ive actually abandoned this psu circuit for my preamp in favor of a bridge rectifier/voltage double circuit after messing it up. But id like to figure out how to fix the circuit so i can learn something.
attached is a schematic of the psu circuit i was using.
While tinkering the -28v broke lose from the board and i didnt notice. It was touching the positive heater coil wire for the first pin when i turned on the amp, and it mustve created an arc. It soldered itself to the pin. And i def didnt do that.
It was on long enough to totally fry R9 til arc, snd it straight up melted Tr4, it killed Tr3.
I replaced r9, and both transistors, and the PSU is working again with stable voltage output, it powered the preamp just fine and sounded fantastic, HOWEVER, there is now a dull hum that stays the same volume no matter where i put the volume knob on the preamp. It sounds like 60hz, but im getting it out of my mid bass drivers too, which xover at 120hz. So it may be both 60 and 120 together im not sure.
im a total novice, so at that point i just built a bridge rectifier and voltage double to power the preamp from a single transformer that outputs both 12v and 26v
If anyone feels like helping me figure out how to diagnose what's wrong, id love to fix the initial PSU so i learn something about what happened. Thinking maybe i damaged a diode and its leaky now? idk
Amplifier section schematic too, but i dont think its totally necessary. But not sure so posting it too anyway
I dont think i really like the original PSU design, seems quite complicated for the result it gives, but it ran much cooler then my current one, and performed identically, and once c2-8 were all swapped with 2200 uf it was DEAD silent until the fireworks lol
Edit:: I did experiment with adding extra filtering caps, nealy 10,000uf no change. Other unused sets of tubes, no change. Gotta be the psu circuit right? I shoved a lot of current through that negative rail pretty sure.
attached is a schematic of the psu circuit i was using.
While tinkering the -28v broke lose from the board and i didnt notice. It was touching the positive heater coil wire for the first pin when i turned on the amp, and it mustve created an arc. It soldered itself to the pin. And i def didnt do that.
It was on long enough to totally fry R9 til arc, snd it straight up melted Tr4, it killed Tr3.
I replaced r9, and both transistors, and the PSU is working again with stable voltage output, it powered the preamp just fine and sounded fantastic, HOWEVER, there is now a dull hum that stays the same volume no matter where i put the volume knob on the preamp. It sounds like 60hz, but im getting it out of my mid bass drivers too, which xover at 120hz. So it may be both 60 and 120 together im not sure.
im a total novice, so at that point i just built a bridge rectifier and voltage double to power the preamp from a single transformer that outputs both 12v and 26v
If anyone feels like helping me figure out how to diagnose what's wrong, id love to fix the initial PSU so i learn something about what happened. Thinking maybe i damaged a diode and its leaky now? idk
Amplifier section schematic too, but i dont think its totally necessary. But not sure so posting it too anyway
I dont think i really like the original PSU design, seems quite complicated for the result it gives, but it ran much cooler then my current one, and performed identically, and once c2-8 were all swapped with 2200 uf it was DEAD silent until the fireworks lol
Edit:: I did experiment with adding extra filtering caps, nealy 10,000uf no change. Other unused sets of tubes, no change. Gotta be the psu circuit right? I shoved a lot of current through that negative rail pretty sure.
Attachments
Last edited:
Is the ground connected as before, the grounding should be exactly as the schematic shows. Also check C7.
So I noticed your recent posts about your continuing adventures with this little preamp. I'm not familiar with those types of power supplies (dual rail with + and - voltages) so I can't offer a post mortem analysis.
But since you've destroyed the power supply and say you don't have much of the original device left I would suggest that this is a perfect opportunity to improve what you had.
The place to start is by looking at the tube data sheet so you can optimize the circuit for the particular tube you want to use. It seems you prefer the sound of the 6AG5 to the original tube. Fortunately, the data sheet for the 6AG5 gives operating points and plate curves for operating the tube in triode mode, which is how these preamps run their tubes.
I'd suggest that you ditch the overly complex dual rail supply for a conventional linear supply. This will be much simpler to build and it will allow you to operate the tube at the operating points specified in the data sheet.
The stock design ran the tubes at a fraction of the recommended voltage. That's the biggest flaw in these. Why repeat that mistake? Since you need to replace the PS you might as well do it right this time.
