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.
@@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!
Please do a safety show. I have been working tubes since the 80s and still need reminders.
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.
Cool video as always :)
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!