12au7 optimum current @ 200 volts

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12au7 optimum current @ 200 volts

Postby joe3rp » Tue Jan 28, 2003 8:51 pm

Advice on the the current setting to get the optimun sound for a simple single triode line amp for a 12au7 with a B+ of 200VDC.

What is the effect of the amount of current at idle through a triode (line amp)? Will higher current result in stronger bass and more detail...while lower current will result in a sweeter....assuming within the operating window.
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Postby mickey » Tue Jan 28, 2003 10:50 pm

Havent gotten to making one but I presume it should be a minimum of 5mA . Lower than that you're operating the tube in a non-linear operation. Some tubes might sound better as you operate it at a higher current level but this could shorten the life of the tube. The lows and highs would depend on the the signal caps and input impedance (log pot. )respectively.
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grid resistance

Postby joe3rp » Wed Jan 29, 2003 4:54 am

What will be the effect of a higher versus a lower input resistance on the grid of triode. A higher cap value equal a lower cut-off frequency correct?
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12AU7 at 200+ V

Postby johnmarc0 » Thu Jan 30, 2003 3:01 am

Joey,

First of all at 200V your max mu will only be at the 17+ to <18 gain region.

Second, Current determines output impedance as you change the slope of the (Imax-Imin) - (Emax-Emin) load line.

Third, tube musicality and it applies to all tubes for that matter has inherent frequency response within their class A envelope, this musicality changes with the plate construction, reactance of the electrodes and the material used in the plate. Current may have influence but very minimal that is totally inaudible. So a Sylvania will sound different from a Tung Sol from an RCA from a Raytheon. 12AU7A will also differ to its electrical equivalents like 5814 and 5963 made by different manufacturers.

Fourth, changing the current will change the output impedance and will mismatch your output network, which in the case of the 12AU7 is an RC Coupled network. This will cause distortion either constructive or destructive, thus eventually sound coloration.

Fifth, Sound coloration is driven by the RC values and the Q of the reactive values (Q fator is a description of the sharpness in frequency response)

So to conclude, if you change the current of your 12AU7, it reacts to new type of coloration to your output but will be truly attributed to your output RC network, thus tweaks go to that direction or roll the tube you use.
:D

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Input grid resistance

Postby johnmarc0 » Thu Jan 30, 2003 3:13 am

Typically input grid impedance is very high for tubes typically in the 1 M or higher region. But because we need to prevent grid excitation current we sink it with a lower value resistance i.e. grid resistor or a transformer.

We also need that resistor so that cathode bias current passes to the grid from the negative side of the supply creating grid bias.

Therefore, we need a resistor value that gives minimal voltage drop (ideally 0 ohms) but maintains a high impedance (maximum 250K to 500K) before grid excitation occurs. So that grid resistance value plays between 100K to 500K, low enough resistance to give low voltage bias drop but high enough to maintain higher impedance (or open circuit).

Depending on the grid resistor you use and the cap to couple it, frequency response cut on and cut off can be determined. To figure out the dynamic AC resistance acting on the grid you can you use this formula.

X(reactance)=1/2piFRC

Now deriving X(r) across 20Hz to 20KHz you should get your dynamic impedance.

Now since you know the voltage input acting on the grid, using simple ohms law you can determine the voltage drops across 20Hz to20KHz.

Converting that to dB using the fromula Vg=20 Log Vo/Vi

You can actually figure out your rate of coloration in decibels.

But again simple listening test will lead you to what capacitor, resistor values to use.
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Postby mickey » Thu Jan 30, 2003 11:46 am

Another thing JM (Guapo!) about lower input impedance is the miller capacitances of the tubes. so when you have a higher input impedance, that means the lower the high frequency response will be. thats why with DACT reccomended a 10K stepped attenuator, for its volume control which gives the system good clarity with the input impedance being in the region of 9K downwards but at the expense of the lows which is dictated by the RC network. the R being the 10K volume control and the C being the signal cap of the source.
some guys use a 100K passive attenuator which automatically give a better low frequency response but at the expense of the high frequency response.
at the end of the day, a good configuration will help bring out the best in a good tube.
by the way have u heard the one about mike tyson? use dictate in a sentence. mike - my girl friend said my dic-tate good

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dictate

Postby johnmarc0 » Thu Jan 30, 2003 12:10 pm

evander holyfield said he heard mike say that his ear tate good. Anyways 10K pots or lower are usually good with trasnformer coupled input connected post secondary winding.

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