1. Joth

    Apr 22, 2007
    Hey guys, got specific questions I hope I can finally solve. Ive been thru and fixed up three SVT's, all very late 70's. They all seem to have been tampered with and badly recapped over the years that leave some small mysteries and inconsistencies in the grounding I've seen that i'd like to discuss.
    The poweramp schem for '75 ive seen has the driver and power tubes grounds going through a 10ohm 1/2w resistor to the chassis.
    Physically on the amp, this resistor is located between one of the effects loop jacks neg terminals, going to its own chassis ground lug. There is a circuitboard trace that connects all the above mentioned grounds, and runs that to the 10ohm resistor via the same loops sheilded wire.
    However, there is also a brown wire from this same circuitboard trace that I believe goes to one of the lugs of the filter capacitors and general power supply chassis grounding, effectively nullifying the 10ohm 'float' above ground.
    A)Is this brown wire at the other end of the trace supposed to connect to chassis ground?
    B) Is the purpose of the 10 ohm resistor to remove some sort of ground loop at one end only? Or is the entire trace really supposed to be 10ohm above ground? The schematic indicates this, since there are no ground connections shown other than THROUGH the the 10ohm resistor.

    Next issue: Preamp grounding
    I have seen the usual thing where the entire preamp chassis and circuitboard ground is connected to the poweramp grounding through the delicate looking shielded wire ground at pin4. However, I have also seen one preamp chassis that had a wire in pin 6 of the interconnect cable, directly connected to the ground leg of the filter caps terminal strip, thereby also grounding the chassis, since pin 6 is grounded inside the poweramp chassis. Again, like the first issue above, this kills the effect of the 10ohm float above ground, since the pin4 ground connection is within that float. Did someone add in that ground wire to chassis on pin6? I know pin6 is involved with grounding the fan.

    I'm hoping someone can instruct me on the correct way to set up the grounding on these for minimum hum and maximum safety.

    Thank you
  2. JimmyM

    Apr 11, 2005
    Apopka, FL
    Endorsing: Ampeg Amps, EMG Pickups
    I can't help, but I'll bump you for the day crowd.
  3. svtb15

    Mar 22, 2004
    Austin,TX - McKinney,TX - NY,NY, - Nashville,TN
    I play it all. Whatever works for the gig
    I have a 72 SVT that has some sort of ground HUM.. I cant seem to fix it.... I will need a Tech. .But then i am selling it... Love it but just too heavy for me to gig locally..
    But im watching this thread. maybe i will get info on how to get rid of my hum issue
  4. beans-on-toast Supporting Member

    Aug 7, 2008
    Best to reference a specific schematic. Sent you a PM.

    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.

    Hope that this makes some sense to you.
  5. Joth

    Apr 22, 2007
    Mr Beans, thanks for the great reply. The 10 ohm resistor is present on the working low hum '78 that I am currently recapping, and is absent on the 78 that I own which is still in refurb process (lots of noise), it looks to be a cream carbon film 1/2w type, and could be original by its appearance, its definitely supposed to be there as both amps have a stock ground lug locarted on the chassis at that point.
    The patch in/out jacks have isolation washers in both amps. So the 10ohm float scheme was in action on the working amplifier, except that I cant recall where the mystery brown ground wire was connected prior to my recapping the amp (there was a badly executed but working recap job present in the amp when we got it), If I connect this borwn wire to chassis ground, I kill the float, if I could figure out where it should go besides chassis ground, that could explain exactly what this flat setup is supposed to be. The connections to the balance control and ground test point are accounted for.

    I am aware that the older amps such as your 71 did not have this setup, and therefore did not have the issues of isolated jacks etc.
    Im familiar with the ground bus setups and have had good results with that in my own guitar amps that I build, if I werent so concerned to figure out what this 10ohm setup is all about, I would gladly put that system in.
    I think when I figure it out the system of the preamp grounding would be clearer to me as well.
  6. beans-on-toast Supporting Member

    Aug 7, 2008
    With the isolation washers and no other ground points on the power amp chassis, since all the cap returns are isolated from the chassis, then the 10 ohm resistor is indeed providing the offset voltage and elevating the ground reference. It seems to have been a short lived addition to the design. It is a good idea and helps eliminate hum on some circuits but the chassis has to be totally isolated for it to work. This normally includes isolating the input jacks as well so it would extend to the pre-amp chassis as well. The service manual shows the input jacks as well as the pre-amp bus grounded to the chassis. Is the hum reduced when you add or remove the 10 ohm resistor? Can you measure a voltage across this resistor when the amp is operating? If it does help with the hum, doing a test by clipping on a 0.1uF cap and a pair of antiparallel 1N4001 diodes across the 10 ohm resistor might prove to help even more.

    To answer one of your earlier questions, grounding the shield at one end only will provide some isolation from noise. Grounding at both ends could create a ground loop. An offset voltage on one end of the shield will not help.

    Although it may be sacrilege to do so, there are modifications to the SVT that help with hum. Twisting the heater wires on the power tubes. The shielding on the gray wires is not the best, a better quality wire can help. In some cases, I've found corrosion on the shield to the extent that there was no shield in parts of the wire. If I see any indication of rust, I like to change them. Boosting the capacitance a bit on node A in the power supply lowers the hum.
  7. Joth

    Apr 22, 2007
    Well, I've fired up the first amp tonight, having given up on the idea of really floating the ground through the 10ohm resistor, and I grounded the brown wire at the other end of the trace to the filter can ground lugs.
    With brand new caps, new diodes, screen, plate and HV dropping resistors and a general cleanup, really low noise, everything working fine but for a light noise here and there from the tone switches. Oh I also replaced the shielded wire on the interconnector.
    Since this is the quietest of the three local amps (all late 70's), I'm using it as the reference to compare to the other amps to find where I'd gone wrong with the little bit of guesswork from cleaning up old filter cap work and bad/weird maintenance.
    The certain error I have made with the others was having a ground wire running thru the interconnector in addition to the shielded on pin 4.
    On my own personal amp there actually is a pin 6 wire in the interconnect going to the ground lug of the preamp chassis filter cap terminal strip, that is absent on the amp I presently worked on. I removed that wire to an unused tab and tested it hooked up to this other poweramp chassis and noted a big reduction in hum, but it still needs more work, funny noises and sudden loss of function when the midrange control is in the 'boost' half. So some more work needed on tha other amp.
  8. beans-on-toast Supporting Member

    Aug 7, 2008
    Sounds like you've made good progress.

    Perhaps some deoxit (I like the 100% liquid with the needle applicator) applied to the pots, switches, jacks, and tube sockets and pins would help with the dropout problem.

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