Scott LK-48-B Bias Voltage Help
- Thread starter njcanuck
- Start date
kvflyer
Super Member
Not sure that is correct. Higher bias would be higher negative. Maybe that is what you mean?
s-petersen
Scott
yes,more negative
njcanuck
aka: MacGyver
I don’t have a definitive document with the LK-48-B bias current spec but there are several threads here on AK that reference .23mA to .24mA per tube for the 7189 based 222 and LK-48 series amps. One general info document references .44mA per channel for the LK-48-B. I settled on .23mA for test purposes.
Yes, there are now two 10Ω cathode resistors per channel where there used to be one. They are not truly in parallel as there is no longer a direct connection between the cathodes. (I forgot to show that on the schematic but will try to correct it). Since these changes only affected the pin 3 cathodes, I didn’t think they would impact the bias voltage at pin 2. I’m willing to bet that I’m wrong about that, though
(remember, I’m new at this :yes
. I lifted legs at one tube and checked the 330K, 390K and 2.2K resistors. They were all slightly high by 2% to 5%. Resistors on the other three tubes all measure very close in-circuit to the first in-circuit so I’m working under the assumption that they are within tolerance as well. Plate voltage at pin 7 measures 390.5V (can’t find ref on schematic) and screen voltage at pin 9 measures 379.5V (schematic = 398V). I believe Scott’s tolerance on these is 15% so should be ok?
Still biasing nicely with 50K in place of the 150K at the bias pots. Can someone confirm that the 10Ω cathode mod would be responsible for this?
s-petersen
Scott
If 23 Ma is correct for a tube ,then everything seems right,I was thinking if it was supposed to have 23 per pair,then then the 50k would make more sense to me,I'm just looking at the possibilities for the radical change.I'm stumped. The cathode resistor change would not change the bias voltage,but would change the current measurement,so you are measuring 230mv across the 10 ohm resistors?
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njcanuck
aka: MacGyver
Yes, measuring 230mv across each 10Ω resistor. From everything I've read, 23mA draw at -12V to -16V bias voltage appears to be normal for these amps.
Well if it has you stumped, I consider myself in good company! :yes: Anyone else got a theory? It's working well but it would still be nice to know why. :scratch2:
Tonight I soldered the temporary resistors in place and let it play for a couple of hours. The bias held very steady and she sounded very nice. I guess I'll go ahead and recap the rest of the power supply since I got those. I'm hoping that Sonicaps will go on sale again soon for the coupling caps.
FWIW the old 100uF bias caps were still spot on! This kinda surprised me because on my 299 they were all toast. This amp also has the cleanest looking ceracaps in it that I've ever seen. Gonna be a shame to yank them all.
Thanks for all the help guys. I learn a little more at each step.
I'll keep you posted as it moves along. :thmbsp:
Well if it has you stumped, I consider myself in good company! :yes: Anyone else got a theory? It's working well but it would still be nice to know why. :scratch2:
Tonight I soldered the temporary resistors in place and let it play for a couple of hours. The bias held very steady and she sounded very nice. I guess I'll go ahead and recap the rest of the power supply since I got those. I'm hoping that Sonicaps will go on sale again soon for the coupling caps.
FWIW the old 100uF bias caps were still spot on! This kinda surprised me because on my 299 they were all toast. This amp also has the cleanest looking ceracaps in it that I've ever seen. Gonna be a shame to yank them all.
Thanks for all the help guys. I learn a little more at each step.
I'll keep you posted as it moves along. :thmbsp:
NJ -- From a theory standpoint (and based on the original design), if the factory specification of -48 volts at the top of the bias controls holds true for any setting of any of the bias or balance controls (which it won't, but should be relatively steady for our purposes here), then with both balance controls perfectly centered electrically, and both bias controls full down, then the grid of each output tube should receive -15.87 volts as the minimum possible negative voltage. This voltage could only grow more negative from there with any other position of the bias controls. Of course, the balance controls could make any one tube go less negative than this, but only at the expense of making its mate go more negative.
The interesting point in all of this is that your negative supply voltage is at least equal to or greater than that shown on the schematic by Scott, but you B+ levels are all lower than Scott shows. As I mentioned earlier, this would tend to strike at the heart of the issue.