Total parts count for a proper linear supply is much lower too. One power transformer will supply both the B+ voltage and the proper heater voltage. You'll also need 2 diodes, 3 electrolytic caps, 1 choke and 1 resistor. Actually, that would be the more "deluxe" version. You could probably even eliminate one of the caps and the choke.
Here's the data sheet: https://tubedata.wernull.com/sheets/093/6/6AG5.pdf
Notice that one operating point runs the tube with 180v on the plate and the other runs it at 250v. Plate voltage is measured from plate to cathode, BTW. Your PS (B+) voltage will be higher, of course, since your plate load resistor will drop some of it. When someone measured the FX version for me several years ago they found that the stock PS ran the stock tubes with just 48v between the plate and cathode. I expect that this version runs very similar voltages.
Bottom line, if you run the tube as it was designed you'll get better results. That holds true whether you use a dual rail supply or a linear supply.
But since you've destroyed the power supply and say you don't have much of the original device left I would suggest that this is a perfect opportunity to improve what you had.
The place to start is by looking at the tube data sheet so you can optimize the circuit for the particular tube you want to use. It seems you prefer the sound of the 6AG5 to the original tube. Fortunately, the data sheet for the 6AG5 gives operating points and plate curves for operating the tube in triode mode, which is how these preamps run their tubes.
I'd suggest that you ditch the overly complex dual rail supply for a conventional linear supply. This will be much simpler to build and it will allow you to operate the tube at the operating points specified in the data sheet.
The stock design ran the tubes at a fraction of the recommended voltage. That's the biggest flaw in these. Why repeat that mistake? Since you need to replace the PS you might as well do it right this time.
Total parts count for a proper linear supply is much lower too. One power transformer will supply both the B+ voltage and the proper heater voltage. You'll also need 2 diodes, 3 electrolytic caps, 1 choke and 1 resistor. Actually, that would be the more "deluxe" version. You could probably even eliminate one of the caps and the choke.
Here's the data sheet: https://tubedata.wernull.com/sheets/093/6/6AG5.pdf
Notice that one operating point runs the tube with 180v on the plate and the other runs it at 250v. Plate voltage is measured from plate to cathode, BTW. Your PS (B+) voltage will be higher, of course, since your plate load resistor will drop some of it. When someone measured the FX version for me several years ago they found that the stock PS ran the stock tubes with just 48v between the plate and cathode. I expect that this version runs very similar voltages.
Bottom line, if you run the tube as it was designed you'll get better results. That holds true whether you use a dual rail supply or a linear supply.
The 6AG5 has a 6.3Volt 0.3 Amp filament, when the filament is at its normal heat (all warmed up) . . .
6.3V / 0.3A = 21 Ohms.
But, when the filament is Cold, it is probably about 5 Ohms; two in series are about 10 Ohms;
42 Ohms After the filaments are warm.
You have a single 1 Amp 1N4007 diode in a Half Wave rectifier mode.
You have 12Vrms on the transformer secondary (17V peak).
17V / 10 Ohms (two 6AG5 Cold filaments in series).
17V / 10 Ohms = 1.7 Amps peak.
Actually, at power up, the uncharged filter capacitor by itself is far less than 10 Ohms.
Ouch!
Capacitor input filters usually draw peak currents that are 5 to 10 times the DC load current (if the capacitor is driven by a full wave rectifier circuit).
Capacitor input filters usually draw peak currents that are 10 to 20 times the DC load current (if the capacitor is driven by a half wave rectifier circuit).
(I am talking about the DC load current After the tube filaments are warm).
If the 1N4007 does not die, the secondary of the power transformer must be getting very hot.
6.3V / 0.3A = 21 Ohms.
But, when the filament is Cold, it is probably about 5 Ohms; two in series are about 10 Ohms;
42 Ohms After the filaments are warm.
You have a single 1 Amp 1N4007 diode in a Half Wave rectifier mode.
You have 12Vrms on the transformer secondary (17V peak).
17V / 10 Ohms (two 6AG5 Cold filaments in series).
17V / 10 Ohms = 1.7 Amps peak.
Actually, at power up, the uncharged filter capacitor by itself is far less than 10 Ohms.
Ouch!
Capacitor input filters usually draw peak currents that are 5 to 10 times the DC load current (if the capacitor is driven by a full wave rectifier circuit).
Capacitor input filters usually draw peak currents that are 10 to 20 times the DC load current (if the capacitor is driven by a half wave rectifier circuit).
