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Post subject: Re: HRD re-bias question
Posted: Sat Mar 16, 2013 9:18 am
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Check the specs on those 6V6 to see the max rating for the Plate voltage. You'll find that with the 6V6 installed in the HRDlx, the Plate voltage will exceed the specs for most any 6V6, over 450 VDC. The only 6V6 tubes that are spec'd for this high of a Plate voltage are either GT 6V6 S (and only the S version), or JJ 6V6, and both sound more like a 6L6 than a 6V6. The Tung Sol might be able to handle the high Plate voltage, and the newer made EH might also. I did put a set of older EH 6V6 in my HRDlx and they sound fantastic. But, I felt that it was a bit risky pushing these tubes that hard so I went back to 6L6. Check the specs though.

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Post subject: Re: HRD re-bias question
Posted: Sat Mar 16, 2013 11:01 am
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The eurotubes web site recommends 70-90 and says you don't have to measure the plate voltage even though they do.

Another friend of mine also thought that was too hot, so I biased it down to 70. It sounds good. I will see what I think.

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Post subject: Re: HRD re-bias question
Posted: Sat Mar 16, 2013 1:37 pm
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lenrabinowitz wrote:
The eurotubes web site recommends 70-90 and says you don't have to measure the plate voltage even though they do.

Another friend of mine also thought that was too hot, so I biased it down to 70. It sounds good. I will see what I think.


I provided you with a detailed explanation of how to properly bias an amp. Without measuring the plate volage you have no clue if the tubes are running too hot or too cold.

These are the meaurements for my BDRI:

CATHODE CURRENT 53 mA
PLATE CURRENT 50 mA
PLATE VOLTAGE 403 VDC
% MAX PLATE DISS 67%

and 5F6A clone:

CATHODE CURRENT 50 mA
PLATE CURRENT 48 mA
PLATE VOLTAGE 429 VDC
% MAX PLATE DISS 68%

Notice that the % of max plate dissipation (using JJ 6L6GC tubes) is almost identical even though the cathode current and plate volatages are very different. Measuring the cathode current alone is meaningless.

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Post subject: Re: HRD re-bias question
Posted: Sat Mar 16, 2013 1:58 pm
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What equipment do I need to get to measure the plate voltage?

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Post subject: Re: HRD re-bias question
Posted: Sat Mar 16, 2013 4:08 pm
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The same multimeter that you use to measure the test point voltage, except that the test point is low, approx 80 mVDC, while the Plate voltage is high, approx 430 VDC. Just change scales on the meter and measure the same way, negative lead on chassis, positive lead at measurement location.

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Post subject: Re: HRD re-bias question
Posted: Sat Mar 16, 2013 4:46 pm
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Thanks! Two more questions then: Where do you attach things to measure the plate voltage?

Why is there such a discrepancy in this? The Eurotubes video quite clearly says you don't have to. My Egnater says nothing about this at all. It simply provides the bias test points and adjustments on the back of the amp. The manual provides an acceptable range of settings- it doesn't have to just be one number.

Thanks!

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Post subject: Re: HRD re-bias question
Posted: Sat Mar 16, 2013 6:07 pm
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lenrabinowitz wrote:
Thanks! Two more questions then: Where do you attach things to measure the plate voltage?

Why is there such a discrepancy in this? The Eurotubes video quite clearly says you don't have to. My Egnater says nothing about this at all. It simply provides the bias test points and adjustments on the back of the amp. The manual provides an acceptable range of settings- it doesn't have to just be one number.

Thanks!


No, you are not understanding the Eurotubes video. They show the simple way to bias an amp (using the Fender test point and a multimeter) and the correct way to bias an amp (using a bias probe that measures current and a voltmeter to measure plate voltage). Fender and Egnator take the simple method of measuring a low voltage at a test point and they assume that the plate voltage will not vary much, which is an incorrect assumption.

Yes, proper bias can be anywhere in a range of values between 50% and 70% of the maximum plate dissipation as I noted earlier.

Plate voltage is measured between pin 3 and ground of the power tubes. See the schematic. However, measuring plate voltage in that manner can be dangerous (trust me, I know). The best way to do this with a bias probe. I use this one:

http://www.amp-head.com/product_info.ph ... ucts_id=70

Shimmilou recommends a different one, but they function the same.

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Post subject: Re: HRD re-bias question
Posted: Sat Mar 16, 2013 8:56 pm
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+1 bluesky636

I measure the Plate voltage at the connection to the circuit board for the blue and the brown wires from the OT primary. These two wires have a push-on connector underneath the tube socket board, and you can access the connectors' solder points on the solder side of the board on top. You can see the three wires, red, blue and brown, that come through a hole in the chassis and the blue and brown wires wrap around the front of the board. I think that the reason for the discrepancy in bias instructions, is as bluesky636 pointed out, the assumption that the Plate voltage will be a specific value for a specific amp. The Plate voltage really should be approx the same from amp to amp for a specific amp model, but it's always best to check to be sure.

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Post subject: Re: HRD re-bias question
Posted: Sun Mar 17, 2013 2:39 am
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Ok that clears it up. Thanks!

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Post subject: Re: HRD re-bias question
Posted: Sun Mar 17, 2013 1:53 pm
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lenrabinowitz wrote:
Ok that clears it up. Thanks!


There's always two ways to do something, the easy way and the right way. :wink:

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Post subject: Re: HRD re-bias question
Posted: Sun Mar 17, 2013 1:59 pm
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Yeah, but part of why I am doing this is to learn. I'm a teacher, and I know that sometimes in order to learn it takes trial and error, risks, mistakes, "stupid" questions, etc... All in a day's fun though.

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Post subject: Re: HRD re-bias question
Posted: Sun Mar 17, 2013 2:51 pm
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lenrabinowitz wrote:
Yeah, but part of why I am doing this is to learn. I'm a teacher, and I know that sometimes in order to learn it takes trial and error, risks, mistakes, "stupid" questions, etc... All in a day's fun though.


Try not to let that "learning curve" become financially prohibitive though.

:mrgreen:

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Post subject: Re: HRD re-bias question
Posted: Sun Mar 17, 2013 3:26 pm
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lenrabinowitz wrote:
Yeah, but part of why I am doing this is to learn. I'm a teacher, and I know that sometimes in order to learn it takes trial and error, risks, mistakes, "stupid" questions, etc... All in a day's fun though.


Just be careful...trial and error in electrical components can result in the ultimate "mistake"

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Post subject: Re: HRD re-bias question
Posted: Sun Mar 17, 2013 4:52 pm
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lenrabinowitz wrote:
Yeah, but part of why I am doing this is to learn. I'm a teacher, and I know that sometimes in order to learn it takes trial and error, risks, mistakes, "stupid" questions, etc... All in a day's fun though.


Asking "stupid" questions goes a long way toward reducing risks and mistakes. :wink:

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Post subject: Re: HRD re-bias question
Posted: Mon Mar 18, 2013 1:58 pm
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Google should be in everyone's tool kit also. :wink:

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