Replacing the original LV DC supply selenium rectifier with a silicon unit would definitely increased the available negative voltage -- but in this case, the increase is basically just bringing it up to "normal". But your B+ is not normal, and in fact, lower than normal.
So why is the B+ lower? Is your rectifier tube good? Are the B+ caps good? Did you install an inrush current limiter? Are you powering the unit by way of an external device that is lowering the otherwise normally higher AC line voltages that exist today? Any of these measures would result in lowered B+ levels -- and also lowered bias levels as well. But with the replaced LV rectifier package, you have effectively increased the available bias voltage over what it normally would be, and in the process, disturbed the original relationship that existed between the negative bias and B+ supplies.
The condition you have is that your B+ -- and more specifically, your screen grid voltages -- are lower than specified, yet your available bias voltage is at least equal to the bias voltage that was specified for a higher B+ level -- a level that currently does not exist. This scenario creates -- or at least aggravates -- the exact scenario you are dealing with. Therefore, it is quite likely that changing the original 150K resistor to 50K is simply compensating for other changes you have make elsewhere in the build.
If anything, B+ levels today run much higher than spec because of the higher AC line voltages -- yet yours are not even up to the original spec based on a lower AC voltage level. Getting to the bottom of your mystery starts with finding out why your B+ levels are lower than spec in spite of higher AC voltages.
Let us know!
Dave
The interesting point in all of this is that your negative supply voltage is at least equal to or greater than that shown on the schematic by Scott, but you B+ levels are all lower than Scott shows. As I mentioned earlier, this would tend to strike at the heart of the issue.
Replacing the original LV DC supply selenium rectifier with a silicon unit would definitely increased the available negative voltage -- but in this case, the increase is basically just bringing it up to "normal". But your B+ is not normal, and in fact, lower than normal.
So why is the B+ lower? Is your rectifier tube good? Are the B+ caps good? Did you install an inrush current limiter? Are you powering the unit by way of an external device that is lowering the otherwise normally higher AC line voltages that exist today? Any of these measures would result in lowered B+ levels -- and also lowered bias levels as well. But with the replaced LV rectifier package, you have effectively increased the available bias voltage over what it normally would be, and in the process, disturbed the original relationship that existed between the negative bias and B+ supplies.
The condition you have is that your B+ -- and more specifically, your screen grid voltages -- are lower than specified, yet your available bias voltage is at least equal to the bias voltage that was specified for a higher B+ level -- a level that currently does not exist. This scenario creates -- or at least aggravates -- the exact scenario you are dealing with. Therefore, it is quite likely that changing the original 150K resistor to 50K is simply compensating for other changes you have make elsewhere in the build.
If anything, B+ levels today run much higher than spec because of the higher AC line voltages -- yet yours are not even up to the original spec based on a lower AC voltage level. Getting to the bottom of your mystery starts with finding out why your B+ levels are lower than spec in spite of higher AC voltages.
Let us know!
Dave
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njcanuck
aka: MacGyver
Dave, thanks for taking the time to type out that great explanation. I know you mentioned the B+ earlier but I didn't really twig or understand. Now I do! :tresbon:
Ok, so this morning I went troubleshooting down the B+ section. In the interests of full disclosure:
Next, I started tracking voltages through the schematic starting with the power transformer primary (hope I'm going about this the right way?). It didn't take long. BINGO! - the secondaries are only delivering 330V versus the 360V spec - tested at the input pins 4 & 6 of the rectifier. So, of course, that is lowering voltages all the way down the line to the screens. It is the original, correct power transformer - I checked! I also tested the secondaries again with the rectifier and power tubes removed and it rose slightly to 339V. Is it possible that bad caps or other components down the line could be dragging it down? With the rectifier out of circuit, the secondaries look kind of isolated from everything else to me?
I suppose worst case is that the power transformer is starting to fail?
Ok, so this morning I went troubleshooting down the B+ section. In the interests of full disclosure:
- Yes, I am lowering normal AC voltage with a variac. Line averages 121V around here.
- Yes, I have installed inrush current limiters - one CL-80 on each input leg. However, I am measuring my 117V input voltage directly at the primary legs of the power transformer.
