Blown Bias Resistor - Did I Mess Up?

Hi all. I sat down tonight to experience my new Genelex Gold Lion KT88 tubes in my ST-120. I'd been running Sovteks KT88 with zero issues since I built the amp a little over a year ago.

I swapped the tubes and reset the bias, which was only a bit off for each and was able to settle all 4 settings in at .550 Vdc pretty easily.

Four songs in, I saw, and heard through the speakers, a brief flash on the V7 tube.

Troubleshooting (if you could call it that), I heard a mild hum and scratchy noise in the right channel (only) and reduced output volume. I did some swapping/removal of tubes to deter kine it wasn't a tube and, after searching this site, determined I have a blown 10 ohm bias resistor for V7. It measures open on my meter and even has a small hole in the case.

So, replacing this free little guy is easy, but before I do I'd appreciate any advice/thoughts about what I might have done wrong.

Might this have been an open pin on the new tube Co kd this have caused the bias resistor failure? Is .550 vdc too high? WI of this failure damage the tube? Should test the new tube before reinstalling it to prevent a reoccurence?

Any help woo kd be much appreciated.

Thx, jasn.

Sorry for the typos. Autocorrect on a cellphone is brutal...

Sounds like the tube that was in that socket shorted internally.  I would consider it defective and not trust it any further even if it seems to work OK now.  You experienced the other function of that 10 ohm resistor.  It works as a fuse to protect the output transformer in the case of a tube short.

A bias of .550 is fine.

A poor socket connection might cause the tube to red plate, but not flash internally.  Replace the tube.

I agree , the tube in question is defective. If bought with warranty then it's time to use it, otherwize
throw it away and replace the 10 ohm resistor with another one. ( 10 ohm 1/2w flameproof)

Thanks to you both for the help. Complete dummy that I am, I mixed up the tubes when I removed them to open the bottom plate. Will a tube test find the bad one?

If the tube has a hard short, the tube test will find it. However, the tube in question may only short after it is good and hot. In that case, keep some spare 10 ohm resistors on hand for when it shorts again.

Try leaving the tubes on the tester with the heater going for a good while to get the tube hot, maybe that will cause the bad tube to short again. Keep in mind that the tube won't get as hot as it does in normal operation.

Same failure happened to me a few years ago.   Output tube shorted and smoked the 10 ohm cathode resistor.  In my case I heard a loud hum when this occurred.

The 10 ohm resistor 2 watt resistor should handle 450 mA of current before reaching its power limit.

I too have experienced the exact same failure of my ST-120 during the first three years after completion of the kit.  I went through 3 sets of  KT88's. First set was JJ, 2nd set was Electroharmonics, and the 3rd set was Gold Lion.  With each failure, the offending tube took out the rectifier as well.  So I would check it as well if you are using tube rectification.

Ditto on the experience. In my case it has been not bad tubes but bad socket maintenance. Here's the thing - at least you didn't fry anything really expensive -- or burn down the neighborhood!

junk both tubes, you will ned a matched pair anyway.

Thank you everyone for the help and advice.  I have some resistors on order (not terribly easy to find) and am working to get a replacement tube (or two).  I'll be sure to re-tension the socket pins before reinstalling.

Stand by...

it's also been recommended by tube guru Jim McShane that when switching to KT120 tubes instead of KT88 tubes,
it's good practice to change the grid bias reisistors (R29-32) to 47K to prevent bias runaway.
Bob says this has rarely happened, but when you are talking about $50+ tubes a 10c resistor change is a no-brainer.

An update:

Turns out I do not have an opportunity to replace a tube under warranty after all. My bad. I was able to test all four tubes using a friends ancient tester, however, and they all looked good. I had mixed up the problem tube one with the three good tubes, so I decided to roll the dice and try to replicate the problem. so I...

Replaced the 10 Ohm resistor
Re-tentioned all socket pins
Reduced bias pots
Re-installed the GL KT88s
Reset bias to 0.500V dc
Sat back, listened to, and watched my amp for almost two hours without blinking
All is well...so far. Not only that, these GL tubes sound sweet.

