I have a little story about what you mentioned in 17:35 That was a really bad mistake I done few days ago, when I was checking my prototyped pedal which didn’t have an output coupling cap on purpose, because, long story short, I didn’t have much of appropriate caps for the tasks I’ve needed and I didn’t want to mess the frequency response I wanted. I didn’t think a DC bias of 4.5V will do that much of a harm to my equipment. After I plugged in into my Orange OR15 tube amp, it sounded dull and hummed pretty strongly, so I knew something ain’t right. I unplugged the pedal, and after some playing without it, the amp died. I opened it, and saw the first (in the chain) power supply voltage divider resistor disconnected at one of its legs. I realized there was probably too much current flowing there. I soldered it back, and the amp returned to work, but with a lot of hum and a bit weird sound. I’ve checked voltages, and all tubes seems fine, expect V3 which is for the FX loop. One of its stage’s plate doesn’t let me even probe it, it pops very loudly even with the volume and gain on zero. And also the power supply resistor which determines the voltage to this tube’s node, drops too many voltage compared to the design. My suspect is that this tube had failed, but I really afraid I did more damage to the amp. What about V1? It probably absorbed the most damage (the amp doesn’t have an input coupling cap), but the voltages there seem fine. How to get into troubleshooting this problem? Do you have some insights or tips?
@@Poly6Groove normally we don't offer troubleshooting advice for amps that we didn't design/build. But in this case I would check the values of v1 resistors against the schematic then replace v1 and see if all is well. On most guitar amps v1 is a high gain tube and they can easily be damaged (even a tiny problem gets amplified 70x + possibly 70x!). We throw out a lot of high gain tubes for this reason. After swapping the tube observe the amp in operation closely to make sure a more significant problem isn't present. Caution! High Voltage is Present, exercise Extreme Caution!
@ Thank you very much! I’ve indeed already checked V1 circuitry, and it looks happy. It’s indeed quite weird it hadn’t absorbed any damage. Maybe the problem with V3 is even doesn’t related to my DC accident, because the DC was really there for a short period of time. However, thank you on the reply!
I know caps in the signal path colour the sound but even the caps in the power supply are in the current return path and can have a deleterious effect on the sound. Also, I have a KT88/KT120 push-pull amplifier with preamp/input 6AQ8 tube (first stage) and a phase inversion/splitter 6GU7 (second) stage before the output stage. These first two stages are directly coupled, meaning there is no coupling capacitor between them. The first stage has a cathode resistor that creates ~84V on the grid and this is passed onto the grid of the phase splitter 6GU7 tube. The main thing I've noted with having these first two stages coupled is that it amplifies any differences between the two sides of the 6AQ8 twin triode. This makes it more important to have a well balanced input tube.
What would be the effect if I replaced C4 in my Universal Preamp with a cap rated at .22 uf 400V? Would It lose top or bottom end? I’m not planning on doing that. Just curious.
@@jimdalto6254 C4 in the Universal 6or12SN7 Kit Preamp is a final output coupling capacitor. It's functions are to block the ~123vdc that's present on the cathode follower circuit as well as safely couple to the next stage. Caution! High Voltage is Present!!! It's in parallel with all the resistors that follow it. That includes R7 220k and whatever you plug the preamp into. Normally the next stage power amp will have a very high input impedance (>470k), but not always. Given that C4 is also a high pass filter, it's important to make sure the entire signal gets safely out. So most designers choose a very high value like 2.2uf. A very low value could end up creating a cutoff point in the lower audio frequencies. To determine your -3db cutoff point, just use any one of the free online filter calculators. We use the one our supplier Digikey makes available. The only reason imo to ever design a coupling capacitor that's close to the audio band is if you have unwanted noise that must be filtered.
@@rickg8015 we use Blue Alps 100kx2 pots for all of our Kits. Because the volume pot and section balance is so important for the stereo image, we also test the impedance of each channel at 4 different volume positions. We typically filter out about 1/4 of the units as not having a close enough channel balance.
Great video as always, guys. Curious if you might be able to offer me some insight. I have a tube preamp, two stage with single gain tube and dual cathode followers with a 5U4 tube rectified power supply, capacitor coupled. When I power up the unit from cold I have measured a big spike of around 15V of DC on the RCA outputs (you read that right, volts, not millivolts) that is sustained for a few seconds and then quickly falls from 15V to almost 0 mv with less than 10 seconds. I don’t power my amplifier up until the preamp has stabilized. If I were to have my amplifier on when I start the preamp I do notice the woofers on my speakers get the high DC and get “sucked in” for a few seconds so I make sure that my amp is off. Why are the 2.2 uf caps on the output of my preamp not blocking the DC? Does this have something to do with it being a tube rectified power supply?
