I blew a Fuse..!! HELP..

[suraj]

Ok so I recently fell in love with the amp I own - Mesa Bass 400 after I loaded it with JJ KT88's and all new F&T filter caps.

My tech installed 1ohm resistors on each tube to check the bias, and replaced the bias resistor with a pot.

Now I got tube depot to send me a matched set of 6 JJ KT88's, but all showed a slightly different reading. Im guessing somethings wrong with the amp.

But anyways, halfway during practice today, theamp went off and the fuse blew. When I opened it up, I saw two burnt resistors..These resistors are 100ohms as my tech said. These two resistors are only on the last two tubes, the brown ones. He said remove them and see if the amp works. I haven't tried that yet, but there has to be something wrong going on that blew the resistors right..?? I mean if I just put new ones, they might pop too. I have been using this amp at home at high volumes with no issues, just today at practice this happened. My master was on 3.5, EQ set flat on 2, 10, 2. and quite a boost on the low end from the graphic EQ.

Please help, I really like this amp. Just want to know why it failed so it doesn't happen again. It could just be parts that are old, or something serious, i'll leave you tube geeks to answer that.

Thanks in advance.

[Caca de Kick]

My D180 had those same old brown resistors on the output tubes, and one burned/cracked popping the fuse. I removed the tube, put in a new fuse and kept playing the amp thay way with one less tube for many months. (my 400 doesn't have those brown resistors...ceramic maybe?)
I eventually called up Mesa who sent me a new modern resistor, I installed it and put in a new set of tubes, and all was good. That was 9yrs ago. Don't know what the failure was, but suspected it was the tube shorting.

So since your KT88 are pretty new, I think I would start by sending them to someplace than could test them.

[suraj]

I have no place to test them at all actually but my amp sounded really really good so i didn't think I need to do anything. Its just the two brown resistors on the tube pins of only the last two tubes. One cracked, and the other burnt taking out the fuse. Im guessing and hoping the life of thoso resistors just got over. Hopefully its not some other problem.

Edit : No they are not ceramic, the are like plain cylindrical small brown resistors with coloured bands on it. And they are only n the last two tubes, the ones NOT on the side of the fan.

Im guessing a swap should be ok.

[Bassmec]

Originally Posted by suraj

I have no place to test them at all actually but my amp sounded really really good so i didn't think I need to do anything. Its just the two brown resistors on the tube pins of only the last two tubes. One cracked, and the other burnt taking out the fuse. Im guessing and hoping the life of thoso resistors just got over. Hopefully its not some other problem.

Edit : No they are not ceramic, the are like plain cylindrical small brown resistors with coloured bands on it. And they are only n the last two tubes, the ones NOT on the side of the fan.

Im guessing a swap should be ok.

If a tube developed an internal short those resistors are sometimes blown.
This is an effect not the cause, I had one of my newer KT 88's
go short in my Hiwatt clone not so long ago, It burnt out the grid resistor too.

[BassmanPaul]

Looking at the schematic, the only 100Ω resistors around the output stage are the pair that ground the heater supply. I see no reason for these to have failed unless you had a severe heater/cathode short. You should also check the individual IΩ resistors your tech installed. Although these 100Ω resistor pass less that a watt I'd replace them with 5W WW units.

Your post is a little unclear about the sequence of events. Your tech installed 1Ω cathode resistors and modified the bias supply to make it variable. I expect that he would set the bias to suit the existing tubes. You then pulled those and replaced them with KT88s. Did you reset the bias to suit the possibly different tube type??

Matched tube are matched to a tolerance that should work for a given bias voltage. They are by no means identical to each other. Add to that the tolerances of the sensing resistors and you have your variance in currents.

Paul

[suraj]

Ya I guess that pair that grounds the heater supply is the pair that blew in my amp. Even my tech thought it was an unusual problem. No my ampwas running on ruby 6l6's with fixed bias, I put in the KT88's and the bias pot and he set the bias to about 480volts to the plates for the KT88's. This all happened at my tech's place. He even changed all the stock filter caps to ones with the same rating.

My amp was working fine all the while, but at practice this happened between a song.

What is a severe heater cathode short..?? Does that mean a tube went out..?? I dont think so cuz I pulled out the blown resistors and the amp is working fine again. All tubes light up and get hot.

[BassmanPaul]

Originally Posted by suraj

Ya I guess that pair that grounds the heater supply is the pair that blew in my amp. Even my tech thought it was an unusual problem. No my ampwas running on ruby 6l6's with fixed bias, I put in the KT88's and the bias pot and he set the bias to about 480volts to the plates for the KT88's. This all happened at my tech's place. He even changed all the stock filter caps to ones with the same rating.

My amp was working fine all the while, but at practice this happened between a song.

