Dynaco MKIII hum at speaker without input.

[Adam N.]

For those that looked at my other thread, this is the same amp that blew 2 rectifier tubes about 80 hours apart.

The offending amp works normally otherwise but emits a very noticeable sound from the speaker audible from feet away. It sounds more 60hz to me than 120 hz but I cannot really be sure.

I tried swapping rectifier tubes. No effect.
Both amps are plugged into the same variac, swapped plugs. No effect.
I moved other equipment away from the amp. No effect.
I removed the small tube that's in the board. No effect.
I tried removing the input RCA, No effect.
Bias adjustment doesn't has no effect.

When powered on the amp is silent until it warms up for a moment then the humming starts. In practice we'd normally might not notice this because we usually start streaming before the amps are on anyway.

Could this be a bad can capacitor which might also explain the blowing rectifier tubes?

Is this dangerous to run? Could it harm other components?

Any help appreciated.

 

[Pio1980]

Could be a number of factors, recapping the power supply is basic preventative maintenance in many cases.
If it gets warm or hotter than the one in the "good" amp, replace it.

 

[peterh]

For those that looked at my other thread, this is the same amp that blew 2 rectifier tubes about 80 hours apart.

The offending amp works normally otherwise but emits a very noticeable sound from the speaker audible from feet away. It sounds more 60hz to me than 120 hz but I cannot really be sure.

I tried swapping rectifier tubes. No effect.
Both amps are plugged into the same variac, swapped plugs. No effect.
I moved other equipment away from the amp. No effect.
I removed the small tube that's in the board. No effect.
I tried removing the input RCA, No effect.
Bias adjustment doesn't has no effect.

When powered on the amp is silent until it warms up for a moment then the humming starts. In practice we'd normally might not notice this because we usually start streaming before the amps are on anyway.

Could this be a bad can capacitor which might also explain the blowing rectifier tubes?

Is this dangerous to run? Could it harm other components?

Any help appreciated.

The best way would be to start measuring. No big thing, any AC voltmeter would do.
How much AC is on the can cap's respective connectors, compared to the quiet one.
It could also be unbalanced power tubes, matched tubes is a must !

 

[primosounds]

For those that looked at my other thread, this is the same amp that blew 2 rectifier tubes about 80 hours apart.

The offending amp works normally otherwise but emits a very noticeable sound from the speaker audible from feet away. It sounds more 60hz to me than 120 hz but I cannot really be sure.

When powered on the amp is silent until it warms up for a moment then the humming starts. In practice we'd normally might not notice this because we usually start streaming before the amps are on anyway.

Could this be a bad can capacitor which might also explain the blowing rectifier tubes?

Is this dangerous to run? Could it harm other components?

Any help appreciated.

If the hum starts getting audible when the B+ is starting to rise that is a good sign that the PS caps is not filtering the AC. Dynaco MK3 are hard on the caps because of the high operating voltages and the poorer quality of the can caps now available. I would replace them with 700 or 800v DC link film caps, they would also be a performance improvement over the can cap.

 

[nj pheonix]

Try swapping driver tubes.
If no change , try swapping output tubes.
I'm thinking you have iffy driver tube.
One other thought.
Make sure tube sockets are tight and clean

One more question. Is the amp with the hum the one that was eating rectifier Tubes?
If so , change that can cap , before you do anything else

 

[gadget73]

My first hunch would also be filter caps.

 

[Adam N.]

My first hunch would also be filter caps.

The liter caps are under the chassis and look like small discs? They are after the fuse? I think there is one per amp? Are the filter caps aka the power supply caps?

I have a can cap on order.

The other news is that amp has always run about 10 degrees hotter than the other amp measured at the transformer with a non contact thermometer.

 

[Adam N.]

can someone post a link to an acceptable film cap? There seems to be a huge price and size range and lots of other specs that I would need to know. Also the number of these per amp.

Thanks

 

[gadget73]

the can capacitor. Its in the power supply after the rectifier. It filters the power supply noise.

