Thanks for your ideas and comments.
Some more observations - I have spent some time listening to the hum and it is more likely a complex signal. I comes out from the tweeter rarher than from the woofer and seems to be 50 Hz modulated with some higher frequency noise.
One thing I tried now was to increase resisotrs in the B+ to GND divider for heater elevation from 100 K + 10 k to 100 k + 20k and funny thing happened - this hum disappeared from the original channel and it is in the other channel nowSo what does it mean?
I will try to elevate the heater voltage further more to 100k + 30 k and see what happens.
Actually is it possible to use different elevation volatage in left and right channels? e.g. 100 k + 10 k in one channel and 100 k + 20 k in the second channel?
Would it make sense to use a kind of hum pot but in this case the CT network is not connected to GND but the divider described above.
Neither your PCB, neither your input valves are appropriate for AC heater supply. IMHO.
Since we told you in many ways, but you're too stubborn, or you do not read every post.
Thomas, thanks for your advises. I created a new divider from B+ to GND and repalced the original one in the channel with buzz. No change.
I have re-solder all points on the PCB in this channel, double checked all screw terminal but no change.
I have knocked all components on the PCB while carefully listening but no efect. I have also moved with all wires going to PCB. Again, no audible difference. Still strange buzz in one channel only.
Actually it is a channel that is farer from the power transformer than the channel that is silent.
What do you mean by leakage between cathode and heater? Is there anything I can do with it? The tubes I use are brand new.
Actually one thinq is not clear to me - original circuit uses CT resistor connected to GND. There was 100 Hz hum in both channels (no buzz). When I replaced GND connection with B+ to GND divider with 100 K + 10K with 220 n paralel this 100 Hz hum disapeared but there apreas a new buzz in one channel only. Isn't it strange?
I have re-solder all points on the PCB in this channel, double checked all screw terminal but no change.
I have knocked all components on the PCB while carefully listening but no efect. I have also moved with all wires going to PCB. Again, no audible difference. Still strange buzz in one channel only.
Actually it is a channel that is farer from the power transformer than the channel that is silent.
What do you mean by leakage between cathode and heater? Is there anything I can do with it? The tubes I use are brand new.
Actually one thinq is not clear to me - original circuit uses CT resistor connected to GND. There was 100 Hz hum in both channels (no buzz). When I replaced GND connection with B+ to GND divider with 100 K + 10K with 220 n paralel this 100 Hz hum disapeared but there apreas a new buzz in one channel only. Isn't it strange?
popilin, I really apreciate your comments and advices. I can imagine that the PCB or tubes are not perfect for AC heating. Anyway it is not my design but a kit and so I'm trying to believe that it should work even with AC heaters.
Maybe I'll end up with DC heaters but before I go this way (recitifiers, elkos, voltage regulator, heatsinks) and I'm trying to find a silution using current configuration.
Maybe I'll end up with DC heaters but before I go this way (recitifiers, elkos, voltage regulator, heatsinks) and I'm trying to find a silution using current configuration.
Hi!
After the modification, one channel went silent, which proofs that this design is ok for AC heaters. As it seems the remaining noise in the other channel is intermittent which points to another issue.
AC heaters are really not necessary in a power amp design. It can create even more problems. An additional rectifier which has to provide heavy charging pulses into the cap to get a clean DC can even cause 100Hz buzz again.
Thomas
After the modification, one channel went silent, which proofs that this design is ok for AC heaters. As it seems the remaining noise in the other channel is intermittent which points to another issue.
AC heaters are really not necessary in a power amp design. It can create even more problems. An additional rectifier which has to provide heavy charging pulses into the cap to get a clean DC can even cause 100Hz buzz again.
Thomas
Hi!
Didn't you at one point have both channels quiet? Try to reproduce that situation and analyse further from there.
There is nothing strange about buzz or hum, these are some of the most common issues. If there is a condition which creates hum/buzz that can take different forms and sound differently. You need to stay patient and track down the cause.
Thomas
Didn't you at one point have both channels quiet? Try to reproduce that situation and analyse further from there.
There is nothing strange about buzz or hum, these are some of the most common issues. If there is a condition which creates hum/buzz that can take different forms and sound differently. You need to stay patient and track down the cause.
