Insight needed from you tube/bias pros!!

[Jorock]

Hey guys I dont post frequently so hopefully the fact that no one here knows me doesnt stop anyone from chiming in

Here's what im asking:

I recently ordered 4 =C= power and 4 preamp tubes from the tubestore.com to go into an Invader 100.

I removed the stock power tubes from my ENGL invader, and on the bottom of the tube they have a number on the label '27'. The four SED =C= tubes I was sent are labelled '41' in the same spot. Pardon my lack of knowledge, I believe this is the gain factor/or tube bias level correct?

Either way, I measured a plate voltage of roughly 420 across all four power tube spots, biased the new tubes at 38 to stay on the conservative side of %70. The amp sounds great, everything I would expect. What I have noticed is the ventilation plate above the tubes after a couple hours of straight playing is hotter than it used to be after the same time playing with the older power tubes.

Are the new tubes burning hotter and can this create any issues?

Is the fact that the old tubes had a '27' compared to the new ones being '41' related to them burning hotter?

And, is this something that will affect the amp or tubes in a negative way in terms of lifespan, or internal components being driven too hard?

Thanks to anyone who can offer some insight!

 

[glpg80]

Tube bias is the current addition of both screen and plate current - everyone and their mother forget about the current through the screen resistors when factoring in their bias current. You also have to account for bias drift due to heat - normally new tubes need careful watch for roughly ~10 minutes to make sure any physical adjustments or shifts do not lend themselves to bias run-away.

The numbers i forget their purpose on SED - sometimes it is used for grading the tubes, other times it is used for matching at the factory. And can also as you mentioned be used to mark the grade of mu in a controlled environment or tube tester.

Tubes need heat in order to work - i would not be concerned with heat output. What you do not want is bias run-away, or biasing a class A/B setup to where it is always on which can cause eddie currents to flow in your OT effectively cooking the primary of your OT. Biasing them colder is actually better in high gain situations, you will get more defined character thumps in the low end and the top end still sounds great. Many guys will bias using their ears, check it with a bit of math and a meter, and call it good.

Do some more searching on the forum - there have been many topics discussed on tube bias on this forum over the years - all of these questions already covered in great detail.

 

[LP Freak]

As long as you don't see any red plating you should be fine.

 

[Jorock]

Ya definitely no red plating happening. I kept my eye on it the entire time. I dont think im even close to being at risk of that. According to the biasrite.com bias calculator, 70% of 420 Plate Voltage is 42ma. I biased to 38ma.

Thanks for the info 'glpg80' re: bias runaway. In all my years of playing guitar I have never been made aware of that. Its a pain in the arse, but I may pull the chassis out of the head shell to re-test in a day or so to check if there is any danger of the bias having shifted. But it certainly doesnt sound like the bias is super hot, it sounds just right!

 

[steve_k]

You are fine. Nothing to worry about. That number is the plate current matching. The SED's will have more headroom. Heat is just a byproduct.

 

[Jorock]

Thanks guys I really appreciate the feedback! Everything seems to be running smoothly!