coupling cap question

[JoshBernstein]

I just had a couple of questions about bad coupling caps. What are the symptoms? Would a leaky cap cause 120 cycle hum? How high does grid voltage have to be to indicate a bad cap? Thanks![/u]

[R.G.]

What are the symptoms?

Shift in the DC bias point of the plate is the biggest one. Grids are very high impedance, much higher impedance than the resistors that pull them to a fixed bias voltage. Any leakage that is noticeable at all will cause a much bigger plate voltage shift.

As an example, if you have a triode gain stage with a 1M grid pulldown resistor, and the coupling cap gets leaky by one microamp, that's a whole volt across that 1M resistor, and the bias on the triode grid shifts up by a volt. However, in a common 12AX7, the negative bias may only be one to two volts in total, so the bias shift is massive. The plate voltage plummets.

A coupling cap that's leaking 100nA will shift the voltage on a grid held by a 1M resistor by 0.1V. That's enough to make the plate drop by many volts.

Worse, your average DMM may not be all that great at measuring grid bias shifts. Many DMMs are only 10M resistance at the inputs, and some cheap ones are aroung 1M. These are low enough to suck down the grid voltage being measured if you parallel the grid leak resistor with the meter.

An old tube service book I read suggested putting your meter on a low volts range and then putting it in series with the coupling cap. Any current at all coming through the coupling cap would show in volts on the meter, and anything bigger than zero volts indicates that the cap is leaking and needs replaced.

Would a leaky cap cause 120 cycle hum?

Not directly, perhaps indirectly. A triode stage, probably not because the plate resistors are so high that it can't pull all that much current through the power supply. A leaky coupling cap to a power tube could increase the ripple voltage on the power supply by causing a highly increased power supply current. If the amp was sensitive to ripple voltage and letting it leak in, then maybe.

How high does grid voltage have to be to indicate a bad cap?

Higher than 0.000000000000000000000001V.

You're dancing around a bunch of things you're not writing down. What amp, what circuit, what are the other symptoms?

It is also reasonably likely that a tube has gone leaky to the grid as well. There is a reason they're in sockets. Have you done tube substitution with known good tubes to eliminate the tube first?

[Stevem]

With a 120 hz hum issue you have three possible causes!
The tube, the bypass cap feeding that gain stage or a bad power supply filter feeding that gain stage.
Swapping in a good known tube is easy as is installing a new 90 cent bypass cap if it is not a circuit board type amp and in regards to the power supply take your meter set for AC volts and read the MV at the input to the plate load resistor and post back what you find.
If this is a early gain stage and you read more than 180 MV ac then you may have a filter issue.

[Phil_S]

Here's something not so technical that I think will address the very basic elements of your question.

A cap is made from a square foil sandwich with a layer of insulation between. Attach a wire lead to each piece of foil. Roll it up like a cigar. This arrangement blocks DC voltage so it doesn't pass from the inner layer to the outer layer. AC voltage, by nature of it's cycling characteristic, is passed from the inner layer to the outer layer (or the other way round depending on how you wired the input/output). A cap is said to be leaky when it passes DC voltage. If it's leaking DC volts, it's also leaking DC amps. IMO, it is easier to measure voltage.

To test for leakage, put the cap in a circuit, set your meter for DCV and place one meter probe on each wire. A few mV of DC is OK. I don't have the technical chops to say where the cutoff for OK lies. I'm guessing a ceiling of 50mV is about it. Ideally, you want zero leakage. Maybe someone who really knows will offer a leakage ceiling?

As R.G. notes, as leakage increases, the bias point shifts. What you can actually tolerate depends on the circuit and what you are aiming for.

[ranjam]

60Hz/120Hz 'hum' is an open filter, or a ground loop. A bad coupling cap will have some DC on the following tube and the grid. Most meters won't spot this, as the input impedance of the meter will just load down the negligible voltage. Well, negligible to the meter.
If you are serious about tinkering, you should spend a few buck and get something like the Sprague TO-5 or TO-6. Will put working voltages on any capacitor you are testing, and show leakage on a meter. Works like a charm.
[img:400:232]http://tone-lizard.com/images/Sprague_TO-5.gif[/img]

[tubelectron]

Would a leaky cap cause 120 cycle hum?

It can, but along with other symptoms like distortion, motorboating or parasitic oscillations, fooling bias of the tube after that cap, high and unstable idle current, glowing plates or screens...

How high does grid voltage have to be to indicate a bad cap?

As stated in the prevoius messages, you should read nearly nothing on the grid (with a good DMM like a Fluke 87 V, at least) if it is a preamp tube or a cathode bias power tube, and the correct bias voltage if it is a fixed bias power tube (with maybe a slight difference lower due to the influence of the DMM).

Try to replace the capacitor by a close value if possible (i.e. a 22n would do for 0.1µ for a test) and see if the problem still arises. If you do not have a bridge, and if your DMM can measure the caps, you should normally find an erroneous value...

And of course, as mentioned in a post above, a leaky control grid tube is always possible (even randomly shorted sometimes) especially with old or used power tubes. Try a substitution first, as advised.

A+!

[JoshBernstein]

First of all, thanks for all of the responses, i appreciate it. This question was about a ceriatone 20 watt lead and bass build that did recently. It is my first build. I had a thread in the marshall section, but it kind of died out.
I have a bad 120 cycle hum and intermittent popping/crackling that goes on. I tried swapping all the tubes with known good ones, and just changed all the filter caps but that had no effect. I have a dmm but its not very good.
Ill post the layout in just a second, i dont have a schematic for it though.
Some other things i have found: the hum is completely eliminated if i short the turret with the 3 220k's and the .0047 on it. (I repleced that cap as well because it was cracked a bit and looked suspicious, but no improvement there). The hum is only impacted by the volume and tone knobs on the lead (left) channel, the bass channel has no effect on it. The hum also stopps if the 2nd preamp tube is removed, but is unchanged by the removal of v1.

[Stevem]

If you flip the amp over onto a sheet of aluminum foil that the chassis can make good contact with does the hum by enlarge go away?

[JoshBernstein]

I just tried that, no diffenence in the hum

[JoshBernstein]

The dc voltage on the 22nf caps from right to left: .3 mv, .5 mv, 15 mv, 21 mv

[Phil_S]

IMO, the dc on the caps isn't the problem. I don't believe there is enough DC present to make any meaningful difference.

AFAIK Ceriatone only gives out layouts, no schematics. So we are stuck.

it would be helpful to see pictures of the build.

120Hz can be lots of things. Here's what I'd do.

1) Pull V1 and see if the hum goes away. If it doesn't pull V2 also. If it doesn't go away pull all tubes. (Power up in between pulling tubes.) Report what you find.

2) If pulling tubes doesn't kill the hum, I'd look at all the ground connections. Tick them off on the layout one at a time and account for all of them. Do it twice, once L to R and again R to L.

3) Use a chopstick or other non-conductive to probe various wires. If the hum changes when you push on a wire, you've probably found a problem. Make a note and report what you find.

Be open to the possibility you have multiple problems. Go slow. Do only one thing at a time or you won't be able to be certain of cause and effect.

[JoshBernstein]

Will post pics in just a second. Just a warning, the routing is pretty bad. When i first started the build about a year ago i had no idea what i was doing and was way in over my head.
Anyway, pulling v1 has no effect, but pulling v2 kills the hum.

[JoshBernstein]

Pics

[JoshBernstein]

Pics

[JoshBernstein]

Also ive done the chopstick test multiple times and i havent noticed and change.