PA-774 transformer buzzing

for the DIY ST35, the Dynakit and every other PP EL84

Postby TomMcNally » Sun Oct 30, 2011 3:53 pm

Right - but the bolts help tighten the laminations, it's like a
big sandwich.
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Postby corndog71 » Thu Nov 03, 2011 9:15 am

Another thing I noticed is that I'm getting 125VAC from the wall as well as .15VDC. The outlet is grounded. Not sure if this makes any difference.
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Postby mesherm » Thu Nov 03, 2011 3:10 pm

If you have a scope you should check the AC waveform. It might not be a true sine wave. If you have ever run a square wave through your amp at near full power you will hear your output trannies whine.
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Postby corndog71 » Tue Jan 10, 2012 6:30 pm

I don't have access to a scope but I picked up an isolation transformer to see if that might eliminate the hum on my PT. It seems to have reduced it a bit but it's still there.

Tripp Lite IS250 250W Isolation Transformer
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Postby cbates55 » Tue Jan 10, 2012 11:00 pm

Silly question, but have you tried plugging it into another outlet in your new place. Maybe that is the cause of your issue and if so you will need to track down why.
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Postby SVinzant » Wed Jan 11, 2012 7:08 am

Corndog71,
You state that as soon as you connect the heater leads, the buzz returns.

corndog71 wrote:So I'm pretty sure the transformer is not bad. I've disconnected all of the secondaries and applied power to the primaries. Dead quiet.

As soon as I connect the filament leads I get buzzing.
(green+brown = A , green/white+brown/white = B)


If it is not too much trouble, could you try isolating the two sets of heater windings?
i.e. separate the green and green/white from the brown and brown/white leads.
Try running the amp with just one set of heater leads connected at a time.
If your original assessment is correct, you *may* hear the buzz without any tubes in the circuit. If the buzz returns with the green-green/white leads, try disconnecting the green set and connecting the brown-brown/white heater leads and see if there is a buzz.
I don't know what the current rating is of the heater windings, but I would not try running the amp with all six tubes in on only one set of heater leads connected. (Somebody chime in here if I am way off base on this).

I would think that if you separate the heater leads and only hear a buzz with one set and not the other connected, you are looking at a power transformer problem. If you hear a buzz with either set, it could the 100R resistors or still a mechanical issue with the transformer.

You also said that it buzzes without any tubes in the circuit. I think that there are four resistors in the circuit (if the tubes are out of the circuit) so that does not leave very many things to check.
Did you check the R36 and R37 resistors?
Maybe double check (with the aid of a strong light) the solder connection for J1-7 and J1-8 and the connection points for the four resistors. Maybe you are chasing a cold / cracked solder joint.

Some big clear pictures may help as well.

Also there was a suggestion of trying a different receptacle in your apartment. Or maybe take you gear to a friends house and listen for the buzz.

Barring all that, play some Metallica really LOUD! (666) (just kidding) let us know what you find with these suggestions.


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Postby corndog71 » Wed Jan 11, 2012 1:38 pm

To answer some of your questions...

I took the amp to my work (office building) and the PT still buzzed audibly.

R36,37 are jumpers so I did not include them.

I haven't tried separating the heater secondaries but I'm not certain how best to do that. I wondered if it might be ok to run one to a pair of EL84s and say the 12AX7 and run the other secondary to the remaining tubes.
But I would think it would have to be connected to the rest of the circuit somehow. Or perhaps add the voltage divider to each heater secondary.
Any experts reading this? Thoughts?

I have checked for cold solder joints but they're all solid.

I really don't know what else to do. I've resigned to the fact that it's just not going to be perfectly quiet. At a moderate or higher volume It's not noticeable and still sounds excellent so I'm mostly happy.
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Postby SVinzant » Thu Jan 12, 2012 11:29 am

Corndog71, what I was meaning about connecting the heater windings one at a time, you were basically there.

corndog71 wrote:So I'm pretty sure the transformer is not bad. I've disconnected all of the secondaries and applied power to the primaries. Dead quiet.


corndog71 wrote:As soon as I connect the filament leads I get buzzing.
(green+brown = A , green/white+brown/white = B)


So, you had all the secondary windings disconnected from the circuit.
You state that when you connected the heaters you hear the buzz return.
So, instead of connecting everything back, only connect the Green to "A" and the Green/White to "B". Is there a buzz?
Disconnect the Green and Green/White then try connecting the Brown to "A" and the Brown/White to "B". Is there a buzz?

Like I stated before, you are basically cutting the amount of current to power the heaters in half, which is probably not enough to run all six tubes. I think you also stated that you heard the buzz even with no tubes in. So, if that is true, it should not be an issue to hear the buzz with only one set of heater windings connected and no tubes installed. (If I am understanding and remembering everything I have read up to this point.)

