Hi everyone,
I'm working on a Scott LK-72-B tube amp and hoping someone can clarify a couple questions about setting the DC balance. The original manual says to remove tube V3 when adjusting the Left Balance then to replace it and remove V103 when adjusting Right Balance. However, I also read this post here at DiyAudio with a good description of setting balance and bias on the LK72B and it doesn't say anything about removing tubes. Then I read this post on AudioKarma that describes the balance/bias process and one of the first steps is "remove the phase inverter tubes (6U8 or others)", which sounds like you remove them all at the same time, rather than one at a time. So I'm not sure whether the 6U8 tubes need to be removed one at a time, all at once, or not at all.
The AudioKarma post also says to attach a 16ohm resistor to the speaker terminals and measure the voltage across the resistor with the meter set to AC Volts, but the DiyAudio link says to attach speakers or resistors to the speaker terminals and measure the voltage "between the plate connections on the two Power tube sockets" with the meter set to DC volts. Is that just two different ways of achieving the same results, one measuring AC and one DC?
I'm going to be replacing the coupling caps and cathode resistors soon and would like to adjust DC Balance and Bias afterwards, so any advice would be much appreciated.
Thanks very much.
I'm working on a Scott LK-72-B tube amp and hoping someone can clarify a couple questions about setting the DC balance. The original manual says to remove tube V3 when adjusting the Left Balance then to replace it and remove V103 when adjusting Right Balance. However, I also read this post here at DiyAudio with a good description of setting balance and bias on the LK72B and it doesn't say anything about removing tubes. Then I read this post on AudioKarma that describes the balance/bias process and one of the first steps is "remove the phase inverter tubes (6U8 or others)", which sounds like you remove them all at the same time, rather than one at a time. So I'm not sure whether the 6U8 tubes need to be removed one at a time, all at once, or not at all.
The AudioKarma post also says to attach a 16ohm resistor to the speaker terminals and measure the voltage across the resistor with the meter set to AC Volts, but the DiyAudio link says to attach speakers or resistors to the speaker terminals and measure the voltage "between the plate connections on the two Power tube sockets" with the meter set to DC volts. Is that just two different ways of achieving the same results, one measuring AC and one DC?
I'm going to be replacing the coupling caps and cathode resistors soon and would like to adjust DC Balance and Bias afterwards, so any advice would be much appreciated.
Thanks very much.
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If yours has current sensing cathode resistors, neither of these is correct. DC balance cannot be measured across the primary of the output transformer because the two halves of the primary winding don't necessarily have the same resistance. Also, it's dangerous to have the DVM floating up at B+ and should be avoided. DC balance can be adjusted by measuring across the cathode resistors, and bias adjusted by measuring across each separately. The 16 Ohm loads are put in place to substitute for a speaker load, to keep the amplifier stable.
Or, you could splurge for two DVMs, each connected across a cathode resistor, and make it easy and fast.
All good fortune,
Chris
Or, you could splurge for two DVMs, each connected across a cathode resistor, and make it easy and fast.
All good fortune,
Chris
Thanks for your reply Chris. I'm glad I asked if neither approach is applicable to my amp. The original schematic and assembly manual shows one 3.3Ω resistor as seen in pics below, but my amp as I received it has four 2Ω resistors as seen circled in the photo below. They all test between 2.0 and 2.2 and I could probably leave them in place, but it was recommended that I replace them with 1Ω 1% resistors so the test points will then measure as 1mV = 1mA.
I would really appreciate it if you could elaborate more on the correct procedure for setting the DC balance and where I should attach my DVM leads to measure "across the cathode resistors". I'm a newbie at this and sometimes needs things spelled out for me. I do have two DVMs that can measure DC voltage if that will make things easier.
Thanks very much,
Andy
I would really appreciate it if you could elaborate more on the correct procedure for setting the DC balance and where I should attach my DVM leads to measure "across the cathode resistors". I'm a newbie at this and sometimes needs things spelled out for me. I do have two DVMs that can measure DC voltage if that will make things easier.
