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Old 27th October 2004, 03:31 PM   #11
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Hello,

So much controversy, but the proof is in eating the pudding!
I have tried 36, 50, 60 then 80 ma! No sign of cherry red tubes!
Soundwise, I found that I've got the nice mids and highs, the pumping bass at 72ma!
any scientific reasons behind that?
Thanks for educating me!
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Old 27th October 2004, 03:40 PM   #12
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Weeeelll... voltage at the grids will tell all.

But, 270 volts is not 335 volts and does not correspond with what the specs show for that tube on page 7, which shows what the tube is really doing with that current and that B+.

If you draw the lines up from 35 mA and over from the right at 335 volts, you will see that the tube is barely conducting (relatively). It is in class AB.

On page 8, the grid is at -25 volts, again drawing a line up from 35 mA and over from 335 volts. The tubes are dissipating 11.7 watts at idle. This is typical of 10-20 watt AB push pull tube amps. 11.7 watt dissipation at a 90 percent efficiency for class AB means 10 watts per tube output times 2, or... 20 watts (rounded up, of course. Your mileage may vary)!

It should be at between -14 and -18 volts for class A at that particular plate voltage. It should be conducting between 170-200 mA, hence dissipating about 50 watts (plates red yet?). At a maximum of class A efficiency of 20 percent, that would be 10 watts per tube times 2, for the rated 20 watts.

The specs don't lie. In fact, they confirm that the amp is AB. Remember please that at idle the tubes draw maximum current in class A. 35 milliamps for that tube is just way too small for 20 watts output at class A.

Again, tube plate characteristics confirm.

http://www.triodeel.com/6l6gc_p8.gif

By the way, did you take a look at the specs on that beast??? Real bad! About 160 to 13.5KHz. It is about 20-12K at full power (unless there is a typo and the 160 is really 16, but still not too good for the money IMHO)! Terrible! Sorry to say, but specs should be better at lower power, not worse (like 10-40K at 1 watt, 30-20K at full power). Money wasted... sorry to say.

But the nice thing about our hobby, we can make it better! Yay!

Gabe
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Old 27th October 2004, 03:50 PM   #13
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Hi Gabevee,

For the price I've paid for those beasts on Ebay, I don't care if something goes wrong.

I am now running them at 72 ma per tubes (I am well away from them, lol) but they are still singing, for how long, only God knows

If the worse happens, I'll still have the nice chassis!

Transformers aren't nice anyway
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Old 27th October 2004, 05:28 PM   #14
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Oh, I apologize for the confusing price statement. I was referring to the orignal sale price of about $400 USD apiece (though their actual page for the amp says $300). Still... $600 for specs like that?

You probably get a sweeter, warmer sound now. I hope the transformers can take the heat, though.

Gabe
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Old 27th October 2004, 10:56 PM   #15
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Hi guys,

Over here I have a question,too.
I've just build a P-P amp with 6L6GC.And I think my tubes are getting too blue.Is this normal?I haven't use before 6L6.So I don't know.
I have bias them at 430V, Vgk=30V and Ia= 65mA.
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Old 27th October 2004, 11:00 PM   #16
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Blue glows are usually normal, generally caused by electrons striking the glass. It can also be ionization from imperfect vacuum. If the plates start showing red, that's a bad thing.
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Old 27th October 2004, 11:13 PM   #17
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Default I was wrong

G'day kooltubes,
I was wrong above to suggest the amp was runnning Class A PP.
Those who said Class AB were right.

When you adjusted the bias up to 72mA per tube what you effectively did was change it from Class AB to Class A. This should give better damping and hence better bass control.

Cheers,
Ian
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Old 27th October 2004, 11:14 PM   #18
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Thanks SY

My tubes are EHX.
No reds.

But some hummmm problems
If you can help on this,I have post a thread.
here
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Old 27th October 2004, 11:15 PM   #19
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Hi gingertube ,
I am enjoying the amp, it never sounded so good.
Thanks to all of you!
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Old 27th October 2004, 11:17 PM   #20
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Hi resident,

My 6L6's are EHX: blue glow but no hum

I am planning to use Valve art KT66 just to see if there's any sonic improvement
But nothing beats a single-ended DHT amp!
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