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Posted: Fri Oct 29, 2010 1:01 pm
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I agree, you don't need to drain capacitors to adjust your bias, even put your fingers within the chassis. On top of that, I DONT recommand to use a resistor with bended wires to drain caps due to the fact that there is no isolation.
For information, here is my drain capacitor:
100K + wires + crocodile AND all fully isolated.
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Post subject: Re: BDRI biasing
Posted: Fri Oct 29, 2010 1:06 pm
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shimmilou wrote:
63supro wrote:
...Without knowing the actual plate voltage the mv values mean nothing....


Excellent point 63supro! I presumed that his amp was close 440 plate volts like mine, and 65 mV at the test point would be about 28 mA of Plate current per tube, about 12.5 watts idle Plate dissipation (about 50% for a 5881), still on the cold side lol. 85 mV then would be about 17 watts idle, or 70% of a 5881 25 watt tube. I guess that the JJs were 30 watt tubes so 85 mV sounds pretty good. For me it depends on the amp for the bias setting. I like the HRDlx at about 50%, but I like the BJr closer to 70%. To me the 50%-70% range is ideal, and within most everyone's specs. :)

I started to post about the Plate current/voltage/wattage, but I thought it might be too much for the OP. :lol:


You can't really presume anything with these amps. Like bluesky636 said, the power supplies on these amps aren't well regulated. Cheap components are cheap components. You get what you pay for. Even the guy who does the biasing video on tubedepot.com mentions how bad the tube socket connections are to the board. This can also effect the voltage readings. Reason #899 as to why I sold my HRDlx. LOL

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Posted: Fri Oct 29, 2010 1:39 pm
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Here are a set of isolated screwdrivers I use for bias setting or RF-IF adjustments:
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Last edited by Tissan on Sun Oct 31, 2010 4:06 am, edited 2 times in total.

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Post subject: Re: BDRI biasing
Posted: Fri Oct 29, 2010 2:42 pm
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shimmilou wrote:
63supro wrote:
...Without knowing the actual plate voltage the mv values mean nothing....


Excellent point 63supro! I presumed that his amp was close 440 plate volts like mine, and 65 mV at the test point would be about 28 mA of Plate current per tube, about 12.5 watts idle Plate dissipation (about 50% for a 5881), still on the cold side lol. 85 mV then would be about 17 watts idle, or 70% of a 5881 25 watt tube. I guess that the JJs were 30 watt tubes so 85 mV sounds pretty good. For me it depends on the amp for the bias setting. I like the HRDlx at about 50%, but I like the BJr closer to 70%. To me the 50%-70% range is ideal, and within most everyone's specs. :)

I started to post about the Plate current/voltage/wattage, but I thought it might be too much for the OP. :lol:


As I noted above, the plate voltage of my BDRI drops quite a bit as bias current increases. I don't know if that is typical of teh amp in general, or just mine. It doesn't seem to cause any problems as the amp sounds great.

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Post subject: Re: BDRI biasing
Posted: Fri Oct 29, 2010 3:00 pm
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bluesky636 wrote:
As I noted above, the plate voltage of my BDRI drops quite a bit as bias current increases. I don't know if that is typical of teh amp in general, or just mine. It doesn't seem to cause any problems as the amp sounds great.


That should be the case for all amps, the more idle current through the primary of the OT, the more voltage dropped across it, which is less voltage left for the Plate. :)

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Posted: Fri Oct 29, 2010 3:14 pm
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I confirm that normal, this is due to Transformer secondary impedance with more current that react on primary, using simply words: output transformer internal resistor multiplied by idle current gives you less voltage out available.
8)

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Posted: Fri Oct 29, 2010 3:57 pm
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shimmilou/Tissan:

Whew. I thought that was the case. Makes sense. Glad to hear that confirmed.

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Posted: Fri Oct 29, 2010 4:40 pm
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Hey guys, using this link http://www.justinholton.com/hotrod/bias.html I learned how to measure the plate voltage at the pin 3 point. Mine measured to 409 V for one tube and 411 V for the other tube. I then entered the data into the calculator, and it came up with these results:

Calculated static plate dissipation of V4: 13.394 W
Calculated static plate dissipation of V5: 13.460 W

Does this sound good?

