Hi,
Also for ampli with bias adjustment, for a fine tuning of bias depending by tubes
we must know plate voltage (Ohm law).
The question I've not undestand is:
I can measure plate voltage with tube out/off (EL34/KT88 pin 3-pin8)
and I can estimate plate voltage a little bit lower (example if I measure 450V WITHOUT tube, estimation of 430-435V as palte voltage).
OR
it makes no sense
and I need to measure plate voltage only with the tube in,
thus it's a must to have a probe/adapter in order to not open the chassis???
Thanks a lot
It's very important
Renis
Also for ampli with bias adjustment, for a fine tuning of bias depending by tubes
we must know plate voltage (Ohm law).
The question I've not undestand is:
I can measure plate voltage with tube out/off (EL34/KT88 pin 3-pin8)
and I can estimate plate voltage a little bit lower (example if I measure 450V WITHOUT tube, estimation of 430-435V as palte voltage).
OR
it makes no sense
and I need to measure plate voltage only with the tube in,
thus it's a must to have a probe/adapter in order to not open the chassis???
Thanks a lot
It's very important
Renis
I don't understand what you are asking. Are you talking about bias adjustment in reality for an amplifier, or determining bias as part of the design process?
I can adjust the bias with a multimeter. The bias suggested is 0.55V, meaning 55mA
But I don't know if the bias suggested by manufacter it's 50%, or 60% or 70% of plate dissipation for the tube.
For example: KT88 SED winged C have max plate dissipation of 44W; if I want the tube works @60%, the bias should be 44/plate voltage*0.60. Assuming the plate voltage is 450V: 44/450*0.60= 0.58 (58 mA for bia in order to have 60% plate dissipation).
Thus I must know the plate/anode voltage to calculate the desidered bias for that specific tube.
So the post I did:
It's enough to have an estimation measure without the tube (450 V is volatage sampled with tube off/out)
OR I need the measure of plate/anode voltage with the tube in, thus I need an adapter/probe to measure in order to avoid to open the chassis????
I hope the post it's more clear
Thanks
PS: the ampli we are working are retubed Yaqin MC100B and Galaxy 34
But I don't know if the bias suggested by manufacter it's 50%, or 60% or 70% of plate dissipation for the tube.
For example: KT88 SED winged C have max plate dissipation of 44W; if I want the tube works @60%, the bias should be 44/plate voltage*0.60. Assuming the plate voltage is 450V: 44/450*0.60= 0.58 (58 mA for bia in order to have 60% plate dissipation).
Thus I must know the plate/anode voltage to calculate the desidered bias for that specific tube.
So the post I did:
It's enough to have an estimation measure without the tube (450 V is volatage sampled with tube off/out)
OR I need the measure of plate/anode voltage with the tube in, thus I need an adapter/probe to measure in order to avoid to open the chassis????
I hope the post it's more clear
Thanks
PS: the ampli we are working are retubed Yaqin MC100B and Galaxy 34
You must have the tube in, otherwise the "plate" voltage is meaningless. Also, you should follow the manufacturer's instruction on how to bias the tube, since the design may call for higher or lower bias than one based on a generic %.
Ok
thus, to know the bias related to % of plate dissipation I need a plate voltage with the tube in.
Anyway depending by the design the bias suggested should be fine.
But maybe that the bias suggested for the KT88-90 (chinese tube) used by manufacter
could be different for 6550??
I have GE 6550A NOS.. and I'd like to use the right bias in order to preserve these extremaly rare tubes.
Thanks
thus, to know the bias related to % of plate dissipation I need a plate voltage with the tube in.
Anyway depending by the design the bias suggested should be fine.
But maybe that the bias suggested for the KT88-90 (chinese tube) used by manufacter
could be different for 6550??
I have GE 6550A NOS.. and I'd like to use the right bias in order to preserve these extremaly rare tubes.
Thanks
Nice! Yes, I would not use the same bias on a NOS tube as the Chinese KT88. If preservation is the goal, then running them cooler makes sense, as long as you are still happy with the sound.
The voltage hardly changes: 450V vs. 435V is only 3% change so not worth bothering about. Valve circuitry only notices 10-20% changes.
No, not without the tube. I'm still not sure what you are asking. For an output valve there are three possibly relevant voltages: supply rail off-load, supply rail on-load, anode/plate. For calculating anode dissipation the correct one is anode/plate voltage under normal conditions. However, assuming a normal OPT then this voltage is not much lower from the supply rail voltage on-load. With a good PSU this is not much lower than the off-load voltage. I don't know whether you have a a normal OPT and a good PSU.
Why not just measure the voltage at the anode if you are worried about minor details? I don't understand the fuss. If you don't feel safe working on live valve circuits then don't work on valve circuits.
Why not just measure the voltage at the anode if you are worried about minor details? I don't understand the fuss. If you don't feel safe working on live valve circuits then don't work on valve circuits.
The problem is to measure plate/anode voltage with the tube in without a dedicated adapter/probe...
I'm asking for a possibile estimation measuring the voltage between pin 3 and 8 of the socket without tube by a multimeter. I measured 450V .... Is this voltage close to idle plate voltage of the tube???
It seems to understand that is not the right way......correct?
If I want to know the operating point of the tube I must get a bias probe/adapter...correct ??
I'm asking for a possibile estimation measuring the voltage between pin 3 and 8 of the socket without tube by a multimeter. I measured 450V .... Is this voltage close to idle plate voltage of the tube???
It seems to understand that is not the right way......correct?
If I want to know the operating point of the tube I must get a bias probe/adapter...correct ??
