Ampeg SVT-VR CH 2 Buzz Help

[Jack W.]

I have an SVTVR in with a buzz (120Hz) that's trouble to isolate. I think I got it down to V5a. Pulled C36 & R44 to separate tube sides. SYMPTOMS :
- No Input
- Turn volume up on Vol 2 buzz grows with Volume & Treble
- Ultra Hi makes it worse (louder & brighter)
- W/guitar plugged in Normal jack, buzz heard with guitar
- plugged into Bright jack Buzz much quieter but still present
- Disconnecting the jacks & grounding J8 header does dim the noise but not eliminate it
- Ch 1 nice and quiet works as expected.
Here's what's done.
- Tube swapped - new tubes
- Resolder Input jack PCB, Jacks are solid & clean
- Voltages look good AC/DC
- Grounds read > 0.3 ohms to chassis
- Resoldered entire preamp PCB
- isolated the +12VDC regulator heatsink as suggested elsewhere in the forums.

I'm kinda thinking IC1 is right next to the tube & it's grid resistors, some kind of leakage? The supply to the IC is +12VDC regulated & -15VDC. The +12V has no ripple but the -15 has a little present.
I know the 2nd channel could be skipped & use only ch1, but It's not mine, any help or ideas is appreciated.

[Jack W.]

No suggestions? I know these type of issues can get complicated, especially when the MFGs are little help. And some you have to watch how much time you can spare chasing gremlins...

[The Dude]

If you ground the grid of V5A does the noise go away?

[Jack W.]

Thanks Dude, Grounding the grid by shorting J8 (w/gray coax cable disconnected) quiets the buzz some but doesn't eliminate it. The only way the buzz is not heard under the normal layout is by turning the CH 2 volume completely off. The Pic shows the tube. The Pilot lite is wired (black wire) to the PCB above the tube w/fastons. The yellow wires go to the standby switch. It just seems like the tube itself is picking up a stray Hz in V5A and amplifing it. Tried several good tubes but results are the same, worse with some. All resistors measure out on a Fluke to 1%. Kinda thinking now maybe a bad tube socket, gotta order this type to see.

[The Dude]

......The only way the buzz is not heard under the normal layout is by turning the CH 2 volume completely off......

So basically grounding the grid of V5B kills the noise. That, at least narrows it down substantially to something related to V5A, the jack board, or the switch board. You say the buzz is 120HZ and also say that the ultra high switch makes it worse. The switch works with a 470pf cap so I can't see how that would have an effect on 120HZ. It makes me think that you might have a connection problem to the switch board (J12 to J9). I would check those pins board to board and make sure you have solid connections, the cable is good, and that pin 2 of that connector carries ground all of the way to the switch board as it should.

[Jecarroll7]

When you say the AC/DC voltages look good, what does that mean? Are you seeing any AC ripple on your supply voltages?

[Enzo]

You may have more than one source of hum here.

So it is before the volume control and after the grid of V5a. Try this: unplug the little switch board, now without that stuff does the hum change? J12 empty.

Now ground pin 1 only of J12, does that kill the hum or just lessen it?

You can also try wrapping a short piece of wire around pins 1 and 3 of J12 to take the place of the switches. Don't use a clip wire because it wil act like an antenna, just use an inch or so of plain wire.

Just for fun, ground the casing of the volume control.

You think the tube is picking something up? Slip a tube shield over the tube. Clip a ground wire to the shield. Your photo shows an open chassis, is there shields over the preamp tubes when the whole thing is assembled?

[Jack W.]

Ac/DC voltages as respect to whats called out on schematics & what you would normally expect to see. AC as far as injecting a 1kHz & 400 Hz sign wave & expected gains thru the stages. Enzo, I have messed with disconnecting the switch board. I have resoldered that PCB as well. I used a .1/630V cap on a wire to chassis to track down the buzz. It would stop when touching any of the pins to the switch PCB (except ground of coarse.) When switch was disconnect,buzz was still present, touching pins on J12 produced same results. Thats when I pull out C36 & R44, Buzz was showing on the plate V5A. Putting the cap/wire on R15 grid lowered the volume of the buzz, but did not eliminate it. I was thinking of dropping a shield over that tube...will do next chance. There is a metal plate that covers the whole preamp chassis several screws hold it on. Off or On the cover makes no difference to the noise, nor does moving yer finger around the tube. ( tryin to induce a change in the noise). I also checked the nuts on the controls (& jacks) after a deoxit cleaning, for contact to chassis. Will check their case grounding as well.

[Enzo]

Wait, so you didn't ground the grid of V5a, you only shunted the grid with that cap? If that is so, try a hard wire ground from the grid to the base of R17. In fact, tack solder a short piece of bare wire across R15. Your ground clip wire - and I use the same technique - is killing the hum on the plate side of V5a, but not the grid side. The grid side has all the tube gain after it, so the clip wire itself might ground off the hum but act as an antenna to reinject some. Possibly.

