Members fiveightandten Posted December 12, 2006 Members Share Posted December 12, 2006 I have a SF Bandmaster head that i'm converting to blackfaced circuit and I got a Zener diode to drop the B+ down a bit. Put it in the amp, and the B+ is right on the money now. Problem is...the neg voltage is acting weird. Before the diode installation, I had it at a steady -47V or so. Now...it's showing a steady -47V when in standby, but when I flip the standby switch it jumps way down to -75V. The tubes are practically in cut-off at that point. Is this normal when you lift the HT CT with a Zener, or is something going on with this thing? Also, FYI, I replaced the 220K bias feed R's with new 220K's, and they check out fine. The neg voltage drop happens all the way down the line back to the bias circuit. Any ideas? -Nick Link to comment Share on other sites More sharing options...
Members Locky Posted December 12, 2006 Members Share Posted December 12, 2006 Hmmm are you sure about the Zener's current ratings. It sounds like you may have fried it, and its now a short. Explain your layout a little more and I might be able to help. Link to comment Share on other sites More sharing options...
Members fiveightandten Posted December 12, 2006 Author Members Share Posted December 12, 2006 Originally posted by Locky Hmmm are you sure about the Zener's current ratings. It sounds like you may have fried it, and its now a short. Explain your layout a little more and I might be able to help. Here's the schematic:http://www.ampwares.com/ffg/schem/bandmaster_ab763_schem.gif Without the Zener in place, the amp functions perfectly. Everything matches that schematic to the T. Actually, I called Dan Torres (where I ordered the Zener from), and he reminded me that the bias tap comes off the HT winding....they have the same CT. I didn't notice this on the schematic before. He says it's not a problem and to just bias the amp up according to the neg voltage I see when the B+ is engaged. The only thing i'm worried about is the tubes seeing a huge spike in bias before the neg voltage stabilizes after the standby switch gets flipped. He seems to think that the tubes won't react fast enough for it to be an issue at all. But it makes me a little unseasy. The only other recommendation he had for me was to completely re-build the bias supply so it just borrows off the B+ supply, instead of using the bias tap. I'd be surprised if the Zener got burned out. I had a 30W iron on it for only a few seconds, and I used some heat-sink tweezers. I made sure to solder it before I bolted it in so I didn't have any issues with it sucking up heat from the iron. And as far as the current ratings go, it's a 50W diode (no current rating listed though). I'm inclined to agree that it's due to the bias tap sharing a CT with the HT taps. Any ideas? Is that voltage change safe, or should I rebuild the bias supply? If I bias it up to normal, i'll probably be seeing close to -20V while the amp is in standby, and in the high -40's when the switch gets flipped. I'll more than likely have to change the bias resistor to get it in the right range. Thanks, -Nick Link to comment Share on other sites More sharing options...
Members fiveightandten Posted December 13, 2006 Author Members Share Posted December 13, 2006 Originally posted by Roccaforte Amps I never liked this scheme. there are better ways to lower your voltage. Regarding bias. As long as its stable when you're off standby,that's all that matters.If you're setting this amp up blackface, and provided you copy the blackface bias circuitry, the 15K (or 27K) resistor that mounts on your bias pot sets the maximum potential of the supply. You could either lower this value until you get the voltage you're looking for, which BTW should be when the bias cotrol is rotated to its lowest setting. (should never go to zero).Or, you could completely revamp the supply, and add a trim pot to replace the fixed bias droping resistor(what I do in some cases). Thanks Doug. This amp actually has a bias pot, as opposed to a balance pot like other SF amps i've seen. So I know I can get it in the range I need. My biggest concern was the fact that the neg voltage was jumping so much, I had never seen an amp do that before. Though...I've also never put a Zener in an amp with a bias tap that shares a CT with the HT winding. I was forseeing the tubes popping if they were seeing, say, -20V for a moment while the bias stabilized (probably accompanied by a nice voltage spike as well). I guess i'll get the neg voltage in the right range so I can get the bias set optimally and call it a day. I have a few 10K trimmers kicking around, but I may just use a fixed resistor in place of that 15K as the amp isn't mine and i'm not sure who's going to be under the hood next. If there aren't going to be any issues with the neg voltage jumping like that, I see no need to reuild the bias circuit to borrow voltage from the HT output. Correct, No? -Nick Link to comment Share on other sites More sharing options...
