120 Hz hum problem

[rfgordon]

Last night I flipped on the amp to do some practicing and it suddenly had horrible 120 Hz hum. That lunch-hour it had been perfect in a recording session.

I know the hum must be originating in the power supply, but I can't seem to kill it. I checked all the filter grounds and other connections, short of pulling the board and checking the power rail connections underneath.

Any ideas?

[Brian Hill]

Last night I flipped on the amp to do some practicing and it suddenly had horrible 120 Hz hum. That lunch-hour it had been perfect in a recording session.

I know the hum must be originating in the power supply, but I can't seem to kill it. I checked all the filter grounds and other connections, short of pulling the board and checking the power rail connections underneath.

Any ideas?

I'd check your filter caps. How old are they?

Brian

[rfgordon]

The filter caps were new in June, when I built the amp. However, one (I'm not sure which one) has always been a bit microphonic, and you could hear it go "tink" when the Standby switch was turned on. I don't have any spares right now, so I reckon I'll have to order some.

I know that the 120 Hz is unfiltered pulsing DC off the rectifier, so it must be a filter cap problem, right?

[Brian Hill]

The filter caps were new in June, when I built the amp. However, one (I'm not sure which one) has always been a bit microphonic, and you could hear it go "tink" when the Standby switch was turned on. I don't have any spares right now, so I reckon I'll have to order some.

I know that the 120 Hz is unfiltered pulsing DC off the rectifier, so it must be a filter cap problem, right?

It's not the only thing that can cause it but it's more than likely the problem and a easy fix and new caps have been known to go or be bad. Check all the conections first , you know, hit everthing in that area with the soldering iron and check your ground points etc.. Is this the Silvertone thats on youe web site?

Brian

[rfgordon]

If one or more filter caps has gone bad, I'm not surprised. I've got 500 VDC Sprague/Atoms in there, but the filters are seeing the following voltages:
451 VDC off the rectifier
450 VDC off the choke
430 VDC to the power tubes
400 VDC to the rail for V3
418 VDC to the rail for V1
342 VDC to the rail for V2

So, if there is surge voltage to speak of when I power up, several of these caps could be at risk.

Is there a way to use multiple caps to get a higher voltage rating? There aren't that many values available in 600 VDC electrolytics.

[Brian Hill]

If one or more filter caps has gone bad, I'm not surprised. I've got 500 VDC Sprague/Atoms in there, but the filters are seeing the following voltages:
451 VDC off the rectifier
450 VDC off the choke
430 VDC to the power tubes
400 VDC to the rail for V3
418 VDC to the rail for V1
342 VDC to the rail for V2

So, if there is surge voltage to speak of when I power up, several of these caps could be at risk.

Is there a way to use multiple caps to get a higher voltage rating? There aren't that many values available in 600 VDC electrolytics.

Those voltages seem very high. Do you have any service literature to refer to? What type and model or amp is it?

I don't recommend connecting capacitors in series to get higher working voltages. There are people out there that say it works but they're wrong. What happens is one capacitor will usually end up getting more voltage than the other. This is because the leakage resistances of two capacitors are rarely the same and the capacitor with the higher resistance will get a greater share of the voltage and could result in the capacitor breaking down.

[rfgordon]

The amp is one I built using a PT from an old Harmon Kardon mono office PA. It ran 2 6L6s in cathode bias, so I built an amp to do the same. The flaw in my set up was not realizing that the preamp and filter section (which were based on a Matchless Lightning) were s'posed to have lower voltages.

As for caps in series, that's the way the big Fenders do it, and many Marshalls. You're correct about the differing voltages, so that's why many designs have bleeder resistors across the caps, so they keep even voltage.

Howsomever, I have decided to do a couple of things to rework my amp. First, I'll have the first filter as two 20 mF 600VDC caps in parallel for 40 mF. Then I'll add a resistor in series with my choke to bump some of the voltages down, though they're perfect at the plates of the tubes. Thus, I'll have to change some of my dropping resistors if I don't like the tone/feel of the lowered voltages. I might put a resistor after the rectifier into the first filter and see how that affects things.

Ya know, when you think about it, since the tolerance on Spragues is 20%, a 500VDC cap could really be only a 400VDC cap. Maybe I'll have to start using those big non-electrolytics the hifi guys use!

[Normster]

As for caps in series, that's the way the big Fenders do it, and many Marshalls. You're correct about the differing voltages, so that's why many designs have bleeder resistors across the caps, so they keep even voltage.

Howsomever, I have decided to do a couple of things to rework my amp. First, I'll have the first filter as two 20 mF 600VDC caps in parallel for 40 mF.

I may be completely wrong, but I think your voltage capacity will stay the same when you run caps in parallel (600WVDC). Much better to do as "the big Fenders" and run two 80uF 350WVDC caps in series and use 2W/220k balance resistors.

[Brian Hill]

The amp is one I built using a PT from an old Harmon Kardon mono office PA. It ran 2 6L6s in cathode bias, so I built an amp to do the same. The flaw in my set up was not realizing that the preamp and filter section (which were based on a Matchless Lightning) were s'posed to have lower voltages.

As for caps in series, that's the way the big Fenders do it, and many Marshalls. You're correct about the differing voltages, so that's why many designs have bleeder resistors across the caps, so they keep even voltage.

Howsomever, I have decided to do a couple of things to rework my amp. First, I'll have the first filter as two 20 mF 600VDC caps in parallel for 40 mF. Then I'll add a resistor in series with my choke to bump some of the voltages down, though they're perfect at the plates of the tubes. Thus, I'll have to change some of my dropping resistors if I don't like the tone/feel of the lowered voltages. I might put a resistor after the rectifier into the first filter and see how that affects things.

Ya know, when you think about it, since the tolerance on Spragues is 20%, a 500VDC cap could really be only a 400VDC cap. Maybe I'll have to start using those big non-electrolytics the hifi guys use!

Ok. Sounds like your on the right track. Let us know how it comes out so this thread has useful conclution. I am curious.

Brian

[rfgordon]

When the parts gome in, I'll update y'all on this. One thing I've learned is that these vintage PTs will jam some volts since my wall voltage is higher than it was in 1960!

I noticed yesterday that Angela Instruments has some nice caps rated at 600VDC, but they're huge-- 60 mm in diameter! Might have to start building with the 3" deep chassis.