Howdy, Everyone:
Let me first apologize for being gone for such a long time. When one does consulting work, it is a paradox that when things are slow, you get busier looking for new work. I'm in the Oil Patch and things have been way slow. Thank Goodness, it's all changing for the better and rather quickly.
OK, since last of any posting, the vision was a pair of 6CB5As with a modest amount of CFB, and the Schade type (I know this name can be a sore point for some of y'all but I'm using because it is short and descriptive, most people know what I mean regardless of what name they use) of local feedback via a resistor from plate of PA to plate of corresponding driver. That was maybe 2 years ago.
After a LOT of research, I have chosen the 6HJ5 as the PA and the 6JV8 as the phase inverter and driver. 2 of each of the types for each channel. The triode sections are the input/phase inverter in a long tail setup using cascaded mosfets as the tail. Running 4mA per triode so of course the sink is 8mA. There is a small amount of degeneration on each cathode. I don't think a balance pot will be needed but testing will tell for sure, the calcs show large immunity to tube imbalance.
I actually wanted to use the 6DQ5 but the wife said she'd rather not have the exposed plate caps since the amp will not be enclosed. Bummer, I think plate caps are cool! The 6DQ5, 6HD5 and 6HJ5 are cousins, based on the data sheets, with the 6DQ5 having a slight edge in terms of maximum cathode current. But all are typical of sweep tubes in having very high G2 transconductance and low maximum G2 voltage.
The 6HJ5s will be driven by the pentode sections of the 6JV8. Swing needed is about 60V pk which appears to be no problem, with a +B of 360. These pentode sections are connected to the -50 supply via common cathode resistor so also long tail.
The plates of the pentode sections are swinging about +/-10mA including the Schade feedback, well within their capability, and idle at about 18mA.
The PAs are idling at about 65mA, staying class A-AB1 all the time. I'm doing something a bit different with the secondary. Instead of the commonly used separate CFB winding, the speaker load winding and the CFB are combined so coupling between them is much tighter. Overall FB is taken from the cathode of the "top" PA and fed back via a proper network to the grid of the input long tail opposite the input signal grid.
The entire amplifier is full differential all the way and could, in fact, accept a fully differential input signal (if I had one). Calculated necessary input for full output is about 700mV.
All voltages will be regulated. For the heaters, the most economical solution just may be a low cost DC power supply (about $50 for a 12V/8A), connect the heaters in seriesparallel sets so the heater string is 12V. I have a scheme for the high voltages, actually three. The +120 for the G2 of the PA, +360 for all the plates, and -50 for long tails and PA fixed bias (adjustable to the PAs of course, and with a bias balance pot). I'll reveal my regulation scheme, kind of different.
Anyway, schematic is mostly finished, waiting to have a bit more time, maybe next week.
Happy designing/building/listening to All
Rene
Let me first apologize for being gone for such a long time. When one does consulting work, it is a paradox that when things are slow, you get busier looking for new work. I'm in the Oil Patch and things have been way slow. Thank Goodness, it's all changing for the better and rather quickly.
OK, since last of any posting, the vision was a pair of 6CB5As with a modest amount of CFB, and the Schade type (I know this name can be a sore point for some of y'all but I'm using because it is short and descriptive, most people know what I mean regardless of what name they use) of local feedback via a resistor from plate of PA to plate of corresponding driver. That was maybe 2 years ago.
After a LOT of research, I have chosen the 6HJ5 as the PA and the 6JV8 as the phase inverter and driver. 2 of each of the types for each channel. The triode sections are the input/phase inverter in a long tail setup using cascaded mosfets as the tail. Running 4mA per triode so of course the sink is 8mA. There is a small amount of degeneration on each cathode. I don't think a balance pot will be needed but testing will tell for sure, the calcs show large immunity to tube imbalance.
I actually wanted to use the 6DQ5 but the wife said she'd rather not have the exposed plate caps since the amp will not be enclosed. Bummer, I think plate caps are cool! The 6DQ5, 6HD5 and 6HJ5 are cousins, based on the data sheets, with the 6DQ5 having a slight edge in terms of maximum cathode current. But all are typical of sweep tubes in having very high G2 transconductance and low maximum G2 voltage.
The 6HJ5s will be driven by the pentode sections of the 6JV8. Swing needed is about 60V pk which appears to be no problem, with a +B of 360. These pentode sections are connected to the -50 supply via common cathode resistor so also long tail.
The plates of the pentode sections are swinging about +/-10mA including the Schade feedback, well within their capability, and idle at about 18mA.
The PAs are idling at about 65mA, staying class A-AB1 all the time. I'm doing something a bit different with the secondary. Instead of the commonly used separate CFB winding, the speaker load winding and the CFB are combined so coupling between them is much tighter. Overall FB is taken from the cathode of the "top" PA and fed back via a proper network to the grid of the input long tail opposite the input signal grid.
The entire amplifier is full differential all the way and could, in fact, accept a fully differential input signal (if I had one). Calculated necessary input for full output is about 700mV.
All voltages will be regulated. For the heaters, the most economical solution just may be a low cost DC power supply (about $50 for a 12V/8A), connect the heaters in seriesparallel sets so the heater string is 12V. I have a scheme for the high voltages, actually three. The +120 for the G2 of the PA, +360 for all the plates, and -50 for long tails and PA fixed bias (adjustable to the PAs of course, and with a bias balance pot). I'll reveal my regulation scheme, kind of different.
Anyway, schematic is mostly finished, waiting to have a bit more time, maybe next week.
Happy designing/building/listening to All
Rene
Those little switch-mode PSUs can have trouble starting into heater loads.
Interesting write-up. I look forward to seeing the whole design.
Interesting write-up. I look forward to seeing the whole design.