Thomas
Hold on, folks.
BUZZ (as opposed to HUM) is almost always caused by "something switching hard".
What could that be? Silicon rectifiers obviously are candidates in the "switch hard" department. Especially since the original poster hears the problem in the tweeter (which would never happen at 50/60 Hz power-mains frequency). The "general solution" is to slow down switching!
How is that done? By adding a low-value resistor in series with the silicon rectifier(s) and the capacitor-inductor/resistor-capacitor filter bank. Give up a few volts for substantially softer switching and quieter operation.
There really isn't much else. If this were a very old unit, I'd venture dissimilar-metal junctions as creating low-bandgap PN junctions and transient trolls ... but with all the fiddling that's been done, it isn't that likely. Again, just "soften the rectification".
Which, ironically, is what vacuum-tube rectifiers do!... having voltage drops in operation measured from a handful of volts to many dozens. They're very "soft". This is why I don't mind at all dropping 5 to 10 volts through a nice wire-wound resistor.
JUST REMEMBER: P = E²/R
So if you're going to drop 10 volts (say with 250 milliamps - but just guessing), then E×I = 2.5 watts (power law) and ohm's law: E = I×R ... R = E/I = 10/0.25 = 40 ohms. See the nice power formula for confirmation: E²/R = 10²/40 = 100/40 = 2.5 watts.
I may be singing to the choir, or maybe singing all by myself on this ... but hey - its cheap, simple, and will definitely deliver results.
GoatGuy
BUZZ (as opposed to HUM) is almost always caused by "something switching hard".
What could that be? Silicon rectifiers obviously are candidates in the "switch hard" department. Especially since the original poster hears the problem in the tweeter (which would never happen at 50/60 Hz power-mains frequency). The "general solution" is to slow down switching!
How is that done? By adding a low-value resistor in series with the silicon rectifier(s) and the capacitor-inductor/resistor-capacitor filter bank. Give up a few volts for substantially softer switching and quieter operation.
There really isn't much else. If this were a very old unit, I'd venture dissimilar-metal junctions as creating low-bandgap PN junctions and transient trolls ... but with all the fiddling that's been done, it isn't that likely. Again, just "soften the rectification".
Which, ironically, is what vacuum-tube rectifiers do!... having voltage drops in operation measured from a handful of volts to many dozens. They're very "soft". This is why I don't mind at all dropping 5 to 10 volts through a nice wire-wound resistor.
JUST REMEMBER: P = E²/R
So if you're going to drop 10 volts (say with 250 milliamps - but just guessing), then E×I = 2.5 watts (power law) and ohm's law: E = I×R ... R = E/I = 10/0.25 = 40 ohms. See the nice power formula for confirmation: E²/R = 10²/40 = 100/40 = 2.5 watts.
I may be singing to the choir, or maybe singing all by myself on this ... but hey - its cheap, simple, and will definitely deliver results.
GoatGuy
OK. I understand.
Try to elevate heaters to about 80 V then, don't worry, no problem for your valves.
Use a resistor divider plus 220uF/160V cap.
You can use a 100 ohm pot. between heaters extremes, and connect CT to divider.
The idea is best cope with a "diode" between heater and cathode, and Rfk, Cfk.
If I understood correctly, the noise/hum/buzz, switching from one channel to another, depending on the reference voltage, this may be due to interactions of fields and ripple from PSU, no symmetrical tracks heaters in PCB.
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I always use DC heater supply, and PCBs, both in valve preamps like in valve power amps, never had problems with hum/buzz.
I find it incomprehensible that with the technology available today, most still uses techniques of early last century, and I do not mean the valves.
Thomas: This is what I'm trying to do last 3 hours to find the situation when it was was without buzz. I happened by accident but there was nothing special about the configuration. As I mentioned I just added 4 screws to fix the PCB and when I powered it on again the hum was again there.
popilin: I already tried to elevate heater to ~60 V but no difference. The buzz is still the same and still in one channel only. Should I go even higher to 80 V you recommended?
Right now I have 220 n capacitor parallel to lower resistor in the divider. Anyway I used 1000 u / 63 V as well and there was no change.
