More glowing tubes question ... pics included

[flemingmras]

EH,

The formula I am using to get the bias is this

(30w/470v) * .7 = .0446

Is this not correct?
Thanks

Hey Jon, no that is not correct. You're taking 70% of the plate dissipation divided by the plate voltage. However, the max bias is 70% of plate dissipation only. On top of which, 30 watts is max plate dissipation for a 6L6, not an EL34. 25 Watts is max dissipation for EL34s.

With your equation using the values shown above, you would actually be biasing at 20 watts, well above 70% max plate dissipation for EL34s, which is 17.5 watts.

So you'd actually want this equation:

(25*0.7)/Plate Voltage=Max Bias Current

To simplify it, 70% of 25 is 17.5. So here is the simplified equation:

17.5/Plate Voltage=Max Bias Current

I know that Groove Tubes claims that their EL34s can dissipate 30 watts, but judging from the fact that this is coming from Aspen Pittman, I wouldn't even try it. Aspen is #1 of people on my shit list since his power tubes don't last for shit and half of what he says is bullshit. Stay with 25W max plate dissipation and ALWAYS use the above equation to figure out max bias current.

Now remember, the above equation just gives you the MAX bias current value. You can bias anywhere below that, depending how the amp sounds to your ears at different bias settings. Mess with the bias until the amp sounds right to your ears. However, DO NOT GO ABOVE THE AMOUNT YOU GET IN THE EQUATION!!! I cannot stress that enough.

I know you're a beginner in all of this so anytime you need to know something just let me know. You can even email me personally as well.

BTW my real name is Jon also.

Jon

[gtrbarbarian.]

Aspen's tubes have that extra plate heat dissipation thingy, that he can't really explain very well, but it make his tubes indestructible

[flemingmras]

OK if you actually believe that idiot I'm gonna have to smack you!

OK I just looked at a JJ and a GT JJ and yes there are these so called "wings" on there that are supposed to increase the surface area of the plate so that the plate can take some extra heat, but judging by the size of them I doubt that they can handle an extra few watts of power like he claims they can. Plus, what's the point of biasing hotter anyway? I run mine pretty close to 17.5 watts(around 16.6 or something) and they sound great there!

Jon

[gtrbarbarian.]

Did you see the after my last statement? No I don't believe what GT says, they are trying to sell tubes

[flemingmras]

I know, I just had to bust your balls a little!

Jon

P.S. You did get the correct bias formula above didn't you?

[gnugear]

Hey Jon,

I measured the voltage at pin 3 and 4 and they were 421V and -35 (for both tubes). When I took a measurement at the HT fuse it was 431 (I've seen it drift as high as 450.

Sound normal?

chris

[flemingmras]

OK I think we found Gnugear's problem.

It seems that the bias probe isn't working right for some reason. I had him measure his bias current via the transformer shunt method and found that he was biased at 91.4mA!!! OUCH!!! Then come to find out the 47K resistor in the bias supply had been changed to a 33K! WAY too low for KT66s!

So I'm having him substitute resistors in that position, starting with higher values and working his way down. We should have this problem licked in no time!

Jon

[gnugear]

Whew ... thanks to Jon, I think I got it worked out.

Using the bias/voltage formula and the shunt method, I arrived at these results:

17.5/430V=Max Bias Current (.0406) for KT66s

My measurement at pin 3 = 3.31mA

I'm not a rocket scientist, but I think that means my max range for setting the bias is 40.6mA and my bias is currently set at 33.1mA

That being the case, I no longer have glowing tubes ... yay!

I also changed my bias resistor to 68k to get everything adjusted properly. Now I just have to figure out why my bias probe is so far off.

I'm still not sure what the hell I was doing, but Jon held my hand. It also scares the shit out of me to probe around in the amp ... especially since I created a fireworks display by accident.

[flemingmras]

Hey Chris, I just got your message.

OK when you're measuring current, on the meter display make sure there is a mA symbol on the display. If there is, the reading 3.31 is too low.

However, yes you are right about the 40mA being the max.

I'll call you tonight and we'll get this thing set right. I know you're a little freaked still, but trust me, I've done this for years so anything I have you do you're not gonna fry your amp(or yourself for that matter LOL).

Jon

[gnugear]

Just to follow up ...

I think I have everything stabalized now. I'm not sure I understand what all the problems were, but this is what I did to fix them;

1. Switched my dual 1M pot to a dual 500k pot for my Rich PPIMV. Jon said this was f*ing up my bias

2. Installed a 1ohm resistor on my power tube socket. This is giving me a much more accurate reading. Not sure what was going on with my bias probe, but I'm not using that anymore. The 1ohm resistor is way easier anyways.

So far I've swapped out 6550s, KT66s, EL34s, and EH6CA7s and all seem to bias in the correct range and work great with no glowing tubes!

Thanks for all the help!