@@sean_heisler for a very short period of time, coupling capacitors need to power up before they can fully block dc. Now the amp topology will definitely affect how much of a problem this is, as well as the rating, quality and condition of the coupling caps. If they're older than 15 years or poor quality, then I'd suggest replacing them as a first step. Caution! Tube amps have high voltage present, if you're not sure what you are doing, I recommend bringing the amp to a qualified technician.
Don't know if this helps but 5u4 warm up pretty fast compared to 5v4 or 5ar4.might want one of those. 5ax4 and 5v4 still reasonable price and work for preamp.may have to ADD resistance to maintain same plate volt.check those 2.2 for leakage and the resistor that follow those caps to ground (bleader resister).there should be one with value of 500 k ohm or less at out put to stop d.c that leak through output couple cap.some pre with solid state rect.use muting relay.
I'm curious why the cathode resistor for the 2nd part of the 12sn7 is lifted off ground with R6.. Is this a solution for ground noise? Assuming... My home built amps are super quiet BUT just may be this is an upgrade I can try :-)
@@AudioFanMan I'm not exactly sure what you are asking. So I'll summarize the CF (cathode follower) circuit. R5 is the cathode bias resistor it works in conjunction with R4 (grid leak) to set the bias at 7v. And because the relative ground voltage is established at the junction, the grid behaves as though it is set to 0v when in fact it's actually much higher. R6 is basically the plate resistor, but it's position is flipped from it's normal position on top at the plate. The junction of R4,R5,R6 is a good place to take off the low impedance signal, but you could also take it off at the cathode pin as well. This circuit gives excellent control of the operating point, uses very few parts and is very stable. Noise really doesn't come into the design topology. Though given the gain will be slightly less than unity, all CF stages are inherently low noise. I think if you tried hard enough you could probably build a noisy one! BTW this circuit is pretty much standard for a CF stage. Hope that helps.
Rca tube manual chapter electron tube applications discuss this type of cathode follower of one want to read about it.good to know jim finding on the performance.
So I apologize, had a not so bright moment, I internally compared the cathode follower shown with a self biased cathode resistor to a plate driven driver tube.. doh! As soon as I pulled up the schematic to my tube preamp I got my mistake.. but ty for the guidance!
I have a little story about what you mentioned in 17:35
That was a really bad mistake I done few days ago, when I was checking my prototyped pedal which didn’t have an output coupling cap on purpose, because, long story short, I didn’t have much of appropriate caps for the tasks I’ve needed and I didn’t want to mess the frequency response I wanted. I didn’t think a DC bias of 4.5V will do that much of a harm to my equipment. After I plugged in into my Orange OR15 tube amp, it sounded dull and hummed pretty strongly, so I knew something ain’t right. I unplugged the pedal, and after some playing without it, the amp died. I opened it, and saw the first (in the chain) power supply voltage divider resistor disconnected at one of its legs. I realized there was probably too much current flowing there.
I soldered it back, and the amp returned to work, but with a lot of hum and a bit weird sound. I’ve checked voltages, and all tubes seems fine, expect V3 which is for the FX loop. One of its stage’s plate doesn’t let me even probe it, it pops very loudly even with the volume and gain on zero. And also the power supply resistor which determines the voltage to this tube’s node, drops too many voltage compared to the design. My suspect is that this tube had failed, but I really afraid I did more damage to the amp. What about V1? It probably absorbed the most damage (the amp doesn’t have an input coupling cap), but the voltages there seem fine. How to get into troubleshooting this problem? Do you have some insights or tips?
@@Poly6Groove normally we don't offer troubleshooting advice for amps that we didn't design/build. But in this case I would check the values of v1 resistors against the schematic then replace v1 and see if all is well. On most guitar amps v1 is a high gain tube and they can easily be damaged (even a tiny problem gets amplified 70x + possibly 70x!). We throw out a lot of high gain tubes for this reason. After swapping the tube observe the amp in operation closely to make sure a more significant problem isn't present. Caution! High Voltage is Present, exercise Extreme Caution!
@ Thank you very much!
I’ve indeed already checked V1 circuitry, and it looks happy. It’s indeed quite weird it hadn’t absorbed any damage. Maybe the problem with V3 is even doesn’t related to my DC accident, because the DC was really there for a short period of time. However, thank you on the reply!
Please do a safety show. I have been working tubes since the 80s and still need reminders.
Cool video as always :)
I know caps in the signal path colour the sound but even the caps in the power supply are in the current return path and can have a deleterious effect on the sound.
Also, I have a KT88/KT120 push-pull amplifier with preamp/input 6AQ8 tube (first stage) and a phase inversion/splitter 6GU7 (second) stage before the output stage. These first two stages are directly coupled, meaning there is no coupling capacitor between them. The first stage has a cathode resistor that creates ~84V on the grid and this is passed onto the grid of the phase splitter 6GU7 tube.
