| Posted by Lydia | January 30, 2012 - 03:35 am |
| Hi Dave, Another question is , how often do you have to bias one's new 6BQ5's? I had spoke to a technician that owns and operates a repair shop for dynaco and other hi fi components for customers, and he said quote: "Why would you want fixed bias in any form!" He said I would have to adjust the bias trimmer pots almost every time I turned it on.... I totally understand the reasoning behind EFB, but he said that it's more hastle than it's worth. He also said that even if you get N.O.S. output tubes one doesn't know the tubes capable output on various conditions and the possible variation in component construction of the tubes themselves. He also implied that you would have to monitor your settings constantly even if you get matched tubes....He said auto-biasing is the way to go!. I don't agree with him because that is what the adjustible regulator's job is to keep current 27mA , going to the tubes the same no matter what the voltage swing is right? anyway I would love to hear your comments on this one! thanks Dave, Lydia |
| Posted by Dave | January 29, 2012 - 11:25 pm |
| Hi Lydia -- I see that Art and Steve have done a great job of helping you with your questions in my absence. I hope their answers resolved the questions you had. If I can be of any assistance with your project, please don't hesitate to ask! Thanks for your interest in the EFB modification. Dave |
| Posted by Steve L. | January 29, 2012 - 08:32 am |
| Hi Lydia, I guess instead of "cathode resistors," you mean grid resistors. These 470K resistors go to ground in the original Dynaco circuit. For the mod, they are disconnected from ground and connected to the wipers of the 2K pots shown in Dave's Fig.10). A good way to do this is to remove the 470K resistors from the Dynaco circuit board and put them (preferably new ones) in your mod board, with the pots. Wire the ends of the resistors to points "8" and "9" on the Dynaco circuit board. Please be very careful about the dangerous high voltage. If the unit has been on and you need to work on it, discharge the filter caps, after powering down. |
| Posted by Lydia | January 29, 2012 - 03:06 am |
| Thanks, Steve! this is starting to make sense now! I appreciate that!. I am a beginner at Hi Fi designing/building, however that's what I got my Degree in Design/Development; so I hope I can do this....The only other question, I have is when I hook up the cathode resistors to the EFB trimmers, I am a little fuzzy on how to wire this, even though I have read the articles, I guess once I get into it, I will go as far as I can and get help by showing examples also to other DIY members that have already built their assemblies and so forth. Again thanks. Lydia |
| Posted by Steve L. | January 28, 2012 - 09:57 am |
| Hi Lydia, Well, Art thought that I should give this to Dave and he can't get to it for a few days, so I will answer. Let's refer to Dave's Fig.10 on the last page of his article ("A New Look..."). The LM337 is a negative regulator. Normally, the IN terminal is at a negative voltage and the ADJ terminal is at a negative reference voltage. The job of the regulator is to keep OUT about 1.2V more negative than ADJ. So OUT "follows" ADJ in voltage, but offset by 1.2V. You could think of the LM337 as a buffer, which lets a weak voltage at ADJ, control the OUT pin. In this circuit, ground is the most negative voltage, so IN is connected to that. The positive supply, through the output tubes, is the load on the OUT of the LM337. The chip will make OUT follow the voltage on the ADJ terminal (offset by 1.2V), as long as the ground is more negative. It will be more negative, because the positive supply, through output tubes, is trying to pull OUT positive. So the voltage set by the 5K pot will set the cathode voltage. To implement the individual tube adjustments, Dave has inserted the 2K pots, which have a small amount of voltage (maybe 1V) across them. This let's you tweak the grid voltages enough to balance the individual tubes. Mostly, the bias is set by the 5K pot, though. Note that the tubes draw almost no current from the wipers of the 2K pots. Thus, the voltage across the pots does not change with the pot settings. This means that the pots cannot interact. We will be happy to help if you have further questions. |
| Posted by Lydia Pawlak | January 27, 2012 - 07:13 am |
| Hi Art, thanks for the info "very detailed as needed" but I want to implement Dave's circuit for the biasing of each channel also. look at the article in dave's and see the #12 modification and see that you can add after the 13k resistor; a parallel (2) 2k trimmer pots to control biasing to a pair each channel 6BQ5's. I am concerned about this circuit because for one there is no voltage/current going into the input of the LM337 Regulator, and the parallel after the 13K resistor with the (2) 2K Trimmers that, if you change the bias on one channel, then it will affect the other at the same time?.... Can you elaborate and help me to understand this further.... I would appreciate that, Lydia |