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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 4:29 pm
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Please explain this statement. I have never heard anything like this before.--Bluesky
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You never hear about that because the web is .....the web ; many incomplete information.

Ok let say you have 400 volts on 6V6 plate, at 23 ma.
You want a colder bias so you turn your bias pot to have 18 ma ( 60 % ).

You think job is finish ? Take a new plate volts mesurement , you see now more than 400 v maybe 425 ! And 18 ma at 420 volts it could no to be the bias you want .

Why you have now 425v ? Because when you change the bias of tube from 23 ma to 18 ma, the internal resistance of the tube change . It come higher because bias is now cool. Higher resistance, more voltage at the plate.

Sometimes m.a. varation is just few m.a. but many times I see going from average ( 60 % ) to cool ( 50 % ), changing the tone.

And if you go to hotter bias ; plate volt drop.

I hope this help


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 4:32 pm
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bluesky636 wrote:
cedarblues wrote:
bluesky636 wrote:
Reread the original post.The OP only measured a plate voltage of 412/411 VDC at those current values. One tube is 9 watts and the other 12 watts.

Still, at 23 and 30 mA that's too hot for 6V6GT in a DRRI.


Agree on the whole. Per tube, 23 mA at 412 VDC is right at 70%. 30 mA at 411 VDC is way too hot.

I'm assuming you're going by 14 watts? At 12 Watts 20 mA is at 70%.


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 4:36 pm
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stratele52 wrote:
Please explain this statement. I have never heard anything like this before.--Bluesky
________________________________________________________________

You never hear about that because the web is .....the web ; many incomplete information.

Ok let say you have 400 volts on 6V6 plate, at 23 ma.
You want a colder bias so you turn your bias pot to have 18 ma ( 60 % ).

You think job is finish ? Take a new plate volts mesurement , you see now more than 400 v maybe 425 ! And 18 ma at 420 volts it could no to be the bias you want .

Why you have now 425v ? Because when you change the bias of tube from 23 ma to 18 ma, the internal resistance of the tube change . It come higher because bias is now cool. Higher resistance, more voltage at the plate.

Sometimes m.a. varation is just few m.a. but many times I see going from average ( 60 % ) to cool ( 50 % ), changing the tone.

And if you go to hotter bias ; plate volt drop.

I hope this help


I understand all of that. It is made quite clear in the Weber bias tables. It is this statement that I do not understand:

"adjust bias many times until no plate voltage change and you have correct bias."

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 4:39 pm
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cedarblues wrote:
I'm assuming you're going by 14 watts? At 12 Watts 20 mA is at 70%.


Plug the OP's numbers into the Weber bias calculator and see what you get.

http://www.webervst.com/tubes1/calcbias.htm

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 5:02 pm
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bluesky636 wrote:
cedarblues wrote:
I'm assuming you're going by 14 watts? At 12 Watts 20 mA is at 70%.


Plug the OP's numbers into the Weber bias calculator and see what you get.

http://www.webervst.com/tubes1/calcbias.htm

I know and understand where the OP got his #s from. All I'm saying is that at 23 mA it's already over 70%.

If I go by 12 Watts per 6V6 tube, then 70% is 8.4 Watts max.


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 5:37 pm
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All of the spec sheets that I've seen show all of these 6V6 tubes are 14 watt tubes, GT, EH, JJ, Tung Sol, GE, RCA. Anyone have a spec sheet showing otherwise? Which 6V6 are 12 watt? :idea:

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 6:19 pm
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cedarblues wrote:
bluesky636 wrote:
cedarblues wrote:
I'm assuming you're going by 14 watts? At 12 Watts 20 mA is at 70%.


Plug the OP's numbers into the Weber bias calculator and see what you get.

http://www.webervst.com/tubes1/calcbias.htm

I know and understand where the OP got his #s from. All I'm saying is that at 23 mA it's already over 70%.

If I go by 12 Watts per 6V6 tube, then 70% is 8.4 Watts max.


The OP said that his tubes are GT 6V6-R which according to the Groove Tube website are "about 15 watts per tube".

http://www.groovetubes.com/power.html

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 6:20 pm
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shimmilou wrote:
All of the spec sheets that I've seen show all of these 6V6 tubes are 14 watt tubes, GT, EH, JJ, Tung Sol, GE, RCA. Anyone have a spec sheet showing otherwise? Which 6V6 are 12 watt? :idea:


The Groove Tubes website says that the 6V6-R tubes that the OP has are "about 15 watts per tube". No spec sheet is provided.