Yes, some SMPS's will not start, shut down, or go into a restart loop when tasked with a tube heater load. Some will start up right into a cold tube load that is near maximum rating, many will not.
I have found that most everything except for some of those plastic enclosed "laptop bricks" will start into a tube heater load that is less that half the supply's maximum rating.
I have found that most everything except for some of those plastic enclosed "laptop bricks" will start into a tube heater load that is less that half the supply's maximum rating.
wow is that PP GK-71? how much power did you squeeze from that pair? what is your driver tubes?
OMG … I almost blew hot tea thru my snout. Made me laugh SO hard!!! Thanks for the 5:45 AM wake up, Wavebourn. … tears … blowing nose …
GoatGuy
With you, all who are cautioning about PS start into heater. In fact, I was more concerned with the supply starting quickly into a heater load and beating the snot out of them. So part of the power supply is a soft start consisting of a simple Pchannel MOSFET in series with 12V with a simple circuit on gate which drives it slowly toward saturation (slowly meaning within a second). This will mitigate the 1:10 change in heater resistance cold to hot, not trip the PS into overcurrent and help with heater life.
Stay tuned, first the amp schematic then the power supply. I'll bet there will be more comments about it than the amp.
Rene
Stay tuned, first the amp schematic then the power supply. I'll bet there will be more comments about it than the amp.
Rene
I've had some issues with mosfet SOA with inrush suppression circuits, you want to make sure that the maximum current flowing at any moment in time until the the mosfet is saturated is well inside the current limit allowed at that value of Vds. (And the power product at those voltages is often highly unintuitive) Many mosfet switches rated for 100A at 1V will be good for less than a couple of amps at 10V Vds - I learned this as I was fixing someone else's design mistake. i.e At a Vds of 1V the transistor mightly safely handle 100A and 100W of dissipation, at 12V it might only handle 1A and the resulting 12W of dissipation.
Thanks, Kevin, I know all that. MOSFETs and switching power supplies and MOSFETs as auxiliary switches and I are old friends (or adversaries, as the case may be!).
The dissipation will short term be high on the MOSFET and it is necessary to make sure the power-time product does not cause the junction to fry. Good news is that once saturated, it's easy going with maybe 1W or less, depending on channel saturated resistance Rds sat.
Rene
The dissipation will short term be high on the MOSFET and it is necessary to make sure the power-time product does not cause the junction to fry. Good news is that once saturated, it's easy going with maybe 1W or less, depending on channel saturated resistance Rds sat.
Rene
I had experienced long time ago with 6JS6 compactron tubes, but the experience was unsuccessful. This was because sweep tubes are designed for switching, not for linear operation. from there, they are good switchers (I made a SMPS with a 33GY7) but they are very nonlinear and have low plate dissipation compared with audio ones.
Thanks, Osvaldo. Please take a look at the curves for the 6DQ5, and the 6HJ5. I think you will change your mind about non linearity.
McIntosh used sweep tubes very successfully in some of their units. Trick is to apply them correctly. And these have Pd of 24W, other members have driven some types to way over published Pds with success. These are very, very ruggedly built tubes.
The proof will be in the building and listening!
Schematic hopefully next week.
Rene
McIntosh used sweep tubes very successfully in some of their units. Trick is to apply them correctly. And these have Pd of 24W, other members have driven some types to way over published Pds with success. These are very, very ruggedly built tubes.
The proof will be in the building and listening!
Schematic hopefully next week.
Rene
What Va are you running the big beasts at?
Sent from my phone with Tapatalk. Please excuse any typpos.
Sent from my phone with Tapatalk. Please excuse any typpos.
Let me refer you to the initial post for any details but to answer your question, the +B is 360V Current will peak to about 350mA at full output, a breeze for the 6HJ5.
Amp schematic hopefully next week, PS schematic may be some week or weeks later.
Amp schematic hopefully next week, PS schematic may be some week or weeks later.
I use 20 Ohm resistor in series with primary shunted by contacts of relay powered from a filament DC. As soon as all voltages are almost up the relay shunts the resistor.
Sounds like a variation of the soft start MOSFET but with a relay. Are you timing it or actually measuring the heater voltages? I'm comfortable with the MOSFET and a delayed gate drive, or even a hybrid with a resistor across the MOSFET and then saturate it, a true variation of your idea. Remember the concern is the heaters. The high voltage regulators have their own soft start 😀
Thanks!
What it has to do with MOSFET? All resistances and capacitances in all rectifiers all together plus cold filaments define the summary time constant. As soon as the voltage is enough for the relay, it shorts the resistor. Simple digital positive feedback by a voltage. Cheap, efficient, reliable.
At least a dozen (that I know of) of the 125 WPC version of Pete's big red board were built with 6HJ5's on 600 to 650 volts into a 3300 ohm load. After about 5 years of abuse including me playing my guitar through it, the original set of tubes are still working just fine in mine.
A new amp design is in the works with several different power levels. The 100 WPC flavor will get 6HJ5's. The bigger ones will get bigger tubes. As with this design the power supply will be rather complicated, but likely for different reasons.
You don't know what you're discussing. Sweep tubes are some of the most linear finals for audio. I did a project using the 6BQ6 as an audio final. Not only was it a helluvalot more linear than the audio final: the 807, it made more than twice the power of the 6V6 -- famous for its sonic performance. The 6BQ6 is like a 6V6 on steroids.
I've run loadlines for other sweep tubes (both horizontal and vertical) and have seen similar excellent harmonic performances predicted. This isn't any great surprise since CRT scanning needs linear finals. The only reason sweep tube specs don't mention audio is because the most linear part of the plate characteristic id well within red plate territory if operating in Class A. Otherwise, these make for excellent Class AB, PP finals.
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