Members gusfinley Posted September 23, 2005 Members Share Posted September 23, 2005 Hi, I just fired up my DIY 50W plexi. It is sounding great. I have been trying to bias it up, but I am having some difficulty. I think that it is operating in triode mode at the moment, because I can only get a reading of 49mA of plate current for both tubes. I installed a 1ohm resistor in the plate circuit to test the bias current.... The resistor actually reads 1.5 ohms. I get a reading of .037 Volts across it. Plate voltage is 460 Volts. Another strange thing is that it doesn't seem that I am getting much of a voltage drop across my choke. Its a 5H 100ohm choke, and I am only getting a voltage drop of about 1 volt. Becuase of this there is virtually no difference between the plate voltage on the EL34 and the screen voltage - hence it seems to be stuck in triode mode. Is there any way that I can correct this? thanks for any input.... Link to comment Share on other sites More sharing options...
Members JamesPeters Posted September 23, 2005 Members Share Posted September 23, 2005 Your screen voltage might not be much lower than the plate voltage. That's normal under a lot of circumstances especially if the tubes aren't doing a whole lot at the moment. Your amp might not be in triode mode. Anyway you can easily check your wiring to figure that out. You can also check the series resistance of the choke (undo one end) to be sure. I'd take that 1 ohm resistor out of the plate path and put it from the cathode to ground instead. If your bias range is inadequate, you might have to alter it. That happens when building amps with somewhat different components from the original circuit. Link to comment Share on other sites More sharing options...
Members JamesPeters Posted September 23, 2005 Members Share Posted September 23, 2005 Also, check your meter's accuracy. It's very rare for a new resistor to be 50% off base value. Your meter probably reads about 0.5 ohms when you touch its leads together, I'd bet. Link to comment Share on other sites More sharing options...
Members gusfinley Posted September 23, 2005 Author Members Share Posted September 23, 2005 My meter is probably not the best..... especially at those low values of R. The choke that I am using a a Heyboer 5H from metroamp. What advantage would I gain by placing the 1 ohm resitor in the cathode circuit?. I would like to read Plate current readings and bias that way, rather than subtract all the screen currents to find it.... I forgot to mention that I did put a pentode/triode switch in the amp. The measured voltages are the same from the plate and screen. after the choke there are two 10K resitors. I was thinking that perhaps I could connect the screen to the midpoint between these two and get a better screen voltage from there, and then increase the bias current as a result. As far as I have seen I am using the original values in the circuits, inlcuding the bias circuits. any other ideas/suggestions? Tommorrow I will take a reading of the votage difference between the plate and screen voltages - I was getting 1.2 V earlier today, but I'll do some more poking around with the meter. Link to comment Share on other sites More sharing options...
Members Duesentrieb Posted September 23, 2005 Members Share Posted September 23, 2005 Cheap meters measure incorrect at lower resistance IMO. So to select real valued 1 Ohm Rs you need a really good one. And I agree to James that I would put them to PIN 8 (cathode) - is your meter able to read amperes? Then why not double check with the shunt and the cathode methode?You need to read the mA's between the inner and one outer leg of the OTs primaries then (don`t forget to set your meter correctly !!!! normally the red probe needs a different input than for reading voltage/resistance). Link to comment Share on other sites More sharing options...
Members gusfinley Posted September 23, 2005 Author Members Share Posted September 23, 2005 I'll have to run the amp to the engineering lab wich has better meters and get a good reading on the 1ohm resistor...... once again, it was suggested to put the 1R in the cathode - why? I haven't heard any arguments to convince me that this would be the best idea. And AGAIN - The plate current doesn't change when I switch the amp from pentode to triode mode and back again - therfore I have determined that it must be running in triode mode. I'm really more concerned about the lack of differences in voltages here.......... Link to comment Share on other sites More sharing options...
Members scottosan Posted September 23, 2005 Members Share Posted September 23, 2005 Originally posted by gusfinley once again, it was suggested to put the 1R in the cathode - why? I haven't heard any arguments to convince me that this would be the best idea. I suggest if you ask for help then accept it from 2 of the most knowledgable people on here. If you aren't willing to take their advice, then why are you asking? http://www.duncanamps.com/technical/lvbias.html Link to comment Share on other sites More sharing options...
Members Duesentrieb Posted September 23, 2005 Members Share Posted September 23, 2005 Hi Scott James is, I`m not. Gus: I`ve said: double check by using the shunt AND cathode methode. Is your meter able to read amperes? Then just try what I have suggested. Since even a standard meter should have "good" R's (when reading amperes) you get a better idea whats going on there . . . A standard 5H choke would have an approx. R of something between 100 and 200 Ohms - calculate with that when you want to know its voltage drop between plate and screen . . . . Link to comment Share on other sites More sharing options...
Members Duesentrieb Posted September 23, 2005 Members Share Posted September 23, 2005 B-U-M-P Link to comment Share on other sites More sharing options...
