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Loud buzz upon turning on the amp

149.5.23.2

I have a no-name Chinese SE amplifier powered with two KT88 tubes. I know everything is relative, but it sounds awesome.

I asked the seller whether I can use the GE 6550 tubes instead of the stock Chinese instead of the stock Chinese KT88-98 tubes. She said yes, and I've been listening to it for approx. a month without any problems - until yesterday.

I turned the amp on and immediately heard a very loud hum and buzz from the speakers - it sounded like a motor sound. The CDP was not turned on. I turned the amp off and noticed that I forgot to set the volume switch to zero. The same thing occurred once again.

I cannot say for sure but as far as I remember, I did not always set the volume knob to zero before turning the amp on, and no such thing occurred when I used the stock KT-88 tubes.

When I heard the buzz, I switched the amp off immediately both times, then set the volume to zero and switched it on again. After that, no such thing occurred again.

Perhaps, this is explained by a different nature of 6550 tubes as compared to KT-88s and by a current in-rush upon turning on? The amp is auto-biased.

Soundwise, I like the GE 6550A tubes better here, though the stock tubes are also great.

Any ideas? Should I always set the volume to zero before turning the amp on?

Follow Ups:

Sounds like you had a grounding glitch; loose connection... - Interstage Tranny 18:23:40 08/24/14 (4)
It should not be necessary to turn the volume down at turn-on...but it is a good caution to turn it up slowly...

Good interconnects, especially gold plated connectors, do tend to oxidize over time. That oxidation causes resistance or an open connection. An open ground connection from preamp to amp, or even cd to amp, can blow fragile speakers.

Also, remember this fact. Low powered amps pushed to high distortion levels, can blow speakers as easy, or even easier than using high powered amps ! That loud ground buzz will easily push a SE amp to high distortion and possible speaker damage...be careful... and ENJOY !
- RE: Sounds like you had a grounding glitch; loose connection... - akolegov 02:51:11 08/25/14 (3)
  Follow-up:
  
  I replaced the GE 6550 tubes with the stock KT-88-98 tubes (as thought that one of the GE tubes might be the culprit).
  
  Nope. There was the same buzz, and I started to think about faulty caps or resistors, but decided to replace vintage Sylvania VT-231 tubes with the stock Chinese tubes, and it was OK.
  
  So, one of Sylvania VT-231 tubes was to blame.
  It is interesting that I purchased them as NOS from a reputable dealer and barely used them. Is it possible that a tube may spontaneously loos vacuum? I don't have a tester to check it.
  
  Now the amp sounds nice (warm, moderately colored and consistent) but it has lost some magic and subtle detail that were present with vinatge tubes.
  Will try the GE6550 tubes again.
  - RE: Sounds like you had a grounding glitch; loose connection... - Michael Samra 14:18:50 09/08/14 (0)
    The old vintage tubes develop oxidation on the pins or sometimes especially on 6SN7 types the tube's pins need to be heated up and solder reflowed..I did this on two of my 6SN7 TungSol round plates and they have been superb ever since. You may just need just need to clean the tube pins with some tuner cleaner or alcohol even but look at the bottom of the tube pins and see if the solder joints look corroded.
    
    Honest amplification is better than excessive 2nd order distortion anytime.
  - noisy VT-231 on fire-up - FenderLover 06:57:07 09/08/14 (0)
    Almost sounds like the power supply maybe at issue and is causing a sensitive tube to manifest noise on fire-up. Can you try those Sylvania VT-231 in another amp, to rule-out the tube being the sole issue?
  - RE: Sounds like you had a grounding glitch; loose connection... - mhardyman 06:12:59 08/26/14 (0)
    "So, one of Sylvania VT-231 tubes was to blame.
    It is interesting that I purchased them as NOS from a reputable dealer and barely used them. Is it possible that a tube may spontaneously loos vacuum? I don't have a tester to check it."
    
    If vacuum had been lost then the getter flash would be a cloudy white. If it still looks like a mirror then the only way is to test it.

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