I forgot to mention that I had to go to the 300ma position on my meter because the 10A setting didn't show in ma. Hopefully the meter didn't add any resistance to affect the reading.You’re welcome,
It’s what this forum is all about..
Regards
M. Gregg
Its normal to adjust the meter to get the sensitivity for the mA required.
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Just for fun info for any interested...
and waffle..The change in resistance if any is minimal. If I said that an ammeter does not exist...then think of the meter as measuring voltage drop across a low value "resistor" (shunt) inside the meter (between the probes when set to amps) and showing it as current. This is what you have in current mode..a low value high wattage resistor between the meter probes. Think of the shunt as almost a short circuit.
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It’s always good practice to unplug your meter leads from current mode and reconnect as voltage to prevent you connecting the meter to measure voltage a few minutes later. The same applies to resistance setting..even I have done it…measure resistance and then forget and connect to voltage and the meter blows a fuse and the resistance range is damaged..even when you know you should have checked the settings...its even worse when you do it a second time in the same day because of pressure of work load..
LOL so the saying goes," its not the equipment change the engineer"..
Think what would happen if you forget that a meter is set to current and put a meter across a high current supply thinking its set to measure voltage (between +/- or L & N. and between the probes is almost 0 ohms) even worse if the meter is not fused on the 10A current range, because the probe leads may become the fuse and melt with you holding them..
Sorry I'm waffling on...
Regards
M. Gregg
For a repair shop or production facility, time is money. Modifications take time and might not be appropriate. Going across two points for each OPT primary half and turning a pot is very simple. With a meter having sufficient resolution (enough digits), the 10 Amp scale provides negligible loading error (0.1% for a 100 Ohm primary coil side across a .1 Ohm internal meter shunt). Also, on a 10 Amp range, time won't be spent changing a blown meter fuse if a tube arcs.
In practice, cheaper meters must be used on a more sensitive scale since 3 1/2 digits won't give enough resolution otherwise. The internal meter shunts are higher resistance on the lower current ranges. For that reason, use the highest current range with sufficient resolution if testing by the across the coil method. Using a 20 mA scale would add more error, I estimated minus 10 to 15% on a Fluke 8020A, but it is a tenth that or less on higher ranges.
The target current changes a bit depending on where you measure. In pentode mode, estimating screen current at approximately 20% of plate, note that the 17 mA target figure at the plate reflects about 20 mA at the cathode.
Fears of tube damage from shorting across primary coils while doing idle tests are unfounded. Without signal, at far below peak operating current, tubes certainly won't be harmed by the small percentage increase in plate voltage seen from shorting the OPT primary. Even line voltage fluctuations can present a larger variation. The dissipation is far less than under operating conditions which see much higher peak currents, and normal operation has audio peaks exposing the anodes to far higher peak voltages. As the conducting side of push pull sees the plate voltage fall, the voltage on the other side rises well above the supply voltage.
Ionization from a brief radiation spike during a solar flare is actually more likely than these tests to trigger an arcing failure event in a marginally gassy tube. There was a M5+ flare yesterday and there have been several of the more intense X-class flares so far this year. (Not to be forgotten, geomagnetic storms that sometimes accompany or follow solar events may cause damaging power grid instability and line surges)
The highest risk of failure during bias adjustment is seen in badly designed amps where bias voltage is lost if the slider of a bias pot goes open. That may happen with a dirty pot. It's better to have negative bias coming in through a fixed resistance with the pot (and series range-limiting resistor) shunting that.
Adding a resistor between the slider and the more-negative side is a quick fix. Then current will fall if the slider goes open.
(sorry, too much coffee)
"The highest risk of failure during bias adjustment is seen in badly designed amps where bias voltage is lost if the slider of a bias pot goes open."
How high are the probabilities of a trimmer pot having this problem and would it be a good idea to replace the adjusted value with a fixed resistor.
How high are the probabilities of a trimmer pot having this problem and would it be a good idea to replace the adjusted value with a fixed resistor.