(I am talking about the DC load current After the tube filaments are warm).
If the 1N4007 does not die, the secondary of the power transformer must be getting very hot.
Thank you both. It won't let me quote for some reason rn. But thank you anyway. All very useful information lol.
And no, ive actually been quite shocked how cool everything was running. even the diodes and everything have stayed cool to the touch. All but a 22ohm 3 watt resistor that had no business being anything over 2ohm and under 5w. I had a 1amp transformer at first. Quickly replaced with a larger one, and it was just warn to the touch.
I already had much higher amperage rated diodes on hand expecting them to fail honestly. But this first board i built not thinking about easy reparability either.
Looks like ill be rebuilding a power supply again this weekend. Really im just using this preamp to figure out wth im doing. Keep finding myself overloaded with information im not looking for trying to find the info i am looking for, so big thanks to both of you! I did build a simpler power supply but im still giving 52v across the plate and plan on improving all that. It already sounds plenty good enough for me to keep moving on. Not like these components are pricey.
And no, ive actually been quite shocked how cool everything was running. even the diodes and everything have stayed cool to the touch. All but a 22ohm 3 watt resistor that had no business being anything over 2ohm and under 5w. I had a 1amp transformer at first. Quickly replaced with a larger one, and it was just warn to the touch.
I already had much higher amperage rated diodes on hand expecting them to fail honestly. But this first board i built not thinking about easy reparability either.
Looks like ill be rebuilding a power supply again this weekend. Really im just using this preamp to figure out wth im doing. Keep finding myself overloaded with information im not looking for trying to find the info i am looking for, so big thanks to both of you! I did build a simpler power supply but im still giving 52v across the plate and plan on improving all that. It already sounds plenty good enough for me to keep moving on. Not like these components are pricey.
Last edited:
Often, it is not necessary to use DC filaments for a preamp.
But if it needs DC for the filaments to keep the hum down, then . . .
A full wave bridge rectifier works better than a half wave rectifier. Of course, details such as the Ohms value of a dropping resistor value will change.
Full wave is easier on the secondary of the power transformer, and full wave also gives lower hum, even if the capacitance of the filter cap is unchanged.
But if it needs DC for the filaments to keep the hum down, then . . .
A full wave bridge rectifier works better than a half wave rectifier. Of course, details such as the Ohms value of a dropping resistor value will change.
Full wave is easier on the secondary of the power transformer, and full wave also gives lower hum, even if the capacitance of the filter cap is unchanged.
the 6ag5 have slightly nosier filaments pretty consistently then the 5654 or 6au6 i just liked the way they sound in this circuit more. I still had to crank my power amp to 100% and turn the volume of the preamp down to just under half to hear any filament noise at all though. its not been enough to bother me.
Im probably going to replace the diodes, see if that fixes anything. And if it does, ill put it in a drawer somewhere and call it my first functioning (technically) handmade circuit. Although ive tinkered with guitars for many years so thats not really true.
Im probably going to replace the diodes, see if that fixes anything. And if it does, ill put it in a drawer somewhere and call it my first functioning (technically) handmade circuit. Although ive tinkered with guitars for many years so thats not really true.
I like the sound of all my amplifiers with lots more than 2 x 28V = 56 Volts across the tubes.
"You should make things as simple as possible, but no simpler" - Albert Einstein
"You should make things as simple as possible, but no simpler" - Albert Einstein
I honestly already feel pretty dumb for spending the last handful of years bashing tubes. I've not been a fan of tube hifi power sections i've heard in person, at all really. until learning more about the amplifiers themselves i had never even separated voltage based and current based amplification in my head. It was all just amplification. And it either sounded good or didnt. (not that good tube power amps arent a thing. ive been rather judgemental in my audio journey until now.
I watched some interviews with Bascom King, and he was raving about solid state power sections (mosfet in particular, which i plan on some point trying to get into), with tube preamps and decided to see if i could get into the diy side of it to see what thats all about for as little monetary investment as possible. Rent in murder these days.
I have a yamaha s801, a ps audio sprout, a couple fosi amps and a couple amazon chip amps. Every one of them, even the weird fizzy class D topologies sound better from a musicality and enjoyment perspective with this thing, and its far from optimized as a line stage preamp. This is probably a new lifelong interest, i can already tell.
