eth2 Posted May 7, 2016 Share Posted May 7, 2016 (edited) Before I start let me apologize for my complete lack of technical expertise. I am trying to bias my VRD's. On the right amp, with the Gardner Denver Digital multimeter model GDT-311 set at 20 volts I get a reading of 55, while on the left amp I get a reading of 0.00. When I switch the meter to 200 ohms (on the amp with the 0.00 reading at 20 volts), I get a reading of 10.6. Therefore, I know it is not the pots or the meter that are bad. I have tried to bias the amp with the 0.00 reading and turning the bias screw does nothing. However, both amps sound fine. What is going on? I have also emailed Craig. Edited May 7, 2016 by eth2 Quote Link to comment Share on other sites More sharing options...
Wolfbane Posted May 7, 2016 Share Posted May 7, 2016 Before I start let me apologize for my complete lack of technical expertise. I am trying to bias my VRD's. On the right amp, with the Gardner Denver Digital multimeter model GDT-311 set at 20 volts I get a reading of 69, while on the left amp I get a reading of 0.00. When I switch the meter to 200 ohms (on the amp with the 0.00 reading at 20 volts), I get a reading of 10.6. Therefore, I know it is not the pots or the meter that are bad. I have tried to bias the amp with the 0.00 reading and turning the bias screw does nothing. However, both amps sound fine. What is going on? I have also emailed Craig. Bad connection between the meter and the bias measurement pot on one channel perhaps? Have you tried cleaning the contact points? Quote Link to comment Share on other sites More sharing options...
eth2 Posted May 7, 2016 Author Share Posted May 7, 2016 I will right now. If the biasing pots were not making contact, why would I get a reading at the 200 ohm setting? Thanks Quote Link to comment Share on other sites More sharing options...
mark1101 Posted May 7, 2016 Share Posted May 7, 2016 You will be measuring DC voltage. Set the meter accordingly. Red lead into the bias pot and black lead on the chassis. Quote Link to comment Share on other sites More sharing options...
Wolfbane Posted May 7, 2016 Share Posted May 7, 2016 I will right now. If the biasing pots were not making contact, why would I get a reading at the 200 ohm setting? Thanks More sensitive to a poor connection perhaps? I know very little of electricity other than to fear it (learned that at age 5 while trying to pry a piece of toast stuck in an old toaster). Quote Link to comment Share on other sites More sharing options...
eth2 Posted May 7, 2016 Author Share Posted May 7, 2016 (edited) The biasing pots are shinny and I took a pin and cleaned them anyway. No change. The 10.6 reading at 200 ohms may tell us something. I have very expensive tubes in there and don't want to burn them out. Edited May 7, 2016 by eth2 Quote Link to comment Share on other sites More sharing options...
USNRET Posted May 7, 2016 Share Posted May 7, 2016 (edited) Earl, perhaps we are saying the same thing but you clean the Bias Test Point not the pot. Craig will be along but it sounds to me that the connection (wire) to Test Point is bad (assuming meter set to DC voltage at correct scale); do not adjust pot without a valid reading. I assume other amp reads correctly. What reading do you get if the meter is connected to both test point (not one lead to ground)? Edited May 7, 2016 by USNRET Quote Link to comment Share on other sites More sharing options...
mark1101 Posted May 7, 2016 Share Posted May 7, 2016 (edited) By the way........Turn the music off. Preamp volume all the way down. Set the meter for the lowest DC scale since you are measuring less than a volt. Insert the black lead into the ground test point, and insert the red into one of the tube test points while watching the meter. You will see when contact is made. Set the bias to the proper setting for your amps. Might be .55 volts DC or up to .70. Repeat for second tube. Check each tube a couple of times as changing the bias on one tube affects the other. Basically, dial them both in one after another. This procedure should be done immediately when you connect a new pair of amps to get them into a safe ballpark. Then, once the amps are good and warm after a couple of hours, check them again and readjust. Edited May 7, 2016 by mark1101 1 Quote Link to comment Share on other sites More sharing options...
mark1101 Posted May 7, 2016 Share Posted May 7, 2016 The 10.6 reading at 200 ohms may tell us something. Yes..........it tells us your meter is set wrong. Quote Link to comment Share on other sites More sharing options...