Members Negative K3 fan Posted December 13, 2006 Members Share Posted December 13, 2006 Originally posted by fiveightandten Is this normal when you lift the HT CT with a Zener, or is something going on with this thing? there you have it, zener's between the HT CT and ground mess stuff up. I used some zeners in order to lower the B+ and i put them right after the rectifier... Link to comment Share on other sites More sharing options...
Members fiveightandten Posted December 13, 2006 Author Members Share Posted December 13, 2006 Originally posted by Negative K3 fan there you have it, zener's between the HT CT and ground mess stuff up. I used some zeners in order to lower the B+ and i put them right after the rectifier... Hmm...You just tie the output of the rectifier to the Zener, and run a lead to the B+ rail (grounding the body of the Zener) or do you put it in series (not grounding the body of the Zener)? I thought they had to be placed at the CT. -Nick Link to comment Share on other sites More sharing options...
Members fiveightandten Posted December 14, 2006 Author Members Share Posted December 14, 2006 bump Link to comment Share on other sites More sharing options...
Members tommythelurker Posted December 14, 2006 Members Share Posted December 14, 2006 Do you know what a diode does? If so, think for a minute and the answer should be obvious Link to comment Share on other sites More sharing options...
Members CliffC8488 Posted December 14, 2006 Members Share Posted December 14, 2006 Are you saying you lifted the transformer center-tap and put a zener between it and ground? If so, that's not a good way to do it. You should put it in series with the standby switch. CC Link to comment Share on other sites More sharing options...
Members tommythelurker Posted December 14, 2006 Members Share Posted December 14, 2006 Originally posted by CliffC8488 Are you saying you lifted the transformer center-tap and put a zener between it and ground? If so, that's not a good way to do it. You should put it in series with the standby switch. CC aw man, you gave away the answer Link to comment Share on other sites More sharing options...
Members fiveightandten Posted December 14, 2006 Author Members Share Posted December 14, 2006 Originally posted by tommythelurker Do you know what a diode does? If so, think for a minute and the answer should be obvious A diode is an component that only lets electricity flow in one direction. The reason why I was hesitant about it being in series is that this diode is specifically designed to mount to the chassis. There's only one lug, the other end is a threaded bolt, and it comes with a nut and a lock washer. Here's the part in question: http://torresengineering.stores.yahoo.net/newtechnology.html I installed it exactly how I was supposed to: -Nick Link to comment Share on other sites More sharing options...
Members fiveightandten Posted December 14, 2006 Author Members Share Posted December 14, 2006 Originally posted by CliffC8488 Are you saying you lifted the transformer center-tap and put a zener between it and ground? If so, that's not a good way to do it.You should put it in series with the standby switch.CC Thanks for the reply, but i'm not sure how I can wire it up like that with this part. -Nick Link to comment Share on other sites More sharing options...
Members tommythelurker Posted December 14, 2006 Members Share Posted December 14, 2006 Originally posted by fiveightandten A diode is an component that only lets electricity flow in one direction. It blocks the current until a certain threashold voltage is reached, then allows current to flow while maintaining that voltage. The voltage of your zener is what, around 50V? If you put that from B+ (which typically runs a few hundred volts) to ground, what's going to happen? The reason why I was hesitant about it being in series is that this diode is specifically designed to mount to the chassis. There's only one lug, the other end is a threaded bolt, and it comes with a nut and a lock washer. Ok, I can understand your hesitation. It's just a physical thing though. You could bolt a solder lug to it and connect it in series that way. Just make sure it doesn't touch ground. I installed it exactly how I was supposed to: To get back to your original question, it's understandable that it would affect the bias which would need to be adjusted, but I'm not seeing why it would suddenly change when you switch it off standby. That seems very strange to me. Link to comment Share on other sites More sharing options...
