Testing Power Tubes

rgalpin · Post by **rgalpin** » Thu Jun 02, 2005 1:47 pm

'74 100W SL - KT88s

My amp became ill on Saturday night. Started blowing fuses left and right. When the dust settled, the amp would hum thru the speaker cab even when the standby switch was set to the standby position. Baffled, i just swapped out all the power tubes (new KT88s) with a set of older 6550s and the problem went away. The amp seems fine now.

KT88s are expensive and I want to test each one in the set to see which one blew rather than trash the whole group. Is there any reliable way to test them other than sticking them back in the amp one at a time to see which one causes the hum?

Thanks!

dynaman · Post by **dynaman** » Thu Jun 02, 2005 7:24 pm

Can you say tube tester?

Albeit, most testers don't run tubes under the same high voltage conditions that an amp does. I've had a few "test" good but run poorly in an amp.

Post by **VelvetGeorge** » Thu Jun 02, 2005 7:32 pm

Without some test equipment, the "one at a time in the amp" method might be easiest.

You could try two at a time first to narrow it down.

George

Billy Batz · Post by **Billy Batz** » Thu Jun 02, 2005 7:50 pm

Use the two at a time method. Looking at the tubes as ABCD then try AB then CD then AC then BD. Take the 2 sets that give you your problem and whatever tube is in both is it.

rgalpin · Post by **rgalpin** » Fri Jun 03, 2005 10:30 am

hmmm...

I'm sure there's a formula for the three at a time method . . .

Beyond that though - it would be cool to be able to know "for certain" if the remaining tubes are still as good as they were before whatever happened. There was a mod in the amp that looked like a DI on the last speaker output. It was an 820K (or maybe just 820 - can't remember) resistor and a cap - the resistor blew - I removed it and the cap and tested the resistor and there was 0 resistance - so the speaker signal was being shorted out - (i think!) - don't know if this is a cause of what happened or a symptom.

Why and how could a hum be going to the speaker cab even when the amp was in standby - just by a bad power tube?

rgalpin · Post by **rgalpin** » Fri Jun 03, 2005 11:37 pm

Tested 'em.
One offending tube.
How can a bad power tube create a hum that is present when the amp is in standby?

Flames1950 · Post by **Flames1950** » Sat Jun 04, 2005 12:01 am

It all depends on what's shorted in the tube I guess.

Can you say tube tester? Albeit, most testers don't run tubes under the same high voltage conditions that an amp does. I've had a few "test" good but run poorly in an amp.

Yeah, like my Dynaco Mullard EL34's -- tested good in my tester, but I was lucky to shut the '75 Marshall off before something blew.
Maybe they'd cut it in my lower-voltage 'Wreck project.....except that I wired the input grid resistors between pins 1 and 5.

flemingmras · Post by **flemingmras** » Sun Jun 12, 2005 1:06 pm

The only way that there could be hum going to the speaker if the standby is off is if the heater internally shorted to something, since the heater in the tube is the only thing running with the amp in standby.

Jon