Thanks very much,
Andy
If you have separate cathode resistors per tube, then adjust for equal DC voltage across each resistor.
Or if you prefer, connect one lead of the DVM to each cathode resistor of the channel, and adjust for 0VDC.
Or if you prefer, connect one lead of the DVM to each cathode resistor of the channel, and adjust for 0VDC.
The old method was basically to adjust for least hum at the output, which would mean the tubes were pretty well balanced. In your case I'd argue for replacing the 2 ohm resistors with 10 ohm 1% matched. I think you'd get a more accurate reading.
I'd also replace those Ceracaps. They don't age well and the last thing you want is a leaky cap taking out a 7591 and possibly an output transformer.
I'd also replace those Ceracaps. They don't age well and the last thing you want is a leaky cap taking out a 7591 and possibly an output transformer.
Thanks rayma and Chris. Just want to make sure I'm crystal clear on this based on your feedback and the other posts I've read. To set DC balance I would attach leads of DVM to each cathode resistor on the side closest to the tube socket as shown in first pic below. Then I would adjust the Balance Pot to get as close to 0VDC as possible.
After Balance is set in both channels, I would adjust Bias by attaching one lead to tube side of cathode resistor, other lead to chassis ground, then adjust Bias Pots to achieve 30-35mV (if Cathode resistors are 1ohm). Original specs call for 35mA per tube, but I've read 30mA may be better.
Hope that is correct, but I'm Still not sure if I need to remove 6U8 tubes when setting Balance? Or was that only applicable to old method described in assembly manual?
Cheers,
Andy
After Balance is set in both channels, I would adjust Bias by attaching one lead to tube side of cathode resistor, other lead to chassis ground, then adjust Bias Pots to achieve 30-35mV (if Cathode resistors are 1ohm). Original specs call for 35mA per tube, but I've read 30mA may be better.
Hope that is correct, but I'm Still not sure if I need to remove 6U8 tubes when setting Balance? Or was that only applicable to old method described in assembly manual?
Cheers,
Andy
Don't remove any tubes, just iterate between DC balance and DC bias until all four readings are correct.
The two channels will interact to some extent.
The two channels will interact to some extent.
I just received a Mouser order yesterday with new Cornell Dubilier MPW caps. I'll start with coupling caps, but ordered replacements for all the Ceracaps. I just wanted to make sure I understood Balance and Bias procedure so I can get things dialed-in after new coupling caps are installed.
Someone else on AudioKarma recommended using 1ohm 1% resistors so 1mV would equal 1mA. Would than mean 10mV = 1mA with a 10ohm resistor and the DVM might be more accurate at 350mV rather than at 35mV, or is there another reason?
Cheers.
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Finally started replacing components. Installed new coupling capacitors last night and will be doing the cathode resistors and a few other caps today.
I replaced the .047uF capacitors near the coupling caps today. Also replaced the 330KΩ grid resistors with 220K based on the recommendation of Jim McShane here to make amp work well with new production 7591A tubes.
I was about the replace the 2Ω cathode resistors with the 1Ω I ordered, but I thought I would try setting Balance and Bias first to get comfortable with the procedure. I figured if I was aiming for 35mV with 1Ω resistors and 350mV with 10Ω, then I would be shooting for 70mV with the 2Ω that are in there now.
I was able to set Balance in both channels to zero no problem, but the highest Bias I could get on the Right Channel was 59mV on V105 and 53mV on V104. On the Left Channel highest I could get was 38mV on V5 and 33mV on V4.
Am I doing something wrong or could this be an indication of another issue somewhere?
Thanks for any advice. Here's a pic of my work so far.
I was about the replace the 2Ω cathode resistors with the 1Ω I ordered, but I thought I would try setting Balance and Bias first to get comfortable with the procedure. I figured if I was aiming for 35mV with 1Ω resistors and 350mV with 10Ω, then I would be shooting for 70mV with the 2Ω that are in there now.
