Tube noise and speaker crackling - SOLVED

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Tube noise and speaker crackling - SOLVED

Postby galael » Mon Dec 28, 2015 9:00 pm

I wanted to share my recent troubleshooting experience with my home-built Dynaco ST-70 (one of Bob Latino's kits) should anyone else encounter this problem. After a couple of years of perfect operation, I started hearing a persistent high pitched whine coming from the unit itself and some speaker crackling after my unit had warmed up. Infrequent at first, but after a few months, the noise would consistently appear just a few minutes after powering up the st70. Another symptom was the center transformer making a separate sort of resonant tone sound that you could dampen by touching it with your finger. After lots of probing with the multimeter, I finally found the root of the problem.

Short answer: It was a bad 5AR4 rectifier tube. The main noise component appeared to have been coming from the 5AR4 tube filament.
The fix: I replaced the Chinese-made 5AR4 with a Telefunken GZ34/5AR4, readjusted the speaker biases, and my st70 is back in perfect operation. Moral of the story: Stay away from cheap Chinese tubes.
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Re: Tube noise and speaker crackling - SOLVED

Postby Geek » Tue Dec 29, 2015 12:17 am

Glad to hear you solved the problem. Sometimes ones like that are really difficult to troubleshoot, even for those of us with 35 years+ in the field.

The ST-70 is very hard on rectifiers. One trick I do is use "PIV relief" rectifiers. 4 x MUR4100, two in series in each leg on the HV AC side of the rectifier to be "kinder and gentler" on the rectifier. Then you can get away with "lesser" GZ34 designs of tube.

Cheers!
-= Gregg =-
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Re: Tube noise and speaker crackling - SOLVED

Postby EWBrown » Tue Dec 29, 2015 6:51 pm

Those Chinese made "Sino" 5AR4s are not much stronger than an average 5Y3GT.

Though it may be far superior to this "Election Tube" 5AR4 which was made in Chian:

Image

This is on a small "test" power supply approx. 350VDC @ 70 to 90 mA, plus 6.3VAC @ 2A, all built on an inverted baking pan.


I'm surprised that it lasted as long as it did in an ST-70!

Even the rather humble Sovtek 5AR4 is much more capable, I've used them in various ST70s, with good results, and a long operating lifetime. [:)
Last edited by EWBrown on Thu Dec 31, 2015 6:17 pm, edited 1 time in total.
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Re: Tube noise and speaker crackling - SOLVED

Postby burnedfingers » Wed Dec 30, 2015 5:56 am

I think the chinese tubes get a bad rap. The first 5AR4 chinese tube I used went up in a ball of fire. After this I employed a trick I used with my Quicksilver 8417's that couldn't keep rectifiers. I simply put a couple of 1N4007's in series going to the plates of the 5Ar4's. I did this first around 2000 with the Quickies and they are on the same set of 5AR4's. I ran one of my S70's over 3 years before selling it and it had the same chinese 5AR4 in it. This is a trick that others have copied off me since I have mentioned it in a number of different tube forums since 2000. The diodes take the rectification and the 5Ar4 simply acts as a delayed B+ turn on. It still sounds like a tube rectifier. No need for a Old 5Ar4 or a high dollar one.
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Re: Tube noise and speaker crackling - SOLVED

Postby galael » Sun Jan 03, 2016 12:15 pm

I just saw the same advice for the 1N4007 diode modification on Bob Latino's forum site: http://dynacotubeaudio.forumotion.com/t1006-tube-rectifier-diode-mod?highlight=rectifier+fuse+st+-70.

I've ordered a couple, thanks for the tip.
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Re: Tube noise and speaker crackling - SOLVED

Postby Geek » Sun Jan 03, 2016 4:03 pm

Bad advice. Don't use the 1N4007 for that trick, use the UF4007... less noisy.
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Re: Tube noise and speaker crackling - SOLVED

Postby burnedfingers » Sun Jan 03, 2016 5:00 pm

BAd advice to use 1N4007's? Kinda like to see the test results on that one. ;)
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Re: Tube noise and speaker crackling - SOLVED

Postby Geek » Sun Jan 03, 2016 6:52 pm

I'm talking about recovery noise.

Had a ST-70 in here once with an aftermarket driver. Very faint in the background.. bzzzzzzzzzz.... thought it was a CFL, so I ditched the lights all through the house and used a flashlight. Noise persisted.

Read about a similar problem on a guitar forum, turned out to be regular diodes used when FRD were better. Replaced the 1N4007 with UF4007 and the noise went away.

Had a couple preamps with the same issue, a Jolida and a L'Audio - replaced with UF diodes, noise gone.

This is what I base my comment on.
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Re: Tube noise and speaker crackling - SOLVED

Postby burnedfingers » Mon Jan 04, 2016 5:37 am

I've heard the comments on the lower noise diodes. Thought you might be able to provide an actual measurement to that effect. To be honest I've heard comments backing up what you are saying and other comments saying the noise from standard diodes wasn't a problem. Personally I have used this mod that I first made public on Diyaudio back around 2000 and have had no noise problems associated with the cheaper 1N4007 diode. I have tried the low noise in ONE amplifier and didn't see a difference. Now, the fact that you used this in a guitar amplifier is a difference in the control group in my opinion simply because of the higher gain stages present in a guitar amp. I don't believe an average stereo 70 or Quicksilver mono block is going to exhibit any noise because they don't contain the additional gain channels. Its kinda like installing them in a preamp verses installing them in a power amp. In my opinion the preamp would be more apt to exhibit a noise problem. I would have to agree that the low noise has its place in guitar amps and preamp but not necessarily power amps. I believe the higher the gain the greater the possibility of noise.
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Re: Tube noise and speaker crackling - SOLVED

Postby galael » Sat Jan 09, 2016 4:52 pm

I have an update:

I had replaced the Chinese 5AR4 with a Telfunken equivalent and things were grand. Then I happened to hot-cycle the amp one evening without thinking, something I have done previously that never seemed to bother the Chinese tube. Well, I flashed my new Telefunken and also blew the fuse - Telefunken was toast. That was all the incentive I needed to make the diode mods which I've just completed. I used 1N4007s and a went with a Mullard 5AR4 this time round. The st70 with diode mod is totally quiet and hum free. Audio is what I remember, pure and smooth with no unwanted harmonics that I can detect. Hopefully, this will be my last 5AR4 purchase for a while. I'd rather spend my tube money on some KT-88s!
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Re: Tube noise and speaker crackling - SOLVED

Postby Geek » Sun Jan 10, 2016 12:32 am

Probably the tube eliminated the Rr noise, come to think of it.

burnedfingers - There was a really nice PDF on the topic I can't find at the moment (was on Geocities, was hoping someone mirrored it).

Cheers!
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