Ampeg SVT-VR&CL ground loop hum solution

[Mars Amp Repair]

hello folks,
I don't know if anyone has encountered this issue, but I have a few times & have finally found a solution to the overly noisey, hummy, front end on some of these Ampeg SvT-VR's & probably the Svt-CL's, too.

Even with the volumes fully down the amp has hum & noise. If you insert an input in the 'Amp in' jack, the hum/noise all goes away.

The first indication you can try to determine if this design snafu is your problem is to remove the green grounding lead that goes to the preamp chassis. If the nasty ground hum/noise is remedied, then continue on.

You might have also noticed on these amps that in attached pic that the jumpers that are supposed to chassis ground the Star ground spot on the board have been left out. This was to remedy an initial ground loop issue. It worked until another ground was created via the heat sink of IC-1 12V regulator. It punctures its way through the paint in the chassis & then becomes the new grounding point. See pic showing heatsink under main pcb. If you loosen the 3 pcb mounting screws & gently lift the pcb up so the heatsink doesn't contact the chassis, the hum should go away.

The solution is of course to electrically isolate the regulator from the heat sink & then isolate the heatsink from the ground trace.

This can be done easiest by replacing the 7812 with the plastic version 7812CP. see pic.

Then you can dremmel out the connection from the center lead of the regulator to the heatsink. see pic.

Since I received this amp second hand from another service center, I don't know if Ampeg had any kind of insulator in there as a counter measure to keep this heatsink from contacting the chassis or not...I just know what I was up against.

Amp was nearly completely quiet after this countermeasure.

Hope this helps someone down the line...glen

[douglas parks hodges]

Thank You

Thanks...I experienced this problem with my new SVT VR and I will try this solution...I will take these directions for fixing to the nearest authorized repair place

[Mars Amp Repair]

Always great to help...glen

[malu]

Hi Glen,
I have an SVT VR with a similar problem (50Hz hum). If I apply a signal to the power amp input, the hum goes away, but if I disconnect the grounding lead that goes to the preamp chassis, the noise remain (and maybe increase).
However, should I try your voltage regoulator mod or it's better to look for something else?
Thank you very much!
I'm sorry for my poor english.

[Mars Amp Repair]

you should first verify that this is the problem. You should loosen the bottom circuit board & move it around to see if the heatsink getting grounded is the problem.
then you could perform the mod if it is indeed the problem. thanx, glen

[malu]

Thank you Glen!
It's a bad idea to put a piece of rubber (like a bicycle tube) between the heatsink and the chassis?

[Mars Amp Repair]

Definitely a bad idea to use anything but something designed to transfer heat. rubber would not only act as a heat insulator between the sink & regulator, but it would probably melt, too.

It's best to use the plastic variety of the regulator if you can get one. If not, then you have to deal with a mica insulator (or teflon sheet) as well as getting the plastic collar to keep the mounting screw insulated from the regulator.

You could also find a way to keep the entire heatsink from touching the chassis. On some models, they have a piece of material wedged between the chassis & heatsink. I think it's a pretty poor way to do it, but would also serve the purpose if you cannot get the insulated regulator IC.

glen

[Vukefal]

I cant see the photos..is it possible to reupload them since I think that I'm having this problem

Thanks

[drewl]

I can see them.

[gilthethrill]

Hello Glen. Just bought a used 2006 USA SVT-VR and have this hum noise which is driving me nuts. Can you tell me what year your VR is?
I will try the simple remove the ground wire from the pre-amp as you said to do first to see if this hum goes away.

[Mars Amp Repair]

To GillTheThrill:
the SVT that i posted about belonged to a customer of my business 'Mars Amp Repair'. At this stage of the game i have no way of determining what year that amp was.
i would suppose given the date of the post that around 2006 would be good to assume.
If your amp had all the same symptoms eg: when you lift the ground the hum improved, etc and it had the heat sink mounted regulator, your chances are good to perform the mood as described.
you may email me directly if you wish.
MarsAmpRepair@gmail.com.
good luck...Glen

[SimTech]

I added two very thin lock washers under each of the 3 standoffs at the chassis. This elevated/lowered the pcb just enough that the heatsink can't touch the chassis. Then to make sure this didn't add any stress to the pots and jacks I reflowed the solder for all the pots and jacks with them fully installed. Infinitely easier fix than drilling out the RIVET they used to mount the 7812 to the heatsink (what idiot rivets a regulator to a heatsink). The heatsink only has to clear by the slightest amount since there's no real voltage difference between the heatsink and the chassis. Arcing isn't gonna happen unless it gets struck by lightning and that's a whole different ball of wax,

[nevetslab]

One of my steady clients sent me their SVT-VR Amp, it too with significant 60Hz hum, which goes away when I dead-patch the power amp input jack with a shorting plug. Having read thru Glen's opening post on this thread (Mars Amp Repairs) which nicely defined where the hum problems appear to be in this model, beginning with, as he found, no jumpers installed at the Power Amp Ground to the Chassis Grounding Standoff (JWOP11 & JWOP12). I've always found the Chassis Grounding Standoff on the SVT-CL Power Amp will hum if the hardware for its standoff is NOT fully tight at the chassis floor end. Here, that mounting screw (underneath the end bell of the Output Xfmr) is inaccessible for use of screwdriver to hold the screw from turning, after removing the screw from the top of the Power Amp PCB of that standoff. After I had fetched suitable large needle nose pliers to hold the chassis screw from the top of the chassis and an open-end wrench tighten up the standoff securely, putting the PCB mounting screw back in and adding those missing jumpers, I still wasn't getting ground continuity to chassis. So, had to remove the standoff and the chassis screw, then file thru the powder coat paint on both top (under screw/ITL washer) & on the underside of the chassis where the standoff fits, scraped that paint surface off to I had exposed metal. Then put the hardware back into place, now having solid ground.

