Humming SVT

[rf7]

When I crank the midrange pot with the midrange selector set to the #1 position, the amp puts out a low freq. oscillation, even with the volume turned down. Any ideas about what could be causing this?

This thing has been sitting for years in a garage, and even had a can of paint spilled into it! I have re-capped the amp already, and it does have a bit more hum than I would like it to have, even without the midrange cranked. Different tubes have not helped, either.

[Mars Amp Repair]

My guess would be really just mismatched power tubes. Have you performed the bias adjustments? These are very sensitive to tube & bias mismatches. After getting the 2 settings close on your meter you can tweek one side or the other for minimum hum........glen

[Enzo]

Plug something into the power amp input jack, that will disconnect the preamp. Now you will be hearing the power amp only. I agree that hum in the power amp is likely from misbalance. One common enough failure is in the driver/bias tube. Even with all 100% power tubes, a bad driver can upset the works.

[loudthud]

Does this version of the SVT have a hum balance pot? If the pot is bad, weird things like oscillations can occur.

[rf7]

Thanks for the suggestions! I did mess with the bias, and it only made the hum worse. I also replaced the hum adjust pot with 2 100 ohm resistors to ground and it did not change the hum level.

It seems to be a preamp problem. I grounded the input into the power amp, and the hum is a lot quieter, not bad at all. I took the preamp output and fed it into another amplifier and heard the same exact symptoms. I'm going to mess around some more with the preamp tubes, I guess. Could a leaking coupling cap cause this?

[Mars Amp Repair]

As Enzo mentioned, you could further isolate the stage by starting with the first preamp tube & just yank them one by one until you get to where the hum goes away.

You could try swapping around the preamp tubes to see if the hum changes. If it does, it could obviously be a tube with Cathode/filiament leakage or some sort of inter-element short or the amp may just need the mod to offset the filiament voltage above ground.

I've never needed to do that mod on an SVT, but perhaps someone else could chime in on that issue. It's usually more of an issue on the V-4's & such.

Glen

[rf7]

I did pull the tubes, starting with V1, and the low frequency oscillation goes away when I pull V3. If I lift C16, the oscillation goes away. I lifted C7 (V3a plate output), C14 (V3b input), and R35 (V4b cathode to V3b cathode) individually, and the oscillation was still occurring. I replaced R27 - 30, but that did not help anything. Again, I've got a low freq. oscillation caused by cranking the midrange control with the 3 position switch in position 1. The other 2 positions of the 3 position switch, cause lesser amplitude oscillations at higher frequencies. I've put different tubes in V3 and/or V4, and the problem still remains.

What is the filament voltage offset mod?

[rf7]

Sorry to bring this back up, but it's such a pain trying to figure this out. The amp has an oscillation when you crank the midrange control with the MR Select switch in the #1 position. Could it be that the MR pot is just amplifying the existing hum in the amp? I noticed that the B+ to the preamp moves up on the scope a few times a second, I thought with new caps it should be rock solid. Any help would be greatly appreciated.

I talked to another tech that I know, and he had a similar problem with a V-4, and he could not figure it out either.

[Amp Kat]

What do you mean by had a can of paint spilled in it ? and where ? If that paint covered a component or cap it is possible that it caused a cold solder condition. Hopefully it was latex so you can just pull it up. How did you replace the caps ? Did you use can caps or Axial caps ? After replacing caps in the SVT the voltage will increse a good bit and it's possible you fried the last 12DW7 as it takes more voltage because it's the first down the line after that dropping resistor. Check your voltage at pin 1 and 6 of the 12DW7 and also across the big dropping power resistor to see what you have there.

[rf7]

Well actually it was spilled on the amp head, there's not much on the inside, and it happened about 20 years ago. It just looks pretty funky from the outside. I used axial Sprague Atoms for the caps along with a couple of Mallory 140uf/475V can caps (to replace the 70uf and 40uf in parallel). I've done several V4's in the past, in a similar manner. Now, I notice that the original has a 100uf on top of a 110uf (70 and 40 in parallel). Maybe I should stick a 100uf in those spots instead.