- No, I haven't recapped or tested any capacitors in the B+ supply yet.
I stuck the rectifier (GE 5AR4) in the tester and it passed with flying colors. Subbed in another one anyway, just in case. No change.- Yes, I have installed inrush current limiters - one CL-80 on each input leg. However, I am measuring my 117V input voltage directly at the primary legs of the power transformer.
- No, I haven't recapped or tested any capacitors in the B+ supply yet.
Next, I started tracking voltages through the schematic starting with the power transformer primary (hope I'm going about this the right way?). It didn't take long. BINGO! - the secondaries are only delivering 330V versus the 360V spec - tested at the input pins 4 & 6 of the rectifier. So, of course, that is lowering voltages all the way down the line to the screens. It is the original, correct power transformer - I checked! I also tested the secondaries again with the rectifier and power tubes removed and it rose slightly to 339V. Is it possible that bad caps or other components down the line could be dragging it down? With the rectifier out of circuit, the secondaries look kind of isolated from everything else to me?
I suppose worst case is that the power transformer is starting to fail?
I'm sure that your power transformer is just fine. Transformers are something that either work correctly, or generally don't work correctly at all. If they are "in between" these states, they don't stay there for long. They simply run hot, smell, and then burn out and die.
What's more likely is that you are over compensating in your efforts to lower your AC voltage -- which can be just as harmful if too low, as if it is too high.
To best address your issue/question/concern, the first order of business is to determine what is in fact the correct AC voltage for your unit to operate from (as measured at the power transformer's primary winding leads). This is easy enough to determine: With the amplifier operating and properly biased, the correct AC line voltage for the primary winding to operate from is that which produces 6.30 vac to the heater circuit (that is, to those heaters that operate directly from the heater winding), as measured at the point where this winding connects into the heater wiring. Once this is determined, the resulting AC line voltage at the primary that produces the correct heater winding voltage at the secondary should be the target line voltage to operate the amplifier from. With the primary winding operating from the target line voltage, then the B+ level will be what it will be -- again assuming the amplifier is properly biased. If at that point the bias circuit does not allow for a proper bias range to be had -- and/or the DC voltage from the DC heater string is off an acceptable mark (because the LV selenium rectifier has been replaced with a silicon rectifier), then adjustments should be made to the components in each of these circuits (or to any components common to both circuits) to achieve their proper operating levels.
Establishing the proper AC line voltage for your unit -- and operating it from that AC voltage -- is the necessary first step from which then all other adjustments (if necessary) should be made. Then you can feel confident that the changes you are making are the proper and necessary ones to compensate for the new rectifier pack you installed.
Once you determine what the proper AC line voltage is for the primary winding of the power transformer in your unit, then if enough voltage difference exists between what that level is and what your actual line voltage is, you can then determine if any line voltage adjusting device is necessary for your amp. Typically these measures should be considered only if the AC line voltage is more than about 4% high.
Dave
What's more likely is that you are over compensating in your efforts to lower your AC voltage -- which can be just as harmful if too low, as if it is too high.
To best address your issue/question/concern, the first order of business is to determine what is in fact the correct AC voltage for your unit to operate from (as measured at the power transformer's primary winding leads). This is easy enough to determine: With the amplifier operating and properly biased, the correct AC line voltage for the primary winding to operate from is that which produces 6.30 vac to the heater circuit (that is, to those heaters that operate directly from the heater winding), as measured at the point where this winding connects into the heater wiring. Once this is determined, the resulting AC line voltage at the primary that produces the correct heater winding voltage at the secondary should be the target line voltage to operate the amplifier from. With the primary winding operating from the target line voltage, then the B+ level will be what it will be -- again assuming the amplifier is properly biased. If at that point the bias circuit does not allow for a proper bias range to be had -- and/or the DC voltage from the DC heater string is off an acceptable mark (because the LV selenium rectifier has been replaced with a silicon rectifier), then adjustments should be made to the components in each of these circuits (or to any components common to both circuits) to achieve their proper operating levels.