Powered back up, and now the hum was FAR WORSE.

Difference between the SVT-CL Preamp and the SVT-VR Preamp chassis is...the SVT-VR Input Jacks have metal sleeve HI-D TRS Jacks, type 114BPC, which grounds the preamp input channels to chassis ground there. In the SVT-CL, which have insulated Cliff Jacks, the signal ground goes to chassis thru a 100-ohm resistor in parallel with 47nF.

Both the SVT-CL and the SVT-VR have a dedicated Chassis Grounding Wire in the harness to ground the preamp chassis to power amp chassis.

I lifted up the two jumpers I had installed at the SVT-VR Grounding Point and was back to where I began.

Before going further, I checked my service notes history on this particular amp to see its past history, after seeing date codes on the two power amp driver tubes and input tube (two Electro-Harmonix ECC82 w/date codes of 0702 and Sovtek 7025/12AX7WA date code of 0702). I replaced these with new J/J ECC82 drivers and ECC83S input tube just received. Installed my clear plexiglass Cover Plate so I had access to all of the test points on the power tube PCB Assy and checked/reset the plate current levels of the output stage to make sure I was still in balance, and not having a power tube way out of balance. Minor tweak to reset them to 23mA. Checked the Grid Voltage from the bias tubes, found those lower than normal, being -37.9VDC rather than the usual -45V or close to that). Reset the Balance control. Listened again to the residual hum, and found nothing had changed. Still has significant hum in the output, and NOT 120Hz, but 60Hz (16.67mS intervals)



So, not a tube issue. The heater voltage was +12VDC, very low ripple, so that didn't appear to be an issue.

Then reading Glen's findings on the second chassis ground path from the power amp GND to Chassis was happening when the heatsink for the 12VDC IC Regulator 7812T is making contact with the inside surface of the power amp chassis, I searched thru my inventory to see if I had any plastic-body 7812T's (lacking metal tab, like that installed). I didn't. But I did have adhesive-backed fish paper on hand, so I cut a matching piece to fit between the top of the 7812T Heat Sink and the chassis, marked the hole pattern for cooling thru the chassis for ventilation from the heat sink, punched the ventilation holes into the fish paper insulator and installed that. As they used a #6 Sheet Metal Screw to mount the regulator, all attempts to get that screw thru a #4 insulated shoulder washer failed. I did order plastic body 7812T's which should come in this week. As I now had the heat sink isolated from chassis and no separate ground path from the heat sink other than the GND of the regulator (middle pin), the PCB layout pattern isn't causing an issue.

Powered back up, but even with the heat sink isolated from chassis, no difference in hum in the output.

I installed a 100-ohm 3W resistor in place of those Ground Jumpers I had installed, added a 47nF/400V cap across it between Power Supply Gnd and the Chassis Ground path via Standoff to chassis. Not hearing any difference with that installed and nothing installed where the jumpers were temporarily installed.

I then disconnected the Ground Wire from the preamp chassis. That made it much worse, so that wasn't a solution either.

When I began, I had first dead-patched the power amp, finding the preamp appeared to be the source of the hum. I swapped one preamp tube at a time to see if it was a particular tube but found no solution there either. I also unplugged the two Input jack cables to the two preamp input channels, removing the grounded input jacks to chassis as a possible ground loop. No difference there either.

I'm not defeated yet, as I know there ARE some SVT-VR amps in our rental inventory that DO NOT have this amount of hum in the output. I DON'T have this sort of hum in the output of our SVT-CL amps, nor in the output of this client's SVT-CL's either.

While waiting for the insulated 7812T Voltage Regulators to arrive, anyone have any helpful thoughts on this issue? I haven't yet severed the Ground trace to the heat sink for the regulator, though with the installed insulator between chassis and the top of the heat sink, that seemed to isolate it from Chassis Ground.

[Greg Robinson]

Why not a mica insulator or silpad rather than the plastic package regulator?

[Mars Amp Repair]

Hi,
This is Glen (Mars Amp Repair). Well, I would assume since Nevetslab couldn't get the mounting screw that Ampeg was using to mount the regulator thru the plastic insulator washer, then using the mica insulator would not deal with the screw connecting the metal tab to the heatsink. I imagine that was just a misunderstanding.
Ya know it was soooo long ago when I posted all that stuff about the SVT-CL, that I'm actually impressed by myself, he said jokingly (;- ) I was a lot sharper & more motivated back when I owned the shop and all this was brand new to me.
Anyway, it's really difficult to translate an issue like this thru text without actually being there. I guess I don't have a whole lot more to offer regarding this issue as I can see Nevetslab has a good handle on the issue.
I don't see many of the higher powered amps anymore. Boise is a weird market that way. Of course in 2024, smaller combos seems to be the way things are going that are miked, as opposed to everyone having a monster rig to tote around as well as all the SMPS Dclass stuff that is available.
Definitely post your findings.
Thanx, glen