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Burning question about two VAC amps and dead tubes

I borrowed a friend's PA 80/80 and like it so much I bought a PA 100/100. While borrowing the 80, I noticed the tube on the far left overheating. I took it in and replaced all 8 tubes, the unit had never been retubed. It was plugged directly into the wall. When I picked it up I did not reinstall it in the system as the 100 had arrived. Everything was fine for a week aster i iinserted the 100. I then noticed a hum in the left channel coming from the speaker. The hum got louder. Tonight while idling, the far left tube went black, the other three overheated. The bias lights were all green. I turned it off immediately. The first set of tubes were original vac tubes in the 80. The tube that died tonight and took the other three with it were gold lion reissues with RCA clear top 12au7. I have another quad of gold lions and Mullards but am afraid to use them until I get some advice......power issue, demonic possession? I'm open to suggestion. It seems like more than a coincidence that the far left tube would fail on more than one amp.
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ebm
9,311 posts
 
Thats why i love my Rowland 8T.Had many tube amps CJ ,ARC etc too much trouble!!
notbananas
17 posts
 
I invested in a Variac just for this type of problem. If you bring up the line voltage slowly, you have a chance to measure the voltages on the tubes, assuming you have the schematics and a digital voltmeter. If no schematic, then compare the voltages of the working side with the one not working.

I also have a tube tester to check all the tubes before powering on the amp. Tube testers are relatively cheap and if you have tube equipment, well worth the investment. Make sure you get one that measures transconductance not just good-bad indicator. I use the tube tester to make my own matched set of output tubes rather than pay a high premium for them.

Plates glowing cherry red indicates there's a huge current draw of that tube only. This could be caused by a defective coupling capacitor which allows plate voltage from previous pre-amp stage to be on the grid of the power amp tube, thus forcing it to conduct max current.

The loud hum also indicates the most common problem: bad electrolytic caps in the power supply. I also invested in a ESR (effective series resistance) capacitor tester. This will measure caps "in circuit" if the cap is bad (leaking, shorted, etc.) and gives you the actual value. Unfortunately it gives false reading if a filter cap is attached to a choke or power transformer, so I have to clip one lead before I measure it.
fplanner2010
283 posts
 
You can continue on with this brain damage or you can just call VAC, explain what happened and ask for a bit of advice. I know that sounds too easy for some of you, but it really is the simplest solution.
lrg36
9 posts
 
anyone know where to get kt88 treasure tubes for the vac phi200,the sellers from china are not listed on ebay anymore and the canadian stores only have pairs to sell
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