The voltages at the last 12DW7 are:
pin1: 273V (schematic shows 235V)
Pin6: 197V (schematic shows 175V)

The voltage after the big dropping resistor is 444V. The schematic shows 430V there.

I did the .1uf cap to ground trick to chase the hum, and it all centers around the C16 to VR7 chain. The cap cuts the most hum at the output of the pot, with less of an effect around C16.

I paralleled a 33uf cap with the preamp filter cap to see if it helped, but no change was observed.

[pdf64]

Hi rf7, the problem was originally described as a low freq oscillation. Could you check what the frequency and waveform is, to give an idea whether it is an oscillation, hum from the heaters, or power supply buzz?
That will tell us whether it is worth putting a dc offset on to the heaters (check out aiken for background on this).
Is there a metal case on the inductor, if so have you confirmed it has a good ground connection?
Have you checked all resistors for drift?
Have you remade/tightened/flowed all ground connections?
Hope that helps - Peter

[rf7]

Could you check what the frequency and waveform is, to give an idea whether it is an oscillation, hum from the heaters, or power supply buzz?

It is 60hz, and it's nasty looking. See pictures/links below.

Is there a metal case on the inductor, if so have you confirmed it has a good ground connection?

No metal case.

Have you checked all resistors for drift?

Yes.

Have you remade/tightened/flowed all ground connections?

Yes, as far as I can tell.

Power amp input grounded (5mv/div) and (5ms/div) Mid-Range is on 0 unless noted:http://i168.photobucket.com/albums/u...IMG_0001_7.jpg
Master Volume on 0:http://i168.photobucket.com/albums/u...IMG_0001_3.jpg
Master Volume on 10:http://i168.photobucket.com/albums/u...IMG_0001_4.jpg
MidRange on 10, master volume on 10:
Master Volume on 5:http://i168.photobucket.com/albums/u...IMG_0001_6.jpg

Master volume on 5 is noisier than on 10.

[pdf64]

As it's 60Hz it's most likely heater/filament induced hum. Confirm that those 100ohm resistors between each heater winding leg and ground are good. If they are then try the procedure on the technical q&a on aikenamps.com-

Q: How can I find out if the hum in my amp is caused by the filament wiring?
A: Disconnect the 6.3V AC wires from the power transformer, and temporarily connect in a 6V lantern battery. If the hum is caused by filament wiring, or by induction from the filament inside the tube, it will go away with the battery connected. If the hum is caused by something else, such as a ground loop, it will not go away when the filaments are run on DC.

Q: How can I get rid of the hum if it is caused by the filaments?
A: The best way to get rid of the hum is to generate a DC filament supply, but this isn't always practical, because of the high dissipation caused by the large current draw of the filaments. First, make sure the filament wires are run as twisted pairs, then make sure they aren't routed too close to sensitive grid wires or next to coupling capacitors. If that doesn't do it, try "elevating" the center tap of the filament winding (or junction of the two filament reference resistors) to a potential above the cathode voltage of the tubes, typically between 10V - 50V or so. You can generate this voltage with a couple of resistors arranged as a voltage divider from the plate supply. Be sure to bypass the junction of the resistors to ground with a suitable filter capacitor, or you may inject some buzz or noise into the amplifier from the power supply.

Q: What if the hum is not caused by the filaments?
A: If the amp still hums when a battery is substituted for the filament supply, the cause is most likely one of four things: either the AC wiring (plate supply, filament supply, or mains input) is running too close to a sensitive stage, the transformers themselves are inducing hum into sensitive stages, there is too much residual ripple on the main power or bias supplies, or there is ground loop hum caused by a poor grounding scheme in the amplifier. The true cause has to be determined by a process of elimination. Sometimes it is advantageous to pull preamp tubes, one by one, starting at the input, to determine if the hum is getting in to a particular stage.

Good luck - Peter

[rf7]

I tried the lantern battery, on the preamp stage only, did not help.

I found that the AC power cord was wired with the hot and neutral reversed, so I fixed that. I also found that the Molex connector needed to be cleaned/re-tensioned. But, of course, the amp still hums. AArgh!