I was able to set Balance in both channels to zero no problem, but the highest Bias I could get on the Right Channel was 59mV on V105 and 53mV on V104. On the Left Channel highest I could get was 38mV on V5 and 33mV on V4.
Am I doing something wrong or could this be an indication of another issue somewhere?
Thanks for any advice. Here's a pic of my work so far.
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Ceracaps are not paper caps. They are polystyrene film. I've never seen them leaking in high voltage leak test (less than 2 uA at full working voltage).
DC balancing procedure is to adjust for lowest hum voltage at speaker output. The principle takes advantage of residual B+ hum of power supply, which is minimal when the PP output stage is balanced. Driver tubes are pulled out to eliminate background hum coming from upstream stages.
Measuring voltage drop across cathode resistors is dumbass and invasive way in this particular case,
Measuring voltage drop across cathode resistors is dumbass and invasive way in this particular case,
I'm more than happy to consider alternative approaches. Could you provide some more details on how you would complete the DC balancing procedure? I'm a real newbie when it comes to tube amps and I think every forum post I've found so far recommends measuring the voltage drop.
The assumption is that 7591 tubes are in good condition and in matched pairs (within 10%). Original schematic with no mods.
1. Pull out 6U8 tubes (or whatever substitutes you have) while the amp is off. Disconnect speakers. Connect 8 Ohm 10 W resistors across 8 Ohm speaker output terminals. Turn the amp on and let it warm up for 20 min.
2. Adjust bias current first. Connect DC voltmeter between test point B and chassis ground. Turn bias pot to obtain reading of 220 mV. This corresponds to about 66 mA combined current of the two tubes.
3. Connect AC voltmeter across 8 Ohm resistor. Adjust balance pot of the corresponding channel for minimum AC voltage.
4. Check bias current again. If it needs minor adjustment, do it and then repeat balance adjustment. If no adjustment needed, you are done.
5. Do same for the other channel.
6. Turn off, re-insert 6U8 tubes, remove dummy resistors, and reconnect the speakers.
Edit: if you have good sensitive headphones, you can use them to adjust for minimum hum.
1. Pull out 6U8 tubes (or whatever substitutes you have) while the amp is off. Disconnect speakers. Connect 8 Ohm 10 W resistors across 8 Ohm speaker output terminals. Turn the amp on and let it warm up for 20 min.
2. Adjust bias current first. Connect DC voltmeter between test point B and chassis ground. Turn bias pot to obtain reading of 220 mV. This corresponds to about 66 mA combined current of the two tubes.
3. Connect AC voltmeter across 8 Ohm resistor. Adjust balance pot of the corresponding channel for minimum AC voltage.
4. Check bias current again. If it needs minor adjustment, do it and then repeat balance adjustment. If no adjustment needed, you are done.
5. Do same for the other channel.
6. Turn off, re-insert 6U8 tubes, remove dummy resistors, and reconnect the speakers.
Edit: if you have good sensitive headphones, you can use them to adjust for minimum hum.
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The amp has already been modified, so I'm not sure this is helpful. And this is the first time I've heard that measuring tube current across a cathode resistance is a "dumbass" idea. ;-) The installation of two matched cathode resistors in a Scott amp is pretty common and a more efficient and sensible way of making sure the tubes are operating properly. Lowest noise should follow if the tubes are closely balanced.
What is the bias voltage at the output tube grids? Also check your voltages in the negative bias circuit:
It is dumbass idea because it requires a) do unnecessary modification, b) the modification does not allow manufacturer-approved bias current adjustment, and c) poking probes into energized amp standing on its side, which is unsafe. The procedure that I describe is manufacturer-recommended. And, as I said, it is not bad idea in general, but "in this particular case", where manufacturer-described method is the one to be followed.
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