Olaf, and other amp builders... I need your assistance.

[SFW]

So according to H&K, I should bias my entire amp for 66ma, to prolong tube life. Now if I am correct, that would bean that each tubes is only getting 16.5ma of current- which to me is starving the powertubes, and not allowing the amp to function at it’s peak. Currently, each tube is reading 27.3ma- still really cold for a 6L6 tube. I would like to get them up to 36ma across the board to allow the amp to get out of the negative feedback area. However, the bias adjustment will only get me to 27.8ma. H&K says:

If the control range of the trimmer is insufficient, replace the R128 resistor with a 3.3 K-ohm resistor. If this change does not increase the control range by a suitable margin, replace the R129 resistor with a 120 K-ohm resistor.

Now my question is this: What wattage 3.3K-Ohm resistor do I need? 1/8 watt, ¼ watt, ½ watt? I tried to ask the Texas based “authorized service center” but those guys were really dicks about the subject. Basically stating that they would not give me the value, because then I would not send them the amp. I’m not gonna pay these clowns $120/hr to change one resistor.

Anyway… any help you guys could be would be greatly appreciated. Thanks!

 

[Sixtonoize]

First, how are you determining your bias target?
If you're not calculating it based on plate voltage, it's a meaningless number.

Secondly - the bias resistor doesn't see a lot of wattage. 1/2 watt should be just fine.

 

[SFW]

My plate voltage is 500VDC. Which would mean that I need each 6L6 to see 35mA to get out of crossover distortion range. Correct?

There are currently two resistors soudered into the space I need to be in. They read a total of 4.97 k-Ohm. So is the 3.3K-Ohm gonna do me any good?

 

[Sixtonoize]

35ma is right around 60% dissipation, but the only way you're going to know whether or not you're eliminating crossover distortion is if you hook the amp up to a scope. But regardless, 35ma @ 500v is a good general setting for 6L6GCs.

I always make sure to ask, because most people have no idea what bias means and throw around the current draw like it's the only measurement that means anything. Obviously, you know more about this than "most people".

Maybe H&K means 66ma a pair. That would make a lot more sense.

You can always use the 3.3k resistor to jumper the 4.97k combined resistance.
That will give you a total resistance of 1.98k. Since 5k is too much, this might get you into the correct range.
Just start with the bias pot maxed (as cold as possible) so that you minimize the possibility of red plating.

 

[glpg80]

i would just install it parallel as well, the wattage requirement of 1 resistor would actually be reduced since the resistors are now in parallel - so you would get somewhat more accurate resistor readings due to shared power load.

this is personal opinion, but i find it is actually better to bias them around 57-62% PD, your cleans will not suffer and you will get better bass response - not as chewy so to speak. this is of course talking high gain amplifiers, blues players will probably prefer a warmer tone, as will low volume players who do not crank their amplifiers much.

to calculate current, use ohms law. a standard 6L6 is 25W max, 22W for calculations, 15-18W to be more realistic with tube quality and tube life.

P=IV, I= P/V

22Watts/500VDC = 44mA at 100% PD.

44mA times the percentage acceptable, in this case 62%, is around 28mA for 1 tube. try it again with 70% which is a much warmer bias is around 31mA.

if you are measuring using the center tap method of the OT, you will need to divide that measurement by 2 because that is for each side/pair in 120W class A/B amplifiers. i posted a pretty lengthly post about biasing a few posts ago with a few other, more involved methods if you are going for accuracy.

another method is to do it by ear, and measure it for safety. i can tell you that a few members here do that instead. just do not play the amplifier while measuring with weber bias probes since current draw fluctuates when a sin wave is introduced which can fry your meter causing it to short and bias to run away.

 

[TX6Strings]

Just buy the Budda Joel

 

[SFW]

Maybe H&K means 66ma a pair. That would make a lot more sense.

You can always use the 3.3k resistor to jumper the 4.97k combined resistance.
That will give you a total resistance of 1.98k. Since 5k is too much, this might get you into the correct range.
Just start with the bias pot maxed (as cold as possible) so that you minimize the possibility of red plating.

So if I solder the 3.3k-ohm on top of the other two it will drop the value? Just want to make sure that I understand this before I pull out the iron.

The other question that I have is: as tubes get older do they loose their ability to get up to the proper bias voltage?

 

[glpg80]

he is just saying instead of replacing the resistors, solder a 3.3k in parallel with 128, or in parallel with 128+129 to give you a better negative bias swing

1/(3.3k) + 1/(5k combined resistance) = 1/Ans, in this case 1.98k = ~2k

 

[glpg80]

The other question that I have is: as tubes get older do they loose their ability to get up to the proper bias voltage?

the bias circuit is not up to proper bias, bias keeps the tubes off. class A/B push pull is not the same as class A - class A is much less effiecient and is always full on 100%, where class A/B is designed to shut the transistor (tube) off when no signal is present.

the whole point of biasing is the adjustment of this off location in reference to its plate voltage supply to optimal operation characteristics. the bias of the tube will drift yes with tube life - electrons will not be able to flow more effieciently from the cathode to the anode/screen as well due to heat and normal wear of the getters inside the tube.

 

[SFW]

Just buy the Budda Joel

I'm already looking at spending too much money on other things... I'm just gonna have to get the triamp back in shape.

 

[Nigel Tufnel]

What is the value of r128 stock?

 

[SFW]

What is the value of r128 stock?

10K-Ohms

 

[Nigel Tufnel]

What is the value of r128 stock?

10K-Ohms

Cruise over to rad shack and get a 4.7k 1/2w and put it in parallel with that 10k. You'll end up with right about 3.2k, close enough for rock and roll mang, and you won't have to pull anything off of the board.

 

[SFW]

Just to give an update. I added the 3.3k bringing the resistance to 1.9K-Ohms. This actually dropped the mA reading down to 11mA to a max of 19mA. I then removed the resistor that I had added, as well as the one that had been added before hand- so that the am was back to its stock form. This allowed me to get the mA reading up to a comfortable 35mA. There is still movement in the trim pot, but once I reached 35mA, I stopped. I will try the amp here, and at the 27mA that was recomended in the earlier post and see which one I prefer.

Thanks for all the help guys!

Joel