One last question, is there still reason to run that much voltage across the plates when gain/amplification isnt exactly the goal and your already getting 6db of clean gain you dont exactly need? The benefits to the soundstage are already ridiculously obvious. Is there a physical downside to running plates low like there is to heaters? I already cant come up with fault to the sound besides extreme worst case scenario heater hum and i definitely dont need any line level gain in any part of my setups. I may not be totally grasping something there though.
Im not trying to say i dont want to, just asking questions lol.
I watched some interviews with Bascom King, and he was raving about solid state power sections (mosfet in particular, which i plan on some point trying to get into), with tube preamps and decided to see if i could get into the diy side of it to see what thats all about for as little monetary investment as possible. Rent in murder these days.
I have a yamaha s801, a ps audio sprout, a couple fosi amps and a couple amazon chip amps. Every one of them, even the weird fizzy class D topologies sound better from a musicality and enjoyment perspective with this thing, and its far from optimized as a line stage preamp. This is probably a new lifelong interest, i can already tell.
One last question, is there still reason to run that much voltage across the plates when gain/amplification isnt exactly the goal and your already getting 6db of clean gain you dont exactly need? The benefits to the soundstage are already ridiculously obvious. Is there a physical downside to running plates low like there is to heaters? I already cant come up with fault to the sound besides extreme worst case scenario heater hum and i definitely dont need any line level gain in any part of my setups. I may not be totally grasping something there though.
Im not trying to say i dont want to, just asking questions lol.
Last edited:
Ill probably leave the amp circuit how it is since im happy with the performance, throw it back in the cigar box and leave it as a conversation piece (if it wont damage itself at 52v anyway). And just go about trying to put an separate design together that makes more proper use of the tubes.
p.s. I replaced the 5 diodes with much much beefier ones from an old dead Panasonic 8 track and its functioning silently again. for now lol. Im not sure i have a way to test diodes individually, and replacing all of them was easier anyway with the convoluted way i built that one.
When i turn it on, no pops, when i turn it off, the sound just fades slowly to nothing. No pops. When its turned on cold, theres about half a second of hum then total silence unless i worst case scenario the heater noise once the tubes are warm. Although in its current form, it does have like 10,000uf of filtration lol. I can also grasp the transformer firmly indefinitely without it burning my hand. based on people's reactions to this circuit, im kind of suprised it works as well as it does.
p.s. I replaced the 5 diodes with much much beefier ones from an old dead Panasonic 8 track and its functioning silently again. for now lol. Im not sure i have a way to test diodes individually, and replacing all of them was easier anyway with the convoluted way i built that one.
When i turn it on, no pops, when i turn it off, the sound just fades slowly to nothing. No pops. When its turned on cold, theres about half a second of hum then total silence unless i worst case scenario the heater noise once the tubes are warm. Although in its current form, it does have like 10,000uf of filtration lol. I can also grasp the transformer firmly indefinitely without it burning my hand. based on people's reactions to this circuit, im kind of suprised it works as well as it does.
Last edited:
As i dig more, even here on DIY audio back through the last 2 decades i do find several gain stage schematics for 6au6 that run 50-70v. simulated and implemented both. 0.16% was the highest thd measurement i've seen in that range so far as well. It least with 6AU6 and 6AK5. both of which perceivably distort sooner as voltage drops then the 6ag5. 6ag5 just has a higher average noise floor.
And i find several implications and claims, that 6ag5, 6au6, and 6ak5 all perform just fine in terms of distortion and output when used for audio use and non RF purposes in the 50-100v range on the plates.
My own measurements of the circuit seem to confirm that as well. Hell when i turn the preamp off with meter hooked up to b+, the voltage is down into the 30s and output WAY down before i can even hear the signal distorting at all with any of my 6ag5. The 6ak5 actually start distorting as plate voltage drops much quicker then the 6ag5 do. It's almost immediate after flipping the switch. Which gives me hints as to why i didnt care for them in this implementation as much maybe? 6au6 and 6ag5 definitely perform just fine in this voltage range to ear and measurement both. At least for audio.
I probably will hang out in the sub 100v area for a while while gathering my bearings on all of this. Out of safety sake. And when i move on to higher voltages ill probably be a bit more interested in the 12a* tubes anyway. that kind of voltage gives me quite a bit of anxiety.
