HIWATT Bias Resistor Question

[flying snake]

Hi---
I just finished building a HIWATT DR103 amp from Tilo Zotolla's book and I am having trouble understanding what to do about the bias resistor. The voltage reading I got with my multimeter on pin 5 of each EL34 socket is -58 which is much more (or is it less negative?) than the desired -41 which Mr. Zotolla says is optimum. As it is now, the amp will not put out a clean signal (crossover distortion). From my research, I gather this is because of the incorrect bias. In the book, Mr. Zotolla says to increase or decrease this 1000 ohm resistor (which is soldered across the 2 positive posts of the cap) in order to obtain the desired bias. Does anyone know if this resistor value should be increased or decreased? And by how much? Could a potentiometer be wired here in place of this 1000 ohm resistor in order to be able to adjust the bias? Also, this is the third amp I have built, but I am still a total amateur...so I am a little worried about touching this cap to change the resistor. I am only 35 and married (with a 15 month old baby) and do not wish to be shocked to death just yet. A separate problem I am having with the amp is none of the tone pots work. Only the normal volume and master volume pots are working. The bright pot makes a weird low sound when turned, but this channel doesn't seem to want to function either. Any suggestions? THANKS!!!

[R ski]

Over biasing will cause some horrible sounds. The fact that Tilo mentions a nominal value at pin 5 is close but not accurate. Two reasons, first the resistance of the transformer, second the gain of the power tube.

The most accurate way of obtaining actual power tube draw, is the surest way to optimize that stage. Let the amp stay off...for a while. Measure from output tranny center tap to each power tube....pin 3. Average the reading. Using ohms law I=E/R. Measure each tube, from center tap to pin 3. A nominal value of 25 mA is good, some prefer to obtain 35 mA to 45 mA...idle. The warmer tube yields more power amp gain....although the idle hum...may increase as well. Wire in a pot to obtain a reasonable reading. Substituting a resistor...is less prone to opening up....causing power tube melt down

Working with high voltages is daunting at first...many ampagers mention this.
I work near potential dangers each day at my occupation. I read a lot of suggestions, on safety precautions. Mine....buy leather gloves...that fit well, soft to handle meter probes. If your skin doesn't come in contact with the B+
no shock will occur. Avoid being well grounded....basement floor....put a mat on the concrete floor...if your work space is in the basement, good idea.

[Amp Kat]

Originally Posted by R ski

Over biasing will cause some horrible sounds. The fact that Tilo mentions a nominal value at pin 5 is close but not accurate. Two reasons, first the resistance of the transformer, second the gain of the power tube.

.

Depends on what your calling over and under. Over being cold and crossover distortion and underbiased meaning hot or approaching maximum dissipation by the tube but yes both can cause freaky ugly noises. All good information you gave snake but you have to understand that he doesn't know what your telling him or at least I don't think he does. I tried explaning this to someone on the other topic and still don't think it quite sank in. Theres so many people building amps right now (which btw is a cool thing with me) that aren't really technically inclined as to exactly what every thing does but anyone can do it as long as you follow the guidelines and don't run in to any problems. I think Snake has a few problems on this one. Snake the bias circuit is a low voltage low current circuit that won't harm you in certain situations. When we work on bias circuits we leave the amp on standby as the high voltage is not present on the tubes but the bias voltage usually is depending on the transformer and if it has a sepearate bias tap but it may not ? You can sub a pot for the resistor pretty easily as the negative voltage coming in is probably -60 to-70 before the resistor. Wire that to pin 3 of say a 10k pot and put a 15k resistor from pin1 to ground. Pin 2 (the wiper) goes to pin 5 of the tube. Adjust the resistor up for less range and down for more range. Remember the less negative as in approaching zero adds more milli-amps (hotter bias) to the tube. As far as the treble pots now your getting into the high voltage stuff as the other side of the coupling cap has preamp high voltage in play mode. I think you have something wired wrong or a connection that looks good that may not be so doublecheck your connections and make sure everything is going where it should be. Very easy to get shocked by an amp when in play mode so never let your guard down and a lot of the things R Ski said are good tips but IMHO even if you did get shocked it wouldn't kill you but it sure hurts like hell.

[flying snake]

Thanks for the quick replies R ski & Amp Kat!
Amp Kat, when you say "Wire that to pin 3...." I am not sure what the "that" is you are referring to. The 1000 ohm resistor is currently attached to the 2 positive terminals of a 50/50 cap. There are two bias tap wires (50V) (two blue wires). So what exactly should be wired to pin 3 of the 10K pot? Also, you say to wire the wiper "to pin 5 of the tube". Which tube do you refer to? And should this be done to each tube? (four 10K pots adjusting bias for each tube, or will one do the trick?)
I really appreciate your help and that you don't think you're wasting your time on giving advice to beginners like me. I signed up for an electronics class at the local community college here in Portland, but a week before the class was supposed to start I got an email from the school that said the class was cancelled due to there not being enough people enrolled. So...Everything I have learned has been from books, advice from awesome people like yourselves on electronics forums, and just by diving right in and making stuff. Anyways.....thanks again...

[Satamax]

[flying snake]

Amp Kat,
Based on your suggestion I paid a visit to Radio Shack and bought a 10K linear pot and some 10K and 5.6K resistors (they didn't have any 15K). I scratched my head a little & poked around with my multimeter. I found the wires that lead to the terminals in front of each pair of EL34s (the bias leads). I rigged-up the pot like you said and it worked! The right pair of EL34s are now running at a much healthier voltage (-41) with some room to spare for going into the -30's. The left pair of EL34s came down to -50 from -60 (All pin #5 voltage readings were at -60 for all 4 EL34s before I rigged-up the bias pot). So I am going to make a bias pot on each side, one for each pair of power tubes.
Like I said...I totally appreciate your help on this! If you were here I would by you a beer! Now to troubleshoot those dang tone pots.....
Thanks again! This forum is awesome!

[Zouto]

Originally Posted by Satamax

Hi Satamax,
I have a similar problem with a DR103: the bias Voltage after the UF4007 is only -46V.The 1K resistor is not fitted.So the bias is too hot.Is there any layout/wiring description of the voltage doubler circuit using the original small tag strip?http://mhuss.com/Hiwatt/Schematics/BiasCkts.gif
Thanks a lot
Zouto