Last edited:
You are confusing us... if you have a voltmeter and is able to measure the voltage between the power tube's socket pins, why can't you make the measurements with the tube in the socket (referenced to ground) at idle? But as DF96 already said, the 3% difference in plate voltage isn't enough to worry about, the tubes are quite tolerant when operating within spec (sometimes even out of spec).
I apologize for the confusion
Yes I performed the voltage in the socket pin by voltmeter (450V)...but how can I do the measure with the tube in?? ( not space where to put the voltmeter' tips without a dedicated adapter/probe)
But of the difference are not significant I can calculate using 450 as anode voltage to calculate bias current to adjust....that's correct????
Yes I performed the voltage in the socket pin by voltmeter (450V)...but how can I do the measure with the tube in?? ( not space where to put the voltmeter' tips without a dedicated adapter/probe)
But of the difference are not significant I can calculate using 450 as anode voltage to calculate bias current to adjust....that's correct????
I see, you measured from the top of the amp using the socket with the tube out. Without a bias adapter, the measurement needs to be made from below the chassis unless the manufacturer has provided the test points on the top of the chassis (seems like it did not). If you have no prior experience working on tube amps, then DO NOT try to bias the amp yourself - it could KILL YOU! So I think you better take it to a qualified amp technician to have it biased.
Last edited:
Yes there are Test points near the tubes to adjust current bias of course, not for plate voltage
Is it usefull to have the measure of plate voltage for a fine tuning of bias??
Is it usefull to have the measure of plate voltage for a fine tuning of bias??
voltages change a lot without the tube in socket...
what i do in my amps is to set the grid bias at the most negative voltage,
this can be done without the tube in socket.
the object of bias setting is so that you know how much your tube
dissipates under idle conditions, sp plate voltage x cathode current is your tube diss...
what i do in my amps is to set the grid bias at the most negative voltage,
this can be done without the tube in socket.
the object of bias setting is so that you know how much your tube
dissipates under idle conditions, sp plate voltage x cathode current is your tube diss...
If you are fanatical, then yes, it would be helpful to measure the plate voltage and the currents (via the test points) through the tubes, But in most cases, adjusting the bias current per the manufacturer's recommendation is usually sufficient, i.e., no fine tuning required.
it would be helpful to know bias setting so that you can choose mode of operation,
classB for cool running amp, classAB(up to about 80% of plate rating) for bigger classA window, and hot(90 to 100% of plate rating)....
run your output tube cool means longest life but maybe not so good sound, run them hot and you get shorter tube life but good sounds...classAB is the middle ground, your choice...
an exception is screen grid drive....
classB for cool running amp, classAB(up to about 80% of plate rating) for bigger classA window, and hot(90 to 100% of plate rating)....
run your output tube cool means longest life but maybe not so good sound, run them hot and you get shorter tube life but good sounds...classAB is the middle ground, your choice...
an exception is screen grid drive....
No, not really. Bias does not need to be fine tuned. I suspect you are doing the usual newbie thing of obsessing over an unimportant detail while being unaware of more important things. Lets say that for some reason you decided to bias for 70%. In that case it doesn't really matter whether you end up with 65% or 75% bias - nothing magic happens at exactly 70%.Achi70 said:
If you want to measure the anode voltage then measure it. Take the covers off, clip a voltmeter probe on the anode pin then switch on and wait for the voltage to stabilise. You must, of course, always have a load applied to the speaker port to keep the OPT safe.
My advice is to forget it. Follow the maker's instructions for now. If in the future you learn enough to be genuinely concerned about bias details then by then you will know how to measure and adjust it to whatever you want.
Performed Plate/anode voltage measures with tube in and off
Hi,
However finally I performed measure of plate voltage not only with tube out in the pin3-8 socket, but also with the tube in (adapting the pointers and wires: DIY)
-As just said the plate/anode voltage resulted 475 in idlewith the tube out/off (volume 0; load speaker present)
-With the tube in, and changing the current bias, as expected plate voltage changed.. but just a little bit; when the bias was minimal the voltage was 475 and with a bias current as suggested by manufacter (0,30V) the plate/anode voltage decrease to 469V
.
Thus I can conclude that the variations are neglectable in order to calculate the bias adjustment. I think it's enough to measure the voltage with the tube out (for octal socket [EL34, KT88] between pin 3 and pin 8) and to calculate the bias according to well known formula
So for working a tube @ 60% of plate dissipation in my ampli Galaxy 34 (power supply in my town is 230V):
25W (EL34)/470*0.60= 0.32 that is close to the bias suggested by manufacter.
This is my experiment, not a rule.
Ciao
waiting for your expert opinions
Thanks
Hi,
However finally I performed measure of plate voltage not only with tube out in the pin3-8 socket, but also with the tube in (adapting the pointers and wires: DIY)
-As just said the plate/anode voltage resulted 475 in idlewith the tube out/off (volume 0; load speaker present)
-With the tube in, and changing the current bias, as expected plate voltage changed.. but just a little bit; when the bias was minimal the voltage was 475 and with a bias current as suggested by manufacter (0,30V) the plate/anode voltage decrease to 469V
.
Thus I can conclude that the variations are neglectable in order to calculate the bias adjustment. I think it's enough to measure the voltage with the tube out (for octal socket [EL34, KT88] between pin 3 and pin 8) and to calculate the bias according to well known formula
So for working a tube @ 60% of plate dissipation in my ampli Galaxy 34 (power supply in my town is 230V):
25W (EL34)/470*0.60= 0.32 that is close to the bias suggested by manufacter.
This is my experiment, not a rule.
Ciao
waiting for your expert opinions
Thanks
Last edited:
- Status
- Not open for further replies.