Hum in SVT power amp?

[Mark]

Thanks Martin, it's largely resoldering. I'll give that a try. What did you read on the earthing issues associated with SVT amps. Mind you I can't believe Ampeg still uses the 22 ohm screen grid resistors bypassed by a diode.

I'Ve included diagrams detailing the earthing inthe SVT power amp chassis.

Various DRWs of SVT earthing.pdf

[martin manning]

I don’t have a link but it was someone’s web page documenting the work. I recall that the external preamp jacks were involved somehow, and that would make sense if they are not isolated and are forming a ground loop. If that’s the case removing them and cleaning the contact areas would be a good idea.

[Mark]

Martin the amp does have a pre-amp out and power amp in Switchcraft jacks mounted on the power amp chassis but they are insulated.

IMG_0706.JPG

EDIT: I got desperate this afternoon and lifted the feedback loop. That didn't help as the hum only got louder. I'm still trying work out how the hum is amplified when I don't see hum on the grid or cathode.

[martin manning]

What is the thing between the jack and the lug? Where is that connected? Also, out of curiosity, can you find a part number on the signal limiting diodes? The schematic doesn’t seem to give that information.

If the hum gets louder without the NFB, that would suggest that the source is inside the FB loop.

[Mark]

Thanks for the reply Martin, the burn component is a 10 ohm resistor. I'm not sure of the part number of the diodes, I can remove one this afternoon and check.

[martin manning]

Does that lug get attached to ground, i.e. it’s the ground for the isolated jacks? Is the 10R open?

[Mark]

Martin the lug gets attached to the chassis. Ground is above chassis ground. I have recently put a 0.1uF cap across the 10 ohm resistor. I may put back to back diodes in to clamp it to 0.7vdc.

This amp is a MIJ SVT, the power amp pcb is identical to the vintage pcb layout. I'm trying to get an idea of the earth wiring is the same.

[martin manning]

Do you know the year this particular amp was made? The 1970 Ampeg schematic I have been referring to does not have the 10R ground lift.

[Mark]

The amp is an 80's model which was made in Japan. The circuit is the one in my attachment in the first post.

SVT-HD_schematics.pdf

EDIT The latest thing I have been looking at is the earthing layout in various SVT amps. Here are some of the pictures I have been looking at. The earthing is different between the older amps and the MIJ SVT.

SVTguts6.jpg

SVTguts15.jpg

75SVT-4-1000.jpg

[tubeswell]

This schematic may be the correct one if its fitted with the 10R fusible resistors

[Mark]

Thanks for your reply. In the case of the MIJ SVT the 1972 SVT circuit diagram isn't the correct one.

SVT HD is the correct circuit diagram. It has the ten ohm ground lift and there are extra power supply filtration stages in this circuit.

[Mark]

Tried disconnecting the 40uF cap that supplies the P.I. stage and use a 100uF cap and a different earth point. No change at all. I also moved the earth point for the hum balance pot, again no change at all.

Back to the drawing board again.

[martin manning]

I thought you had it isolated to the input stage? Maybe just the first tube, meaning when you pull that the hum stops. Can you learn anything by momentarily grounding the input grids, plates, and cathodes of that tube through a 100n cap? Nice .pdf btw, thanks for posting that!

[Mark]

Thanks for the reply Martin. Yes it is in the first stage, but I'm asking myself where is the origin of this hum. I'm thinking if I eliminate it, it won't be amplified. I'm thinking it is an earthing issue but I could be wrong.

I think I need to take more measurements with the CRO.

[Merlinb]

Short out R5 (the feedback shunt resistor). Yes the hum may increase but it is much easier to trace the hum when the amp is open loop.
Also short R2 (input grid leak).
Now connect a big cap (1uF or more) from the input triode place to ground. What happens to the hum?