1970s Ampeg SVT refurb

[cbass]

I don't think you need any balancing resistors to parallel caps.

[xtian]

Of course. That's for series. Right. I'm no closer to finding this nasty hum.

[Leo_Gnardo]

I'm no closer to finding this nasty hum.

Those SVT hum balance pots are often carbonized at least or vaporized more likely - by accidentally sending 700 volts through 'em in an output tube short which practically every SVT must have had by now. Any pot from 100 to 500 ohm will do, recommend 1 or 2 watt. Extra points for wirewound.

[xtian]

I'm no closer to finding this nasty hum.

Those SVT hum balance pots are often carbonized at least or vaporized more likely - by accidentally sending 700 volts through 'em in an output tube short which practically every SVT must have had by now. Any pot from 100 to 500 ohm will do, recommend 1 or 2 watt. Extra points for wirewound.

Cool. I guess we could test the theory by replacing the pot temporarily with 100R resistors?

EDIT: lifted the legs, and pot measures very nicely, very smooth.

Keep the idea coming, though. The hum is not just in the power section; gets twice as loud when I put the PI tube back in. Does not seem to be in the preamp, though; grounding the output of the preamp does not remove much hum.

[xtian]

The rectifier diodes look ancient. Has that ever been a source of hum?

[Leo_Gnardo]

The rectifier diodes look ancient. Has that ever been a source of hum?

If one or more are open or shorted, yes. Can't say I've seen it happen much in SVT's.

Back to hum, yes if the main hi voltage 700V or screen grid supply 350V OR don't forget bias supply, any have failing filter caps, that'll get you hum with just the output tubes in place.

If I'm not mistook it takes both the 12BH7's and 12AX7's to be in place in order to settle the bias voltage as well as signal into the output tubes.

I wouldn't risk the new tubes while you're trying to solve this. What I do is just put in one pair of "pulls" - used good 6550's or KT88 - while troubleshooting the power amp on SVT's.

At least it's nice to know the hum balance pot isn't at fault.

[xtian]

Leo, I replaced ALL of the e-caps with CE multisection cans and F&Ts.

There is only one 12ax7--it is the first two triodes of Channel 2. The PI is a 12dw7, which feeds a pair of 12BH7 before hitting the power tubes.

I'll take your advice about using second-string power tubes for testing.

[Stevem]

What is the ma each output is actually idling at?

[xtian]

I set it to Ampeg's spec, which is 0.072v per side across 1R cathode resistors, meaning 0.024v per tube.

As one would expect, as I turn the bias voltage down, and the current across the cathodes drops to zero, so does the hum.

[xtian]

Stumped. At this point, I have pulled the preamp off and grounded the grid of the PI, and am trying to find the cause of the hum in the power section.

My scope shows it's a combination of 120 and 60 Hz. The ripple on my reservoir cap group is sawtooth 60Hz, and same also for the bias supply.

Here is an interesting observation. Note that the standby switch is in the PT PRIMARY, and allows 120vac into the PT. The amp's power switch turns on the heater transformer first, and the standby switch then turns on the H+, the bias supply, etc.

If I turn the standby switch off, the hum continues for a couple of seconds as the reservoir drains. However, if I turn the power switch off first, the hum stops immediately!

This suggests to me that the hum is coming from the heaters. Agree? How to go about confirming or denying?

Combined schematic attached, FYI.

[martin manning]

The ripple on my reservoir cap group is sawtooth 60Hz...

This says to me that only half of the FWB is working.

[xtian]

The ripple on my reservoir cap group is sawtooth 60Hz...

This says to me that only half of the FWB is working.

Interesting! Diode replacement, then?

Waitaminnit, isn't that sawtooth wave typical after the reservoir?

http://www.circuitstoday.com/full-wave-bridge-rectifier

[img:493:435]http://www.circuitstoday.com/wp-content ... filter.jpg[/img]

[martin manning]

Yup. But you can test them to confirm.

[xtian]

This is the ripple at the reservoir. That's 3v p-p or about 1v RMS at 60Hz. Isn't that typical?

[martin manning]

If all four sides of the bridge are working you should have 120Hz ripple.