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Posted: Sat Oct 30, 2010 12:31 am
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Hey guys, thanks for all the advice. I forgot to mention that the "perfect pair" number of the tubes from the tube store is 28. I know this is important but I can't remember why :oops: .

Anyways, looks like I should bias a little hotter b/c they're 30 watt tubes.

Thanks


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Post subject: BDRI biasing
Posted: Sat Oct 30, 2010 6:43 am
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Yes you can run the 30 watt tubes a little hotter if you want (at least 15 watts idle or higher). Keep in mind that it's the tubes that you are biasing, not the amp, different tubes will probably require an adjustment. I talked to someone at thetubestore when I had a mismatched pair of tubes, if I remember correctly, they said that that number was the actual current that the tubes ran when they were tested. But, those numbers are really used just to match the tubes, because the Plate voltage that thetubestore uses, could be very different from the voltage on a users amp.

You have to be one of the fastest learners I've ever seen. You have learned a lot in just a couple of days, good job! :)

Edit:
Forgot that you had mentioned the meter that you have was from a Physics lab. When I went to College, Physics II had about 1/4 to 1/3 of the semester dedicated to electronics, that would be a good class for an intro to electricity, caps, coils, and resistance. It is just the basics, but a lot was taught in just one semester. :idea:

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Last edited by shimmilou on Sat Oct 30, 2010 6:54 am, edited 1 time in total.

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Posted: Sat Oct 30, 2010 6:46 am
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dgcobb wrote:
Hey guys, thanks for all the advice. I forgot to mention that the "perfect pair" number of the tubes from the tube store is 28. I know this is important but I can't remember why :oops: .

Anyways, looks like I should bias a little hotter b/c they're 30 watt tubes.

Thanks


I have no idea what that number means either. :lol:

The important thing to remember is to keep the bias within a safe range: between 50% and 70% plate dissipation. To do that, you need some idea of the tube type (Like is it a true 6L6GC rated at 30 watts per tube or a lesser version rated at only 25 watts), and you need to measure the plate voltage (Danger Will Robinson!). Use the proper tools, study up on the subject and you should be good to go. Within that safe range you will find a point where the amp sounds really good. That should be your bias point.

Here is another good link for reference:

http://www.aikenamps.com/

Click on the "Tech Info" link and read:

(Under the "Introductory" link)
"A Glossary of Common Amplifier Terms"
"Safety Tips for Working On Tube Amplifiers"
"What is Biasing?"

(Under the "Advanced" link)
"Idle Current Biasing - Why 70 percent?"
"The Last Word On Biasing"

Lot's of other good stuff in there too.

It may be more info than you wanted and it won't make you an expert, but it is good stuff to know. Not everyone agrees with Mr. Aiken's ideas, but it is good information and may help you understand better why all the stuff talked about in this thread is important.

Good luck. Have fun. Stay safe.

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Posted: Tue Nov 02, 2010 12:39 am
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Power dissipation is a mater of thermal resistor between the valve heating (heatflow is equivalent to a current source) and ambiant temperature that is constant (equivalent to a voltage source), using "Cauer" models, you have a current source that delivers current (heatflow) through a thermal resistor connected to a voltage source (Tamb). Then the "voltage" (Delta temperature) at the resistor is added to the voltage source (T ambiant) given the temperature of the heatflow source (example 400°C + 25°C).
To be safe, you have to guaranty that the valve internal temperature will never reach a value that reduces drastically its life. The "70% safety" could be different from 1 amp to another one.

There are three manners to do that:
1. Play on power --> with idle current ...
2. Decrease thermal resistor --> fan, additional heatsink ...
3. Decrease ambiant temp --> the less easier (play in Goenland ...)

Note the power is driven by the filament heating + power due to idle, playing with idle current affects life time and amp classAB region of use (more idle = more A in AB).

Then a good design should be in this way:
1. Choose a amp class idle
2. Choose valve, deduct temperature and deduct a life time (coould be impose life time, chose a valve and deduct temperature ...)
3. Design for thermal resistor
4. Validation and Operating Life Time test (ageing test)

The reality is I've components and it has to work with ... and it has to sound!

By the way, more B class you design less Idle current you need but more feedback you need to avoid non linearity at 0 crossing.

To much heating within the valve will also favor electrod poisoning.


It is just supply conversation about the subject

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Posted: Tue Nov 02, 2010 11:14 pm
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Tissan, that is a very interesting observation and way of stating the interaction of the valves and power section. I think BMW2002Ti or Shimmilou, also spoke of temperature as being the primary concern for regulating power sections. You guys are all over this stuff ! Never liked the 70% dissipation mantra, but I think it's just a ballpark(general) figure for the sake of simplicity. At any rate I'm with you on the electrode poisoning. IMO The cold electrode stripping and poisoning is a myth. I like your earthy-realistic concept, of having these components in this environment and they have to work and sound good to us, even if it's not perfect ! Art

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Posted: Mon Nov 08, 2010 1:51 am
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So I just rebiased the tubes to 73 mV cathode current. I'm now right at 50% plate dissipation. Do these amps come biased stock at 50% plate dissipation? Haven't gotten a chance to play it yet (it's late) but I think I'll be satisfied.

Thanks for the support guys. I'm really having a lot of fun learning all this stuff.


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Posted: Mon Nov 08, 2010 5:54 am
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50% seems safe and good for valve life time.

From my point of you, the important thinks for good matching are:

- Idle vs grill voltage and amplification coefficient.

For good sound:
- at what level is the switch A to B class. What is the none linearity ratio due to 0 crossing in classe B.
:wink:

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Posted: Mon Nov 08, 2010 6:28 am
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dgcobb wrote:
So I just rebiased the tubes to 73 mV cathode current. I'm now right at 50% plate dissipation. Do these amps come biased stock at 50% plate dissipation? Haven't gotten a chance to play it yet (it's late) but I think I'll be satisfied.

Thanks for the support guys. I'm really having a lot of fun learning all this stuff.


I assume you mean 73 mV as measured at the Fender test point which would translate to about 36 mA cathode current per tube. Slightly less for the actual plate current. What is the measured plate voltage? That is a critical number to know.

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Last edited by bluesky636 on Mon Nov 08, 2010 10:30 am, edited 1 time in total.

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Post subject: BDRI biasing
Posted: Mon Nov 08, 2010 6:52 am
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Remember dgcobb, mV is millivolts, and mA is milliamps (current is in amps not volts). 73 mV at test point = 73 mA Cathode current for both tubes, approx 36 mA Cathode current per tube, or approx 31 mA Plate current per tube. Your earlier measured Plate voltage = 410 V (409 and 411). So, 410 volts X 31 mA = 12.71 watts idle. That is quite a bit less than 50%, how did you get 50%? (maybe you used the Cathode current instead of the Plate current for the math) That is still on the cold side, should be at least 15 watts idle for a 30 watt tube. You want at least 37 mA of Plate current per tube, which is 42 mA Cathode current per tube, which is about 85 mV at the test point, considering your Plate voltage. My amp came with 5881s, 25 watt tubes, and were biased right at 50% when new. My plate voltage was much higher than yours also. :idea:

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Post subject: Re: BDRI biasing
Posted: Mon Nov 08, 2010 10:30 am
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shimmilou wrote:
Remember dgcobb, mV is millivolts, and mA is milliamps (current is in amps not volts). 73 mV at test point = 73 mA Cathode current for both tubes, approx 36 mA Cathode current per tube, or approx 31 mA Plate current per tube. Your earlier measured Plate voltage = 410 V (409 and 411). So, 410 volts X 31 mA = 12.71 watts idle. That is quite a bit less than 50%, how did you get 50%? (maybe you used the Cathode current instead of the Plate current for the math) That is still on the cold side, should be at least 15 watts idle for a 30 watt tube. You want at least 37 mA of Plate current per tube, which is 42 mA Cathode current per tube, which is about 85 mV at the test point, considering your Plate voltage. My amp came with 5881s, 25 watt tubes, and were biased right at 50% when new. My plate voltage was much higher than yours also. :idea:


shimmilou,

Your numbers are about dead on to how I have my BDRI biased:

Plate voltage = about 410 vDC
Fender test point = 85 mV (5% tolerance 1 ohm resistor)
Measured cathode current = about 39 mA (1% tolerance 1 ohm resistor)
Calculated plate current = 36 mA
Plate dissapation = 15 watts

dgcobb,

Use the "Calculate Plate Dissipation Based On Plate Voltage And Cathode Current Readings" from the Weber bias tables to calculate yuour values. You can use "Jim Jones' Bias Tables" on the same page to find your safe range.

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

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Posted: Mon Nov 08, 2010 10:48 am
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Hey, the plate current is actually 420, not 410. I used the calculator on this page to determine my plate dissipation.
http://www.justinholton.com/hotrod/bias.html

It says just enter the number read at test point and the plate voltage, which is what I did. With vales of 73 mV and 420 V, I got a plate dissipation of 15.329, which is 50% of the 30 watt tube. Is this not correct??

I didn't enter the voltage across R62 or R61 b/c it said it was optional. I assume this is the screen current. I'm not sure were R62 and R61 are, what value should I enter?

Bluesky, how are you measuring your cathode current? I ask b/c your bias is at 85 but your cathode current is only 39 per tube.. I thought the cathode current of the two tubes should add up to the bias test point.

Thanks


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Posted: Mon Nov 08, 2010 11:30 am
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dgcobb, the 420 number is Plate volts, you're getting there, the terminology is very important. :wink: 420 volts does make a little difference. I'm not sure, but the Justin Holten info might be for biasing the 25 watt tubes that are stock in the HRDlx, the numbers will be different for your tubes. But, if we are on different pages, we'll never get this right. :idea: I think that bluesky636 is measuring the actual current with the meter, and the voltage drop across the resistor in the bias probe, will be different than the one in the amp used for the test point. The tolerance of different components will give slightly different numbers when measured vs calculated. Measured is always best, I calculated(estimated) the current, bluesky636 actually measured it (go with his numbers).

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Posted: Mon Nov 08, 2010 1:20 pm
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shimmilou wrote:
dgcobb, the 420 number is Plate volts, you're getting there, the terminology is very important. :wink: 420 volts does make a little difference. I'm not sure, but the Justin Holten info might be for biasing the 25 watt tubes that are stock in the HRDlx, the numbers will be different for your tubes. But, if we are on different pages, we'll never get this right. :idea: I think that bluesky636 is measuring the actual current with the meter, and the voltage drop across the resistor in the bias probe, will be different than the one in the amp used for the test point. The tolerance of different components will give slightly different numbers when measured vs calculated. Measured is always best, I calculated(estimated) the current, bluesky636 actually measured it (go with his numbers).


Correct.

As prviously noted, the Fender test point uses a 1 ohm resistor with a 5% tolerance to measure the sum of the two cathode currents. The bias probe uses individual 1 ohm, 1% toerance resistors to measure each cathode current individually.

The 73 mV measured at the Fender test point would break down into a cathode currect of about 36 mA +/- 1.8 mA for a range of about 34 mA to about 38 mA.

36 mA measured by the bias probe would have a range of about 35.6 to 36.4 mA. That is a noticable difference

Then you have to subtract grid current or use one of the caclulators to get actual plate current.

Isn't this fun? :lol:

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Posted: Mon Nov 08, 2010 4:02 pm
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Thanks guys. Has anyone checked the link to the calculator I posted? I just want to know if that thing is correct, b/c it's what I've been using and I've been entering the right numbers into it.


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Posted: Mon Nov 08, 2010 4:55 pm
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I suppose that most of the bias calculators are going to give a higher number (using Cathode current), and you will be actually running a little less wattage than the calculator shows. An actual Plate current and voltage measurement with a meter will be the most accurate, the voltage reading at the test point will be the easiest (you want accuracy or ease?).This is from the link that you posted from Justin Holton. ( I think that he's saying that the calculator is "close enough")

Entering values for D and F will add screen current in the equation. Cathode current (measuring at the bias test point) is the combination of plate AND screen current, even though screen current is usually very small in comparison to plate current—usually a few milliamps. Ignoring D and F will result in biasing slightly more modest, so it can be safely ignored.

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