Blown KT88 or 6550?

Icedsounds · « **on:** 30 Jan 2010, 08:18 pm »

I was wondering if I could get a little help. I am fairley new to tubes and not to sure of what happened to my amp.
Background- I have built a few tube preamps and modified my power amps with no ill effect. I know how to measure and test inside an amp safely.
I have been modifying my jolida 801a in a few stages. I just put new caps in it and did some testing. I got about 30 hours on the new caps.
2 Nights ago I had the amp running great for about 15 hours straight. Last night I turned the amp on and in about 5 seconds of start up there was some crackling from 1 speaker then a bright flash in 1 of my tubes. When it flashed it took out the fuse in the amp.
I have gone through the amp and there is nothing wrong inside. No componets were shorted and the circut tests good.
My question is, did I just simply blow a tube? I have no idea if I can test it with no equipment other then a multimeter. I dont want to put a new fuse in the amp and turn it on to check the tube and risk damaging something else or worse a speaker. I did bias the tubes just before the 15 hour run.
I must also confess I may have bumped the bad tube when I biased the amp.

I also will add when I bought this amp the tubes came with it, There are no markings on the tubes as to what they are. I will post a pic if that helps.

Niteshade · « **Reply #1 on:** 30 Jan 2010, 09:01 pm »

It sounds to me like a tube shorted out and nothing else. Try these steps:

1. Test voltages without any tubes installed (If good, go to step 2)
2. Install power tubes, including the replacement one(s). Do NOT install pre & driver tubes***
3. Re-check your voltages.

*** This way you can check voltages w/ out worrying about oscillations, etc... due to not having an input connected or speakers.

Icedsounds · « **Reply #2 on:** 30 Jan 2010, 10:21 pm »

ok I will do. I have to wait till monday to get a new fuse since I dont have the correct value on hand.
I also dont have a replacement tube yet. Can I test each channel seperatley (two tubes in amp instead of 4) I will buy a new matched quad to try. I just dont want to blow a new tube if its an amp problem.
Is there any chance of damaging the output transformers without any load on the amp?

Niteshade · « **Reply #3 on:** 31 Jan 2010, 02:29 pm »

Yes you can test each channel separately, but the B+ will be higher with a lighter load. This could throw off your readings. As long as the phase splitter/preamp tubes are not installed, there will not be a problem testing the amp when all of the power tubes are installed with the speakers disconnected. There will not be an outgoing signal from the power tubes.

Icedsounds · « **Reply #4 on:** 1 Feb 2010, 07:08 pm »

Ok I tested the amp and everything is in normal range. Pins 3 and 4 are between 511 volts and 518 volts on all tubes. Including the bad one. Pin 5 has -58 volts on the bad tube and the rest are -62.
What would be the next step to checking this out?

Niteshade · « **Reply #5 on:** 2 Feb 2010, 11:01 am »

If your voltages are all normal at the tube's pins- it sounds like no damage was done.

Icedsounds · « **Reply #6 on:** 2 Feb 2010, 07:31 pm »

What do you figure would cause the fuse to blow and the tube to flash. Just a tube that went bad?

Niteshade · « **Reply #7 on:** 2 Feb 2010, 07:33 pm »

I would suspect it's a simple tube failure. Either gas or a wire broke loose inside it.

If the tubes are not putting out an extraordinary amount of heat, then the amp isn't stressing them enough to promote early failure.