my jcm800 KT88 tube get blue color lightning

[kwm488]

hi

i build the marshall 2203 2 year before, and i haven't visit here for a long time.

my amp KT88 power tube get lightning blue color. and my amp no sound now. the HT fuse always blow. and i find out the outer power tube socket 1 and 8 1 ohm resistor is dead. anyway, i replaced it. but what problem i will get if the tube pin 1 and 8 can't ground ? will the OT blow? will the 50uf + 50uf cap destroy? will the tube dead? also, i check all the power tube pin, all voltage correct

Best Regard

[rzenc]

When you play a chord, do you hear the O.T./Power tubes ringing?? If so, check speaker cable/connections on O.T. secondary.
Also, be sure your tubes are good. The other day I had an amp in for repair which keeps blowing fuses as soon as you hit stand by to play... 1 of the 4 EL34's had a small fracture on the glass, pretty close to the base. I noticed the silver getter was going away... replaced tubes and no more problems...

Hope this helps
Best Regards

Rzenc

[kwm488]

hi
thank you for your quick reply.

 i remeber i just hear a very high volume crack sound. so i turn it off immediately. i can't hear guitar sound even i plug guitar and connect to cab.

and the power tube voltage correct ((i haven't install the power tube))

Also, can I use install 6l6gc for testing?
Do anyone can let me know what happen if power tube pin 1 and 8 can't ground
sorry, i haven't build amp for a long time. if the power tube can't read voltage of the 1 ohm resistor to ground. is it mean the tube is dead?

Best regard

thank

[jojokeo]

Pins 1 & 8 must go to ground or no bias, no bias = no play. You fix, test tubes or install new, you bias, then you play and all likely fixed.

[jerrydyer]

you prob have a simple tube failure. replace all the 1 ohms for the hell of it. pull two tubes one of the ones you suspect is bad and its counterpart. fire it up tell us what happens. 

if a power tube cant ground its not critical because  some of us use a switch there as a standby. you will get no sound if its not grounded so, in essence youve ansewred your question and prob. its a simple poower tube failure.

[plexi50]

The cathode pin #8 / 1 ohm resistor must be changed if it is bad (open). You can just put pin #1 & #8  to ground if you dont have a 1 ohm resistor laying around. Check your plate voltage on pins 3 of the power tube sockets and the grid voltage pin #5 before just putting in new tubes

[JayB]

What wattage are you guys using for those 1 ohm resistors?

[kwm488]

thank you all of your reply.

i test all the 1 ohm resistor, just the outer two power tube 1 ohm resistor is dead, i replaced it. the inner power tube 1 ohm resistor is no problem. as the 1 ohm resistor, i use daly 1/2 watt. i need 1 ohm resistor to set the bias.

i also think it should the KT88 tube problem. but it same all KT88 is dead. i have no more now. can i use 6L6WGC (5881) for testing? in fact, i have some EL34. but it is SED wind C, i am scare that it demage my EL34 before i make sure the amp is no problem

[kagliostro]

Blue color in the tube ..............

if I remember well that happen with low vacuum inside

Kagliostro

[kwm488]

the tube look like have blue color bomb inside. so i am scare when i turn it on now.

[jojokeo]

i also think it should the KT88 tube problem. but it same all KT88 is dead. i have no more now. can i use 6L6WGC (5881) for testing?

Yes, as long as your B+ isn't too high exceeding 6L6wGC specifications. Install tubes and re-bias for dissipation factor. Go here  http://www.dreamtone.org/Calculate_Bias_Form.htm  enter info for type of tubes you are using, nice and simple.

[kwm488]

I am happy to come here every time . I feel very warm and I can learn a lot here.

As this jcm 800 , I will install 6l6 and try. But I just install two kt88 in outer tube socket for test. One tube get blue color Fire (like bomb) inside the tube when I put standby on. And the ht fuse blow immediately. Is it mean this tube is short?

The result is 1 tube dead ( no voltage in 1 ohm resistor), 2 tube get blue bomb , 1 is ok. All tube are new gold lion kt88. $200 usd.

I am sorry to ask more, because I want to learn more.

1.  The jcm 800 used 6 lcr 50 + 50 uf can cap. It almost 15 - 20 years. (not use 15 year, but product is made 15 year already). If the cap is dead or almost dead. What problem will come out?

2. I own fender twin reverb silverface. It is 1978. It used 33 years. It is older than me, I bought it second hand. All the parts are original. But my amp volume sometime normal, sometime very weak. What problem? I will fix it after the jcm 800. The power tube is RCA 6l6gc. I bought it from eBay. I guess the problem may the power tube or electrolytic cap. Am I correct?

Best regard

[kwm488]

hi
good news. i install the 6L6 tube to test the amp. all problem is fix. thank everyone advice.

and i plan to order new 6L6GC for my twin reverb. new tung sol or SED 6L6GC better?

[jojokeo]

Q1 - If the cap is dead or almost dead. What problem will come out?

noise and little or no output due to no filtering from your power supply.

Q2 - same as above, replace your electrolytic caps and check bias

*glad your jcm amp is back running good! 

[Packerswin14]

In the Fender Twins, I really like the Tube Amp Doctor 6L6GC-STR tubes.  Each individual amp will react differently to each individual tube, but for Twins, the TAD's are the ones I most often like the best, well over half the time.  I've had surprising success with the Chinese-Made Coke Bottle style 6L6GC's as well.  Probably a quarter of the time, they're either the best-sounding, or they sound great and it's a matter of preference.  After those two, it's about even between Sovtek, EH, Groove Tubes, JJ, Ruby, Tung-Sol, they're all occasionally the best-sounding.  I've put the TAD's against vintage RCA blackplates a few times now, and they're either dead even or pretty close, with the TAD's beating the RCA a couple of times.  Good luck!

[kwm488]

hi
thank everyone here again.

sorry to repeat the question. the volume sometime normal , sometime weak. this is the problem come out from electrolytic cap, right? i ask my friend, he said that power tube error will not course this problem. is it correct?

and really really sorry for more question. i hope to understand my amp more.

1. where is the bias cap in my twin reverb? it is silverface with master volume.
2. In the cap board, it is 3 of 20uf 500v and 2 of 220uf 285v. but i see some model is 3 of 20uf 500v but 2 of 100uf 300v. what different (sound) of 220uf and 100uf?
3. the blue color cap, is it very cheap and bad? because i plan to replace it to sozo BlueMolded Vintage Capacitors. have anyone try it ? should i change it. i think these cap in my amp can work 10 year more.
4. some electrolytic is 25uf/25v. i think i don't need to change it. just change the 20uf and 220uf. because very little voltage pass to 25uf/25v. can i don't change it?
5. normally, if the amp with reverb get problem (no reverb), it should be the 12at7 tube , reverb transformer or reverb tank problem? ( i help my friend to ask this, because his verb king reverb get error. if i know this , i can help him repair)