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Old 1st April 2012, 01:43 AM   #1
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Default Blown Output Tubes

Output tubes in an EICO amp (uses EL34s in P-P) and output tubes in a Fisher amp (uses 7868 in P-P) arc internally, then blow out.

All tubes tested very good with no leakage prior to blowing up.

Bias voltages in both units were set to recommended levels.

Diodes, filter caps and coupling caps replaced.

All resistors within tolerance.

All B+ voltages are within specs, to a bit high. Maybe voltage is a bit higher with line voltage higher and new diodes installed. Not sure if this could be causing the problem?

This occurs after 15 minutes warm up with no input audio signal.

What is causing the tubes to self destruct??
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Old 1st April 2012, 02:57 AM   #2
tomchr is offline tomchr  United States
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Did you convert this from a tube rectifier to a silicon diode rectifier? That would make B+ quite a bit higher than its nominal value. Probably 50 V higher... That could be the cause.

Are the amps loaded properly with speakers or are you powering them up unloaded?

Have you checked the bias levels after, say, 10 minutes of warm-up? I find my amp (300B SET) takes a good hour to fully stabilize. Keep an eye on the bias points after powering up the amp. Check/adjust them periodically for the first 15 minutes and then every 15 minutes thereafter.

~Tom
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Old 1st April 2012, 07:18 AM   #3
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Quote:
Originally Posted by tomchr View Post
Are the amps loaded properly with speakers or are you powering them up unloaded?


~Tom
Without sound on the amp, it does not make any difference...
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Last edited by Blaireau; 1st April 2012 at 07:19 AM. Reason: Text forgotten
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Old 1st April 2012, 12:48 PM   #4
godfrey is online now godfrey  South Africa
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Some tube amps oscillate badly with no load.
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Old 1st April 2012, 01:26 PM   #5
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I replaced the diodes.

This was with speakers connected.

Did check the bias. It changed a little on the amps so I adjusted it to the set point.
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Old 1st April 2012, 02:57 PM   #6
DF96 is online now DF96  England
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Who made the valves?
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Old 1st April 2012, 03:02 PM   #7
avp1 is offline avp1  United States
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Quote:
Originally Posted by atwaterkent911 View Post
I replaced the diodes.

This was with speakers connected.

Did check the bias. It changed a little on the amps so I adjusted it to the set point.
If B+ value is within specs of a particular tube, then only other explanation is oscillation at high frequency. Did you check that speakers are not damaged after tubes arked? High frequency oscillation can fry tweeters within seconds.
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Old 1st April 2012, 07:40 PM   #8
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The 7868 tubes are mfg'd by Fisher for the Fisher unit and are original to the amp.

The EL34s in the EICO amp are mfg'd by Svetlana.
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Old 1st April 2012, 08:12 PM   #9
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You might put 50 ohms 5W between the transformer and the rectifier. Will drop your B+ voltage. If that helps, it was too high. Remember wall voltage is 10-15% higher than 1960. A choke of the same value would also help. I mount my resistors on TRW Cinch terminal strips. Who calibrated your voltmeter on the 600 V scale? You can check calibration at up to 30 volts with a zener diode, but I don't have a method above that.
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Old 1st April 2012, 09:13 PM   #10
DF96 is online now DF96  England
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I don't know if Fisher actually made any valves.They are probably just Fisher-branded, but made by someone else.

Svetlana had a good reputation, but whether modern 'Svetlana-branded' valves are reliable may be less clear. These days the brand name tells you even less about the actual design and factory than it did 50 years ago.
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