Members gusfinley Posted September 23, 2005 Author Members Share Posted September 23, 2005 I just ran a few tests... I removed the 1ohm resistor from the plate circuit and used my multimeter to read the current running through the plate. I got a reading of about 40mA. For both tubes that would give me 80mA of plate current. For the 3200 ohm primary of my output transformer that gives me a power output of 20W = triode connected..... The choke reads 100 ohms on the meter..... I do have another 1 ohm resistor and tested it in the univerity's fancy LCR meter. It read .9987 ohms - close enough to 1 ohm for me. I'll be putting this back into the plate circuit BECAUSE I want to bias my amp based on plate current alone. Eventually, I will be adding a switch that will convert the amp to cathode bias, so I Will need to take some readings from the cathode end of the circuit. I may have to recheck my wiring job, but I thought I had already double checked it. Do you think this could be something to do with the choke? Can you wire one backwards? any suggestions would be very helpful.... Link to comment Share on other sites More sharing options...
Members gusfinley Posted September 24, 2005 Author Members Share Posted September 24, 2005 BumpinItUp Link to comment Share on other sites More sharing options...
Members Duesentrieb Posted September 24, 2005 Members Share Posted September 24, 2005 Originally posted by gusfinley I just ran a few tests... Can you wire one backwards? Nope. Check your wiring, Gus. Link to comment Share on other sites More sharing options...
Members JamesPeters Posted September 24, 2005 Members Share Posted September 24, 2005 At 460V each tube is drawing 40mA--that means each tube, at idle, is dissipating about 18W. If anything the bias is a bit hot, but that doesn't point to the circuit being in triode mode. The static DC power dissipation at idle does not equal maximum AC (signal) power output when in use. You're making "a leap in logic". I've said this before and I'll have to say it again--AX84.com is a great place for stuff like this. If you're wondering why, it's because I don't like explaining every single little thing I say in the finest detail. If you don't understand why I recommend something, I find it hard to bring myself to explain it when it's been explained a lot on AX84. It seems like you want me to do the work for you, and you're not willing to do any reading on your own. Link to comment Share on other sites More sharing options...
Members Voodoo Amps Posted September 24, 2005 Members Share Posted September 24, 2005 once again, it was suggested to put the 1R in the cathode - why? So that you can obtain accurate readings and therefore be able to bias the amp properly. You cannot install a 1 ohm resistor from the plate to ground read the current through the cathode, unless you install it in series between the cathode and ground (IE: you do not install a 1 ohm resistor from the plate to ground). You do need to know what the plate voltage is to crunch the math in order to properly bias but that does not require a 1 ohm resistor to ground off from the plate pin. You can simply set your DMM for DC voltage, place the black lead on ground and the red lead on the plate pin and that will give you the plate voltage. Keep in mind there is high voltage when doing this so do not slip off from pin 3 and connect with the heater pin, screen pin, etc. Also check the wattage rating on the resistor and see what the max voltage rating is as you may have exceeded the max voltage rating and if so I would recommend using new resistors for the most accurate readings. Coffee hasn't kicked in yet and I played a show last night so I hope this all makes sense. Have a great SaturdayTrace Link to comment Share on other sites More sharing options...
Members gusfinley Posted September 24, 2005 Author Members Share Posted September 24, 2005 Okay, I think I get it now. The 1 ohm resistor isn't going from the plate to the gorund, it is in series with the wire headed to the ouput transformer from one of the plates. Link to comment Share on other sites More sharing options...
Members gusfinley Posted September 24, 2005 Author Members Share Posted September 24, 2005 Originally posted by JamesPeters At 460V each tube is drawing 40mA--that means each tube, at idle, is dissipating about 18W. If anything the bias is a bit hot, but that doesn't point to the circuit being in triode mode. The static DC power dissipation at idle does not equal maximum AC (signal) power output when in use. You're making "a leap in logic". Right. I was using the other Power Equation, P=I^2*R, to find the Idle Dissipation throught the ouput transformer. Running my equations for bias current at 460V I do get around 40mA of plate current in a High AB configuration. But what of this 18W per tube dissipation at idle, is this normal for a 50W amp to have this kind of setup? Oh and about AX84. I can't stand the formatting of that place. I posted here to get some advice from people here, if I wanted advice form AX84 I would have posted there in the first place. I look into biasing a little more. Still one odd thing persists--- Why is there no difference when I engage my amp into triode mode? It's either stuck in pentode or triode. I will triplecheck my connections, but this is the one thing that has struck me as odd. Link to comment Share on other sites More sharing options...
Members scottosan Posted September 24, 2005 Members Share Posted September 24, 2005 Originally posted by gusfinley Right. I was using the other Power Equation, P=I^2*R, to find the Idle Dissipation throught the ouput transformer. Running my equations for bias current at 460V I do get around 40mA of plate current in a High AB configuration. But what of this 18W per tube dissipation at idle, is this normal for a 50W amp to have this kind of setup? Oh and about AX84. I can't stand the formatting of that place. I posted here to get some advice from people here, if I wanted advice form AX84 I would have posted there in the first place. I look into biasing a little more. Still one odd thing persists--- Why is there no difference when I engage my amp into triode mode? It's either stuck in pentode or triode. I will triplecheck my connections, but this is the one thing that has struck me as odd. Can you either snapa picture or post a schematic you are using to switch from Pentode/Triode. Link to comment Share on other sites More sharing options...
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