Tube questions - 0 bias

ChopperChas

Super Member
I bought 8 "factory second" Teslovak KT88 tubes off of eBay, and got four that appear to work. I've got them in my tube amp now, burning them in at a low bias. (as low as my amp goes, which is between 0.19v and 0.43v, depending on the tube... with one running at 0.69v. Not socket dependent, the bias voltage follows the tube)

First, the questions about the working tubes.

1. Why does one have an idle bias voltage of 0.69v (or 69 ma, I guess, but my manual says to use a voltmeter to measure bias)

Now questions about the bad tubes:

2. Two have heaters that don't light up. What causes this? No orange glow, no nothing.

3. Two have perfectly working heaters, but I never get bias voltage across the test points. Always 0.000v. What is wrong with these two tubes?

Thanks!

Charles.
 
On the tubes with no filament light, what's the resistance between pins 2 and 7? If it's open, there's your problem.
 
It could be a broken weld inside the tube,(not repairable) or a poor solder joint in the tube pins(this also applies to the open filament as well)(possibly repairable)
 
I bought 8 "factory second" Teslovak KT88 tubes off of eBay, and got four that appear to work. I've got them in my tube amp now, burning them in at a low bias. (as low as my amp goes, which is between 0.19v and 0.43v, depending on the tube... with one running at 0.69v. Not socket dependent, the bias voltage follows the tube)

First, the questions about the working tubes.

1. Why does one have an idle bias voltage of 0.69v (or 69 ma, I guess, but my manual says to use a voltmeter to measure bias)

Now questions about the bad tubes:

2. Two have heaters that don't light up. What causes this? No orange glow, no nothing.

3. Two have perfectly working heaters, but I never get bias voltage across the test points. Always 0.000v. What is wrong with these two tubes?

Thanks!

Charles.
I hope it was a bargain. Or that you still have time to complain.
Face it, 4 of them are broken, and none are matched.
Return them and get 4 ( or 8 ) matched new tubes from a reputable dealer.
 
For $28 I can't really complain. The seller said they were seconds and many were gassy or broken and were being sold for parts. I took a gamble and got four good tubes for $28. That's a pretty good deal, if you ask me. Also, note that I don't need matched tubes - my amp (Yaqin MC-100B) has individual bias settings for each tube. That saves me a lot of money in the long run!

I'm going to bring up the bias and start playing some music in a couple hours, and will hopefully be a happy camper.

Charles.
 
For $28 I can't really complain. The seller said they were seconds and many were gassy or broken and were being sold for parts. I took a gamble and got four good tubes for $28. That's a pretty good deal, if you ask me. Also, note that I don't need matched tubes - my amp (Yaqin MC-100B) has individual bias settings for each tube. That saves me a lot of money in the long run!

I'm going to bring up the bias and start playing some music in a couple hours, and will hopefully be a happy camper.

Charles.
The possibility to set individual bias is only part of the story. Tubes unmatched Gm is like using different size piston
in the car.

Matched tubes is tubes with similar idle point AND similar Gm , both measured close to the actual operating conditions. Anything else
is unmatched, it _might_ be partially compensated by the NFB.
 
Well, they're sounding pretty nice now. All biased to 0.70 volts, just rocked out some Iron Maiden at moderate volume with no problems.

Charles.
 
That's too high for a 100B! Should be no greater than 0.55V and I run mine myself between 0.45 and 0.5V
 
The one that idles very hot I'd be a little worried about. Gassy tubes tend to run like that, and they also tend to run away with themselves. Keep a close eye on that one. If it starts glowing pink or purple inside the plate structure, its pooched.
 
I got a little bias-happy with the Teslovaks, because the bottom end kept tightening up the higher I dialed in the bias. Sounds really good up there, but I dialed back to 0.55v. Going to replace the hot tube.

Charles.
 
On the tubes with the bad heaters, you could heat the filament pins and add a little solder in the end of the pin to see if it can be fixed
 
Nope, no dice. I can look at one tube and see the heater filament connection flapping inside the tube. It's come disconnected. The other one, heating the pins and flowing solder on them didn't do anything, so back into the trash it went.

I got another 8 of these tubes, and low and behold I have two full working quads now, and two extra tubes that bias hot. I'm a happy camper! Plus I have a bunch of extra tube boxes to store all my other power tubes in now :)

Charles.
 
Personally I would not let any tubes that sketchy anywhere near my amp.
It is not worth the risk to me. What if a tube flames out and takes other components with it? Not such a great deal then eh?
 
Back
Top Bottom