Im not going to damage the tubes at all doing this if heaters are dialed in. In fact they'll probably actually last longer as far as my research is informing me.
*I have made a MUCH simpler full wave bridge/doubler DC supply circuit running from 26.5vac providing stable 53v, and a full wave rectification circuit for the heaters from the 12vac output on the trans. Is a single shielded EI transformer, 26vac and 12vac outputs.
I used 3a diodes as well, and my transformer is indeed running much cooler to the touch now without the janky circuit i was using..
I also flipped around the positions of the 100k and 470k resistors in the gain stage, As im pretty sure i misunderstood which 100k resistors were being referenced as needing a value change once i began actually researching this and digging up circuits lol.
And i find several implications and claims, that 6ag5, 6au6, and 6ak5 all perform just fine in terms of distortion and output when used for audio use and non RF purposes in the 50-100v range on the plates.
My own measurements of the circuit seem to confirm that as well. Hell when i turn the preamp off with meter hooked up to b+, the voltage is down into the 30s and output WAY down before i can even hear the signal distorting at all with any of my 6ag5. The 6ak5 actually start distorting as plate voltage drops much quicker then the 6ag5 do. It's almost immediate after flipping the switch. Which gives me hints as to why i didnt care for them in this implementation as much maybe? 6au6 and 6ag5 definitely perform just fine in this voltage range to ear and measurement both. At least for audio.
I probably will hang out in the sub 100v area for a while while gathering my bearings on all of this. Out of safety sake. And when i move on to higher voltages ill probably be a bit more interested in the 12a* tubes anyway. that kind of voltage gives me quite a bit of anxiety.
Im not going to damage the tubes at all doing this if heaters are dialed in. In fact they'll probably actually last longer as far as my research is informing me.
*I have made a MUCH simpler full wave bridge/doubler DC supply circuit running from 26.5vac providing stable 53v, and a full wave rectification circuit for the heaters from the 12vac output on the trans. Is a single shielded EI transformer, 26vac and 12vac outputs.
I used 3a diodes as well, and my transformer is indeed running much cooler to the touch now without the janky circuit i was using..
I also flipped around the positions of the 100k and 470k resistors in the gain stage, As im pretty sure i misunderstood which 100k resistors were being referenced as needing a value change once i began actually researching this and digging up circuits lol.
Last edited:
Im assuming there's nothing wrong with feeding the heaters from an isolated 6v dc supply in parallel and listening to the amp as the psu discharges to see when the perceivable distortion starts, yea? If thats valid in some way, then 6ag5 can indeed provide usable amplification of a line level signal as low as about 38v. 6ak5 about down to about 48v. And 6AU6 wont give any amplification, but still sounds good down to 30v flat on the plates.
Procedure was dc meter in parallel on the B+. Flip the preamp off with heaters remaining powered, then watch the voltage drop while listening to music from a strong source. Preamp 50k pot at about 75%, the loudest i ever have it.
There genuinely appears to be nothing at all wrong with running these particular tubes at these voltages in triode in audio bands according to my ear and interface both. Cannot speak for RF band although without the rf bypass cap added this thing is flat to basically 300k and amplifies to nearly 500k. Would seem the chinese 6K4 often put in circuits like this is just their modern variant of the 6au6 according to the translation charts. Spec is same same. Does not surprise me these chinese companies offer 6K4 tubes as "upgrades" for 6j1 in many of their products. They both sound better, and handle the lower plate voltages with less distortion. They do indeed perform better in a circuit like this then 6j1/6ak5 if my voltage drop test, my interface input, or my ears that have been mixing and mastering for years, say anything accurate. I would assume my overly excessive use of filter caps is whats making such a slow fade possible lol. theres 4 4700uf and 4 680uf nichicon pw on the new bridge doubler. (i can probably remove a few, i just wanted to guarantee zero ripple first go with it)
These tubes are very usably clean at low plate voltage, you just can't get much line gain from them. Which is pointless for me anyway lol
I'd probably be using the 6AU6 if i didnt think the 6AG5 sounded better (I also have more of them)
I promise no more multi posts 😅 And im not just trying to be a contrarian. There's no way there aren't other beginners like myself who would have otherwise turned away from the DIY side if 200V+ charged capacitors were their only option to gain hand on experience.