Establishing the proper AC line voltage for your unit -- and operating it from that AC voltage -- is the necessary first step from which then all other adjustments (if necessary) should be made. Then you can feel confident that the changes you are making are the proper and necessary ones to compensate for the new rectifier pack you installed.
Once you determine what the proper AC line voltage is for the primary winding of the power transformer in your unit, then if enough voltage difference exists between what that level is and what your actual line voltage is, you can then determine if any line voltage adjusting device is necessary for your amp. Typically these measures should be considered only if the AC line voltage is more than about 4% high.
Dave
njcanuck
aka: MacGyver
Ok Sensi, I have more data. 
As I wasn't expecting that answer, I proceeded to pull the 4x20uF and restuff it. I know it adds another variable into the mix and wouldn't have done it yet had I known. Seems to have ended well with no sparks and everything operating as before.
With the amp operating: To produce exactly 6.302vac at the heater windings I need 116.7vac on the primary windings. This gives me 326.5vac on the secondary windings. Bias is set at .22mA current draw per tube with bias voltages measuring from -13.7 to -14.4.
Something still isn't right. :scratch2:
I'm really appreciating the time you are taking for me on this!
Rich
As I wasn't expecting that answer, I proceeded to pull the 4x20uF and restuff it. I know it adds another variable into the mix and wouldn't have done it yet had I known. Seems to have ended well with no sparks and everything operating as before.
With the amp operating: To produce exactly 6.302vac at the heater windings I need 116.7vac on the primary windings. This gives me 326.5vac on the secondary windings. Bias is set at .22mA current draw per tube with bias voltages measuring from -13.7 to -14.4.
Something still isn't right. :scratch2:
I'm really appreciating the time you are taking for me on this!
Rich
Rich -- happy to help.
It would appear now however that you are facing misinformation from ol' H.H. himself -- or at least his draftsmen anyway.......
OK. So the nominal voltage to appear at the primary leads of the power transformer is 117 vac. That would certainly seem reasonable for equipment of this age. If your line voltage more than 121 volts, or you just want to target 117 vac closer, then use your in rush current limiter(s) as necessary to achieve that target voltage.
As to the AC voltages listed for the HV supply, one fact you can count on is that a 5AR4/GZ34 rectifier tube (assuming it is true to specification) will have a DC output that is basically 1.25 times the RMS value of the AC voltage applied to its plates in a cap input filter design. Your amplifier deviates ever so slightly from this scenario with a 20 ohm resistor between the rectifier and the first filter cap, but the value of this resistor is so small as to be of no consequence in this exercise.
We know that 117 vac is the target voltage for your amp to operate from, and this voltage will produce 327 vac at the plates of the rectifier tube from your power transformer with the amplifier properly biased. Now 327 vac X 1.25 = 409 vdc -- or a result that is within 1 volt of the main B+ voltage listed on the Scott schematic. Not bad. This implies that with a target line voltage of 117 vac provided to the power transformer, not only will the AC heater voltage be correct, but so will the B+ voltage as well.
But what of that 360 vac listed at the rectifier tube plates? Well 360 vac X 1.25 = 450 vdc, which is much higher than the B+ voltage listed. Still, if you assume that is a correct figure, then to produce the correct B+ voltage, the AC line voltage would need to be lowered by some 10% below the target value, which in turn would lower the AC heater voltage by the same amount, which would not be good for the tube heaters. Therefore, it does not seem plausible that the 360 vac figure is correct.
Running all of these thoughts out then, it is virtually a certainty that the AC voltage listed on the Scott schematic at the plates of the rectifier tube is an error. It would hardly be the first time that a Sams or factory schematic was wrong. As final proof of this, I pulled my own LK-48 out from storage, applied 117.0 vac to the primary, and obtained -- you guessed it -- exactly 327 vac at the rectifier tube plates under load. Therefore, you can correct your schematic accordingly.
Finally, as to the voltage required at the control grids to achieve the correct output tube current draw, if the heater voltage is correct, and the B+ voltage to the plates and screens is correct, then the negative voltage at the control grids will be whatever is required by the particular tubes you are using to produce the target current draw with the target heater and B+ voltage levels in place. That your tubes are requiring -14 vdc on average to produce the correct current draw with these parameters in place now is exactly normal for 6BQ5 class tubes.
