KT88 Cathode current...

[gearbreaker]

Hi, Newbie here. I just biased a pair of KT88s and set them to idle at 120ma cathode current. The plates began to look a little cherry. I thought that the cathode current of the KT88 was 230 and I was well below the rated cathode current. I can run EL34s in the same amp and the tubes wont start to glow until about 130ma. I normally bias them to run at 65ma and they are perfectly happy. I figured that the heavier tube with all higher specs would run cooler, not hotter. Help please! I just don't get it.

[xtian]

Gear Breaker! A descriptive handle!

Check out this nice bias calculator: https://www.amplifiedparts.com/tech-art ... calculator

You have to know your plate voltage (fixed bias), or the difference between the plate and cathode voltages (for cathode bias).

For 400v plates and KT88, the value PER TUBE is about 60mA. So if the value you gave (120mA) is for the PAIR, that's in the ballpark. Please confirm your plate voltage.

Also, while the bias calc says, "Recommended bias point for Class A/B amps = 70%", that's old news. I commonly get nearer 50-60%. Still sounds good, and tubes live longer and cooler.

[gearbreaker]

Gear Breaker! A descriptive handle!

Check out this nice bias calculator: https://www.amplifiedparts.com/tech-art ... calculator

You have to know your plate voltage (fixed bias), or the difference between the plate and cathode voltages (for cathode bias).

For 400v plates and KT88, the value PER TUBE is about 60mA. So if the value you gave (120mA) is for the PAIR, that's in the ballpark. Please confirm your plate voltage.

Also, while the bias calc says, "Recommended bias point for Class A/B amps = 70%", that's old news. I commonly get nearer 50-60%. Still sounds good, and tubes live longer and cooler.

Ah, Gearbreaker can be taken a few ways but it's always good for a smile. Your answer is effin perfect! Thank you. That calculator is the greatest thing since color coded resistors! It explains a lot, including how much I don't know. I'll be poking values into it as well as trying the formulas on my own. I'm no mathematician but when something interests me I find a way to learn. On the lighter side, I love the sound of KT88's and will figure out what I need to do to get my bias control voltage lower to make things run cooler.

[tubeswell]

The plate dissipation (Pmax), which is measured in Watts, is the critical variable. Do not exceed the tube's maximum rated dissipation. Consult the relevant data sheet. (Pmax = 42W for a KT88)

Plate dissipation (Pmax) = plate voltage (Vp) x plate current (Ip).

(You have one measurement (Ip). Now you need to know Vp.)

If running a single-ended tube in Class A mode, you don't even need to be at Pmax (unless you want maximum power). Depends what result you want. The KT88 in the AX84 20W SE*** power amp only runs at 36W.

(But the KT88 in the AX84 20W power amp has a large grid leak resistance, and so you don't want the KT88 to run flat out in that case anyway. But, as that case shows, you also need to understand the impact of grid current (Ig1) on tube current (Ik)*, and how to manage it. So that's more stuff for you to think about).

You also need to understand the impact of screen current (Ig2) on Ik and the operating point of the tube. The operating point in an output tube that is connected to a reactive load**, is primary determined by the bias, but also by Ig2. The more Ig2 you have, the more negative Vg1 you need, to keep the tube happy.

*Tube current is equivalent to cathode current (Ik)

**i.e. a speaker and output transformer

***As xtian has already said, PP is different again.

[Stevem]

Also in a simple way of looking at it , the better/ stronger testing a tube is the sonner it will Red plate.

[bal704]

Here's a site I use....

http://www.tedweber.com/webervst/tubes1/calcbias.htm

[gearbreaker]

The plate dissipation (Pmax), which is measured in Watts, is the critical variable. Do not exceed the tube's maximum rated dissipation. Consult the relevant data sheet. (Pmax = 42W for a KT88)

Plate dissipation (Pmax) = plate voltage (Vp) x plate current (Ip).

(You have one measurement (Ip). Now you need to know Vp.)

If running a single-ended tube in Class A mode, you don't even need to be at Pmax (unless you want maximum power). Depends what result you want. The KT88 in the AX84 20W SE*** power amp only runs at 36W.

(But the KT88 in the AX84 20W power amp has a large grid leak resistance, and so you don't want the KT88 to run flat out in that case anyway. But, as that case shows, you also need to understand the impact of grid current (Ig1) on tube current (Ik)*, and how to manage it. So that's more stuff for you to think about).

You also need to understand the impact of screen current (Ig2) on Ik and the operating point of the tube. The operating point in an output tube that is connected to a reactive load**, is primary determined by the bias, but also by Ig2. The more Ig2 you have, the more negative Vg1 you need, to keep the tube happy.

*Tube current is equivalent to cathode current (Ik)

**i.e. a speaker and output transformer

***As xtian has already said, PP is different again.

Great information. Its a great guide for me to learn more. I'm going to take each section and research it. I am just learning. Thank you.

[gearbreaker]

Here's a site I use....

http://www.tedweber.com/webervst/tubes1/calcbias.htm

Thank you.

[Roe]

Also in a simple way of looking at it , the better/ stronger testing a tube is the sonner it will Red plate.

really?