Ampeg SVT-CL hum

[Randall]

I just did the filter caps on this beast because of a rather annoying 120 Hz hum. Now that I power it up, I have a little 60 Hz hum. I also have one of the 6550C's redplating independent of socket. I really want to get this right, and not sell this guy a $250 sextet of tubes only to find it still hums from some other issue. Can I plug in six good 6L6GC's in place to see if that cures the hum?

[nevetslab]

YES, you can do that. For just checking the hum issue, you can also install just one pair or two pair, making sure to NOT have that tube that is redplating regardless of which tube socket it's in.

You do need to have the working sextet of tubes all relatively close in plate current per half. The balance of the upper and lower plate current levels DOES affect the amount of hum that results.

[The Dude]

...... you can also install just one pair or two pair, making sure to NOT have that tube that is redplating regardless of which tube socket it's in......

^^^^^^Yes, that. I'd just pull a pair that includes the red plating tube to test the amp. For test purposes, the amp will work fine with just 4 tubes. You'd be surprised how many "working" SVT's there are out there with a burnt screen resistor or two and are running on less than 6 tubes.

[Randall]

I have grouped them to try to balance out the sides, but it didn't really make much of a difference. The hum is definitely affected by twiddling the bias controls. The spread in current draw is 16.9mA to 29.5mA.

I put in a hodge-podge of 6L6's from my used stash, and it still hums, and is still affected by the bias pots. One thing is interesting, bias 1 LEDs act a bit differently than Bias 2 on both sets. Bias 1, after it has warmed up a bit has the red LED creep up repeatedly, sometimes having both green and red on until I adjust it, but there is a fine line between just green and both red and green. Bias 2 does not do this.

[Jon Snell]

Have you replaced the 12BH7, V2 & V3?

[nevetslab]

C

I have grouped them to try to balance out the sides, but it didn't really make much of a difference. The hum is definitely affected by twiddling the bias controls. The spread in current draw is 16.9mA to 29.5mA.

I put in a hodge-podge of 6L6's from my used stash, and it still hums, and is still affected by the bias pots. One thing is interesting, bias 1 LEDs act a bit differently than Bias 2 on both sets. Bias 1, after it has warmed up a bit has the red LED creep up repeatedly, sometimes having both green and red on until I adjust it, but there is a fine line between just green and both red and green. Bias 2 does not do this.

I misspoke.....I misread your asking about installing 6L6GC's in place of the 6550's. Those are NOT rated for the high 650VDC plate voltage (spec limit is 500VDC typically) with half that for the screens. I thought you had a different set of 6550's. I gather you don't have any 6550 pulls in your stash to substitute. I would then try using four of the six 6550's, as one is red-plating, and juggle the tubes for best plate current balance. In this operation, I always have both chassis standing on the power xfmr end, with a block under the power xfmr to stablize the power amp chassis. Then, you can measure the plate/screen current across the cathode resistors.

I've had the benefit from managing a large rental stock of SVT-CL's, SVT-VR's and SVT-AV's at CenterStaging, LLC in Burbank, CA, where over the years, I've accumulated 6550's and KT88 pulls from amps that have had one or more tubes fail. If you ARE able to get acceptable results with four of the five remaining working 6550's, you may have to bring in a new matched Sextet of KT88's or 6550's. I've been using J/J KT-88's now for several years, instead of the 6550's. Though I have both types from many vendors to select from in the pulls. Your Bias LED issue may be from using the 6L6GC's here. I've never tried 6L6GC's in an SVT.

Now, another item that controls hum in the amp is the Main PCB's front corner Standoff (on the Output Xfmr side). THAT standoff has to be solidly mounted both to the chassis (often gets loose) as well as the screw holding the PCB down to that standoff. If it's loose, the amp WILL HUM. Then, what will also get you is fractured solder joints along the back edge of the main PCB, as well as the input jacks & pot terminals on the preamp PCB. While you have it open, I'd be looking at ALL of the solder joints on the main PCB, as well as on the AC Mains PCB and the Output PCB. Over time, from the sheer weight of the beast in transit, I find solder joint fractures on a regular basis.....one of the first things I'm looking for once I've pulled the two chassis from the cabinet.

[Randall]

I pulled the 6L6s and now have four of the 6550s, balanced within a couple of mA from side to side. I can't get the green LEDs to come on, only red, but I can null out the lion's share of the hum with the pots. These things normally hum SOME, don't they? If so, how much is normal? I used one tube at a time in the same socket with a socket bias probe to find the tubes to pair up. Even with one tube in it, there was a little bit of hum, not enough to complain over, but the same sound as when four tubes is used, just less.

I pulled the chassis out again and looked for bad solder all around, and found none. The grounds are all tight. This one lives in a studio and doesn't get moved much. I don't have any good pulled 6550s to try. I just want to make sure he needs a fresh set before I order them, but I'm leaning that way.