Just for the hell of it, try measuring resistance from the base of R21 (V2b cathode resistor) to the ground terminal at J8 Since J8 takes the input jack, it is possible a ground trace got burnt open somewhere on the board.

[Jack W.]

I did do a hard ground on J8 at first which only muted the buzz not eliminate it, before I used the cap/wire (with the same results). The ground measurement from R21 to J8 read 0.08 ohms (with or without the j8 coax to input jacks). Yep it's an odd ball thing goin on here. Won't get the chance to work on it more till tomorrow and try out the rest of these suggestions. I'll keep ya posted.

[Jack W.]

Ok back on the bench. Tried all the suggestions. No change. Still buzzes. Tried messing with chassis grounds, no change. When I removed the green chassis to chassis wire it got worse a bit ..mostly more hum.
Pulled it apart, looked for contamination, fractures, etc. I couldn't find anything conclusive. Just to sleep tonite pulled the tube socket clean, tensioned, back in...no change. Here's what I'm thinking now..swap out the controls. Even though grounding the cases didn't seem to help any.
Well here's some DSO shots at what I'm seeing & hearing.
Conditions: CH1 & CH2 : Ultra Hi - ON , Ultra Lo - OFF, Treble - Full, Bass/Mids - OFF, Volume - FULL

Pic9 : CH 1 - taken across output load
Pic10 : CH2 - with J8 header hard shorted. Taken across output load
Pic16 : CH2 V5A output taken @ J12 Pin 1 input J8 hard short

Some kind of grounding issue,(or maybe voodoo..?) I just can't isolate it...The two channels share so much that I can't figure why it doesn't affect Ch1...again any help is greatly appreciated.

[nevetslab]

I had one of our SVT-VR's with the same problem. In looking close at the layout of the PCB, there's actually an induction loop in the layout for Ch 2, and I ended up running a shielded cable from the input jacks along the front of the PCB, if memory serves me, and cut the traces that had routed it to that input tube for that channel. It did improve matters, though didn't completely cure it. I didn't have the same problem with the other SVT-VR's for some reason. I don't have the documentation in front of me on that repair for further clues.

[nevetslab]

Long Ch 2 Input Cable source of hum?buzz pickup

I found my notes on the same issue you're having on the SVT-VR.

...now disconnecting (at the input tube stage) the long shielded input cable from the jack PCB, and shorting those two pins out, to see if the cable is picking this field up.

That’s the culprit…..input cable is really noisy. Single-conductor shielded cable, so the shield is coupling in noise from the stray magnetic field. Because both channels are tying their shields from the input jacks, which are metal bushings and are getting grounded to the chassis, one cable connects at the far left side ground, the other to the far right side, you have a hum loop. I did try and break that loop by lifting one end, using the shield as just a shield, but that didn’t solve the problem. Now, I get to try and replace the cable & connectors with another cable. Wonder if I can pull the connector apart and tack-solder new Belden 8761 2-cond shielded cable into place?

I checked to see if I had any small diameter tubing to insulate the drain wire of the Belden cable. No Teflon tubing on hand. Checked to see if Kerry had any….nope. Guess I’ll have to use 1/8” heat shrink to couple it and the black lead.

Checked to see if I had any 0.10” spaced Molex-style crimp connectors…I did. So, made up a new input cable, tying shield at the input end, let it float at the other end, while connecting the signal and ground lines inside the shielded cable. Installed the new cable, put the preamp PCB back into the chassis and reconnected to the amp chassis. That cured most of it. There’s still a little bit of buzz, but whether or not it will go away if I remove and reverse the cable, tying the shield at the input jack end, I’d have to tear the preamp apart again. I’ve made note of that, and if it comes back, I’ll deal with that issue then.

I found photos of the install the next time I went in to service the amp. I had also made some revisions to the grounding. 1st image shows the trace cut on the ground line, just above V5 tube pattern. Also, trace cut to route the plate VDC line from the filter cap, instead of from the dropping resistor, as the board was laid out. The revised grounding eliminated the hum loop around Ch 2’s circuitry. Also seen is the routing of a new 2-cond shielded cable to route the Ch 2 input signal along the front of the PCB, above the ground trace over to the input connector. Shield is connected at the jack PCB, & dropped at the input circuit connector.

[Jack W.]

nevetslab, thanx for the great info & pics. seems we both came to similar conclusions. It really seemed to be a case of induction. I replaced the caps associated with V5 as I've seen the caps in some Fender's act like antenna's to "catch a buzz" into the ckts. But there was only minor improvement here. (the tone is better). Ended up isolating the ground/shield as you have at J8. The whole amp is chassis grounded at the input jacks which is carried by those coax cables...not ideal IMO. Signal ground & dirty grounds are tied at the PS filter caps then ran to the preamp PCB. Run the tube traces a bit more direct & away from those IC PS voltages as well. As you described not a "cure" but much more acceptable. Thanks again to all for the help, really appreciated.