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Fixed Bias: How to build it conveniently to use?
Fixed Bias: How to build it conveniently to use?
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Old 5th February 2018, 06:02 AM   #1
Blitz is offline Blitz  Germany
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Default Fixed Bias: How to build it conveniently to use?

I want to build a PP-Amp with fixed bias. I tried different autobias modules like tentlabs, but they smoked after a while tgem self to death. So, in a perfect world, a switch beteen the measuring resistors, a stationary voltmeter and round knobs to be used without screwdriver would be cool...or is there a better way ? It would be nice to see some pictures ...?
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Old 5th February 2018, 10:19 AM   #2
Eli Duttman is offline Eli Duttman  United States
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Build the bias supply "simply", which makes it reliable. Don't rely on "gadgets"; instead, use your multi-meter to periodically check O/P tube "idle" current.

If you're worried about bias supply failure, use combination bias and size the wattage of the cathode resistor such that it acts as a fuse. If the bias supply fails and runaway sets in, the cathode resistor "fries" and the O/P tube(s) are spared.
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Old 5th February 2018, 10:43 AM   #3
dotneck335 is offline dotneck335  United States
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How 'bout one o' these?
BR-2 Bias Rite
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Old 5th February 2018, 10:49 AM   #4
Miniwatt is offline Miniwatt  Netherlands
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I've used several Tentlabs autobias units daily for years, so far none of them went up in smoke, it's a reliable product.
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Old 5th February 2018, 11:41 AM   #5
Lingwendil is offline Lingwendil  United States
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Fixed Bias: How to build it conveniently to use?
Use the fixed bias arrangement that Pete Millet uses on the DCPP (engineer's) amplifier. Simple enough and it will work very well.
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Old 5th February 2018, 12:16 PM   #6
kstagger is offline kstagger  United States
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What Eli said - though I like to add in current meters so I can easily see the mA flowing through the tube. Make biasing really easy. And given the price of current meters it isn't cost prohibitive.
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Old 5th February 2018, 01:54 PM   #7
kodabmx is offline kodabmx  Canada
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This module does full auto grid bias like the tent labs design but it's a different circuit and costs less than 1/2 as much. Bias Supply Modul for four Tubes (Single pcb to Control 4 Power Tubes) | eBay


This board makes it quick and easy to set the bias, but it's manual, and must be adjusted periodically. Electron Tube Amp Negative Grid Bias Power Supply Adjustable 4channel for PP SE | eBay


I've used them both and they work great. I prefer the "set it and forget it" approach of the automatic module, but the manual version works fine but should not be used to A2 or AB2 due to the pots being very low power.
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Old 6th February 2018, 11:35 AM   #8
Blitz is offline Blitz  Germany
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I am using all of these already plut the shunt reg of pmillet and the bias regs of Rod Coleman.

The question is more how to physically realize stuff.

So, what Amperemeter or Voltmeter ? Are there difference in terms of audible ? Are amperemeter in series with the electron flow of the cathode ?

So, a multimeter across a 1 ohm resistor best of all or something like this YAMAMOTO PANEL METERS
or is an old voltmeter fine as well ? Switchable ?

or simply banana plugs everywhere for the multimeter...which is not convienent...

I smoked three gen 1 boards of Mr. Tent. Zhey run first perfect, but after 3-4 minth burn down badly. Always with Svetlana el34 sed.

My current Philips El34 change their bias as well nearly each time they start up, so it has become a standatd procedure to check their bias once a week.
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Old 6th February 2018, 01:42 PM   #9
goldenbeer is offline goldenbeer  Germany
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Use a voltmeter across a cathode resistor (precision 10R or 1R). You can then make it switchable across all four cathode resistors.

Edit: Or you could hack up a window comparator with led output instead of the voltmeter.

Last edited by goldenbeer; 6th February 2018 at 01:52 PM.
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Old 7th February 2018, 03:26 AM   #10
PRR is offline PRR  United States
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Quote:
Originally Posted by Blitz View Post
...PP-Amp with fixed bias. ....smoked after a while tgem self to death. .... My current Philips El34 change their bias as well nearly each time they start up ...?
Use Self-Bias Whenever Possible!

When fixed-bias works you can get some/lots more power out of a pair of tubes. But when it does not work things can get very ugly. A 250 Ohm 10W cathode bias resistor is very reliable. After decades of blowing-up tubes (I used to do that a lot in my job) I have mellowed and will accept a reliable 25 Watts rather than 50 Watts on good days and major repairs on other days. (My last major P-P project was down-rating an unreliable 60W amp to a 20W which may out-live me.)
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