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Old 16th October 2010, 03:48 PM   #11
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Quote:
Originally Posted by Frank Berry View Post
Is the bias voltage correct?
is the bias voltage the -35 volts? i measured it at -37v. i guess that is pretty close. the heater voltage is good too. i changed out C8 that didnt make a diff. try C7 next
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Old 16th October 2010, 04:01 PM   #12
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Originally Posted by Brit01 View Post
Give us some measurements.

Do you have a DMM?

without the tube the caps charge up good, C1 and C2 charge up to 160 volts and C3 up to about twice that much. the voltage on the B+ line is 330 volts.
the bias supply voltage is -37.5volts. heater voltage is 14.25

with tube plugged in B+ is 255v, bias is -37volts and filament voltage is 13.5v.

i will install a 1ohm resistor on pin 3 to ground to determine the current that is going through the tube. will a .5watt resistor suffice?
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Old 16th October 2010, 05:08 PM   #13
Brit01 is offline Brit01  United Kingdom
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What is the voltage on the plate?

What tube are you using? 13.5 volts for the heater? Is it a 13 series?

To measure current, just measure across the cathode resistor and divide that voltage by the value of the resistor (if it hasn't drifted: measure the resistance of R3 with power turned off to verify).

How did you measure/calculate the bias?
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Old 16th October 2010, 05:42 PM   #14
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Originally Posted by Brit01 View Post
What is the voltage on the plate?

What tube are you using? 13.5 volts for the heater? Is it a 13 series?

To measure current, just measure across the cathode resistor and divide that voltage by the value of the resistor (if it hasn't drifted: measure the resistance of R3 with power turned off to verify).

How did you measure/calculate the bias?
voltage at the plate is 230v on pin 2

pin 1 is 35.6v which is the bias voltage

its a 13em7.

i cant measure the current because there is no resistor from pin 3 to ground. someone said i should mod it with a 1ohm resistor to measure that current.
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Old 16th October 2010, 05:52 PM   #15
Brit01 is offline Brit01  United Kingdom
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Yes you can add a 1R resistor to measure current on Pin3 to ground.

I'm doing the same on the cathodes of my OTL in work with a fixed bias.

What current is flowing through R3? Has this changed?
This could have also altered the B+.
If the resistor has drifted down in value then it's going to be pulling more current dropping the B+.
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Old 16th October 2010, 06:05 PM   #16
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Quote:
Originally Posted by Brit01 View Post
Yes you can add a 1R resistor to measure current on Pin3 to ground.

I'm doing the same on the cathodes of my OTL in work with a fixed bias.

What current is flowing through R3? Has this changed?
This could have also altered the B+.
If the resistor has drifted down in value then it's going to be pulling more current dropping the B+.

i think the voltages are actually within tolerance if i measure them right at the pins. maybe i can increase R1 by a few values.

i will put that 1ohm resistor from pin 3 and monitor the current there. .5 watt is good?

maybe stalker was right

Last edited by chopchip; 16th October 2010 at 06:19 PM.
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Old 16th October 2010, 06:20 PM   #17
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The current through the input section of the tube shouldn't affect the B+ to this extent since the plate of the input stage is fed through a 47k resistor.
The bias voltage is a bit high. I noticed that the 470uf filter capacitors on the bias supply are rated at 35 volts. They're under-rated. Should be at least 50 volts. You might want to bring the bias voltage down a little bit. That will increase the plate current and bring down the B+ voltage a bit.
This is very puzzling.
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Old 16th October 2010, 06:26 PM   #18
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Quote:
Originally Posted by Frank Berry View Post
The current through the input section of the tube shouldn't affect the B+ to this extent since the plate of the input stage is fed through a 47k resistor.
The bias voltage is a bit high. I noticed that the 470uf filter capacitors on the bias supply are rated at 35 volts. They're under-rated. Should be at least 50 volts. You might want to bring the bias voltage down a little bit. That will increase the plate current and bring down the B+ voltage a bit.
This is very puzzling.

if i measure the B+ voltage right at pin 2 it is about 230 volts so i guess that is within tolerance. i was measuring it before the transformer so i guess the transformer drops the voltage by about 30v. kinda silly of me.

i guess it is actually operating properly, i just wasnt measuring the voltage at the right spot. it didnt occur to me that there would be voltage drop across the OT.

the sound was probably distorted because it was turned up too loud and i played some head games with myself.
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