All that hullabaloo said. Im actually planning my first tube rectified 12a*7 build preamp now 😅 🤣🤣
Procedure was dc meter in parallel on the B+. Flip the preamp off with heaters remaining powered, then watch the voltage drop while listening to music from a strong source. Preamp 50k pot at about 75%, the loudest i ever have it.
There genuinely appears to be nothing at all wrong with running these particular tubes at these voltages in triode in audio bands according to my ear and interface both. Cannot speak for RF band although without the rf bypass cap added this thing is flat to basically 300k and amplifies to nearly 500k. Would seem the chinese 6K4 often put in circuits like this is just their modern variant of the 6au6 according to the translation charts. Spec is same same. Does not surprise me these chinese companies offer 6K4 tubes as "upgrades" for 6j1 in many of their products. They both sound better, and handle the lower plate voltages with less distortion. They do indeed perform better in a circuit like this then 6j1/6ak5 if my voltage drop test, my interface input, or my ears that have been mixing and mastering for years, say anything accurate. I would assume my overly excessive use of filter caps is whats making such a slow fade possible lol. theres 4 4700uf and 4 680uf nichicon pw on the new bridge doubler. (i can probably remove a few, i just wanted to guarantee zero ripple first go with it)
These tubes are very usably clean at low plate voltage, you just can't get much line gain from them. Which is pointless for me anyway lol
I'd probably be using the 6AU6 if i didnt think the 6AG5 sounded better (I also have more of them)
I promise no more multi posts 😅 And im not just trying to be a contrarian. There's no way there aren't other beginners like myself who would have otherwise turned away from the DIY side if 200V+ charged capacitors were their only option to gain hand on experience.
All that hullabaloo said. Im actually planning my first tube rectified 12a*7 build preamp now 😅 🤣🤣
Last edited:
I'm not sure why these low voltage preamps do not use tubes that were designed for battery operation, like for example a DL96, they are designed to work at low plate voltages. I understand those are not produced anymore, but there are plenty NOS around for the DIYer.
baudouin0,
You are probably correct.
The commonly used quote is likely just a Redaction of his lecture.
Here is an interesting one:
KISS_principle
Albert Einstein, correctly, or incorrectly was quoted as saying: "There are limits to Genius, but no limits to stupidity"
From the Book of Lists:
Barths distinction: "In this world there are two types of people, those who divide people into two types, and those who don't"
You are probably correct.
The commonly used quote is likely just a Redaction of his lecture.
Here is an interesting one:
KISS_principle
Albert Einstein, correctly, or incorrectly was quoted as saying: "There are limits to Genius, but no limits to stupidity"
From the Book of Lists:
Barths distinction: "In this world there are two types of people, those who divide people into two types, and those who don't"
Tha(t)oneGuy,
"It is better to have Built, than to never have tried"
Happy building and Happy Listening!
"It is better to have Built, than to never have tried"
Happy building and Happy Listening!
I don't think that listening to the momentary sound differences that you hear when you turn off the power and the voltage steadily drops as the caps discharge is a valid way to determine much of anything but especially sound quality. You really need to be able to listen carefully at a particular voltage / operating point for an extended period of time. Simply knowing that it can pass an audible signal at a given voltage doesn't really tell you much.
When I played around with the audio portion of this circuit years ago I was surprised that the stock lower voltage op point sounded as good as it did. But I also found that I much preferred the results when running the tubes at higher voltages that were closer to what the tube data sheet recommends.
Perhaps you would come to a different conclusion and prefer different operating points. But until you are able to listen at different voltages and evaluate them in the same manner you won't know.
What you have heard is based on running them at voltages that are significantly lower than what the data sheet recommends and, when you turn off the power, at even lower voltages that are not stable but continuously dropping. Maybe it's time to listen to them at some higher voltages.
Data sheet operating points don't have to be strictly adhered to but they're usually a good place to start. The people who designed the tube probably knew a little bit about what they were doing. The only reason these preamps run such low voltages is because they are cheaper to manufacture and there's more profit to be made. If they can be run on a wall wart power supply the price is kept very low and they can sell many more units, mostly to people who are curious about tubes and are willing to take a chance since the price is so low.
If you study tube data sheets you'll see that in most cases a tube's mu (amplification factor) is the same at different operating points. Perhaps this doesn't hold true if you run them at op points that are extreme. The 6AG5 seems to vary a tiny bit but if you look at the data sheet you'll notice that the mu is actually higher at 180v (mu 45) than at 250v (mu 42).