 

[Erhard-Audio]

my recommendation is that you have a tech, or a friend who has worked on high voltage tube gear, look at your amp. Going by your comments and questions, it does not seem as though you have had a lot of experience with working with tube gear.
Since we are dealing with very high AC and DC voltages here, this is not something you would want to work on if you've not had any prior experience.
These high voltage can be lethal!

 

[Adam N.]

I think if I discharge the can cap I can solder in a new one. One thing that's nice is I have the second amp to copy from.

I've done quite a bit of work on motorcycle harnesses but don't know this kind of electronics.

 

[primosounds]

Using

can someone post a link to an acceptable film cap? There seems to be a huge price and size range and lots of other specs that I would need to know. Also the number of these per amp.

Thanks

The use of the film cap is more of custom installation than a replacement can cap. For the MK3 the B+ voltage can be around 500vdc+ depending on the wall voltage and the health of your tubes. Solen makes a 630vdc film cap but there are large and i am not sure if 2 of them can fit the top of the MK3 chassis. I have included a pic of a Solen 100uf x 400vdc film cap that is epoxied to the top of the chassis on the end. The hot end is down and the top wire is to ground. Whatever you decide in terms of the replacement caps, you should also buy enough for the other amp if you have a pair of mk3. If one cap is bad the other is probably ready to blow as well.

 

[nj pheonix]

I think we established a while back.
Both caps were replaced at some point. They appear to be CE. Without being hands on and testing no real way to know.
Based on description, I (and some others feel that cap is suspect.

 

[primosounds]

I think we established a while back.
Both caps were replaced at some point. They appear to be CE. Without being hands on and testing no real way to know.
Based on description, I (and some others feel that cap is suspect.

I replaced the can caps on a MK3 way back. I forget the brand , but after a few weeks they got really hot and 1 blew its vent . I was not experienced back then and did not insure that all aspects of the amp were properly checked and the B+ was too high causing the failure.

 

[Lavane]

Just a thought, one cap can section failed and shorted in a console amp I got several years ago. It caused the choke to basically melt down. It cooked the varnish right out of it. I had to replace it as well. Could the choke have been damaged when the original can failed maybe?

 

[Adam N.]

I tested the cap every which way round for DC volts with a multimeter and a continuity tester and couldn't find anything but microvolts in there. Seems like it's discharged. It'll be at least a week before I get to it. It was interesting to follow the incoming power through the choke, rectifier and out to the tubes and other components.

It'll be good to spend a little time with it.

Even so I may have a local tech do it for $45. It's too bad the can caps don't plug into a socket like the tubes do.

 

[Pio1980]

I tested the cap every which way round for DC volts with a multimeter and a continuity tester and couldn't find anything but microvolts in there. Seems like it's discharged. It'll be at least a week before I get to it. It was interesting to follow the incoming power through the choke, rectifier and out to the tubes and other components.

It'll be good to spend a little time with it.

Even so I may have a local tech do it for $45. It's too bad the can caps don't plug into a socket like the tubes do.

Some did, not these.

 

[AdamAnt316]

Just a thought, one cap can section failed and shorted in a console amp I got several years ago. It caused the choke to basically melt down. It cooked the varnish right out of it. I had to replace it as well. Could the choke have been damaged when the original can failed maybe?

I didn't have this exact thing happen, but the C354 choke in one of my Mark IIIs showed signs of having been stressed before I got it. The bottom cover of the chassis had a dollop of wax sitting on it underneath where the choke sits, and the amp would start to stink after being turned on, even after I replaced the can cap (used the stock value can from DynakitParts, though they also sell ones with uprated values/voltages). Replacing the stressed C354 got rid of the smell for the most part.
-Adam

 

[GordonW]

One output tube conducting a lot more (higher bias current) than the other, can also cause hum like this.

Does this amp still have a single bias pot for both output tubes? If so, did you try swapping in the output tubes from the other amp, and re-biasing this amp for those tubes?

If that solves the hum- then the original output tubes need to be tested, and/or replaced.

Regards,
Gordon.

 

[damacman]

When you say without input do you mean that you have a shorting plug in the input at present? If not, the amplifier could be oscillating, which can cause rapid self destruction.