I've been reading about biasing amps different ways and some refer to the shunt method which seems to place the meter across the xformer like H&K suggested. They say part of the current flows through the meter and some through the xformer. Maybe that's what they meant to do. Most say it's a dangerous and unreliable method. They say the better way is to measure the resistance of each side of the xformer and then measure the voltage across it and with ohm's law that will give you the most accurate bias reading. Do they mean to disconnect the xformer for an accurate resistance measurement or could it be done in circuit. It would be interesting to see how it compares to my setting by breaking the primary circuit. I did not see any suggestions anywhere on breaking the primary circuit to get a current reading, just the shunt method.
Well,
You already know the current through one side...measure the volt drop across the Tx and.... Voltage divided by current = Resistance of the Tx...
They are inter linked its impossible for your current reading to be any different using this method...except for rounding up or down on the values..That would give an error...the resistance you calculate from the above will be the resistance of one side of the OP Tx. If you measure the resistance with your meter its possible to have an error from lead resistance or bad connection or meter error.
The value of current you have measured Will be the value of current flowing in the anode of each tube its the most accurate way to do it other than putting the meter in the cathode circuit to allow for any grid current. Any variation with the current value measured across the Tx will be less accurate with the Tx in circuit. If you disconnect the OP Tx from the board and use the meter in circuit across the Tx connections you have done exactly the same as before with the meter now in circuit with the anode of the tube. (I don't like this Idea, however again its not my amp)
Regards
M. Gregg
You already know the current through one side...measure the volt drop across the Tx and.... Voltage divided by current = Resistance of the Tx...
They are inter linked its impossible for your current reading to be any different using this method...except for rounding up or down on the values..That would give an error...the resistance you calculate from the above will be the resistance of one side of the OP Tx. If you measure the resistance with your meter its possible to have an error from lead resistance or bad connection or meter error.
The value of current you have measured Will be the value of current flowing in the anode of each tube its the most accurate way to do it other than putting the meter in the cathode circuit to allow for any grid current. Any variation with the current value measured across the Tx will be less accurate with the Tx in circuit. If you disconnect the OP Tx from the board and use the meter in circuit across the Tx connections you have done exactly the same as before with the meter now in circuit with the anode of the tube. (I don't like this Idea, however again its not my amp)
Regards
M. Gregg
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I'm surprised no one measures it the way we decided on earlier. It seems so much easier and safer than the shunt way and easier than measuring ohms and resistance and if the meter adds resistance like you said and iit most definately would with the crappy ones I own, it seems like breaking the circuit is the way to go, even if it means unsoldering the transformer to do so.
I have a Peavey 50/50 that I was going to ask you about that read really high when I tried the bias tool on it. I was wondering if I could test it the same way . It has 4 el84's each side but it's basically the same design except the transformer is encased and it would require unsoldering the primary leads to break the circuit.
The cathode is always the safest option,
Because it’s usually closer to Gnd and lower voltage.
If you have an amp with four tubes per side you can measure in this way, however it’s always better to measure at the cathode if you can get at it.
And at each tube rather than a set of tubes.
Again you would need to be able to measure each tube or set the bias equally for the individual tubes. I would normally use manufacturers instructions for bias setting.
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The main thing is not to get complacent or to relaxed remembering that this is at B+ values.
The tendency is to get over confident “the more you do it” the easier it seems until it bites.
I notice I'm dominating the thread so I'll step back it is always interesting to hear other ideas..
Regards
M. Gregg
Because it’s usually closer to Gnd and lower voltage.
If you have an amp with four tubes per side you can measure in this way, however it’s always better to measure at the cathode if you can get at it.
And at each tube rather than a set of tubes.
Again you would need to be able to measure each tube or set the bias equally for the individual tubes. I would normally use manufacturers instructions for bias setting.
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The main thing is not to get complacent or to relaxed remembering that this is at B+ values.
The tendency is to get over confident “the more you do it” the easier it seems until it bites.
I notice I'm dominating the thread so I'll step back it is always interesting to hear other ideas..
Regards
M. Gregg
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Maybe the others are also learning by your expertise. Better to say nothing if you 're not sure when it comes to early grave measurements.The cathode is always the safest option,
Because it’s usually closer to Gnd and lower voltage.
