Standby with no power tubes?

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MississippiMetal

MississippiMetal

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I've been peeling through documentation and DIY articles regarding various biasing techniques, and one thing really piqued my interest with respect to finding plate voltage of an amp. I keep reading about measuring pin 3 of the power tube socket while the amp is on standby, with the power tube(s) out. Is this safe to do? I'm of the understanding that standby simply keeps the tube heaters going. But even so, can you just leave the tubes out with the amp in standby, and then plug them in with the amp still in standby?

As a side note, I measured the plate voltage on my Triamp with the tubes in, and it topped out at 495V. That's considerably higher than the average 460PV I read about, and this is with tubes. It'd be even higher without them. I'm supposing as long as the primary resistance is high enough, it should be okay, then right?
 
The will be no B+ voltage applied with the amp on standby, so you should not get any reading on pin3. If all of the power tubes are pulled, it it safe for your transformer.
 
Alright, so another, smaller question, so I don't have to start another thread. If I'm measuring current, is it polarized, or can the leads be placed either-or?
 
you cant have negative current. just as long as the meter is inserted into the circuit (series) and not parallel you will be fine.
 
glpg80":2d63asx8 said:
you cant have negative current. just as long as the meter is inserted into the circuit (series) and not parallel you will be fine.
Click to expand...


So touching the probes to the OT primary and then to the power tube without disconnecting the Primary wire would be inserting it Parallel (bad) right?

It's strange, some H&K Triamp documentation I have tells me I should bias the amp by connecting Pin 5 on the OT to Pin 2 on the Power tube. Pinout of an EL34 tube tells me Pin 2 is a heater pin. Why the hell would a heater give me a bias reading?
 
what are you trying to do?

measure the platecurrent, or the plate voltage?

olaf here on the forum posted a very informative post about using the OT as a biasing method. search the rig-talk forum and see what you come up with - it was a great post and very informative.

normally the center tap of the OT is your (really high) plate voltage - then each outter tap of the primary of the OT are the 1/3, and 2/4 tube plate pairs - since these amplifiers are class A/B push-pull.

be careful poking around in the HV sections you are.
 
Yeah I'm tremendously careful, which is why I research and clarify like mad before I commence anything. I'm trying to read the plate current to adjust the bias for 30ma per tube. I've already determined plate voltage at 495v from Pin 3 to ground.
 
just install a 1 ohm 2watt resistor from pin 8 to ground which measures the cathode current.

measure the voltage across the 1ohm resistor with the meter set to mV. since ohms law says that I=V/R, as long as the resistor is 1ohm, then ohms law says that the current through the 1 ohm resistor is the same as the voltage read across the resistor.

once you do that, you can determine the plate current with a little bit of math.

the cathode current is the sum of the screen grid current and also the plate current. to determine the screen grid current for each tube, measure the voltage drop across the screen grid resistors. the way you do this is to measure one side of it, then the other side of the resistor. so if the resistor is 1K and you get if you get 5V difference, ohms law says that I=V/R. divide that voltage by the value of the screen grid resistor and that is your screen grid current (5mA in this instance)

now simply subtract the screen grid current from the cathode current and you have your plate current.

hope that helps :thumbsup:
 
BTW the method where you insert the meter in parallel in the circuit id like to explain. you cannot measure current in parallel but with these amplifiers there is a small exception.

when you measure from pin 3 of X tube to the correct corresponding side of the OT primary, you are putting the meter in parallel with the circuit. but because the shunt resistance of the OT is much higher than most of the meter's (when set to measure current) you can buy out there - most of the current goes through the meter instead of its normal path.

its a controlled shunt measurement.

i personally dont like this method because one slip up and you are screwed. the other thing is that "most" of the current goes through the meter. the other reason is that the quality of the meter plays in effect - i dont have a fluke yet, so the internal resistance of the meter would differ the bias reading. the fourth reason is that you can only measure the plate current of each PAIR of tubes this way, and not each individual tube. so each measurement you get is for each pair, and would need to be divided by 2 for 120W amplifiers. you can however measure it for each tube this way for 50 watter's, but the meter quality still matters.

but yeah thats why you can measure plate current in parallel this way. make sure you grab the correct side of the OT (not the center tap!!!!) when measuring with the controlled shunt method the side you grab does matter. but the first method i posted is better and more accurate as you can get it down to each individual tube.
 
glpg80":j4r11zn3 said:
BTW the method where you insert the meter in parallel in the circuit id like to explain. you cannot measure current in parallel but with these amplifiers there is a small exception.

when you measure from pin 3 of X tube to the correct corresponding side of the OT primary, you are putting the meter in parallel with the circuit. but because the shunt resistance of the OT is much higher than most of the meter's (when set to measure current) you can buy out there - most of the current goes through the meter instead of its normal path.

its a controlled shunt measurement.

i personally dont like this method because one slip up and you are screwed. the other thing is that "most" of the current goes through the meter. the other reason is that the quality of the meter plays in effect - i dont have a fluke yet, so the internal resistance of the meter would differ the bias reading. the fourth reason is that you can only measure the plate current of each PAIR of tubes this way, and not each individual tube. so each measurement you get is for each pair, and would need to be divided by 2 for 120W amplifiers. you can however measure it for each tube this way for 50 watter's, but the meter quality still matters.

but yeah thats why you can measure plate current in parallel this way. make sure you grab the correct side of the OT (not the center tap!!!!) when measuring with the controlled shunt method the side you grab does matter. but the first method i posted is better and more accurate as you can get it down to each individual tube.
Click to expand...


Cool info. Yeah I read about shunting, but I'm not entirely sure if my meter's internal resistence is low enough to yield a proper reading. For $60 I hope it is. lol. It's a fluke knockoff made by BK. The unfortunate part about the cathode resistor method is I can't readily access all the power tube sockets on the PCB. Three of them sit directly under FX loop circuits and such. Another weird thing is the ground wires from each cathode pin go to different points, and some points are farther away than others. To complicate matters, the power section has a tube circuit breaker feature where each tube has its own fuse that will allow the amp to continue operating at reduced output with a blown tube.
 
the shunt method still works - just isnt my favorite. its also the method i mentioned in my first post about olaf's topic - its what he recommended to the guy with a 50W amplifier.

seems like in your instance you dont have much of a choice though - but the other method is much more accurate. learn it and try to use it in the future if you can.

BTW the farther your meter is off, the hotter you will be setting the bias - just an FYI. its because of this:

cheaper meter = higher internal resistance = lower rating than what it actually is = setting it to an incorrect number (going by what the meter says)

so i would purposely bias it a little lower to make up for the difference in your instance - how much though im not sure. like i said, depends on the meter. :lol: :LOL:
 
The stock bias trim pot only goes from 28ma-38ma, so I don't think I could redplate the tubes, but that is good info to have. I set the bias trim pot about 3/4 of the way wide open, so it's around 30ma.

This is the DMM I use.

http://www.tequipment.net/BK2704B.html
 
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