You should also get familiar with using the plate curves in the data sheets to draw load lines. I'm not an expert at this but a load line will show you all the operating points that are possible for a given supply voltage and load resistor. If you read and study this you'll learn why some are better than others. There is an online calculator which has data for some tubes but you might have to do it by hand for others that aren't in the database.
I agree. Preamps like the FX have introduced a lot of people to tubes and, in the case of the kits, to DIY. I'm in favor of anything that gets people into tubes and I encourage you to continue experimenting. If you improve your procedures you'll have much better basis to determine which tubes and operating points work best for you.
Working around high voltages just requires some basic safety procedures. When measuring voltages you won't get shocked as long as the only thing you're touching is the plastic grips on the meter probes. (Assuming you're not playing around with transmitting tubes that run at 1000v!) It's really not any different than plugging something into the wall socket without touching the metal blades of the plug. Caps will often discharge partially on their own as you know. To complete the process I just use a 100 ohm 10w cement wirewound resistor and a clip lead between the cap and ground. After a few seconds I dispense with the resistor and connect the cap to ground using the clip lead. No more voltage. But double check with the meter and check all the caps.
You've said that you don't really need any significant gain and the tubes you've been using produce more than you need. If you already have more gain than you need it doesn't make much sense to use a tube with even more gain. The more excessive the gain, the more you shunt to ground (typically via the volume pot) and this makes it more difficult to make fine adjustments with your volume control.
Most of the 12a*7 family of tubes have a higher mu than the 6AG5. One, the 12AY7, has a mu that's the same as the 6AG5. The 12AT7, 12AZ7 and similar 5751 are all higher. The 12AX7 is the highest with a mu of 100. Only one has less than what you're using (12AU7 mu 20) and most people think other tubes with similar mu as the 12AU7 are far better choices when used as a preamp tube. See 6SN7, 6CG7, 12BH7, for example, and there are others.
Most modern amps can be driven to full power with a line level signal and no additional gain. Still, many people, myself included, often prefer using a tube with some gain rather than using a "passive preamp" (which is just a volume control) or a tube configured as a cathode follower (which has no gain). If you don't need any gain at all then even if you're using a mu of 20 you will still end up shunting some signal to ground.
I didn't experiment with the FX tubes extensively because I knew I didn't want that much gain. I was just curious so I just tried a couple of operating points and moved on. I just wanted to hear what all the fuss was about.
I was breadboarding tubes with a mu of 10 or less and I probably tried 10 or 12 different types over an extended period of time. The ones I ended up using, based on my subjective listening preferences and needs, have a mu of ~5. The basic circuit topology (the audio portion, not the power supply) is exactly the same as the FX and your kit, only the parts values are different.
Your preferences and needs may be different but if you want to experiment I would encourage you to set up a breadboard. Doing so makes experimenting much easier since you're not constrained by using a particular circuit, as you are when using a circuit board. Parts can be changed quickly and typically without even soldering anything. Different tubes can be used, even if they have different pinouts or have different bases that require different sockets. Your circuit board allows you to use similar 7 pin tubes that have the same pinout but if you want to venture beyond that and try a 12AU7 or a 6SN7, for example, you're out of luck.
If you move to a setup that gives you more options I think you'll have even more fun and success!
Moving on to 12a*7 is mainly just because i am thoroughly enjoying the whole tube audio thing, and im debating on whether or not i want to maybe work towards getting a power amp of some kind together to take a look see what goes on there too. Ive got a schematic last night of a tested 300b single ended class A power amp, and i already have everything i'd need to build aside from transformers, sockets, and the tubes. Wont take much convincing for me to pull that trigger with what im reading about 300b tubes. I have this growing idea to build a tube phono stage, to run into this 6ag5 preamp however it ends up, into a 300B power amp for my turntable setup.
And ive been enjoying this enough and gaining some confidence so i just want the experience of having done it but also still reserve the right to change every plan i have, as i have already been doing constantly as i learn more. I have bothered to learn what im actually looking at on the data sheets as well over the last couple days, ive had the sylvania 6ag5 data sheet for a bit, but that RCA one has a lot more info then the one i had. Sylvania one Didnt have any graphs just specs.
