Hi All,
As my first post I would like to introduce myself.
I'm Simon From Arnhem, The netherlands and working with electronics for about 35 years. first only tube electronics and after that digital circuits. But now, after 20 years without tinkering with tube circuits actively, I have the restoration of a Klemt M40 amplifier on my hands. Swapped all the small caps and elcos but now i want to adjust the base current of the four EL84 end tubes.
But what should the current be. at the moment I've set them to 16.8mA but i have no idea if this is too much or to low. as this is a ultra linear setup with separate power supply for the G1 bias, I'm at a loss.
BTW. tihs amp sounds terrific, even on a separate 50W 8ohm speaker without a box (I have no passive speakers at my place, only powered monitors)
Next up will be the caps on my Telefunken M3.
Klemt M40 bias current
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pompeiisneaks
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Re: Klemt M40 bias current
Welcome!
Basically it depends on a few factors. The B+ voltage, and the specific setup.
Are they set as fixed bias or cathode biased? Do you have a schematic of this amp to assist?
Basically I'd recommend using this page to plug in the B+ voltage and then calculate the right bias depending on the specific type of bias used
https://robrobinette.com/Tube_Bias_Calculator.htm
~Phil
Basically it depends on a few factors. The B+ voltage, and the specific setup.
Are they set as fixed bias or cathode biased? Do you have a schematic of this amp to assist?
Basically I'd recommend using this page to plug in the B+ voltage and then calculate the right bias depending on the specific type of bias used
https://robrobinette.com/Tube_Bias_Calculator.htm
~Phil
tUber Nerd!
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Stevem
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Re: Klemt M40 bias current
Hi!
The first question that needs to be asked is in relation to the amps V+ voltage.
This voltage is the key to how much current each tube can idle at.
With the amp in standby what is the voltage seen on the hot side of the standby switch, or on the center tap of the output transformers primary.
All this being said I worked on one of these 25 years ago if I recall right.
If I am recalling right the output section had 300 volts or so on it.
With 300 volts on the plates each tube should idle at 20 ma for a 50% disappation rating.
With 325 volts they should idle at 18 ma.
I think I recall that in that amp that the setting of the screen voltage is what controls the idle current.
The best way to confirm each tubes idle current is to install a 1 ohm 1 percent 2 watt resistor in series with each tubes cathode to ground.
Due to ohms law the tube side of resistor will then read the current directly with a meter set for DC volts.
I would not idle 84s at any more then 60% disappation for ok tube life.
The first question that needs to be asked is in relation to the amps V+ voltage.
This voltage is the key to how much current each tube can idle at.
With the amp in standby what is the voltage seen on the hot side of the standby switch, or on the center tap of the output transformers primary.
All this being said I worked on one of these 25 years ago if I recall right.
If I am recalling right the output section had 300 volts or so on it.
With 300 volts on the plates each tube should idle at 20 ma for a 50% disappation rating.
With 325 volts they should idle at 18 ma.
I think I recall that in that amp that the setting of the screen voltage is what controls the idle current.
The best way to confirm each tubes idle current is to install a 1 ohm 1 percent 2 watt resistor in series with each tubes cathode to ground.
Due to ohms law the tube side of resistor will then read the current directly with a meter set for DC volts.
I would not idle 84s at any more then 60% disappation for ok tube life.
Last edited by Stevem on Wed Jan 20, 2021 6:36 pm, edited 1 time in total.
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
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pompeiisneaks
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Re: Klemt M40 bias current
I did a bit of digging, not sure if this is it, but found an Echolette M40 that seems to be made by Klemt.
It looks to be cathode biased from this schematic, and that means that it can be biased near 90% without any major issues. There's no listed B+ voltage, but it shows the first stage filter caps are at either 350 or 385V (can't quite say for sure which)
I'd measure the B+ and then go from there:
~Phil
It looks to be cathode biased from this schematic, and that means that it can be biased near 90% without any major issues. There's no listed B+ voltage, but it shows the first stage filter caps are at either 350 or 385V (can't quite say for sure which)
I'd measure the B+ and then go from there:
~Phil
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Re: Klemt M40 bias current
Cathodes tied to ground and a negative voltage on grids. I'd call it fixed bias.
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pompeiisneaks
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Re: Klemt M40 bias current
Oh man you're right! Sorry.
That's what I get for trying to read a schematic after about 5 hrs of sleep lol.
~Phil
That's what I get for trying to read a schematic after about 5 hrs of sleep lol.
~Phil
tUber Nerd!
Re: Klemt M40 bias current
Hi all,
the schematic shown above is not the one my M40 used. I was confused as well.
mine uses this one:
please note the four bias resistors with the separate diode feeding them. klemt was smart enough to give every cathode a measurement lug on the pcb.
there is already a 1ohm resistor connected to the cathodes and they measure about 1.2ohm. from measuring the voltage over the rectifier after it has been running for a few minutes, there is 342 volts across the system.
this one is actually not so difficult to service, almost all parts are replacable without taking things apart. unfortunately for the echolette NG51 this is not the case.
the schematic shown above is not the one my M40 used. I was confused as well.
mine uses this one:
there is already a 1ohm resistor connected to the cathodes and they measure about 1.2ohm. from measuring the voltage over the rectifier after it has been running for a few minutes, there is 342 volts across the system.
this one is actually not so difficult to service, almost all parts are replacable without taking things apart. unfortunately for the echolette NG51 this is not the case.
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Stevem
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Re: Klemt M40 bias current
Ok, so it’s still fixed bias with a trim pot for each tube.
With over 340 volts on there plates I would say that 16 ma is the max you would want to idle them at.
According to the fixed bias tube data I found on line at the Duncan amps site, the specs state that with 300 volts on both the plate and screen they should idle at no more then 15 ma, and this should come about with a -14.7 volts on the grid.
Indeed according to my bias chart for 50% dissipation and with 350 volts on the plates 17 ma is plenty safe!
With over 340 volts on there plates I would say that 16 ma is the max you would want to idle them at.
According to the fixed bias tube data I found on line at the Duncan amps site, the specs state that with 300 volts on both the plate and screen they should idle at no more then 15 ma, and this should come about with a -14.7 volts on the grid.
Indeed according to my bias chart for 50% dissipation and with 350 volts on the plates 17 ma is plenty safe!
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Re: Klemt M40 bias current
Hey thanks! ik will keep them like that. these are still the original tubes I think. readjustment will be necessary when new ones are inserted.
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Stevem
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- Joined: Fri Jan 24, 2014 3:01 pm
- Location: 1/3rd the way out one of the arms of the Milkyway.
Re: Klemt M40 bias current
Yes, if the new tubes or any tubes you install are stronger they will pull more current which will pull down the plate voltage.
Your main check out if you have a scope should be to confirm that the output wave form when driving the proper impedance load while low levels of input signal are driving the amp input is free from showing crossover distortion .
Your main check out if you have a scope should be to confirm that the output wave form when driving the proper impedance load while low levels of input signal are driving the amp input is free from showing crossover distortion .
When I die, I want to go like my Grandfather did, peacefully in his sleep.
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!
Not screaming like the passengers in his car!
Cutting out a man's tongue does not mean he’s a liar, but it does show that you fear the truth he might speak about you!