GoatGuy: This was actually my idea as well that it is maybe coming from rectifier - switching. The strange thing is that the buzz is in one channel only. There is a common rectifier for B+ that is use for both channels so I would expect the buzz in both channels. I have tried to increase series resistanc from 100 R to 200 R (R5) but no difference:
http://analogmetric.com/download/6L6 SE Amplifier Schematic.pdf
Actually wat I have tried now was to replace volatage elevation in buzzing channel connecting it back to GND and I expected 100 Hz hum as it was before. But this didn't happen. There was still buzz and no 100 Hz hum.
Next thing I tried was to use hum pot between heaters pins with center tap connected to B+ to GND devidier descriobed above. No change while turning the pot - still the same buzz in one channel.
popilin: I already tried to elevate heater to ~60 V but no difference. The buzz is still the same and still in one channel only. Should I go even higher to 80 V you recommended?
Right now I have 220 n capacitor parallel to lower resistor in the divider. Anyway I used 1000 u / 63 V as well and there was no change.
GoatGuy: This was actually my idea as well that it is maybe coming from rectifier - switching. The strange thing is that the buzz is in one channel only. There is a common rectifier for B+ that is use for both channels so I would expect the buzz in both channels. I have tried to increase series resistanc from 100 R to 200 R (R5) but no difference:
http://analogmetric.com/download/6L6 SE Amplifier Schematic.pdf
Actually wat I have tried now was to replace volatage elevation in buzzing channel connecting it back to GND and I expected 100 Hz hum as it was before. But this didn't happen. There was still buzz and no 100 Hz hum.
Next thing I tried was to use hum pot between heaters pins with center tap connected to B+ to GND devidier descriobed above. No change while turning the pot - still the same buzz in one channel.
Popilin & Thomas: AC vs. DC heaters - Thomas is right that if one channel is without buzz with AC heater it shouldn't be an issue to use AC voltage. On the other hand I have read some discussions that 12AX7 is relatively high gain tube so it can be sensitive to AC hum/buzz.
I don't want to give up with AC heater and maybe there will be an idea what to do to get the rid of this buzz in one channel.
Anyway if I will not find a solution maybe I will end up with DC voltage. What are the disadvantages of DC heaters? It would be easier to use DC voltage for both input and output tube (common PCB traces) but it will take ~1.3 + 0.3 = 1.6 Amps which is relatively high current and some heatsink will be needed for regulator. Actually is it mandatory to use regulated heather DC power supply?
I don't want to give up with AC heater and maybe there will be an idea what to do to get the rid of this buzz in one channel.
Anyway if I will not find a solution maybe I will end up with DC voltage. What are the disadvantages of DC heaters? It would be easier to use DC voltage for both input and output tube (common PCB traces) but it will take ~1.3 + 0.3 = 1.6 Amps which is relatively high current and some heatsink will be needed for regulator. Actually is it mandatory to use regulated heather DC power supply?
Before you give up on AC, try this, in the noisy channel, carefully disconnect the heaters, if the noise disappears immediately, go with DC.
Circuits which work fine with AC heaters and NOS valves may need DC heaters if modern valves are used. I suspect you have a combination of problems:
poor PCB layout - as I said, it is difficult to do decent AC heater wiring on a PCB
modern valves - sometimes have poor heater-cathode insulation
You need to take a step back and diagnose the problem, not just keep changing things. I'm still not sure if you have 50Hz or 100Hz or both, yet this is a crucial part of the diagnosis.
poor PCB layout - as I said, it is difficult to do decent AC heater wiring on a PCB
modern valves - sometimes have poor heater-cathode insulation
You need to take a step back and diagnose the problem, not just keep changing things. I'm still not sure if you have 50Hz or 100Hz or both, yet this is a crucial part of the diagnosis.
Hi!
DC heaters with indirectly heated tubes is completely unnecessary in power amps unless it is a bad design with bad layout. Even then using DC heaters is a crutch. I'd rather get the problem sorted out that DC heaters are supposed to fix. They target the symptom, but do not cure the problem.
This has nothing to do with ancient technology. Indirectly heated tubes are designed to work hum free with AC heaters. So why add unnecessary complexity? I even think it's silly to use DC heaters in power amps.
