JJ Kt77 Bias?

[sled108]

Any thoughts on what is a good Bias Point for KT77's in a ST-70 style Amp? I've heard that they can take more than an EL34, so right now I have them at .5A & .55V...They sound Fine like this, but I'm wondering how Hi can they Go... Anybody else using JJ KT77's in a P-P Amp?

 

[Jim McShane]

Sorry to be the bearer of bad news, but the JJ KT-77 is a very poor quality tube. I would not even push them as hard as you do an EL34.

Honestly - if the budget allows get rid of them.

 

[sled108]

Yeah, I know...That's why I've ended up with 8 Small Pins for the price of 4...But they Still do Sound Good, and if I Red-Plate them...I'll look into Others...I still would like to know what a Good Bias is for a KT77....

Now, I've just Checked...For under $75 a Match Quad, they Really Sound Nice..For how long they Sound Nice, well I guess that's a Roll of the Dice..

 

[quadtech]

I am running them in a VTA board setup at 40ma and they sound
excellent.
They are the later ones with the good pins - about 150-200 hours on them
and no issues at all.

 

[sled108]

I've been running my JJ's for years at 46ma, but I just figured why not make it an even 50ma? I've seen that the Guitar Guys really like these, and they Play them Long and Hard...

 

[danFrank]

They are the later ones with the good pins - about 150-200 hours on them
and no issues at all.

How can you tell if they have the good pins or the bad pins? What is the difference? I have a quad and run them at respectable levels (475v) and have had no problems with them. I don't know why so many people complain about the JJ KT77; maybe I have just been lucky.
Daniel

 

[costis_n]

Sorry to be the bearer of bad news, but the JJ KT-77 is a very poor quality tube. I would not even push them as hard as you do an EL34.

Honestly - if the budget allows get rid of them.

I'm sorry, but the ones I have are very good. They bias almost the same as my NOS EL34's and sound better than them. (ok, it's a guitar amp, but still..)

 

[sled108]

The Small Pin ones will be a bit loose in the Sockets...

 

[rknize]

It's been a few years now, but JJ had some quality control issues with their octal tubes. They had issues with undersized pins, weird misshapen "square" pins, and other problems with the build quality of their internals.

The first quad of KT77s that I put into my freshly-rebuild VTA-70 arced over the rectifier due to an internal short. The cathode had slid down, out of the mica rested on one of the straps below. I gave the tube a few good smacks in my palm, which slid the cathode back up. They worked fine after that, but AES exchanged them and even replaced the GZ34 that had arced as a result. The new set have been in there ever since and sound and work great. I think I bias them at 40mA.