• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

miniblok 1 voltage problem

i built this amp a while ago and all the voltages checked out, i had 215 volts B+ but now the voltage has bumped up to 250 for some reason and i cant determine why that is. i guess there isnt enough voltage drop across R1 so that means that the tube isnt pulling up enough current, im not sure where to go from here. maybe i can just increase the value of R1 but that doesnt really solve the mystery...

any ideas?
thanks
 

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Brit01

Member
2008-07-04 10:35 am
I guess if the tube's parameters are within optimum working levels then all should be ok?

Any change in current or sound? Has R2 or R3 drifted?


Have you measured the cathode voltages/current?

If R3 is pulling less current, the B+ is likely to rise.
 
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I guess if the tube's parameters are within optimum working levels then all should be ok?

Any change in current or sound? Has R2 or R3 drifted?


Have you measured the cathode voltages/current?

If R3 is pulling less current, the B+ is likely to rise.


could it be the bypass cap (c8) next to r3? maybe it's voltage rating is too low because i have the same problem with the other channel's amp.

the sound has become distorted. im going to install a 1ohm resistor from the cathode to ground to measure current.
 
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?
 

Brit01

Member
2008-07-04 10:35 am
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?
 
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.
 

Brit01

Member
2008-07-04 10:35 am
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+.
 
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
 
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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.
 
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.