Help!

Hi Gang,

So..I replace 2 of the tubes with KT88's (these are HUGE mothers!),and after 5 minutes of playing....crackling sound and the tube failure light comes on .... it's a 400ma fuse that reads "f400\250V". It actually shattered in the pot.

Should it have? Do I need a 400v fuse instead to handle it? Also, is it normal to remove the fuse "clamps" to accomodate the larger base?

*&^$$%^&!!! :lol:

Pedro

Sounds like it was Biased too hot. Yes you need to remove the bottom rings to get the KT-88's in there. Those suckers are HUGE!!
Just be paitent while biasing them. I went thru about 6 fuses doing so. Once I got the good balance it was fine. Get a good supply of extra fuses though!! :wink:

No kidding on the extra fuses! :lol:

So I do need to stay with the 250? It's unclear because the back of the head says 400mA, yet, the manual says to use nothing other than the 250mA?? :? hmmmmm....

it should be safe to go up to 400ma.
I wouldnt go higher than that. Just start biasing at a low level. When I put the KT-88's in mine i had to leave 2 EL34's in then bias the 2 KT-88's. Then shut it down....pull the el34's....put the 2nd pair of KT88's in....and it went fine. After about 10 blown fuses that is!!
Just be patient and expect them to pop. Those KT-88's draw ALOT of power! :wink:

After killing a marginal pair of tubes by just swapping and then going to adjust, I've taken to turning the bias pot right down before powering up again.

I think what I did when I put two of those in the outside positions is biased the E34L's that were already in the outside positions down to a cold setting like 15mA, shut the amp down and pulled the 2 E34L's, and then installed the 88's and biased them up from there. You definately want to turn your bias down before putting the 88's in there or you are gonna go through some fuses for sure and possibly risk damage to your new tubes. Hope that helps man.

Thanks for all the input guys Much appreciated!

NOW...they didn't have any 400mA fuses...they had 300 and 500. When I spoke to the guy in general about "current", he said that....logically...if the 34L's use a 400 because they max out at around 40mV, then "logically" the 88's withe the higher current should use a higher rate fuse. Or I bias the 88's so they don't exceed 40mV.

So based on that, would the head explode if I used 500mA fuses where the KT88's are? leaving the 400mA's for the 34L's?....or is this a big NO NO! :? :lol:

Just kick the BIAS all the way down before doing any of this. The pair of el34's should bring the heat to the tubes letting them "light up" and allowing you to bias. do it very slow though since if too fast you might blow the Tubes or again the fuses. Just be patient and take your time. Those KT-88's pull the juice I know that! :wink:

Thanks! ...I ended up getting the .500mA fuses because they didn't have 400's, and I certainly was going to use the 300's! :shock:

I'll do as everyone has stated so I don't turn it into a frying pan, but I hope using 500's doesn't do anything to head itself. The way I se it, if other KT88 users biased them over 40mV with a 400mA fuse, it would be lights out all the time! :lol:

Stay tuned! :shock:

I have the stock fuses in multiple MTS amps and KT-88s ran just fine at 48-52mA :?

The amp tech at Chicago guitar and amp, put the KT88's in v1 and v4 for me. I told him to set the bias to 50ma for the KT88's but he said the frequency wave dirty so he set each differently but around there to make it look clean. Apparently the dirtiness would make it sound like **** and take away from the overall tone.

Tony H.

Yup..I think if you bias properly, using a single generator and o'scope...you bias until the 'notch' in the sine wave is minimized/goes away to remove crossover distortion....but I've only read the theory, never done it that way.

UPDATE!!!

so....or should I say "so far", the 500mA fuses in v1 and v4 are doing allright. :shock:

88's in v1 and v4. Bias setting to 42mA, and the 34L's to 39mA. No tube failure light (so far) and everything seems to be working good! logically, I could run them hotter, but I need to be certain that the 500mA fuses won't compromise the health of the amp :? ..but it sounds wicked!

...One other thing I thought I would mention. Boost pedals...I have an EH LPB1. The nice thing about this pedal? no volume or tone. it's just a power booster that drives the signal to give it more edge..and it sounds sweeeet! 8)

It sells for about $35 up here in the north pole, so you's can probably get it in the states for $10 or less :lol: ....seriously though, great drive pedal without changing the natural tone of the head. :wink:

I've heard that if you were using say, KT88s and 6L6s then you should bias both of them around the middle of their range to keep everything running smooth. I would try that next time, but congrats on getting everything to work!

Shinozoku said:

I've heard that if you were using say, KT88s and 6L6s then you should bias both of them around the middle of their range to keep everything running smooth. I would try that next time, but congrats on getting everything to work!

Click to expand...

You know something....the 34's were biased to 42 before I reset them all....I think I'll try that next..keep everything at 42. Good point! .and thanks!

P

I never had any issues at all when I tried this setup. JJ KT88's at 50mA on the outside and E34L's at 38mA on the inside. Never blew a fuse or had any other problems.

I tried 42 across the board, then 39 for the 34L's and 45 on the KT88's...some differences here and there but not really mind blowing. It definitely took away some of that "tubby" base sound that drove me crazy :?

The 500mA fuses are working great, so I should be able to drive them at 50mA without even worrying about....but haven't tried at jam volume yet.

What's driving me crazy now is that all my axes that have Tone Zone pickups produce SO MUCH bass it's gross! :lol: ...the one guitar I have with the SD Invader sounds priceless! 8) Just the right amount of bottom end...nice and balanced. So now it's either EQ the TZones or spend the money to get all my pickups changed :shock: :? ...never freaking ends! :lol: