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PostPosted: January 26th, 2017, 9:56 pm 
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Hey All,

I'm just trying to figure out something.

MY amplifier says
" Bias Target=50 mA, which will read 500mV on multi meter, After proper bias adjustment, each tube should operate with 50 mA cathode current (i.e., plate and screen grid current combined) under no signal. The 50 mA current will develop a 500 mV or 0.05V voltage drop across a 10 ohm resistor. A multi meter is used to monitor the 0.5V voltage drop"

If my tube says 45 mA, does that mean I tune to 450 mV.

What's confusing me is the target is mA but the meter is supposed to read mV. And there is this 10 ohm resistor which I'm not sure exists.

I always bias the amp with the volume all the way down, and my speakers connected (8 ohms), but the previous owner suggested to me the 10 ohm load was built in to the amplifier.

Am I supposed to wire a 10 ohm resistor across the amplifier leads when I bias?

Is there a simple explanation for this confusing phenomena


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PostPosted: January 27th, 2017, 10:47 am 
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Location: Parkville, Maryland
You need to provide more information. Brand of amplifier -- configuration -- power tubes, etc.

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PostPosted: January 27th, 2017, 11:00 am 
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chris1973 wrote:
Hey All,

I'm just trying to figure out something.

MY amplifier says
" Bias Target=50 mA, which will read 500mV on multi meter, After proper bias adjustment, each tube should operate with 50 mA cathode current (i.e., plate and screen grid current combined) under no signal. The 50 mA current will develop a 500 mV or 0.05V voltage drop across a 10 ohm resistor. A multi meter is used to monitor the 0.5V voltage drop"

If my tube says 45 mA, does that mean I tune to 450 mV.

What's confusing me is the target is mA but the meter is supposed to read mV. And there is this 10 ohm resistor which I'm not sure exists.

I always bias the amp with the volume all the way down, and my speakers connected (8 ohms), but the previous owner suggested to me the 10 ohm load was built in to the amplifier.

Am I supposed to wire a 10 ohm resistor across the amplifier leads when I bias?

Is there a simple explanation for this confusing phenomena


:character-oldtimer:
If you look at the schematic for your amplifier, you should see that each output tube has a 10ohm resistor between the cathode & ground. That's where your measuring the 500mV from. The 45mA that may have been included as data with your tubes isn't a target current, it's a measurement of the current through the tube in some specific test conditions when the tubes were matched. Useful for amps that have multiple tubes using the same bias resistor in a self-biased application, but that doesn't apply to your amp. Start with the 50mA recommendation from the amp manufacturer and play with it up and down a bit to find the best sounding bias for your tubes in your amp...

Roscoe


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PostPosted: January 27th, 2017, 5:02 pm 
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Joined: October 21st, 2013, 6:53 pm
Posts: 270
Thanks Roscoe,

After I set them like you said to the 500mV, and played around a little it tried this:

Turn all the bias adjustments all the way down. turn on amplifier and let warm up. Measure all four tubes, and note the measurement of the highest tube. I round this up to an even #, and it's usually around 300mV. Then, I match all the other tubes to this figure. And, If I don't hear anything wrong with it, That's where I leave it! Sometimes it can sound a little bit lifeless, and not as musical, so I will turn it up. On the other end, to me the 500mV sounds like the tubes are a little over-driven( don't know if that's what is actually happening, but that's my take on it), so it's usually somewhere in-between the 300 and the 500.

Is this a science , or an art?

Chris


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PostPosted: January 27th, 2017, 6:13 pm 
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chris1973 wrote:
Is this a science , or an art?


Yes. :obscene-drinkingcheers:


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PostPosted: January 27th, 2017, 6:33 pm 
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SoundMods wrote:
You need to provide more information. Brand of amplifier -- configuration -- power tubes, etc.


Hey Walt,
I don't really need a technical answer, but : Pacific Creek P34I-2, four output tubes EL34, four 6SN7 for preamp and predriver, Roscoe says there is a 10 ohm resistor built in, which I think I have located on the drawing.

Pretty simple looking amp. I was just curious: If 500mV=50mA, then 450mV=45mA, 400mV=40mA, etc. etc?

I have been told two things. One is to set bias according to the amplifier manufacturer recommendations, the other is to set bias according to the paperwork with the tube.

Then, there is my own system, which I explained in the other reply.

I'm just trying to determine the merits of each.

Thanks,

Chris


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PostPosted: January 27th, 2017, 7:30 pm 
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Location: Parkville, Maryland
OK. Got it. To set bias you look at voltage drop across 10-ohm cathode resistors.

My experience and confirmed by others is that 50-ma cathode current is too aggressive. I found that the sound with the bias set at that quiescent current was dark and closed down.

After much fiddling I found that 38-ma per tube is the sweet spot. More open mid-range and sweeter and more transparent. Since most EL-34s out there are pretty close to one another it didn't matter which brand. Besides -- a the lower cathode current you get longer tube life as a bonus. Out of the EL-34s that gave the best sound I found that the vintage Tesla EL-34s from Eastern Europe were the best.

These findings were established with a modified Dynaco ST-70 and a Music Angel integrated amp. Since you can adjust bias you might want to try some nice KT-88s in those slots. If the suppressor grid is already tied to the cathode -- the KT-88s will drop right in -- no muss - no fuss. BTW -- the KT-88s like 36-ma cathode current each.

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PostPosted: January 27th, 2017, 8:02 pm 
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So back to my obviously idiotic question:

Can I get 38mA cathode current from a 380mV drop across the 10 ohm resistor?

Thanks,

Chris


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PostPosted: January 27th, 2017, 8:05 pm 
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chris1973 wrote:
So back to my obviously idiotic question:

Can I get 38mA cathode current from a 380mV drop across the 10 ohm resistor?

Thanks,

Chris


Yup. V=IR...


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PostPosted: January 27th, 2017, 8:16 pm 
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Roger that.

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