Blown Output Tubes

atwaterkent911 · 1st April 2012, 01:43 AM

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??

tomchr · 1st April 2012, 02:57 AM

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

Blaireau · 07:19 AM

Quote:

Originally Posted by tomchr

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...

godfrey · 1st April 2012, 12:48 PM

Some tube amps oscillate badly with no load.

atwaterkent911 · 1st April 2012, 01:26 PM

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.

DF96 · 1st April 2012, 02:57 PM

Who made the valves?

avp1 · 1st April 2012, 03:02 PM

Quote:

Originally Posted by atwaterkent911

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.

atwaterkent911 · 1st April 2012, 07:40 PM

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.

indianajo · 08:14 PM

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.

DF96 · 1st April 2012, 09:13 PM

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.

1st April 2012, 01:43 AM	#1
atwaterkent911 diyAudio Member Join Date: Apr 2012	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??

1st April 2012, 02:57 AM	#2
tomchr diyAudio Member Join Date: Feb 2009 Location: Greater Seattle Area	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 __________________ 21st Century Maida Regulator, Universal Filament Regulator, 300B Driver PCBs, and more... Neurochrome : : Audio - http://www.neurochrome.com/audio - Engineering : : Done : : Right

1st April 2012, 08:12 PM	#9
indianajo diyAudio Member Join Date: Jan 2010 Location: Jeffersonville, Indiana USA	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. __________________ Dynakit ST70, ST120, PAS2,Hammond H182(2 ea),H112,A100,10-82TC,Peavey CS800S,1.3K, SP2-XT's, T-300 HF Proj's, Steinway console, Herald RA88a mixer, Wurlitzer 4500, 4300 Last edited by indianajo; 1st April 2012 at 08:14 PM.

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1st April 2012, 12:48 PM	#4
godfrey diyAudio Member Join Date: Nov 2009 Location: Cape Town	Some tube amps oscillate badly with no load.

1st April 2012, 01:26 PM	#5
atwaterkent911 diyAudio Member Join Date: Apr 2012	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.

1st April 2012, 02:57 PM	#6
DF96 diyAudio Member Join Date: May 2007	Who made the valves?

1st April 2012, 07:40 PM	#8
atwaterkent911 diyAudio Member Join Date: Apr 2012	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.

1st April 2012, 09:13 PM	#10
DF96 diyAudio Member Join Date: May 2007	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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