USNRET Posted May 7, 2016 Share Posted May 7, 2016 On the right amp, with the Gardner Denver Digital multimeter model GDT-311 set at 20 volts I get a reading of 55, while on the left amp I get a reading of 0.00. I just re-re-read your OP. So one amp reads correctly on both tubes but the other amp reads zero volts DC on both tubes? Quote Link to comment Share on other sites More sharing options...
eth2 Posted May 7, 2016 Author Share Posted May 7, 2016 do not adjust pot without a valid reading Now you tell me...I have it so screwed up I have no idea where it should be. I will not turn it on again, however. There is a lot to be said for solid state. I have a 1975 Pioneer receiver that I bought in college, never had serviced, ran the hell out of, and it still sounds like it did when I bought it. (Pioneer SX550) Quote Link to comment Share on other sites More sharing options...
eth2 Posted May 7, 2016 Author Share Posted May 7, 2016 On the right amp, with the Gardner Denver Digital multimeter model GDT-311 set at 20 volts I get a reading of 55, while on the left amp I get a reading of 0.00.I just re-re-read your OP. So one amp reads correctly on both tubes but the other amp reads zero volts DC on both tubes? I am not sure what you mean by "both tubes." the bias points only measure the amp in its entirety, correct? Quote Link to comment Share on other sites More sharing options...
mark1101 Posted May 7, 2016 Share Posted May 7, 2016 (edited) I looked at a picture of your meter. Do you have it set at about 8 o'clock.......20V DC? That's where it should be. Just turn on the preamp and only 1 VRD and adjust each output tube to .55. The black lead stays in teh ground test point and the red lead gets moved from the left tube test point to the right tube test point to make the adjustments. Edited May 7, 2016 by mark1101 1 Quote Link to comment Share on other sites More sharing options...
A1UC Posted May 7, 2016 Share Posted May 7, 2016 (edited) do not adjust pot without a valid reading Now you tell me...I have it so screwed up I have no idea where it should be. I will not turn it on again, however. There is a lot to be said for solid state. I have a 1975 Pioneer receiver that I bought in college, never had serviced, ran the hell out of, and it still sounds like it did when I bought it. (Pioneer SX550) McIntosh MA-8000 http://www.stevehuffphoto.com/all-reviews/hi-fi-reviews/the-monster-mcintosh-ma-8000-integrated-amp-review/ Edited May 7, 2016 by A1UC Quote Link to comment Share on other sites More sharing options...
Wolfbane Posted May 7, 2016 Share Posted May 7, 2016 You can do this. Can you post a picture of your meter? With the settings you have selected. Quote Link to comment Share on other sites More sharing options...
NOSValves Posted May 7, 2016 Share Posted May 7, 2016 Earl...in the future I would suggest you wait more then 10 seconds for an answer from me via email or just give me a call... Asking for technical support on a public forum could result in some real bad information... So others benefit from this thread the chances are very good that Earl has a dead output tube.In most cases this means the output tube had a defective internal solder connection that shorted open which causes the no current condition. Infant mortality...its too bad this rarely seems to happen in the 6 month warranty period from the tube manufacturers... 1 Quote Link to comment Share on other sites More sharing options...
eth2 Posted May 7, 2016 Author Share Posted May 7, 2016 Sorry, Craig. I didn't mean to seem impatient. My bad. I thought perhaps I had set the meter incorrectly or had done something stupid which would have not required your knowledge of the amp. In any event, thank you for responding. I will swap the tubes with the other VRD and follow your advice about the KT88's if the swap fixes the problem Quote Link to comment Share on other sites More sharing options...
NOSValves Posted May 7, 2016 Share Posted May 7, 2016 If the meter was setup wrong you would to be able to read any of the tubes. Before you proceed give me a call 810-287-6135 I need a bit of clarification. Quote Link to comment Share on other sites More sharing options...
mark1101 Posted May 7, 2016 Share Posted May 7, 2016 OK, so what is the verdict? Quote Link to comment Share on other sites More sharing options...
eth2 Posted May 7, 2016 Author Share Posted May 7, 2016 Two issues. First, I was not setting the meter up correctly, and secondly, I have a bad KT150. At Craig's direction I replaced the KT 150's with a quad of SED C's I had. The VRD's sound much better with the KT88's (even when the K150's were working. Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.
Note: Your post will require moderator approval before it will be visible.