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Old 17th June 2010, 04:37 AM   #1
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Default Bias Circuit Calculation

hi ,

i would like to make a modification to change my kt88 bias circuit to el34 bias circuit
assume i have made measurement in my amp with bias pot is adjusted to minimal and :
kt88 plugged to socket :
Vp88 = plate voltage
Iv88 = plate Current
Rc88 = cathode resistor
Vc88 = cathode voltage

el34 plugged to socket :
Vp34 = plate voltage
Iv34 = plate Current
Rc34 = cathode resistor
Vc34 = cathode voltage


Will replacing Rc88 with the right Rc34 value do the mod done ?
How can i get the value of Rc34 from the other variable .

Thanks in advance
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Old 17th June 2010, 01:59 PM   #2
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Do you have a schematic that you can post? EL34's typically need less plate voltage and less bias voltage for proper operation. There are also different requirements for grid leak R but I think that the KT88 requirement is more stringent than the EL34 requirement.

EL34's typically need around 35V +/- or so for bias voltage; depending on the design, the existing bias pot may have enough span to get you there. It's easy enough to measure. If not you'll have to change an R value in the bias circuit.

Can you measure & post your existing plate voltage and the voltages at each end of the bias pot range? Remove the output tubes when measuring the min bias voltage.

In in typical fixed bias arrangement the cathode R is only used to measure bias current.

Also keep in mind that an EL34 is rated for around 24W and the KT88 is rated at about 42W dissipation. So your Vplate X Iplate = power dissipated needs to be less depending on how hard you are driving the KT88's now.

Last edited by boywonder; 17th June 2010 at 02:03 PM.
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Old 17th June 2010, 03:20 PM   #3
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Hi ..
with the output tube removed
the plate voltage is 460 v ,
the (4) bias pot voltage is 0 .

grid resistor is 1 k , feed bias resistor is 120k

i notice (2) 10 ohm 2 watt resistor ,
(2) 46k resistor and (1) 22k near the bias pot .

this bias circuit is used for 2 output tube(s) .

Please Advise ,
Thanks .
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Old 17th June 2010, 05:54 PM   #4
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So the minimum bias voltage is zero? What's the max bias voltage?

What is the value of the cathode R?

What nominal bias current have you been using with the KT88's?

Last edited by boywonder; 17th June 2010 at 05:59 PM.
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Old 18th June 2010, 04:47 PM   #5
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Hi ,
bias voltage is 0.38 vdc ,
nominal bias current is 26 mA ,
cathode resistor is 1k ohm

please advise ,

thanks for your response
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Old 18th June 2010, 07:15 PM   #6
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I'm no expert but now you have me confused.......the cathode R's seem very high for fixed bias..................they are usually 1 ohm or 10 ohms for fixed bias, something convenient to measure voltage across to calculate current.

26ma seems way low for KT88 current also..........

Also, your bias voltage is very low; it should be somewhere in the range of 35-55VDC or so.

typical fixed bias operating conditions for EL34's are 400VDC@60ma (around -35VVDC bias or so), this equals 24W dissipation.

typical fixed bias operating conditions for KT88's are 500VDC@80ma, which is about 40W dissipation.

I wouldn't suppose that you have a schematic that you can post?

When you have the bias adjusted properly, what voltage reading are you getting? With the amp up and running properly, what is the DC voltage at pin 5 of the KT88's at idle?

What is the DC voltage drop across the cathode R's?
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Old 19th June 2010, 08:36 AM   #7
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Hi boywonder ,

you're right i was wrong .
the cathode resistor is actualy 10 ohm , where 1k resistor is grid leak resistor .
bias voltage and current was measured in bias trim pot (0.35vdc , 26.8 mA)

did you mean measuring power tube anode (pin 3 ) voltage and current for
--> fixed bias operating conditions for KT88's are 500VDC@80ma ?

i'll should give you this value tomorrow as i'm out of town today -> dc voltage at pin 5 kt88 idle and dc voltage drop across cathode rs

thanks so much
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Old 19th June 2010, 02:52 PM   #8
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Also, very carefully (use clip leads if you have them, or use one hand and put the other in your pocket) measure the anode voltage of the KT88's at idle. (pin 3 to ground).

Do you already have EL34's?
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Old 19th June 2010, 03:20 PM   #9
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You must use Ua/Ia curves to find out! Ie you should draw the loadlines. Nothing else is worth using.
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Old 20th June 2010, 12:56 PM   #10
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hi ,
i got 400vdc on pin 3 (plate)
and 0.35 vdc on pin 8 (cathode)

can't get current reading in both of them , why is that ?

thanks .
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