1970s Ampeg SVT refurb

[xtian]

Yes! mS, not µS.

Amp is back in its head cab now, client comes tomorrow to check it out.

[KellyBass]

Congratulations! I would never, ever, attempt a major SVT project like this. They are way beyond a simple tube circuit. But, i gotta tell ya, there is NOTHING in this world like plugging an old Pbass into one of these and letting it rip. Intoxicating is the word.

Thanks for keeping another one going strong!

[martin manning]

So what happened to the hum? Did you find the source?

[Stevem]

The last svt I when thru 2 years ago had this same issue and it when back to the owner unresolved.
I would love to know what the cause was, did you figure out yours?

[xtian]

No, haven't solved it yet. I don't want to leave it unsolved, but I've put many hours in that I feel I can't charge the client for. So we'll take a step back, do more reading/searching the intertubes, and decide how to proceed.

[KellyBass]

Are your cap cans well isolated from the chassis? Has this SVT had many mods done on it...ground-wise? (Aside from the 3 wire power cord install) Is the gray shielded cable that runs from pre to pwr amp still there? These beasts are very picky when it comes to grounding...or so I've been told.

[xtian]

In order: No-they are carefully grounded to the chassis (metal cans, metal mounting plates, ground tabs soldered to metal plates)--these cans are not made to be isolated from the chassis. No mods. Yes, shielded cable intact.

Certainly some work could be done on the ground scheme.

[KellyBass]

Hmmmm....there seems to be many ground schemes depending largely on the date of manufacture and mods. Here's some info from a talkbass forum that I frequent...from a guy with a lot of experience with SVT's:



I see the 10 ohm resistor (R53) on the 1977 schematic grounded to the chassis but I've not seen one installed. Does the resistor in your amp look original to the amp or does it look like someone added it?

One approach you see for providing a ground isolation offset is to use two antiparallel diodes parallel to a capacitor and a resistor. The resistor provides a ground lift, the capacitor serves as a low pass filter removing high frequency noise and the diodes clamp the voltage to 0.7 V. When you implement something like a ground isolation, everything else needs to be isolated from the chassis. Looking at the SVT-VR schematic I see that they have a 475K resistor in parallel with the small signal diodes, D1 and D2.

On your amp, are the pre-amp in and out jacks isolated from the chassis with insulating washers? If they are not, the 10 ohm resistor is doing nothing and I don't know why it is there. As a test, I would disconnect the lug to the chassis, removing the 10 ohm resistor from the circuit, and see if it helps with the hum.

I have my 71 open in front of me and there is no 10 ohm resistor and there is no insulators on the jacks. They connect directly to the chassis. I believe that this is the only point grounded on my power amp chassis. The jacks probably should be isolated but they weren't.

All the cap cans are isolated from the chassis. If there are FP type cap cans installed in the amp, there are normally fiber washers providing the insulation. Otherwise you will have a ground loop and hum. These caps should not be grounded to the chassis with a wire so check for this as well.

I don't have the brown wire running to ground that you described in the amp that I looking at. There is a wire going from the circuit trace to the center lug of the balance pot. The lug from the ground test point (that you use to set the bias) connects to this trace. The ground test point is isolated from the chassis. This connects to (-) of C9, a capacitor in the power supply. As I mentioned, all these capacitors are isolated from the chassis.

As for the pre-amp question, there are several different ways that the 12 pin molex connector is wired in these amps, depending on the SVT revision. It is hard to discuss this in a general way since things like grounds, the fan, and polarity switch connections were different.

Some schematics show pin-6 as a chassis ground. Some people change the connector wiring when installing a three-conductor power cord since the polarity switch was connected to the power amp capacitor ground bus.

My amp is very quiet but I have made some other changes. This is how it is set up.

I removed pin-2 in the chassis molex connector. It had a gray wire going to the cap can bus. This was removed.

All the power supply cap returns are tied together on a bus and connect to the shield on the gray wire as you described. This shield connects to the J1 pre-out/power-in jack at the sleeve tab so is grounded to the power amp chassis at that point. Another shielded gray wire runs from the J2 pre-out/power-in jack to the circuit board ground trace. The input jacks on the pre-amp are grounded to the chassis. The ground buss from the pre-amp connects to this through the molex connector. It's nuts but it works!

The hum pot center terminal connects to the capacitor bus. Check that you don't have a bad hum pot. The amp will hum if the pot is broken.

The common from the secondary of the output transformer connects to the capacitor bus.

So does the orange wire from the power transformer secondary center tap.

If J1 and J2 are not isolated from the chassis (I'm not saying that they should be), the 10 ohm resistor isn't doing anything. No harm in leaving it in place if it is original to the amp.

Pin-6 on the chassis molex has a white wire that connects to the AC input black wire group of three on the terminal strip. C24, the cap in the polarity circuit is disconnected in the pre-amp.

In general, it has been my experience that when you have a single ground bus in an amp, it is most quiet when you order the connections from low to high. In this case the input jacks, with the lowest current signal, start at one end and the bus runs to the highest current circuit component, the first power supply capacitor return. The bus is grounded only at the input, not at the power supply end. The output transformer common can connect close to the phase inverter ground connection. This formula has proven to be very good at keeping noise down. Sometimes you have no choice so you order the connections as best as you can.

In the case of the SVT, here is what the layout of the ground bus is: connect the circuit board ground, the hum pot center tab, and the ground test point at the far end, node C ground; the power transformer center tap orange wire connects to the node A ground bus point; the output transformer secondary common ground connection goes to the node A ground point.


I hope this helps.

[xtian]

Wow, KB, thanks for the extensive post! I will definitely return to this post if the amp comes back....because,

The owner came to check out the amp, said he was happy with the level of hum, paid me in full (though I did not charge him for the hours I spent trying to track down the hum) and took the amp home.