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 6:46 pm
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:) Close enough. As long as someone isn't using an oddball 12 watt tube, we can all be on the same page when biasing (no metal cans please :lol: ). I have no doubt that the GTs can handle 15 watts or more, but I'm not sure what they are trying to say on the GT site, I would still count the GTs as 14 watts myself though. Nice to see that they are releasing a little info on the new "S" versions; "These handle plate voltages in excess of 475 plate volts....." I'm going for a pair of GT 6V6 S for my HRDlx.....so we'll see.

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 7:47 pm
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shimmilou wrote:
I would still count the GTs as 14 watts myself though.


That's "about 15 watts". :lol:

I know you like GTs, but their lack of specs is what bothers me the most. At least JJ publishes a limited data sheet:

http://www.jj-electronic.com/pdf/6V6.pdf

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 9:34 pm
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True. It's a pain having to dig for info that should be on a simple data sheet.

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 9:57 pm
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shimmilou wrote:
All of the spec sheets that I've seen show all of these 6V6 tubes are 14 watt tubes, GT, EH, JJ, Tung Sol, GE, RCA. Anyone have a spec sheet showing otherwise? Which 6V6 are 12 watt? :idea:

I'm no expert, but I've been told by amp techs and probably some tube resellers that 6V6GT were 12 W and 6V6GTA were 14 W. If they can handle more then that's a good thing :)

Even the Weber calculator uses 12 W for 6V6, that's usually what I go by...

http://www.webervst.com/tubes1/calcbias.htm

If you scroll down to the first table you'll notice that it says 12 W next to EL84/6V6GT. And if you use the very first calculator and select class AB, 6V6, 420 plate voltage and click calculate bias you get 20 mA, that's exactly 70% of 12 W.

12 / 420 = .028
.028 X .70 = .02

If you select 6V6GTA instead and same plate voltage then it becomes 23.3 mA, which is 70% of 14 W.

I just looked up the spec sheet on the TAD I've been running in my DRRI, bias is at 20 mA right now... not sure what it all means but it looks like 15 W.

http://www.tubeampdoctor.com/images/File/TAD-Datenblatt_6V6GT-STR%20Version1.0-05.07.11.pdf

Until someone convinces me that it's safe to run my tubes hotter I'll stick with these safe #s :wink:

TAD 6L6GC-STR spec sheet for anyone interested... sound wonderful in my SR!

http://www.tubeampdoctor.com/images/File/TAD-Datenblatt_6L6GC-STR%20Version1.0-05.07.11.pdf


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Tue Jul 05, 2011 11:05 pm
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That's odd. :? The spec sheet for a GE 6V6 GT says 14 watts, Tungsol 6V6 GT 14 watts, among others as I mentioned before, and both the RCA GT and GTA are 14 watts. So why would they say only 12 watts for a 6V6 GT?

The Weber calculator site is worthless to me, I can already multiply two numbers. And, if you notice, the Weber calculator keeps changing the entered number for the Cathode current, dropping 2 mA from any number entered. Is this their way of subtracting the screen currents? The screen current(s) are closer to 5 mA than 2 mA for a 6L6. :?:

Again, is there a spec sheet that shows only 12 watts for a 6V6? Some sheets show 12 watts or 14 watts depending on the configuration, for the same tube, which means max 14 watts. :idea:

But, I agree with erring on the conservative side if unsure. :idea:

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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Wed Jul 06, 2011 2:24 am
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I understand all of that. It is made quite clear in the Weber bias tables. It is this statement that I do not understand:

"adjust bias many times until no plate voltage change and you have correct bias."
_Bluesky

Yes you are right Bill

Louis


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Post subject: Re: DRRI Bias - from Stock GT to EH
Posted: Wed Jul 06, 2011 6:01 am
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I use the RCA tube manual,but naturally there's nothing about the new production tubes,so I've been skeptical about a "spec sheet" on tubes manufactured overseas and available only on the internet from the seller,one of our forum members has been trying to work with the new Tung-Sol 6V6s and has not had much luck biasing them without them red-plating,I had bought some to try but after reading some of his troubles,will not use them in one of my amps....it's not worth it.
I don't use an online bias calculator either,I take measurements and do the math myself.


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