[nevetslab]

I pulled the 6L6s and now have four of the 6550s, balanced within a couple of mA from side to side. I can't get the green LEDs to come on, only red, but I can null out the lion's share of the hum with the pots. These things normally hum SOME, don't they? If so, how much is normal? I used one tube at a time in the same socket with a socket bias probe to find the tubes to pair up. Even with one tube in it, there was a little bit of hum, not enough to complain over, but the same sound as when four tubes is used, just less.

I pulled the chassis out again and looked for bad solder all around, and found none. The grounds are all tight. This one lives in a studio and doesn't get moved much. I don't have any good pulled 6550s to try. I just want to make sure he needs a fresh set before I order them, but I'm leaning that way.

With only four of the tube installed, the Bias LED's WON'T work. The Range for GRN Bias LED's lighting is 147mV thru 294mV (14.7mA to 29.4mA) in the window comparator. As I recall, the diode logic on the output of the comparators is such that all three in each half have to be turned on so the LED's will light. The LED's conduct from Ground thru the 2.2k resistors to the +15V rail. With all three comparators toggled, that second 2.2k that ties to the diode AND circuit goes high. Otherwise, it goes to -15V, and the LED won't light. The circuit is adjusted for all six tubes. As to the residual hum related to the Bias Balance of the upper and lower halves of the output stage, Balance is usually quite close to minimal hum. Should still be that way with four tubes. What do you get if you dead-patch the Power Amp input jack on the rear (shorting plug)? That will at least tell if the hum is in the power amp or the preamp.

Power Amplifier PCB Schematics (419xxh2).pdf

For some reason, the software is preventing attaching the Power Tube Schematic to mate with the Power Amp PCB schematic, which shows all of the output stage AND the connections to the Window Comparator LED Bias Indicator circuit. I can have either schematic uploaded, but when both are uploaded, neither can be read. No idea why this is occurring.

Normally, I'm not used to hearing any substantial hum from the amp, but am aware that the balance between upper half and lower half of the bias circuit will influence HOW much hum will be there.

[Randall]

I was guessing the LEDs were only going to work for all 6 tubes, I just didn't know why.

When I plug in a shorted plug in the power amp in jack, the sound changes a little bit, but I wouldn't say it is any louder or softer. I sometimes find sound a challenge to accurately describe. I would pull the phase inverter tube if I could figure out which one it is. This schematic is challenging in places. Why do they not include the power tubes?

I am now not even sure this hum is objectionable. My little shop is very quiet. I can't hear it if my refrigerator is running on the other side of the wall. I am going to order a set of tubes and see what that gets me. Stay tuned.

[nevetslab]

I was guessing the LEDs were only going to work for all 6 tubes, I just didn't know why.

When I plug in a shorted plug in the power amp in jack, the sound changes a little bit, but I wouldn't say it is any louder or softer. I sometimes find sound a challenge to accurately describe. I would pull the phase inverter tube if I could figure out which one it is. This schematic is challenging in places. Why do they not include the power tubes?

I am now not even sure this hum is objectionable. My little shop is very quiet. I can't hear it if my refrigerator is running on the other side of the wall. I am going to order a set of tubes and see what that gets me. Stay tuned.

Here's the Power Tube schematic, which wouldn't display when I added it to the last post. Hopefully it will display on this post:

Tube PCB Schematics (41941h3_).pdf

[nevetslab]

This photo is of my clear lexan test panel for working on SVT-VR amps, but the tube compliment placement IS THE SAME as that for the SVT-CL & -AV, as far as their positions in the chassis and relate to the schematic's tube numbers. Driver tube V8 (J8) feeds power tubes V1, V2 & V3, while V9 (J9) feeds V4, V5 & V6. Bias controls, looking from the rear panel....the right one (P2) is for the upper half V1, V2 & V3, while the left bias pot (P1) is for the lower half V4, V5 & V6.

[Randall]

Man, that service plate is cool beans!

[nevetslab]

Man, that service plate is cool beans!

Thanks! This is worlds better than the cardboard binder cover I had made one from before, as you can now see where your test leads are going. SVT-VR plate current measurements are across the 5 ohm plate resistors at 650VDC potential, which always made me nervous.

[Randall]

Well the sextet of JJ KT88s finally came in. Shipments are sure taking a lot longer these days. I installed and biased them, and now this amp is as quiet as you could ask for. Re-cap, re-tube, and factory bulletin updates, and this generated a $465 bill! These beasts are expensive to maintain!

[nevetslab]

Well the sextet of JJ KT88s finally came in. Shipments are sure taking a lot longer these days. I installed and biased them, and now this amp is as quiet as you could ask for. Re-cap, re-tube, and factory bulletin updates, and this generated a $465 bill! These beasts are expensive to maintain!

YES, regrettably they are. Did you keep the tubes removed, or return them to the client? I mostly have two consistent clients for all the SVT's I service, so I'm able to maintain a stash of 'pulls' allowing potential replacement of one or two power tubes that failed or went way out of balance with respect to the rest of the working tubes. Over the years, that has saved spending the nominal $250 per matched sextet of tubes. Other clients, as you had, don't get that benefit when you don't have spares on hand that match the remaining good tubes.