In the end then, it still goes back to you changing out the LV selenium bridge rectifier for a modern silicon unit. A good move to be sure, but one that also quite likely will require slight adjustments to the bias and DC heater circuit dropping components if the original heater voltage and bias control range is to be maintained. I would again emphasize that while minor adjustments will be likely, you can have confidence that they will be made most accurately when they are made with the unit operating from the target AC line voltage it was designed to operate from.
Best of luck with your amp!
Dave
It would appear now however that you are facing misinformation from ol' H.H. himself -- or at least his draftsmen anyway.......
OK. So the nominal voltage to appear at the primary leads of the power transformer is 117 vac. That would certainly seem reasonable for equipment of this age. If your line voltage more than 121 volts, or you just want to target 117 vac closer, then use your in rush current limiter(s) as necessary to achieve that target voltage.
As to the AC voltages listed for the HV supply, one fact you can count on is that a 5AR4/GZ34 rectifier tube (assuming it is true to specification) will have a DC output that is basically 1.25 times the RMS value of the AC voltage applied to its plates in a cap input filter design. Your amplifier deviates ever so slightly from this scenario with a 20 ohm resistor between the rectifier and the first filter cap, but the value of this resistor is so small as to be of no consequence in this exercise.
We know that 117 vac is the target voltage for your amp to operate from, and this voltage will produce 327 vac at the plates of the rectifier tube from your power transformer with the amplifier properly biased. Now 327 vac X 1.25 = 409 vdc -- or a result that is within 1 volt of the main B+ voltage listed on the Scott schematic. Not bad. This implies that with a target line voltage of 117 vac provided to the power transformer, not only will the AC heater voltage be correct, but so will the B+ voltage as well.
But what of that 360 vac listed at the rectifier tube plates? Well 360 vac X 1.25 = 450 vdc, which is much higher than the B+ voltage listed. Still, if you assume that is a correct figure, then to produce the correct B+ voltage, the AC line voltage would need to be lowered by some 10% below the target value, which in turn would lower the AC heater voltage by the same amount, which would not be good for the tube heaters. Therefore, it does not seem plausible that the 360 vac figure is correct.
Running all of these thoughts out then, it is virtually a certainty that the AC voltage listed on the Scott schematic at the plates of the rectifier tube is an error. It would hardly be the first time that a Sams or factory schematic was wrong. As final proof of this, I pulled my own LK-48 out from storage, applied 117.0 vac to the primary, and obtained -- you guessed it -- exactly 327 vac at the rectifier tube plates under load. Therefore, you can correct your schematic accordingly.
Finally, as to the voltage required at the control grids to achieve the correct output tube current draw, if the heater voltage is correct, and the B+ voltage to the plates and screens is correct, then the negative voltage at the control grids will be whatever is required by the particular tubes you are using to produce the target current draw with the target heater and B+ voltage levels in place. That your tubes are requiring -14 vdc on average to produce the correct current draw with these parameters in place now is exactly normal for 6BQ5 class tubes.
In the end then, it still goes back to you changing out the LV selenium bridge rectifier for a modern silicon unit. A good move to be sure, but one that also quite likely will require slight adjustments to the bias and DC heater circuit dropping components if the original heater voltage and bias control range is to be maintained. I would again emphasize that while minor adjustments will be likely, you can have confidence that they will be made most accurately when they are made with the unit operating from the target AC line voltage it was designed to operate from.
Best of luck with your amp!
Dave
njcanuck
aka: MacGyver
Hi Dave,
Ok, I'm much more comfortable with this amp, now. I follow the calculations and it all makes sense to me. (I'll have to file this info for future use). NOS Valves has posted many times that you can't take Scott's schematic voltages as gospel - looks like we now have two examples as proof.
I understand what you are saying about the change to the LV silicon rectifier but I now have doubts that this amp was ever biased correctly.
Before changing the rectifier, I installed the 10Ω cathode sensing resistors and tip jacks. At that time, I measured only 0.11mA draw with the bias pots at their limits. I neglected to measure the actual pin 2 bias voltages. I do seem to remember measuring about -39V to the first 12AX7 (versus -46V on the now suspect schematic). You would think that would be low enough to produce a workable bias voltage for the outputs but I now believe that that 150K resistor was keeping the adjustment floor too high to achieve proper bias. Probably been that way since it was built.