And i do actually like this circuit set up with a little gain, even though i dont need it, rather then a buffer. I found a schematic that very easily changes this circuit to a buffer setup and played with that a bit too. The THD is literally an order of magnitude lower as a buffer, but i prefer the sound with actual gain, even though i dont need any gain. Slowly but surely im being converted to the ways of the tube i feel like.
I should be able to scale my bridge double psu up quite a bit too without any modification at all. I used 160v vastly overrated nichicon caps, and if i understand the circuit correctly, if i double up on the caps in series i can prob push well over 200vdc flat with no issues from it thanks to the high UF values and 3a diodes (could be wrong there, but thats my current understanding of the circuit). If running it at a higher voltage gives me a more satisfying result then i already have... Man, i cant really conceive of that, but its something i indeed want lol. I have dug up a few circuits that run 6au6 and 6ag5 in the 120-200v range, and id be surprised if i dont start playing with those soon.
I actually have a few breadboards, and a tone of these dual sided point to point boards. They're what i was playing with to decide on the gain stage circuit as they sit right now. Which imho, is a pretty damn legitimate circuit sonically now. As well as thd and noise wise. But if better, is better, ill find out first hand, because i want better. I like better. Better is good.
@jcalvarez
thank you for that tidbit of info, because ill likely be looking into playing with those now too lol. My guess is they didnt know about them, or its because they still make 6j1, 6j3, and 6k4 tubes, which are the modern equivalent of 6ak5, 6ag5, and 6au6. They may not make a modern equivalent of those. Wouldn't surprise me if these tubes were chosen because these companies can get them for free or next to nothing, and they were aware they have acceptable performance in these voltage ranges.
I can definitely feed 40, 50, 60v, etc dc regulated to the plates after appropriately changing component values to play nicely and let the circuit settle in for some time at given voltages. Assuming someone doesn't tell me why i would blow myself up doing this.
Now i gotta refresh my memory on forum commands, i dont remember how to setup a multi quote on a forum, and i cant figure it out in the GUI lol
And ive been enjoying this enough and gaining some confidence so i just want the experience of having done it but also still reserve the right to change every plan i have, as i have already been doing constantly as i learn more. I have bothered to learn what im actually looking at on the data sheets as well over the last couple days, ive had the sylvania 6ag5 data sheet for a bit, but that RCA one has a lot more info then the one i had. Sylvania one Didnt have any graphs just specs.
And i do actually like this circuit set up with a little gain, even though i dont need it, rather then a buffer. I found a schematic that very easily changes this circuit to a buffer setup and played with that a bit too. The THD is literally an order of magnitude lower as a buffer, but i prefer the sound with actual gain, even though i dont need any gain. Slowly but surely im being converted to the ways of the tube i feel like.
I should be able to scale my bridge double psu up quite a bit too without any modification at all. I used 160v vastly overrated nichicon caps, and if i understand the circuit correctly, if i double up on the caps in series i can prob push well over 200vdc flat with no issues from it thanks to the high UF values and 3a diodes (could be wrong there, but thats my current understanding of the circuit). If running it at a higher voltage gives me a more satisfying result then i already have... Man, i cant really conceive of that, but its something i indeed want lol. I have dug up a few circuits that run 6au6 and 6ag5 in the 120-200v range, and id be surprised if i dont start playing with those soon.
I actually have a few breadboards, and a tone of these dual sided point to point boards. They're what i was playing with to decide on the gain stage circuit as they sit right now. Which imho, is a pretty damn legitimate circuit sonically now. As well as thd and noise wise. But if better, is better, ill find out first hand, because i want better. I like better. Better is good.
@jcalvarez
thank you for that tidbit of info, because ill likely be looking into playing with those now too lol. My guess is they didnt know about them, or its because they still make 6j1, 6j3, and 6k4 tubes, which are the modern equivalent of 6ak5, 6ag5, and 6au6. They may not make a modern equivalent of those. Wouldn't surprise me if these tubes were chosen because these companies can get them for free or next to nothing, and they were aware they have acceptable performance in these voltage ranges.
I can definitely feed 40, 50, 60v, etc dc regulated to the plates after appropriately changing component values to play nicely and let the circuit settle in for some time at given voltages. Assuming someone doesn't tell me why i would blow myself up doing this.
Now i gotta refresh my memory on forum commands, i dont remember how to setup a multi quote on a forum, and i cant figure it out in the GUI lol
Last edited: