, no hum. And it is not my first MV mod.

[Leevi]

There is no reason to have the "hum" in a well-built circuit.

Yes, I agree.

The master vol is not causing the hum since it was there before.

I'm using a circuit like this without 47p cap.

http://taweber.powweb.com/store/mv_schem.jpg

And amp need to be fix first

Yes of course, my fix is a quick and dirty workaround.

/Leevi

[stratele52]

You used a good MV circuit same as mine . I used 500K pots it work better for me than the 1 Meg

You said  the hum was there before the MV mod ?

To mod an amp or to built an amp you need to know how to keep is quiet by proper grounding , wiring dress .....

[Leevi]

Yes, I have made several trials like
- changed the grounding schema
- heaters grounding with resistors
- tried with matched tubes
- etc

There are 1 Ohms resistors on the cathodes in order to make the biasing easier.
/Leevi

[stratele52]

If your amp have some hum , sure you forget something or you do some mistake . An AB763 is a quiet amp.

Some pictures of your amp , will help us to tell you where to look .

[Leevi]

I found the root cause with help of oscilloscope. I couldn't believe
that the filtering was too low (or bad cap) in the bias circuit. I changed the electrolytic (22u)
to 47u which helped. Lot of hours spent again for troubleshooting but I'm happy now.
/Leevi
 

[stratele52]

I found the root cause with help of oscilloscope. I couldn't believe
that the filtering was too low (or bad cap) in the bias circuit. I changed the electrolytic (22u)
to 47u which helped. Lot of hours spent again for troubleshooting but I'm happy now.
/Leevi
 

Filtering for bias cap , best is  a 100 MFD , 100 volts . 47 not bad , but it is a minimum.

Bad bias filtering send some noise direct on output tubes . This have nothing to do with the master volume  which is before !!!!  If you decrase MV , noise will always be there anyway with a bad bias cap.

Or your MV circuit is wrong ; did you copy exactly the shematic you show ? And put C19, C20,C21 and C22 ?

[Leevi]

did you copy exactly the shematic you show ? And put C19, C20,C21 and C22

Actually I copied this. The second cap is 1u which keeps the capacitance close to the original value.

http://www.amparchives.com/album/Marshall/Schematics%20&%20Layouts/PPIMV's/slides/Improved%20PPIMV.jpg

I put 100u electrolytic to the bias circuit which made the amp even more silent.

Then I have added a cap over the stand-by switch in order to eliminate popping there.

Thanks for the help
/Leevi

[plexi50]

Maybe a ground loop?

[stratele52]

did you copy exactly the shematic you show ? And put C19, C20,C21 and C22

Actually I copied this. The second cap is 1u which keeps the capacitance close to the original value.
 

1 mfd ????  need a 0.1 mfd if you look at schematic . 1 is too big.

[stratele52]

Maybe a ground loop?

I ask for some photos.  Often the time is a bad wiring . With some photos we can see.

[HotBluePlates]

I found the root cause with help of oscilloscope. ...

This is where pulling preamp tubes helps. If you pull the phase inverter tube and there's still hum, it has to be from the fixed bias supply (if present), heater-to-cathode leakage in the output tubes (but would have to be severe to cause audible hum) or failed/bad filter caps.

You might still need the o'scope to help narrow down. Or you could use a multimeter to measure a.c. volts on the B+ (you really want reference values from a known-good amp) and bias output.

But it seems the hard way is always the best way to learn! 

[Leevi]

I ask for some photos.  Often the time is a bad wiring . With some photos we can see.

There are some photos, but i have made changes afterwards so they are not up to date.

http://i218.photobucket.com/albums/cc183/kz14100/RikstoneC25bottom_zps2faaa270.jpg
http://i218.photobucket.com/albums/cc183/kz14100/RikstoneC25top_zps67feec4a.jpg
http://i218.photobucket.com/albums/cc183/kz14100/RikstoneC25inside_zpsd759d81c.jpg

1 mfd ????  need a 0.1 mfd if you look at schematic . 1 is too big.

Check this (by Kevin O Connor):

http://www.amparchives.com/album/Marshall/Schematics%20&%20Layouts/PPIMV's/slides/Improved%20PPIMV.jpg

Leevi

[SILVERGUN]

Do you have some AC wires tied to some DC wires here?

[SILVERGUN]

I'm also concerned about this heater wiring laying up against the cap in this area:

[Leevi]

Do you have some AC wires tied to some DC wires here?

Yes 6.3VAC but it's not causing any problem there. The amp is quiet now.

I'm also concerned about this heater wiring laying up against the cap in this area:

It's not anymore there. Did not cause problem either but moved.

/Leevi

[SILVERGUN]

OK.....sorry I was a little late to the party....

Looks like a nice amp  

I took a look at your website and EVERYTHING looks nice,,,and I realize now that you are obviously a professional