The sound of a failing tube rectifier?

[tgorham3]

I have an Audio Note preamp kit, M7 variant, that operated quietly at my old house. After a move it has developed an impressive hum. The overload indicators on my ss amp light up with no signal.

I have checked wiring and can find no grounding issues. I have put shorting plugs on all unused inputs and that seems to help (but not cure) the hum. While the audible hum drops with the shorting plugs, the power amp overload lights remain on making me think something sub-sonic is going on..

While the obvious culprit is a ground loop, I haven't been able to chase it down. This is my first piece of gear with tube rectification so I wondered... could what I'm experiencing be explained by a failing rectifier? What would that sound like?

Any thoughts would be greatly appreciated.

Cheers...TG

 

[Johan Potgieter]

Hi Tgoram3,

Your question is difficult to answer - a failing rectifier is best checked by measuring the H.T./scope-on-the-rails.

But the problem appears to have arised as a result of moving, thus shorting inputs and other basic things may not be going in the right direction. I would start (as you mentioned) by checking gound issues: Check your wall outlets (with the necessary care!) for broken ground leads, plugs, sockets, etc - things that might have been damaged during the move. It may be that by sheer misfortune the rectifier is starting to fail just when you moved, but I consider that unlikely.

Good luck!

 

[Burnedfingers]

I have an Audio Note preamp kit, M7 variant, that operated quietly at my old house. After a move it has developed an impressive hum. The overload indicators on my ss amp light up with no signal.

Does the SS amplifier act correctly when it is unhooked from the preamp?

Have you taken a voltage measurement at the output of the preamp's RCA jacks?

 

[tgorham3]

Burned Fingers,

1) Good question. When the amp in my test rig (a sturdy old Apt Power One) is driven directly it is well behaved and the overload LEDs do not come on.

2) Without a great deal of confidence in my multimeter (millivolts just plain aren't part of my day-to-day) it looks like I have about 4-8 mV (dc with no signal) at my outputs.

 

[astouffer]

You want to be measuring for AC on the output.

 

[tgorham3]

AC measures about 45 mV.

 

[Duo]

I suspect oscillation being caused by something. It's not easy to get an SS amp to show overload unless somehow you are causing DC offset (only detected in some designs and even then, coupling caps in most designs will be eliminating the DC offset from ever happening.) or you are inputting enough high frequency stuff to cause the output stage to have shoot-through current (both devices in a push-pull pair turning on at the same time becaues things are going too fast.

It is best to use an oscilloscope to determine what's going on here.

 

[Burnedfingers]

You want to be measuring for AC on the output.

Actually I look for both AC and DC on the output. The AC out of 45mv looks somewhat high to me. I would have suspected it to be less than 3-4 mv. The DC out you measured tells you that your coupling caps look to be good.

If you feel ok about opening up the preamp I would look on the power supply rail. If I were to make a guess I would think a power supply cap may be suspect.

 

[Duo]

If the frequency is high enough, a DMM may read the voltage quite erroneously. There could be a lot more AC voltage in reality.

 

[Burnedfingers]

It is best to use an oscilloscope to determine what's going on here.

I agree

 

[Duo]

In fact, yesterday I was testing the frequency response of a solid state amplifier and found the output stage to do exactly what I mentioned earlier when going up into the 100KHz and up region. An amplifier without output protection can be destroyed by this.

 

[tgorham3]

My thoughts based all of your input

1) While the problem has to be something shared by both channels, a failing rectifier isn't high on the list of candidates.

2) 45mV AC on the outputs is higher than expected. Is there anything about that level that points toward a missed grounding problem or toward a component gone bad?

Unfortunately I don't have an oscilloscope on my "bench" so I may be stuck until I work out a beg, borrow or ebay strategy. In the mean time I'll crawl through my build once again and see if I overlooked somethings that got jarred loose in the move.

Thank you all for sharing your knowledge.

Cheers...TG

 

[Johan Potgieter]

Tgorham3,

This is really hit-and-miss if one cannot determine with a scope what exactly is coming out. You have my sympathy. Radio repair shop in the vicinity that could help out?

To keep focussed, I read that the power amp behaves correctly when fed directly (i.e. from something else). You also mention an "impressive hum", which to me would seem than more than 45mV output. Something makes the power amp overload indicators go on.

Funny things can happen (and do), but for a move to have been the cause of all this, I would place component failure, oscillation etc. low on the list. If you are comfortable with it, try shorting the signal path to ground from the pre-amp output backwards. (You will have to make sure that you do not short any d.c. paths.)

Also, I could not find whether the hum is (equally) there on both channels? Here I run out of further suggestions until it could somehow be determined (scope) what is coming out.

 

[Duo]

The hum is probably due to the high current in the amplifier output stage caused by the high frequency input. This would cause such a heavy load on the amp's power supply as to induce a great deal of power supply ripple which may then get out to the loudspeaker.

 

[tgorham3]

> Radio repair shop in the vicinity that could help out?

Hmm... I'm trying to convince myself that the trade off between paying someone to take a look at the preamp and acquiring a used scope leans toward the scope. I've done a search and have found some good threads on oscilloscope options. Maybe its time to learn something new.

Cheers...TG

 

[Duo]

A scope is a very important piece of equipment to have if you do any audio electronics work, especially projects and such. The scope will tell you things that no other instrument can.

 

[tgorham3]

If you were to pigeon hole me, I would say that I am primarily a constructor. I can do the math and understand topologies to a degree but for the most part I connect point a with point b and choose my components wisely. Cognitively, I am very visual. I suspect that a scopes ability to "see" into a design would help take me from concepts to more fact-based pragmatics.

There is good chance you've all nudged me toward a new leg of the journey. I'll report back.

Cheers...TG

 

[kuroguy]

So, what would one be looking for in a scope? There's ones on ebay for less than $50. Will they do?

 

[Duo]

Generally a good choice is a 20MHz or higher dual channel scope with DC input capability (any good general purpose scope does DC).

The best thing to get is Tektronix, but those fetch higher prices. Many other brands are perfectly fine for most work.

Show us what you find on ebay so that we may comment.

 

[tgorham3]

Kuroguy, I found the following thread helpful when I began to explore my options

Oscilloscope for Home Duty