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Old 9th November 2014, 03:51 PM   #1
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Default Distortion in 1 channel

I have a vintage magnavox tube with 6QB5 in PP(I think) There are four power tubes total, one rectifier and two 12ax7. My problem is that I am hearing (increasing) distortion in one channel; that is to say that it is getting increasingly worse. I tried to eliminate the cause by swapping out the tubes to the other channel thinking it may be a dying tube, but this isn't the case. I had followed several links that applied to my amp and changed out all of the 'caps' but left all of the resistors as-is! Where do I begin to sort this out?
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Old 9th November 2014, 04:10 PM   #2
rayma is offline rayma  United States
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Quote:
Originally Posted by kingfisher View Post
I have a vintage magnavox tube with 6QB5 in PP(I think). My problem is that I am hearing (increasing) distortion in one channel;
Post the schematic if you can. Check the power supply voltages, and the 6BQ5 cathode currents. If self biased, check that voltage.
Measure all the resistors for correct values, especially the output tube cathode resistor.
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Old 9th November 2014, 05:33 PM   #3
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Try to put "worst" tubes to good channel.
Do You sure that Your tubes are matched?
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Old 10th November 2014, 04:42 PM   #4
DF96 is offline DF96  England
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Yes, check DC voltages. You need to be systematic in swappng valves between channels. Keep a careful note of what is where, when the problem occurs (i.e. how long after switching on), when the problem doesn't occur etc. It is easy to get confused!

Swapping all caps may be overkill, and can introduce new faults which may be harder to diagnose. Are the 'new' caps new or NOS?
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Old 10th November 2014, 09:44 PM   #5
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Try change interstage caps, maybe some is bad.
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Old 11th November 2014, 03:14 AM   #6
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I am hoping that my simple 'Volt meter' will do the job here: but Thanks DF96, I'll proceed with the tube swap!
Searching for the schematic, I had, used it to rebuild this amp from a dust covered gem!
About the caps, Yes all are NEW and not NOS.
One thing I did notice when changing input tubes (12ax7) was the sound came on fairly strong but faded into distortion again. I thought the cause would be corroded sockets! Make sense? Where to order brushes? Cleaning will at the least, give piece of mind!
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Old 11th November 2014, 03:20 AM   #7
rayma is offline rayma  United States
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Originally Posted by kingfisher View Post
One thing I did notice when changing input tubes (12ax7) was the sound came on fairly strong but faded into distortion again. I thought the cause would be corroded sockets! Make sense? Where to order brushes? Cleaning will at the least, give piece of mind!
This sounds quite possible, so replace all the small sockets and see.
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Old 11th November 2014, 09:55 AM   #8
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Hmmm, strong sound that fades away... Reminds me of a loose grid leak resistor in the input section. With everything of and dischared, check the resistance of the grids to ground.
About the sockets: some types can be tightened if they are too loose.
Gently tap or wriggle the valves when playing music to check for bad connections.
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Old 11th November 2014, 10:20 AM   #9
DF96 is offline DF96  England
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Yes, missing (or disconnected) grid leak resistor will do this. How quickly the sound fades will depend on several things, including how hot the valve is, how gassy the valve is, how clean the valveholder is (dirtier the better!).
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Old 11th November 2014, 10:29 AM   #10
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Check the anode voltages (pins 1 & 6) with no signal. A damaged or old load resistor can cause this effect as the anode load reduces with a faulty load resistor.
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