If you have an amp with four tubes per side you can measure in this way, however it’s always better to measure at the cathode if you can get at it.
And at each tube rather than a set of tubes.
Again you would need to be able to measure each tube or set the bias equally for the individual tubes. I would normally use manufacturers instructions for bias setting.
-----------------------------------------------
The main thing is not to get complacent or to relaxed remembering that this is at B+ values.
The tendency is to get over confident “the more you do it” the easier it seems until it bites.
I notice I'm dominating the thread so I'll step back it is always interesting to hear other ideas..
Regards
M. Gregg
I'll try to post a schematic of the Peavey 50/50 amp. I don't think they offer any info on biasing it. When I tried my bias probe it kept creeping up just like the H&K did. The plate is also 400v and pin 9 is 400 too so I wonder if these high voltage el84 amps are too sensitive for the probes unlike the ones that set the plate way lower. It's also too difficult to unsolder pin 3 to insert a resistor.
here's an image of the peavey. I'm thinking of unsoldering the transformer at the primary j115 points and inserting the meter there. Would the measurement have to be 4 x the current for each tube since there's 4 tubes?The cathode is always the safest option,
Because it’s usually closer to Gnd and lower voltage.
If you have an amp with four tubes per side you can measure in this way, however it’s always better to measure at the cathode if you can get at it.
And at each tube rather than a set of tubes.
Again you would need to be able to measure each tube or set the bias equally for the individual tubes. I would normally use manufacturers instructions for bias setting.
-----------------------------------------------
The main thing is not to get complacent or to relaxed remembering that this is at B+ values.
The tendency is to get over confident “the more you do it” the easier it seems until it bites.
I notice I'm dominating the thread so I'll step back it is always interesting to hear other ideas..
Regards
M. Gregg
An externally hosted image should be here but it was not working when we last tested it.
Well,
I can only see part of the drawing Is there only 4 power tubes in this amp or 8. If 4 then there would be two on each side of the OP Tx?with 8 then as you say current would be for 4 tubes,if you were going to do this then you would put the meter in each side of the OP Tx so you would have two or four Tubes in the one side..So current from each side of the OP Tx would be current for two tubes or 4. You will have to work out where the bias control is..the drawing is not complete..So be carefull.
A full circuit would be better...You can't tell whats going on with the the OP Tx..whats the top part of the drawing?
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And If you look at the Bias on the other amp the adjust is in the Gnd side as I said in a previous post so if it went open the bias would be on full voltage so the tubes should shut down. ie its already safe.
Regards
M. Gregg
I can only see part of the drawing Is there only 4 power tubes in this amp or 8. If 4 then there would be two on each side of the OP Tx?with 8 then as you say current would be for 4 tubes,if you were going to do this then you would put the meter in each side of the OP Tx so you would have two or four Tubes in the one side..So current from each side of the OP Tx would be current for two tubes or 4. You will have to work out where the bias control is..the drawing is not complete..So be carefull.
A full circuit would be better...You can't tell whats going on with the the OP Tx..whats the top part of the drawing?
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And If you look at the Bias on the other amp the adjust is in the Gnd side as I said in a previous post so if it went open the bias would be on full voltage so the tubes should shut down. ie its already safe.
Regards
M. Gregg
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I tried to post the pdf schematic and this site only allows small attachments and I can't figure out how to post it.
is R69 the safety resistor to keep it from going to zero since it's in series with the pot.
heres a link to the Peavey, scroll down to the schematic and click it.
http://www.guitarvictim.com/
http://www.guitarvictim.com/
R69 is just to stop you adjusting and shorting to Gnd..
The pot TR2 is connected so its track is in series with R69 so the supply does not go through the wiper..ie if the wiper went open then you would still have the track resistance in series with R69..however even if they went open you would still have bias voltage on the tubes..
Regards
M. Gregg
Looks like,
Two OP Tx each driven from 4 OP tubes via concertina PI. So two tubes each side of each OP Tx..I haven't got time now to trace the Bias the supply its on the PSU drawing.
Regards
M. Gregg
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