The main thing I've noted with having these first two stages coupled is that it amplifies any differences between the two sides of the 6AQ8 twin triode. This makes it more important to have a well balanced input tube.
What would be the effect if I replaced C4 in my Universal Preamp with a cap rated at .22 uf 400V? Would It lose top or bottom end? I’m not planning on doing that. Just curious.
@@jimdalto6254 C4 in the Universal 6or12SN7 Kit Preamp is a final output coupling capacitor. It's functions are to block the ~123vdc that's present on the cathode follower circuit as well as safely couple to the next stage. Caution! High Voltage is Present!!! It's in parallel with all the resistors that follow it. That includes R7 220k and whatever you plug the preamp into. Normally the next stage power amp will have a very high input impedance (>470k), but not always. Given that C4 is also a high pass filter, it's important to make sure the entire signal gets safely out. So most designers choose a very high value like 2.2uf. A very low value could end up creating a cutoff point in the lower audio frequencies. To determine your -3db cutoff point, just use any one of the free online filter calculators. We use the one our supplier Digikey makes available. The only reason imo to ever design a coupling capacitor that's close to the audio band is if you have unwanted noise that must be filtered.
Thanks for explaining the preamp in more detail.. What Volume Pot value do you use for it btw?
@@rickg8015 we use Blue Alps 100kx2 pots for all of our Kits. Because the volume pot and section balance is so important for the stereo image, we also test the impedance of each channel at 4 different volume positions. We typically filter out about 1/4 of the units as not having a close enough channel balance.
@@tubelab194 Alps churns out more these days from their factories with less tighter tolerances.. And fakes abound too..
Great video as always, guys. Curious if you might be able to offer me some insight. I have a tube preamp, two stage with single gain tube and dual cathode followers with a 5U4 tube rectified power supply, capacitor coupled. When I power up the unit from cold I have measured a big spike of around 15V of DC on the RCA outputs (you read that right, volts, not millivolts) that is sustained for a few seconds and then quickly falls from 15V to almost 0 mv with less than 10 seconds. I don’t power my amplifier up until the preamp has stabilized. If I were to have my amplifier on when I start the preamp I do notice the woofers on my speakers get the high DC and get “sucked in” for a few seconds so I make sure that my amp is off. Why are the 2.2 uf caps on the output of my preamp not blocking the DC? Does this have something to do with it being a tube rectified power supply?
@@sean_heisler for a very short period of time, coupling capacitors need to power up before they can fully block dc. Now the amp topology will definitely affect how much of a problem this is, as well as the rating, quality and condition of the coupling caps. If they're older than 15 years or poor quality, then I'd suggest replacing them as a first step. Caution! Tube amps have high voltage present, if you're not sure what you are doing, I recommend bringing the amp to a qualified technician.
Don't know if this helps but 5u4 warm up pretty fast compared to 5v4 or 5ar4.might want one of those. 5ax4 and 5v4 still reasonable price and work for preamp.may have to ADD resistance to maintain same plate volt.check those 2.2 for leakage and the resistor that follow those caps to ground (bleader resister).there should be one with value of 500 k ohm or less at out put to stop d.c that leak through output couple cap.some pre with solid state rect.use muting relay.
I'm curious why the cathode resistor for the 2nd part of the 12sn7 is lifted off ground with R6.. Is this a solution for ground noise?
Assuming... My home built amps are super quiet BUT just may be this is an upgrade I can try :-)
@@AudioFanMan I'm not exactly sure what you are asking. So I'll summarize the CF (cathode follower) circuit. R5 is the cathode bias resistor it works in conjunction with R4 (grid leak) to set the bias at 7v. And because the relative ground voltage is established at the junction, the grid behaves as though it is set to 0v when in fact it's actually much higher. R6 is basically the plate resistor, but it's position is flipped from it's normal position on top at the plate. The junction of R4,R5,R6 is a good place to take off the low impedance signal, but you could also take it off at the cathode pin as well. This circuit gives excellent control of the operating point, uses very few parts and is very stable. Noise really doesn't come into the design topology. Though given the gain will be slightly less than unity, all CF stages are inherently low noise. I think if you tried hard enough you could probably build a noisy one! BTW this circuit is pretty much standard for a CF stage. Hope that helps.
Rca tube manual chapter electron tube applications discuss this type of cathode follower of one want to read about it.good to know jim finding on the performance.
So I apologize, had a not so bright moment, I internally compared the cathode follower shown with a self biased cathode resistor to a plate driven driver tube.. doh! As soon as I pulled up the schematic to my tube preamp I got my mistake.. but ty for the guidance!
👍✌️