I don't know, maybe it was an open pin ? Will have to give it some more time.

Time will see, but there is a risk that the tube will flash again.
If it does, make shure you mark it ( and it's twin on that side) and buy a new matched pair.

Tubetesters won't provoke flash-over ( and generally don't give any usable results)

Agreed Peter. Glad I bought extra 10 ohm resistors.

Hi

I had a similar experience with my recently completed ST120.

When I switched it on for the first time, everything seemed fine.

I removed the valves several time. I gripped the kt88 at the base and rotated the tube in a clockwise motion as I tried to lift the valve from its socket. I would then rotate it anticlockwise until it was removed fr9m the docket.

After I placed the valves back in their respective sockets, there was no sound! I should have left it alone.

I didn’t make any changes to the wires, so I decided to adjust the pins. This seemed to solve the problem.
The following day, I decided to check the bias voltages.
Three tubes were ok, V2 was reading Volts instead of millivolts.
I decided to check underneath the chassis.
I switched the amp on and checked the volts feeding into the driver board. It was approx 6.9 volts, which was fine.
I switched the amp off, and proceeded to adjust the pins for all valve sockets.
I’m not certain what happened, but I was adjusting the pins of V7 when I received a mild shock!
I was ok, although there was a sound and two pin point burn marks on the tip of my finger.
I checked the 10ohm resistor and it was blown!
I was unable to source a 10 ohm 2 watt metal oxide resistor at my local store.I
I placed an online order and decided to replace the damaged resistor with a 10 ohm 5 watt wire wound resistor, as a temporary measure.
Will this damage the amp and why did I receive a shock, considering the amp was switched off and I was nowhere near the quad capacitor.
The amp sounds fine…..for now..

Thanks

Sounds like you may have gotten shocked by stored charge in the capacitors. Did you turn on the system with the rectifier installed and the other tubes removed?

As for the 5 watt 10 ohm resistor, the amp can be damaged if the tube in that socket shorts internally. Make sure you replace it with the correct resistor as soon as possible.

I've always used 1 watt metal oxide 10 ohm resistors on all amps I've build.
50ma across 10 ohms is 0.50v which gives you 25 mw, so even a 1/4w resistor is 10X bigger than you need.
I'd rather blow up a 5c resistor than a $50 output tube anyday.

Hi

Yes, the rectifier was installed when I switched the amp on. The other tubes were removed.
Thanks.

Tubes4ever wrote:Sounds like you may have gotten shocked by stored charge in the capacitors.  Did you turn on the system with the rectifier installed and the other tubes removed?

As for the 5 watt 10 ohm resistor, the amp can be damaged if the tube in that socket shorts internally.  Make sure you replace it with the correct resistor as soon as possible.

Hi

Is it ok to use a 1 watt resistor, instead of the 2 watt resistor included in the ST120 kit?
Thanks.

tubes4hifi wrote:I've always used 1 watt metal oxide 10 ohm resistors on all amps I've build.
50ma across 10 ohms is 0.50v which gives you 25 mw, so even a 1/4w resistor is 10X bigger than you need.
I'd rather blow up a 5c resistor than a $50 output tube anyday.

Roy Mottram wrote:it's also been recommended by tube guru Jim McShane that when switching to KT120 tubes instead of KT88 tubes,
it's good practice to change the grid bias reisistors (R29-32) to 47K to prevent bias runaway.
Bob says this has rarely happened, but when you are talking about $50+ tubes a 10c resistor change is a no-brainer.

What is the value of the original R29-32 resistors?

Dolphins wrote:

Roy Mottram wrote:it's also been recommended by tube guru Jim McShane that when switching to KT120 tubes instead of KT88 tubes,
it's good practice to change the grid bias reisistors (R29-32) to 47K to prevent bias runaway.
Bob says this has rarely happened, but when you are talking about $50+ tubes a 10c resistor change is a no-brainer.

What is the value of the original R29-32 resistors?

For EL34 output tubes: 150K 1/2w
For 6550/KT88 output tubes: 100K 1/2w