Also, tape up the ends (NOT TOGETHER!) of the heater windings that are not connected to the circuit so they don't short to each other or other parts of the circuit.

I guess all this is assuming that your two sets of heater wires are all on separate ring terminals. If not, you will need to unsolder them.

Please try to post some pictures of the power transformer leads connected to the circuit board.

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Postby corndog71 » Thu Jan 12, 2012 3:02 pm

SVinzant wrote:Corndog71, what I was meaning about connecting the heater windings one at a time, you were basically there.

corndog71 wrote:So I'm pretty sure the transformer is not bad. I've disconnected all of the secondaries and applied power to the primaries. Dead quiet.


corndog71 wrote:As soon as I connect the filament leads I get buzzing.
(green+brown = A , green/white+brown/white = B)


So, you had all the secondary windings disconnected from the circuit.
You state that when you connected the heaters you hear the buzz return.
So, instead of connecting everything back, only connect the Green to "A" and the Green/White to "B". Is there a buzz?
Disconnect the Green and Green/White then try connecting the Brown to "A" and the Brown/White to "B". Is there a buzz?

Like I stated before, you are basically cutting the amount of current to power the heaters in half, which is probably not enough to run all six tubes. I think you also stated that you heard the buzz even with no tubes in. So, if that is true, it should not be an issue to hear the buzz with only one set of heater windings connected and no tubes installed. (If I am understanding and remembering everything I have read up to this point.)

Also, tape up the ends (NOT TOGETHER!) of the heater windings that are not connected to the circuit so they don't short to each other or other parts of the circuit.

I guess all this is assuming that your two sets of heater wires are all on separate ring terminals. If not, you will need to unsolder them.

Please try to post some pictures of the power transformer leads connected to the circuit board.

Scott


Ok, I understand you now. I never tried to separate the 2 pairs of heater secondaries. I guess I could try that.

I'll have to dig up my pics of the undersides of the power supply board. Or possibly take new ones. Everything is point to point wired.

Here's the top side:

Image

Edit:
Ok, I found a pic of the underside.

Image
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Postby corndog71 » Thu Jan 26, 2012 9:42 am

Update!!!

So it looks like the AC coming into my apartment is pretty crappy. I've been swapping amps and have noticed my Mk IVs hum slightly and the tripplite isolation transformer I got for the ST35 hums too! The latter has reduced the hum from the ST35 slightly. I can also hear a slight hum from my Outlaw receiver and even my sub amp.

Makes me want to invest in a ps audio power plant.

...or move.
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Postby TomMcNally » Thu Jan 26, 2012 10:25 am

All transformers hum, some are just louder than others.
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Postby corndog71 » Thu Jan 26, 2012 11:20 am

They didn't hum at my last apartment. They were dead quiet.
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Postby TomMcNally » Thu Jan 26, 2012 12:07 pm

I would venture a guess that your old apartment had more ambient
noise that masked the sound, making it inaudible.

Every transformer emits 60 Hz hum, you can hear it and feel it,
obviously levels are different.

I always listen when I flip a power switch for the hum to tell
if something has power applied.
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Re: PA-774 transformer buzzing

Postby corndog71 » Fri May 31, 2013 2:13 pm

I just wanted to follow up on this thread. I'm still working on getting a scope. Hopefully some time in the next month or two. But I'm also moving at the end of June - Same building but different apartment. It should be interesting to see if anything changes. One new thing I've tried was I bought a Humdinger from Audio by Van Alstine. It did nothing for my ST35 but did eliminate the hum on my sub amp.

I never did respond to Tom's last post but it was not a case of more ambient noise. My previous apartment was in the suburbs with buried power lines. All of my amps were dead quiet there.

In my current apartment I measure about 5dB of noise coming from the PA774 with the SPL meter mic about 1" away. I still havent tried splitting the heater secondaries but it doesn't seem like that's going to matter. What purpose do the 100R resistors serve? Could I run one heater secondary to 2 EL84s and one of the driver tubes and then hook the other heater secondary to the other pair of EL84s and driver tube and then connect both of them to the EL84 cathodes like in the stock circuit?

...still learning. *)
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Re: PA-774 transformer buzzing

Postby Shannon Parks » Sun Jun 02, 2013 10:09 am

corndog71 wrote:What purpose do the 100R resistors serve? Could I run one heater secondary to 2 EL84s and one of the driver tubes and then hook the other heater secondary to the other pair of EL84s and driver tube and then connect both of them to the EL84 cathodes like in the stock circuit?


It serves to provide a voltage divider for phase cancellation (think hum pot). Indeed, you can separate the filaments.

Re-reading the thread, is your amp working and it just has a noisy power transformer? Search the forums - it seems this topic comes up a lot. I'd be tempted to cook in an oven to "reseal" the varnish. I'm sure it's been done before, but mind any odors! (sick)

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