Members Kyle DiSanto Posted November 8, 2005 Members Share Posted November 8, 2005 I need some help here. I am sending my Weber Mass Bias Rite back to them and they are going to see what is wrong with it...fix it or send me a new one. I figured that was very nice of them. However I have my new tubes now and I dont want to wait however long it is gunan take to get my bias rite back. I have a Digital Volt Meter and I want to bias the Framus with the new tubes. It measures ac/dc voltage up to 500volts (framus never went higher than 500v) and MEasuresAC/DC Current up to 200 mA. My question is: Where do I put the probes to make my readings? I am pretty good with electronics and realize the dangers in such a task, but again if you take the right precautions, then I should be ok. Thanks For ANy HELP!KYLE Link to comment Share on other sites More sharing options...
Members tommythelurker Posted November 8, 2005 Members Share Posted November 8, 2005 Three ways: 1. get some 5W 1 ohm (10 ohm can work too) 1% resistors and connect one between each power tube cathode and ground. Then measure the voltage across the resistor. This is basically the same thing the bias rite does and you woudn't even need it if you did this. You can even mod the amp to bring ground and each cathode connetion out to a test point on the outside of the amp. The cathode voltages in a fixed bias amp are low and aren't dangerous. 2. remove the OT primary from one of the plates and put the meter in series with the plate and OT. Measure mA. Do this for each tube and make sure they're all fairly close and one exceed the max safe current. This is the most dangerous as it's high voltage but also the most accurate. 3. With your meter set to read mA, connect the probes between one of the plates and the B+ supply, shunting the meter across the OT. For this to be accurate, the internal DC resistance of the meter has to be much less than the DC resistance of the OT primary. Link to comment Share on other sites More sharing options...
Members ragnvald Posted November 8, 2005 Members Share Posted November 8, 2005 Originally posted by ROTINPAIN I need some help here. I am sending my Weber Mass Bias Rite back to them and they are going to see what is wrong with it...fix it or send me a new one. I figured that was very nice of them. However I have my new tubes now and I dont want to wait however long it is gunan take to get my bias rite back.I have a Digital Volt Meter and I want to bias the Framus with the new tubes.It measures ac/dc voltage up to 500volts (framus never went higher than 500v) and MEasuresAC/DC Current up to 200 mA. My question is: Where do I put the probes to make my readings? I am pretty good with electronics and realize the dangers in such a task, but again if you take the right precautions, then I should be ok. Thanks For ANy HELP!KYLE It's quite easy actually. I have my Cobra open at the moment. Plate voltage: I think it is safe to use the grounding directly from the socket as there's no isolated design. Anway, turn of stand-by and let the voltage settle for some minutes.You see the two thich blue cables going from the OT to the powertubes? Meassure of those and you have your plate voltage. Current: Try to find the 1 Ohm resistors between the powrtubes to ground. If in doubt you can always look at the schematics. You need to meassure above these, for each tube, and you'll have the bias current for each tube respectively, because V / R = I, i.e. x volts / 1 ohms = x amps Link to comment Share on other sites More sharing options...
Members tommythelurker Posted November 8, 2005 Members Share Posted November 8, 2005 Oh, I didn't know that amp already has the 1 ohm resistors. Well there you go. Link to comment Share on other sites More sharing options...
Members Kyle DiSanto Posted November 9, 2005 Author Members Share Posted November 9, 2005 Originally posted by ragnvald It's quite easy actually. I have my Cobra open at the moment.Plate voltage: I think it is safe to use the grounding directly from the socket as there's no isolated design. Anway, turn of stand-by and let the voltage settle for some minutes.You see the two thich blue cables going from the OT to the powertubes? Meassure of those and you have your plate voltage.Current: Try to find the 1 Ohm resistors between the powrtubes to ground. If in doubt you can always look at the schematics. You need to meassure above these, for each tube, and you'll have the bias current for each tube respectively, because V / R = I, i.e. x volts / 1 ohms = x amps Thanks Alot guys! Ok so those two thick blue wires comming from the OT, I measure them at the OT? Does it matter which is black and which is red? That is Plate. Im having trouble findin the 1 OHM resitors between the powertubes and ground. I will look for a shcedmatic. THANKS ! Link to comment Share on other sites More sharing options...
Members Kyle DiSanto Posted November 9, 2005 Author Members Share Posted November 9, 2005 Well i tried and no luck I might just wait for the weber to come back and return this volt meter. Link to comment Share on other sites More sharing options...
Members tommythelurker Posted November 9, 2005 Members Share Posted November 9, 2005 Originally posted by ROTINPAIN Well i tried and no luck I might just wait for the weber to come back and return this volt meter. what happened? Link to comment Share on other sites More sharing options...
Members Kyle DiSanto Posted November 9, 2005 Author Members Share Posted November 9, 2005 I was having trouble trying to find where to put the mult meter probes...I dont wanna {censored} anything up. I just played the amp before and I think one of the power tubes is going so Im just gunna wait to get these new ones in with the weber bias rite. I heard a coffee maker sound and i got my pencil out and it wasnt the preamp tubes but it was one of my power tube making all the noise..when I would tap it, id hear a pop sound....I hope it is just the tube and not the amp! thanks for your help though ! Link to comment Share on other sites More sharing options...
Members tommythelurker Posted November 9, 2005 Members Share Posted November 9, 2005 If it's got the cathode resistors, shouldn't be that tough. Just look for the big resistor connected to pin 8 of the power tube sockets and measure mV across it. Do you have the schematic?When you have the meter set to read volts (or mV) you can't really screw anything up by putting the probes in the wrong place. There's a very high resistance between the probes and almost no current flows when you're measuring voltage so you can put them pretty much anywhere. Just don't touch two different wires with one probe at the same time or it will short them out.Now, if you're measuring current, then you have to be more careful... I've blown a few fuses by slipping and hitting the chassis with one probe while the other was on B+ Link to comment Share on other sites More sharing options...
Members Craggin Posted November 9, 2005 Members Share Posted November 9, 2005 I'd wait for the bias rite to get back...this thread has that trainwreck scenario written all over it. Link to comment Share on other sites More sharing options...
Members Kyle DiSanto Posted November 10, 2005 Author Members Share Posted November 10, 2005 Originally posted by Craggin I'd wait for the bias rite to get back...this thread has that trainwreck scenario written all over it. LOLOLOL You know man I know that I could do it, but the bias rite just makes it soo much easier that Id just rather wait until I was sure. Down the road maybee Ill have a tech show me the volt meter way, but i duno wanna {censored} nething up!!!! THanks for all your help guys..the train has stopped ahah Link to comment Share on other sites More sharing options...
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