When I installed the silicon rectifier, I also changed the first 10Ω dropping resistor to a 33Ω to achieve -44.7V at the first 12AX7. This naturally increased the bias voltage even further, exacerbating the original problem. (For those following: dropping the 150K to 50K brought the adjustment floor down into usable range).
The CL-80s that I installed on the mains input bring the operating voltage right in line with the target 117V (with minor fluctuations due to fluctuating mains voltage).
Thanks again for the mentoring! Onward and upward!
Rich
Ok, I'm much more comfortable with this amp, now. I follow the calculations and it all makes sense to me. (I'll have to file this info for future use). NOS Valves has posted many times that you can't take Scott's schematic voltages as gospel - looks like we now have two examples as proof.
I understand what you are saying about the change to the LV silicon rectifier but I now have doubts that this amp was ever biased correctly.
Before changing the rectifier, I installed the 10Ω cathode sensing resistors and tip jacks. At that time, I measured only 0.11mA draw with the bias pots at their limits. I neglected to measure the actual pin 2 bias voltages. I do seem to remember measuring about -39V to the first 12AX7 (versus -46V on the now suspect schematic). You would think that would be low enough to produce a workable bias voltage for the outputs but I now believe that that 150K resistor was keeping the adjustment floor too high to achieve proper bias. Probably been that way since it was built.
When I installed the silicon rectifier, I also changed the first 10Ω dropping resistor to a 33Ω to achieve -44.7V at the first 12AX7. This naturally increased the bias voltage even further, exacerbating the original problem. (For those following: dropping the 150K to 50K brought the adjustment floor down into usable range).
The CL-80s that I installed on the mains input bring the operating voltage right in line with the target 117V (with minor fluctuations due to fluctuating mains voltage).
Thanks again for the mentoring! Onward and upward!
Rich
NOSValves
Super Member
Hey guys. The restore of my 299 went so well, I decided to tackle my LK-48-B. Of course I hit a hitch right away: I can’t seem to get low enough bias voltage to pin 2 of the 7189s.
First I installed 10Ω cathode resistors and test jacks so that I could easily see how the tubes were biased. To do this, I replaced the common 10Ω 2W resistor on pin 3 of each channel with 2 10Ω 1/4W resistors and clipped the leads to the old bias switch. I immediately noticed that the highest possible bias voltage across the sensing resistors was only about 0.11V (I was expecting 0.20V and higher).
So I went ahead and replaced the selenium rectifier, dropping resistor (put in a 33Ω 5W) and the 100uF bias caps. At 117V input, this gives me -58.1V after the bridge, -44.7 at the first 12AX7 and -48.7V up to the bias pots. However, the lowest that I can dial the pin 2 voltage is -16.5V resulting in only 0.11mA current draw per tube. If my understanding is correct, I want approx 0.23mA draw and need -15V or less to get there. Right?
I’ve attached the relevant part of the schematic with my changes in red and the voltages in blue. I can see two possible approaches to lower the -16.5V at pin 2: a) raise the value of the 10K resistor (R211) to lower the voltage up to the bias pots, or b) lower the value of the bias-pots-to-ground 150K resistor (R214) that I believe will lower the bottom end of the pin 2 range.
Or is there something else going on here? Did I mention that I’m pretty new at this.
Any advice will be much appreciated!
Edit: corrected schematic
The output tube negative control grid voltage on the LK48B and all the last versions of Scott tube amps do not pull ithevoltage of the last section of the heater supply. It has it's own single resistor and cap right off the bridge rectifier so upping the value to 33 ohms will fix the heater voltage but does nothing to the bias supply voltage. I suggest adding a second resistor and another filter cap. You can also lower the value of the ground reference resistor of the actual bias voltage pots to get further low negative voltage adjustment.
Craig
njcanuck
aka: MacGyver
Hi Craig,
Thanks for chiming in. On my schematic and in my amp the bias supply voltage does have its own resistor and cap but its voltage tap is directly after the first 10 ohm dropping resistor (ie, not directly off the bridge). Upping that 10 to a 33 did lower both the heater and bias voltages. Another Scott variation? I still needed to lower the bias pot ground ref resistor to get the final voltage low enough.