I use AC heaters in linestages without any hum. Phono stages or mic preamps are the only situation where DC heaters are sensible and thats where I use them.
Different story of course with directly heated tubes. If these are used as driver or in preamps, DC heaters are a must, but that is a completely different story.
Best regards
Thomas
DC heaters with indirectly heated tubes is completely unnecessary in power amps unless it is a bad design with bad layout. Even then using DC heaters is a crutch. I'd rather get the problem sorted out that DC heaters are supposed to fix. They target the symptom, but do not cure the problem.
This has nothing to do with ancient technology. Indirectly heated tubes are designed to work hum free with AC heaters. So why add unnecessary complexity? I even think it's silly to use DC heaters in power amps.
I use AC heaters in linestages without any hum. Phono stages or mic preamps are the only situation where DC heaters are sensible and thats where I use them.
Different story of course with directly heated tubes. If these are used as driver or in preamps, DC heaters are a must, but that is a completely different story.
Best regards
Thomas
DF96: I had 100 Hz in both channles but this was solved (removed) by elevating the AC heaters. Now I have "buzz" - it is clearly not pure harmonic signal but rather buzz, maybe modulated on 50 Hz but mostly audible in twweter so it seems to be hgiher frequency.
One more thing I tested now - this buzz appears ~10 seconds after power on and disapperas immediately after power off.
popilin: I have already tried it - if I disconnect heater voltage the buzz disapperas immediately
Maybe it is a combination of "modern"tube where AC heating is not good and "poor" PCB layout. What makes it strange for me is that it is really in one channel ony. PCB layout is pretty symmetrical for both channels and they use the same tubes so if this is the case I would expect either both channels buzzing or none of them.
One more thing I tested now - this buzz appears ~10 seconds after power on and disapperas immediately after power off.
popilin: I have already tried it - if I disconnect heater voltage the buzz disapperas immediately
Maybe it is a combination of "modern"tube where AC heating is not good and "poor" PCB layout. What makes it strange for me is that it is really in one channel ony. PCB layout is pretty symmetrical for both channels and they use the same tubes so if this is the case I would expect either both channels buzzing or none of them.
Regardless of a given design/layout, our beloved valves, has an internal transformer, so hum is induced from heater's AC magnetic field, to the grid, and this is an observable fact.
Not to mention the effects of Rfk, and Cfk, another route for hum.
Not all valves, eg 7025, has a heater with more "turns" in order to increase the transformation ratio then the AC volataje induced in the grid is smaller.
Then I must be pretty dumb !
In directly heated valves, the problem is worse, because in the "primary" you have only one "turn"
OK. before making a final assembly, test again with DC heater supply, if there is no some other problem, do the final assembly.
Remember that one end of DC heaters PSU must also elevate to about 70-80 V.
Good Luck !
I don't want to give up on using AC heaters but I don't know what else I should try to sovle the buzz in one channel I still can try to use different type of 12AX7 tubes but this is probably not the root cause of this buzz.
Regarding DC heaters - will it work without regulator or should I use it? (with respect to sound quality, tube's life etc.) As there is a 6.3 V AC on the transformer I guess the DC will be ~ 9 V so probably series resistor or regulator should be used.
I still can try to use different type of 12AX7 tubes but this is probably not the root cause of this buzz.Regarding DC heaters - will it work without regulator or should I use it? (with respect to sound quality, tube's life etc.) As there is a 6.3 V AC on the transformer I guess the DC will be ~ 9 V so probably series resistor or regulator should be used.
I don't want to give up on using AC heaters but I don't know what else I should try to sovle the buzz in one channel
The easiest and cheapest is to use DC heaters supply.
You must use regulators, unless you want other problems.
I as a rule, use the same kind of regulation, both for heaters and for B+
You have a poorly regulated B+, but I think you can live with it.
Note that there is no amplifier better than its power supply.
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It's class A with nearly constant load, so the B+ regulation doesn't matter much, but B+ filtering is critical because it doesn't cancel AC on the b+ like push-pull does, and of course you want to keep all signal off the supply so it doesn't affect other stages.
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