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Old 24th October 2004, 12:25 AM   #1
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Question Help needed for a 6L6 Push pull amp

Hello,

I've just bought a pair of monoblocks made by Antique Sound Lab.
The amp is a pushpull of 6l6's ; the manual recommends to bias each tube at 35ma; I want to use 6L6 GC tubes instead; should I keep the same biasing or what is the optimized operating point per tube for a plate supply of approx 335V?

Is there any advantage to modify the amp for cathode bias?

Thanks for your help
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Old 26th October 2004, 03:45 PM   #2
Giaime is offline Giaime  Italy
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Dear Kooltubes,

First and foremost you have to tell us if the amp is in class A or AB. What's the output power? If it's from 30 to 60W it is probably an AB. In this case, you should bias your amp according to the rule of the 70% max dissipation. (and, obviusualy, if the tube is still a 6L6 there is no difference in characteristics if the name is 6L6GC, but you need ALWAYS a rebias if you want performance and reliability).

Example: 6L6 max plate dissipation 30W

bias current for each tube = 30/Plate voltage (with no signal applied). Remember to check this for both tubes, and if something's different, use a conservative setting.
But if you aren't experienced in dealing with high voltages, refer to a qualified tecnician.

And no, I won't change the bias to cathode bias. Cathode bias is for little power poor designed old amps.
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Old 26th October 2004, 03:53 PM   #3
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Hello Giaime,

The output transformer is 5k; the amp is rated 20W.
I cannot tell whether it's class A or class AB.

Thanks for your help.
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Old 26th October 2004, 06:25 PM   #4
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Hi There,

What would help is to find out what the cathode resistor is... if any.. or check the voltage at the grid.. or wherever the bias is being adjusted... then one can determine where the tubes are supposed to be biased.

My guess is that with 335 volts B+ for 20 watt output, it is class AB.

BTW, if memory serves, 35 mA is about half of the full recommended current of the tube for class A. That actually sounds class AB to me. (check the spec sheets for 6L6 in class AB operation. Class A calls for 70-80 mA).

If you want to try going to cathode bias, try it. Bass will not be as good, though.

Gabe
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Old 26th October 2004, 06:30 PM   #5
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Hi Gabevee,

The cathode resistor on each tube is 10R and there are 2 test point and 2 pots to adjust the current.

Any tip to improve the bias supply? I have already improved the main psu and neglected the bias supply


Thanks
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Old 26th October 2004, 06:39 PM   #6
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Hello,
Your amp is class AB. I am assuming you are changing the original Chinese tubes out. These are the same as GC version even though they're not marked as such. Your bias will stay the same with the replacement tubes.
I would also recommend changing the input tube as well. JJ's work particularily well.
There is no real advantage to hanging thes amps to cathode bias, except convenience.

Dan
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Old 27th October 2004, 07:38 AM   #7
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Default I disagree

G'day from Oz,
Sorry to disagree BUT then we are a disagreeable lot here downunder.

5K Primary Impedance, 335V at the anodes, 35mA PER TUBE and 20W Output all stack up with Class A Push Pull (fixed bias)

Class AB would be delivering a lot more power.

Since its Class A there is not a lot of point in biasing the tubes harder. Found limited data on 6L6GC in class A PP BUT for:

5K Primary Impedance
270V rail
Cathode Biased version (125 Ohms shared)
18.5W Output
The tube bias current should be 36mA per tube.

Since its fixed bias and slightly higher rail voltage you may achieve some benefit by increasing bias per tube to say 40mA.
Not much point in going higher.

At this setting the tubes are running at around half their maximum capability and should outlast you.

Cheers,
Ian
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Old 27th October 2004, 11:25 AM   #8
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Hi Ian,

Thanks for your reply. The best sonics were achieved at 36/38 ma.
Transformers are cool!
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Old 27th October 2004, 01:20 PM   #9
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IAN,

Noooooo. Me disagree. Just look at the spec sheets. Bias for class AB for the 6L6 is actually 40-50 mA. Here he says 35mA. At 335 volt B+, 20 watts AB is just right. Might actually be 17 to 18 watts. According to the spec sheets, for class A one the tubes should be biased with about 70 mA. What would solve this problem is knowing what the bias voltage at the grids is at the nominal current.

Kooltubes,

Please check the voltages at the grids. If it is -14 to -30 volts, then you are class AB. Yours is probably closer to B than AB.

Here:

http://www.triodeel.com/tubedata.htm

is where you can find typical specs for typical applications. Note that on the third page it specifies 120mA and above for class A for a 15 to 18 watt output with 270 volts B+.

Hope this helps.
Gabe
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Old 27th October 2004, 02:42 PM   #10
Giaime is offline Giaime  Italy
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I think I disagree. On the third page of that site are showed the EXACT conditions of kooltube's amp.

5k plate to plate
270V B+
Power output 17W or something

But the bias current is 130/150mA for the tubes! That means about 65/75mA per tube. And with that little higher B+ (335V) you should bias each tube at no more than 55/60mA. AND THESE ARE VERY CONSERVATIVE VALUES! Tubes will last looooooong time.

With my experience, I think that the amp IS actually rated at 20W, but the bias is set to half the value needed to obtain more lifespan from the tubes.

My suspect is that these tubes that are you changing were the original ones. Isn't it? The manufacturer deliberately overbias (I mean too little current) the tubes at the factory, because they're using cheap tubes that will fall to the Dark Side of the Force very quickly. So, to avoid a bad reputation ("oh, that amp EATS tubes!!") they deliberately sacrifice some power.

I suggest you to recheck everything in the circuit (or post a schematic of it, with the actual voltages) and try a bias point of 45/50ma just to start. Play loud at full volume, and notice if the plate of the 6L6 becomes of a cherry-red color. If everything stays cool, repeat with a bias of 60mA. That would be an optimum compromise between output power and lifetime.

But if you make the math,

MaxBiasCurrent for class A, 335V, fixed bias:

30 / 335 = 89mA! (take 80 just for safety)

So you still are overbiased: remember that class A means that the maximum dissipation occurs at idle, and it nulls to zero at maximum peak plate voltage.
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