Cheers,
Rich
Thanks for chiming in. On my schematic and in my amp the bias supply voltage does have its own resistor and cap but its voltage tap is directly after the first 10 ohm dropping resistor (ie, not directly off the bridge). Upping that 10 to a 33 did lower both the heater and bias voltages. Another Scott variation? I still needed to lower the bias pot ground ref resistor to get the final voltage low enough.
Cheers,
Rich
NOSValves
Super Member
Yea they made all these amps in different versions its almost mind boggle ling to remember all the variations... but regardless the drop experienced is not enough for the bias supply with just 33 ohms.. add a second resistor step and cap to get the max down (this also will stabilize the bias voltage and clean it up better, I'd guesstimate about 3 to 5k should do it(I know it may sounds like allot but the bias circuit draws very little current so it takes more resistance to drop the voltage Ohm's law baby) if it doesn't drop it enough then adjust that reference to ground resistor on the bias voltage pots downward in resistance to get the bottom voltage when the pots are turned all the way down to -12V the top voltage should be about -22V...this can all be tweaked with the rectifier removed so the amp has no high voltage present. Yes you need all other tubes in place to create the current draw.
Rich -- Within a specific tube type, the bias requirements can vary as much as +/- 20% from one tube to the next. Buying matched tubes (in part) pairs tubes together with similar bias requirements. This is also the reason many tube vendors who sell the musical instrument crowd "grade" their tubes by the bias they require, with tubes requiring less bias showing more gain (and therefore and easier overdrive ability), while tubes requiring more bias are harder to overdrive.
My LK-48 came from the factory with Telefunken 7189 output tubes, which are still installed in it. It is my experience however, that these tubes require more bias than other tubes in the 6BQ5/7189 family do to operate at the correct bias point. Therefore, it may just be that Scott developed their bias circuit around these particular tubes, since they were what was being supplied with their product. Using tubes in the same class with more conventional biasing requirements then could very well require an alteration to the bias circuit -- whether the bridge rectifier was changed out or not.
Dave
My LK-48 came from the factory with Telefunken 7189 output tubes, which are still installed in it. It is my experience however, that these tubes require more bias than other tubes in the 6BQ5/7189 family do to operate at the correct bias point. Therefore, it may just be that Scott developed their bias circuit around these particular tubes, since they were what was being supplied with their product. Using tubes in the same class with more conventional biasing requirements then could very well require an alteration to the bias circuit -- whether the bridge rectifier was changed out or not.
Dave
njcanuck
aka: MacGyver
Craig: Is there any particular reason to add a second resistor and cap versus simply upping the resistor and cap that are already there? Ie: raise the 10K to 15K and the cap from 50uF to say, 100uF. (It biases ok the way I have it setup now but the range is a bit compressed at -12.3V to -18.5V).
Dave: I think the only way to verify that theory is for you to send me your Telefunkins!
Dave: I think the only way to verify that theory is for you to send me your Telefunkins!
NOSValves
Super Member
I'm confused what happened to the the lowest negative voltage achieved being -16.5 in the first post in this thread?
Stepping voltage down via resistor cap resistor is always better at filtering then single resistor and one larger cap. That is why so many steps exist in the heater supply.
But after this latest revalation its sound like you do not need to step the voltage down.
I don't get why we are even posting about this if your last post is correct.
Stepping voltage down via resistor cap resistor is always better at filtering then single resistor and one larger cap. That is why so many steps exist in the heater supply.
But after this latest revalation its sound like you do not need to step the voltage down.
I don't get why we are even posting about this if your last post is correct.
NOSValves
Super Member
Ahhh now I see where the improved voltages came from I missed this post. The 150K was indeed a mistake by the kit builder. Scott made random undocumented changes and it looks like this kit builder did also!
If you can bias your tubes and end up near the mid point between -12 and -18 I;d suggest just adding a 50uF @ 100 cap to the center wiper lug of each bias voltage pot to add some filtering and better bias stability and call it good.
Craig