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Old 1st February 2014, 03:21 PM   #1
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Location: Elroy, WI
Default KT88 SE developed a buzz

I would really appreciate any advice. I have been troubleshooting for days and have already searched this site and others for an answer. Nothing has been helpful yet. I am kind of new to tubes, and have basic electricity and basic circuit education. I have built 4 amps now.

This is an Abdellah KT88 SE amp. It has been buzz-free for almost a year now. Recently, a faint buzz started in both speakers. At first, it was volume dependent, now it doesn't matter what the volume is set at. Assuming it was a failed component, I have tried replacing some components to no avail.

Here is more info:
- all new circuit components. F&T filter caps, had been JJ
- Edcor xfmrs & choke were new, less than a year old
- KT88EH power tubes, Sovtek 6N1P driver and tried both EH & Sovtek rectifier tubes.
- star grounding is used and basically follows the schematic
- shielded signal wire from jacks to tube socket, grounded on input side.
- short wiring routes and twisted pairs wherever possible AC or DC
- shielded interconnects

Here is what I've already tried:
- unplugged interconnects and also shorted input jacks together
- swapped pre amp tube
- swapped rectifier tube
- tested PTX out of the amp (secondaries isolated) all voltages OK.
- removed volume pot (have a preamp)
- re-wetted ALL ground solder joints and verified continuity
- installed a terminal strip for ground point for input jacks
- replaced 30uF and 50uf filter caps
- swapped LED cathode bias with RC cathode bias
- physically separated signal from power wires, even better than before
- tested .1uF cap signal ground to chassis ground
- tightened the speaker binding posts
- tried different speakers
- turned off electric baseboard heaters and all lights nearby


I thought I had it fixed last night. There was a tiny acceptable buzz for about 30 min, then the louder buzz came back.

Since the buzz is common to both channels, I did not replace the power tubes. I suspect a solder joint or some component that is common to both channels is heating up and failing. Or, is it possible for a PTX to be shot and still have correct voltage on the secondaries?
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Old 1st February 2014, 03:58 PM   #2
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What is the base frequency or the buzz? That would rule out smoothing issues (twice mains frequency). Try removing the 6n1p.
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Old 1st February 2014, 04:01 PM   #3
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Am I reading this right? Your C5, C6, C7 power supply caps are rated at 30uF@370VAC, 50uF@370VAC, 50uF@370VAC...???? AND your B+ is at 400VDC?
If this is indeed true, you have over-stressed these caps!
These operating voltages should be well over your B+ by at least a one to one point five ratio. These voltages should be a minimum of 500VAC ratings. For safeties sake they should be the common 630 volt.
If indeed those are 370 volt caps, it is surprising they have lasted as long as they have.
Check them, they might be bulging out the top....as in, they have failed.




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Old 1st February 2014, 04:27 PM   #4
dinoc is offline dinoc  United States
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i dont think this is a problem with 370VAC X 1.414=523V DC . I used this kind of motor run capacitor for my project over 10 yeasr without any problem.
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Old 1st February 2014, 04:33 PM   #5
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Yes, it is a bit confusing as most voltage values are written as VDC, or the old moniker WVDC (working volts Dee Cee).....yes it would go square root "X" for AC....It just seems it is a PS problem....We'll see....


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Old 1st February 2014, 04:35 PM   #6
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Quote:
Originally Posted by JonSnell Electronic View Post
What is the base frequency or the buzz? That would rule out smoothing issues (twice mains frequency). Try removing the 6n1p.
60hz, I would guess.
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Old 1st February 2014, 04:35 PM   #7
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Quote:
Originally Posted by Richard Ellis View Post
Am I reading this right? Your C5, C6, C7 power supply caps are rated at 30uF@370VAC, 50uF@370VAC, 50uF@370VAC...???? AND your B+ is at 400VDC?


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Sorry, didn't mention that the caps are rated 500V. They are F&T https://www.tubedepot.com/products/f...r-32fx32f-500v
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Old 1st February 2014, 04:40 PM   #8
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Originally Posted by JonSnell Electronic View Post
Try removing the 6n1p.
I tried that. No sound from the speakers with the driver pulled out.
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Old 1st February 2014, 04:47 PM   #9
osscar is offline osscar  Latvia
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I also had a similar problem- small hum in my SET with nos russian 6n6p lamps, clean pins to be sure there is no oxidation and listening them for 12 - 24 hours - then hum gradually disappears. I thing NOS (35 years old) tubes have some vacuum problems.
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Old 1st February 2014, 05:07 PM   #10
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Tubes and sockets are all new production. No evidence of corrosion on the pins or in the socket. I've pulled and replaced them several times the last few days.
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