Members fiveightandten Posted December 14, 2006 Author Members Share Posted December 14, 2006 Originally posted by tommythelurker It blocks the current until a certain threashold voltage is reached, then allows current to flow while maintaining that voltage. The voltage of your zener is what, around 50V? If you put that from B+ (which typically runs a few hundred volts) to ground, what's going to happen? It's a 50V Zener, correct. Isn't the voltage at the CT supposed to be 0? I guess I don't know the answer to your question. Admittedly, i'm blindly following the directions I got with the part, without knowing the electronics theory behind how it works. Originally posted by tommythelurker Ok, I can understand your hesitation. It's just a physical thing though. You could bolt a solder lug to it and connect it in series that way. Just make sure it doesn't touch ground. I just don't have any way of keeping it off the chassis. It's not a small part. I guess i'd have to get creative. The other thing is that I obviously already drilled the chassis for it. Originally posted by tommythelurker To get back to your original question, it's understandable that it would affect the bias which would need to be adjusted, but I'm not seeing why it would suddenly change when you switch it off standby. That seems very strange to me. When I talked to Dan Torres, he said that was normal and they've never had a problem with it...as long as the neg voltage stays steady while the B+ is engaged, the amp is fine. I swapped the bias resistor out to get it back in the right range and the amp seems to be functioning fine other than the fact that the neg voltage shifts a good 20V when I flip the switch. I've gone back and forth with it, and it does the same thing every time. No arching in the tubes, no big pops. Though I haven't hooked it up to something faster (O-scope) to see if i'm getting any large voltage surges, it seems fine. No redplating, no flashes or anything. -Nick Link to comment Share on other sites More sharing options...
Members tlp123 Posted December 14, 2006 Members Share Posted December 14, 2006 Forget the yanks. Contact Kevin O'Conner @ http://www.londonpower.com/ His work and solutions to many more things regarding tube amps is perhaps far, far more than could be articulated by anyone on this forum. His info is superior. And he uses no bull{censored} cliches' or "gobble gobble babble" in a stupid signature. Link to comment Share on other sites More sharing options...
Members tommythelurker Posted December 14, 2006 Members Share Posted December 14, 2006 Originally posted by fiveightandten It's a 50V Zener, correct. Isn't the voltage at the CT supposed to be 0? I guess I don't know the answer to your question. Admittedly, i'm blindly following the directions I got with the part, without knowing the electronics theory behind how it works. Normally the CT is at 0 (connected to ground). But that's why you're putting the zener in there, to drop 50V between ground and the center tap, to lower the B+ by 50V. The answer to my question is that, with the diode connected between B+ and ground, the diode will be shut off until the B+ charges up to 50V, then turn on and shunt all current to ground, effectively reducing your B+ from 400V or whatever to 50V. That's a pretty steep drop and the tubes probably wouldn't function at that voltage, if the diode and other power supply components could even stand up to the current. If you put it in series with the standby switch, the B+ to the tubes will always be 50V lower than the supply voltage since you need that 50V for the diode to turn on and allow the current to flow. I just thought of a possible explanation for what you're seeing. If the bias circuit draws more current than the B+ circuit in standby, that would cause exactly what you are seeing. When you take the amp out of standby, the B+ current jumps and the zener kicks in dropping the voltage. I don't know what the bias current is without measuring it though. It would make me a little nervous, but to be honest I'm not sure if it's really that big of a problem or not. I certainly wouldn't expect arcing or redplating, the voltage isn't any higher and the current surge would be too short to heat up the plates that much. Might reduce the tube life a little bit but the difference could be too little to even notice. Link to comment Share on other sites More sharing options...
Members CliffC8488 Posted December 14, 2006 Members Share Posted December 14, 2006 The reason the bias jumps when you go out of standby is because the zener changes from forward to reverse biased. In standby the current is going from the zener's anode to its cathode. Flip the switch and the B+ draws current from the anode and the center tap drops 50 volts. Eventually the bias should stabilize though. And since it's a negative going transient I wouldn't worry about it. CC Link to comment Share on other sites More sharing options...