What is a severe heater cathode short..?? Does that mean a tube went out..?? I dont think so cuz I pulled out the blown resistors and the amp is working fine again. All tubes light up and get hot.

I'm sorry but your post makes no sense to me. "set bias 480V to the plates"? KT88s need a completely different bias setting from 6L6GCs

You could still have a bad tube because your heater supply is now floating. That's not really a good thing.

What exactly did you do to make the bias supply variable??

Paul

[suraj]

See I beeing a noob installed 1ohm resistors to each tube. My tech, replaced the bias resistor with a pot, and thats it..!! Ignore the 6l6's for now cuz i got only 4 tubes in the power section when i bought it used.. After putting in the KT88's, and checking the voltage across the 1ohm resistors, one of the tubes was showing a much higher reading, turned out the 1 ohm resistor on that tube was not really 1 ohm. So we replaced that.
The 2.7K resistor on that tube was showing 2.2K or something while all the others showed 2.7K, so we replaced all those resistors with new ones, but since 2.7K wasn't available, he put 2.2K ones instead and said its fine.

What do you suggest I do..??

[suraj]

Oh pual btw thanks for the quote, which is my sig now

[BassmanPaul]

Order up a set of 2K7Ω 5W WW resistors and install them. They control the screen dissipation.

Redo the bias supply. If you just stuck a pot in there instead of the "adjust" resistor you could adjust to zero volts. You need to be able to get -68V at the top, or whatever the raw supply turns out to be. About -50V on the bottom with a resistor of appropriate value to ground. Use a 20 turn pot of the highest quality. I suggest a 50KΩ pot at max. The cap across the "adjust" resistor should be moved to the pots slider which is your bias supply.

Adjust to full negative, install your tubes and set to about 30-35mA per tube. Even better use the notch method with a signal generator and a scope.

Paul

[suraj]

Ok i'll put the 2.7k resistors.
My tech installed a pot, I dunno what kind of pot, but it says, B100K. So its a 100K pot. He said the bias resistor was about 108K so he replaced it with the pot which is a similar value. I'll show my tech what you've told me to do. Also What should I replace those 100ohm resitors with?? 100ohm 5W WW resistors..??

How do I check the mA per tube?? across the 1ohm resistors..?? and what setting should I put my multimeter to?? Sorry I dont know anything and feel very stupid but thanks a ton.

[BassmanPaul]

OK I'm concerned. You do not seem to know enough about tube amps to be inside one with any safety.

Yes 100Ω 5W WW for the heater supply. You check each tubes current by measuring the VOLTAGE across the 1Ω resistors. You need to read in millivolts. 30mV equates to 30mA.

The problem with just replacing the adjust resistor with a pot is that you can only go less than the designed voltage. The bias circuit was designed for 6L6GCs and 6550s. You have to make it suitable for the KT88.

Paul

[suraj]

Ya thats true. I really don't know much, but I know I have to wear slippers and keep one hand in the other pocket, and not to touch anything..!! specially the caps..!!
I know how to solder and thats about it..

I'll talk to my tech about the problem. Thanks so much.
Just one last thing. How do I know if somethings wrong with any of the tubes..??

[suraj]

Ok I just checked the current across the 1ohm resistors. They were between the 47 and 53 mA range. I turned the bias pot to get them to be about 30-36 mA range.

Do you think the tubes were running too hot..?? Which might have taken out the 100ohm resistors..??
My tech agrees with redoing the bias, but din quite understand what you meant by moving the cap across the "adjust" resistor to the pots slider.

[Bassmec]

According to my info the jj kt88 maximum plate dissipation
50% to 70% figures are for a plate voltage of 450 volts
50% is 47mA 70% is 65mA.
For a plate voltage of 500V its 50% 42mA and 70% 59mA
But like bassman paul says you should be looking at the notches on a sine wave with a scope for exact bias.
He is telling you to move the the bias voltage smoothing capacitor legs from across the original fixed resistor now possibly represented in the circuit by the whole bias pot carbon track?
to pick up the wiper of the potentiometer as the output leg of the capacitor.
Unless you have it wired this way, the bias voltage smoothing will suffer.
What is your actual measured plate voltage with the tubes in and running.
My Simms Watts AP200 4 KT88 amp runs well at these figures 671 plate volts biased at 40mA on Winged C's
So you might be able to work out a setting from that and the other figures if you cant scope it at full power into a load resistor.
480 plate Volts is not much to drive KT88's
Scope it for watts and tell us what you have actually gained
if anything.
What is the heater current difference between the tube sets
that might account for the 100 ohm resistors in that part of the circuit getting a bit hot apart from the heater cathode short idea put forward by paul.
PS that is another valid consideration as its a common fault in new production KT88's.