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PostPosted: Tue Jul 08, 2014 5:11 pm 
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Hi all, I recently finished my build and wanted to make sure my voltages are where they should be, also I am having issues with my bias adjustment. Currently when I test the bias voltage, there is a difference of about 10mv between V5 and V6. Currently my V5 bias is 31.2 and V6 is 20.6. BTW I am using 6v6 tubes and a 12at7 on V4. Is the difference in bias something to be concerned about?

My voltages are as follows:

AC Main 118.6
B+ with tubes: 444

Pin 1 2 3 4 5 6 7 8 9
V1 120 0 1.26 - - 106.2 0 .97 --
V2 167.6 0 1.18 -- --- 94.2 0 .73 ---
V3 257 46.8 64.9 --- --- 161.6 0 1.68 --
V4 254 64.8 68.9 --- --- 244 32.4 68.5 ---
V5 0 -- 441 405 -32 -32 --- .031
V6 0 -- 441 405 -31.8 -32.6 -- .02

Thanks in advance for your help!


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PostPosted: Wed Jul 09, 2014 8:20 pm 
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The difference in bias map earns the tubes aren't matched closely, but that is a rather large.
You need to flip the switch to 6v6 B+ Which a the lowest setting and re measure.

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PostPosted: Thu Jul 10, 2014 9:45 am 
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I believe I have it set to 6v6 and if I flip the switch, the bias readings increase , however, there is still about a 10mv difference between the 2 tubes. Do you think it could be an issue with the bias section of the board? I re-checked the connections and everything looks exactly like the v17 layout. Could it be something with the transformer? I played the amp and it sounds great, however I want to make sure that it is running as it should be.

Thanks!


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PostPosted: Thu Jul 10, 2014 1:04 pm 
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With that b+, i would go on the low side. Is that the lowest b+ you can get?
The bias is set for both tubes so if you can get into the ballpark the circuit is working fine.

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PostPosted: Thu Jul 10, 2014 1:16 pm 
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The B+ reading of 444 was at the positive terminal of the 50uf can cap. I am not sure where else I can get a B+ reading from (I apologize for my lack of understanding.) Is there somewhere else I should be reading the B+ voltage? So should I turn the bias down so that the higher reading V5 is around 18 - 21?


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PostPosted: Sat Jul 12, 2014 11:44 am 
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Swap the tubes 5 and 6 and see if the current difference follows the tube or stays relatively the same. you have like a 10 ma diff. If the difference stays with the socket power everthing off pull the power tubes and measure the 2 100k grid leak resistors and see how close they are in value .


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PostPosted: Sat Jul 12, 2014 6:54 pm 
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So i switched the tubes around (v5 & v6) and it looks like the higher current follows one of the tubes. Does this mean I have a bad tube?

Here are the readings I got, I labeled the 6V6 power tubes A and B:


With A in V5 and B in V6: Tp1 = 11.2 TP2 = 19.7
With B in V5 and A in V6: TP1 = 19.9 Tp2 = 11.4

Looks like tube B consistently reads 10ma higher than tube A. I ordered these tubes from Tubedepot.com and they were supposed to be matched. They even came with a sticker on the base of the tubes stating the bias point for both tubes was 21 ma. I guess I just got a bad tube.... Ill give you guys an update when the new tubes come in next week.

Thanks!


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PostPosted: Mon Jul 14, 2014 10:06 pm 
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So new JJ 6v6's arrived today. It's a little late to test how they sound, but the good news is that now the bias is the same for both power tubes. The new tubes are a "matched pair" and came with a bias point of 29ma printed on them. Now I have 2 new questions: 1. The tubes printed bias point is at 29 which is higher than the max per the build manual. Is there an issue with this? 2. With my selector on the6v6 setting the max bias I can get is 19.6. Any reason for this? If I flip the switch to he kt66 side then I can get the bias up to 33. Should I leave the amp in the 6v6 setting and max the bias at 19, or should I switch it to kt66 and set it to 29ma so it matches the tubes bias point??

Thanks!


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PostPosted: Thu Jul 17, 2014 10:32 am 
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Anyone?


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PostPosted: Thu Jul 17, 2014 5:10 pm 
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The 29 on the box doesn't mean you have to bias them to that.

When you got 100's of tubes as a vendor you number your sets (or quads) 1, 2, 3...etc. Then when a customer orders a pair all you have to do is grab two "29" tubes or two "74" tubes.

The Triwatt biases the output tubes on the cool side. If your plate voltage on the 6V6 is 450V you want around 22mA of bias current. So 19 is fine. Google calculating bias current if you don't already know.

Check your resistor values, 6V6 setting should be able to get more than 19mA iirc.

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PostPosted: Sat Jul 19, 2014 7:58 am 
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Plate voltage on 6v6 should be closer to 400 than 450 and you should be able to get the bias there as well if the bias circuitry is in correctly. As Kurlives mentioned, check the bias resistors and Zener and switch. You want the lower of the voltages feeding the 6v6 and180K bias resistor only.

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