Members fiveightandten Posted December 14, 2006 Author Members Share Posted December 14, 2006 Originally posted by tommythelurker Normally the CT is at 0 (connected to ground). But that's why you're putting the zener in there, to drop 50V between ground and the center tap, to lower the B+ by 50V.The answer to my question is that, with the diode connected between B+ and ground, the diode will be shut off until the B+ charges up to 50V, then turn on and shunt all current to ground, effectively reducing your B+ from 400V or whatever to 50V. That's a pretty steep drop and the tubes probably wouldn't function at that voltage, if the diode and other power supply components could even stand up to the current.If you put it in series with the standby switch, the B+ to the tubes will always be 50V lower than the supply voltage since you need that 50V for the diode to turn on and allow the current to flow.I just thought of a possible explanation for what you're seeing. If the bias circuit draws more current than the B+ circuit in standby, that would cause exactly what you are seeing. When you take the amp out of standby, the B+ current jumps and the zener kicks in dropping the voltage. I don't know what the bias current is without measuring it though.It would make me a little nervous, but to be honest I'm not sure if it's really that big of a problem or not. I certainly wouldn't expect arcing or redplating, the voltage isn't any higher and the current surge would be too short to heat up the plates that much. Might reduce the tube life a little bit but the difference could be too little to even notice. Ahh, I see. I understand now. Thanks, I appreciate you taking the time to explain things a little so I can see what's going on with it. Makes sense. At any rate. I'm just worried about weather or not it will be a relability issue. The amp isn't mine, i'm doing the work for a friend who wanted me to blackface it and recap it. I'm not a stranger to being inside an amp, i've built a few and I actually have quite a bit of experience from fixing amps for alocal shop around here for a while. I do very clean work, but admittedly, there are areas of electronics theory that i'm completely ignorant to. Thus is the case here, and I thought it would be a simple case of buying a part that does what I need it to do, and following the directions that came with it. So I guess my options are to either think of a clever way to get the thing off the chassis and keep it nice and secure (it won't be pretty, I know that), or completely rebuild the bias supply so it's not using the bias tap, or leave it the way it is. If it's not going to be a reliability issue, i'm inclined to leave it instead of changing the amp even more if it's un-needed. -Nick Link to comment Share on other sites More sharing options...
Members tommythelurker Posted December 14, 2006 Members Share Posted December 14, 2006 Originally posted by CliffC8488 The reason the bias jumps when you go out of standby is because the zener changes from forward to reverse biased.In standby the current is going from the zener's anode to its cathode. Flip the switch and the B+ draws current from the anode and the center tap drops 50 volts. That's exactly what I was thinking, and would mean that the bias circuit was drawing more current than the B+ in standby. That's the only thing I can think of that would cause the zener to be forward biased. Link to comment Share on other sites More sharing options...
Members CliffC8488 Posted December 14, 2006 Members Share Posted December 14, 2006 The B+ will draw very little in standby. The bias will draw more causing the CT to be one diode drop above ground. Out of standby full current goes through the zener and pulls it 50V below ground so you get a net 51V or so delta on the CT. CC Link to comment Share on other sites More sharing options...
Members fiveightandten Posted December 14, 2006 Author Members Share Posted December 14, 2006 Originally posted by CliffC8488 The reason the bias jumps when you go out of standby is because the zener changes from forward to reverse biased.In standby the current is going from the zener's anode to its cathode. Flip the switch and the B+ draws current from the anode and the center tap drops 50 volts.Eventually the bias should stabilize though. And since it's a negative going transient I wouldn't worry about it.CC I see. Will this damage the Zener over time? -Nick Link to comment Share on other sites More sharing options...
Members CliffC8488 Posted December 14, 2006 Members Share Posted December 14, 2006 Originally posted by fiveightandten I see. Will this damage the Zener over time? -Nick No. The only thing that will kill a zener is too much current. How many watts is it rated at? CC Link to comment Share on other sites More sharing options...
Members tommythelurker Posted December 14, 2006 Members Share Posted December 14, 2006 Originally posted by CliffC8488 The B+ will draw very little in standby. The bias will draw more causing the CT to be one diode drop above ground. Out of standby full current goes through the zener and pulls it 50V below ground so you get a net 51V or so delta on the CT. CC Ya, I know the B+ current is small in standby, but I thought the bias current would be pretty small too. Surprises me a little that the bias current is more, but I can accept it Originally posted by fiveightandten I see. Will this damage the Zener over time? Naw, normal operation for a zener. Link to comment Share on other sites More sharing options...
Members fiveightandten Posted December 14, 2006 Author Members Share Posted December 14, 2006 Originally posted by CliffC8488 The B+ will draw very little in standby. The bias will draw more causing the CT to be one diode drop above ground.Out of standby full current goes through the zener and pulls it 50V below ground so you get a net 51V or so delta on the CT.CC Hmm, I see...The first filtering section (2x80mfd in series) is before the standby switch, but that's about it. Right now, i'm getting about -23V off the bias supply while in standby...and about -47V after the switch is flipped. -Nick Link to comment Share on other sites More sharing options...
Members fiveightandten Posted December 14, 2006 Author Members Share Posted December 14, 2006 Originally posted by tommythelurker Naw, normal operation for a zener. If it's not going to hurt the tubes, and it's not going to hrt the amp, and the Zener won't self-destruct, I think i'm just going to leave it. That amp is wired up so neatly right now and sounds great. Thanks for all your help by the way...all of you. -Nick Link to comment Share on other sites More sharing options...
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