Thanks!


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Post subject: BDRI biasing
Posted: Fri Oct 29, 2010 5:03 pm
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That is pretty good for the 25 watt tube, maybe a little on the cold side for a 30 watt tube. I am not sure of the rated wattage of your tubes, but whichever you have they should be adjusted somewhere between 50-70 percent of the max rated value. So, a 25 watt tube would be between 12.5 and 17.5 watts. A 30 watt tube would be between 15 and 21 watts idle. :)

Be very careful if you're going to measure voltage at the pins of the tube, it is very easy to slip and short some things out. I measure my plate voltage from ground to one of the output transformer primary leads. The connector on the circuit board has a little groove at the top to steady the meter lead on. It might be a good idea to get a bias probe while you are learning. I like the one that Eurotubes sells, you can measure the Plate current directly, instead of the Cathode current translated from a voltage drop across a 1-ohm resistor, like many other probes use (and the test point in your amp). :idea:

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Posted: Fri Oct 29, 2010 5:28 pm
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dgcobb wrote:
Hey guys, using this link http://www.justinholton.com/hotrod/bias.html I learned how to measure the plate voltage at the pin 3 point. Mine measured to 409 V for one tube and 411 V for the other tube. I then entered the data into the calculator, and it came up with these results:

Calculated static plate dissipation of V4: 13.394 W
Calculated static plate dissipation of V5: 13.460 W

Does this sound good?

Thanks!


That's one of the tables I use. I also use this one:

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

Use the top calculator to determine the maximum (70%) safe bias setting. Select Class AB, your tube type, and your measured plate voltage.

Then use the next calculator down with your measured plate voltage and cathode current (Use 1/2 of the voltage measured at the Fender test point. At that point, mV = mA since you are measuring across a 1 ohm resistor). This will give you your tube power rating and will calculate the approximate plate current.

Finally, you can use "Jim Jones' Bias Tables" for your tube type to determine the safe range of bias for your tubes and amp.

Works great.

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Post subject: Re: BDRI biasing
Posted: Fri Oct 29, 2010 5:32 pm
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shimmilou wrote:
Be very careful if you're going to measure voltage at the pins of the tube, it is very easy to slip and short some things out.


Who would do something as dumb as that? :shock: :shock: :shock:













Me. :oops: :oops: :oops: :oops:





I use this bias probe now:

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

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Posted: Fri Oct 29, 2010 5:49 pm
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:lol: Yeah, you, me, and baby makes three,... at least. I can probably even tell you which screwdrivers make the best welding rods. :lol:

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Posted: Fri Oct 29, 2010 6:24 pm
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dgcobb, here is what thetubestore has about your tubes. Looks like they are 30 watt tubes.

"...To my surprise I tried the 6L6WGC-STR and it was magic. I got the same tone as the old G.E. tubes but the biasing was colder than what I used for NOS tubes in the past, closer to 55% or about 17 watts at idle as opposed to 20 – 25 watts (70-80%) that most people seem to like for rock or blues.

http://thetubestore.com/6l6templeton.html#30

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Posted: Fri Oct 29, 2010 8:21 pm
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I know this sounds anal, but what the heck. To measure the idle wattage dissipation of the power tube you need to subtract the ground to cathode wattage from the ground to anode wattage. That is, measure not only ground to anode voltage, but also ground to cathode... then subtract the difference to calculate the actual cathode to anode idle dissipation, in watts.

Ok, off the soapbox for me...


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Post subject: BDRI biasing
Posted: Fri Oct 29, 2010 9:08 pm
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Hi BMW2002Ti,

Really, the rating of the tube is only Plate dissipation (Anode), not tube dissipation. The Cathode to ground measurement would be the 1-ohm resistor, outside the tube. The 1-ohm resistor is only there for the convenience of a test point, because normally the Cathode would be grounded (no voltage difference in ground and Cathode). Remember that the current through the Cathode is the Plate current plus the grids currents. And if you really want to be anal, don't forget the wattage of the filaments (JK, it doesn't matter). :lol:

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