Power tube squeeling on standby?! - diyAudio
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Old 5th June 2012, 01:46 PM   #1
Jclay85 is offline Jclay85  United States
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Default Power tube squeeling on standby?!

I'm having a very strange issue with an old National amp I picked up. It's an old Westwood model, with 2 12ax7's, a 5y3, and a 6v6. When I got it I opened it up and noticed one of the coupling caps leaking so I went ahead and replaced it, along with another coupling cap and the filter caps. It used one of those old Mallory can caps so I found a suitable replacement from tube depot. I also went ahead and added a 3 prong cord. I grounded it to a power tranny lug.

I also got new preamp tubes and a new 6v6. I purchased the vintage 6v6 used off eBay from a reputable seller who claims it tested strong.

Now, when I turn the amp on, it plays ok, but with an annoying buzz. It does get louder when I turn the amp up, and I'm thinking this is just typical dirty power, etc?

What really concerns me is the fact that when I put the amp on standby the power tube starts squeeling. Not through the speaker, but noise emitting from the actual tube. I looked at it and it was emitting a bluish slightly flickering light like it was shorting or something? I have no idea. Any ideas?
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Old 5th June 2012, 02:01 PM   #2
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How do the stand by switch acts? Cuts +B, place a large negative bias on 6V6s...?
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Old 5th June 2012, 02:06 PM   #3
Jclay85 is offline Jclay85  United States
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Thanks alot for taking the time to respond. I'm not exactly sure how to go about determining how it acts. How would I go about doing that?
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Old 5th June 2012, 02:16 PM   #4
Jclay85 is offline Jclay85  United States
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Default pics

If it helps, here is a picture of the inside of the amp:

Click the image to open in full size.

And here is one of the exterior configuration:
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Old 5th June 2012, 04:16 PM   #5
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Squealing what? Power transformer, speaker, output transformer?

I ask the way the stand by switch is wired, because in this manner it is easiest to identify where is the issue, after or before that switching.
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Old 5th June 2012, 05:15 PM   #6
Jclay85 is offline Jclay85  United States
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Its literally the power tube that is squealing. Its a terrible noise. Not extremely loud or anything, about the decibel lever of a meowing cat... Strange comparison, but it's the first thing that came to mind...

How do I tell which way it is wired?
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Old 5th June 2012, 06:29 PM   #7
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How many time does the tube do it sound. Some tubes, and any other stuff makes noise when heating (because of dilatation of materials), and when cold (by contraction), like a rain noise on a steel sheet. but for few seconds.
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Old 5th June 2012, 07:07 PM   #8
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You can see from the picture that there is no switch between the transformer, rectifier and output transformer. Is there a model number anywhere? I can not find a reference to a "National Westwood" in any of my usual sources.
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Old 5th June 2012, 07:20 PM   #9
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How are you putting it on standby? I don't see a standby switch.
If the output tube is squealing, it seems to indicate that the loudspeakers may not be connected and the stage is oscillating.
We really need more information.
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Old 6th June 2012, 12:23 AM   #10
Jclay85 is offline Jclay85  United States
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Well, Im certainly learning a little about this amp through the process. First off, my apologies. They amp was already put back together when I initially posted and I just grabbed a picture of the inside of this amp off the internet. Upon close inspection, I realized mine has has some modification and apparently someone installed a standby switch of some sort. I took my amp back apart and took some pictures. I hope I didn't wasted to much of anyones time the first go around! Again, my sincerest apologies.

What I would really appreciate your opinions on at this point is:

1. Being that this amp didn't even come with a standby switch,should I just disconnect it altogether and fix this problem that way? If so, and idea which wires I connect when I remove them from the switch terminals?

2. Also, any idea what the hum/buzz might be caused by?

Here is my actual amp. Its a National Westwood Model N 6416 T by the way.

Click the image to open in full size.
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Thanks a lot for any additional help on this guys.
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