Hi everyone! I'm an EE, but pretty newbie to DIY vacuum amplifiers. After weeks of researches on books, forums, and youtube videos, I've some idea of how hi-fi schematics work. I'm now trying to finalize a schematic decision in order to order parts that I need. However, I've got few questions.
First of all, I can hear you all asking "What do you want?". Here's what I want:
Now, here are my questions.
Power Supply
I'm planning to use a transformer with no tap (because it's cheaper) and diode bridge. I know I can use higher capacitance values w.r.t. tube rectifier and this will improve my ripple rejection. However, is it ok if I use no choke in that case (they are expensive)?
Preamp Tube Selection
In preamp stage, I've read that 12AU7 is not as linear as 12AX7. That's why I'm preferring 12AX7 even though it has unnecessary amount of gain for me. Would you suggest using 12AT7 or should I go with 12AX7?
Preamp Design
I've seen something as 4S universal preamp for 12A*7. Many schematics around the web uses a variation of this preamp. However, I've read somewhere that it is a "fiasco" and there shall not be something called universal preamp. I've seen many references on how to design output stage, but I don't really know how to design preamp stage. What do you think about this 4S Universal Preamp? What should I consider for an 12AX7 preamp?
Here is the schematic I'm talking about.
Negative Feedback
I've seen some very good sounding design that has no NFB. Would you suggest NFB or not? Why?
Some Designs I've Found
This one also uses the "4S Universal Preamp"
This one is from kleydejong of tdpri.com. I'm planning to build something like this, maybe without NFB. What do you think about this?
Thank you! 🙂
First of all, I can hear you all asking "What do you want?". Here's what I want:
- 6V6 SE
- 8-10 Watts Output
- Ultra-Linear (Maybe switched triode)
- 12AX7 preamp
- Diode rectification
- Mid-level volume operation
- Input will be line level (from pc or turntable with line level amp)
- Pure-natural sound
Now, here are my questions.
Power Supply
I'm planning to use a transformer with no tap (because it's cheaper) and diode bridge. I know I can use higher capacitance values w.r.t. tube rectifier and this will improve my ripple rejection. However, is it ok if I use no choke in that case (they are expensive)?
Preamp Tube Selection
In preamp stage, I've read that 12AU7 is not as linear as 12AX7. That's why I'm preferring 12AX7 even though it has unnecessary amount of gain for me. Would you suggest using 12AT7 or should I go with 12AX7?
Preamp Design
I've seen something as 4S universal preamp for 12A*7. Many schematics around the web uses a variation of this preamp. However, I've read somewhere that it is a "fiasco" and there shall not be something called universal preamp. I've seen many references on how to design output stage, but I don't really know how to design preamp stage. What do you think about this 4S Universal Preamp? What should I consider for an 12AX7 preamp?
Here is the schematic I'm talking about.
Negative Feedback
I've seen some very good sounding design that has no NFB. Would you suggest NFB or not? Why?
Some Designs I've Found
This one also uses the "4S Universal Preamp"
An externally hosted image should be here but it was not working when we last tested it.
This one is from kleydejong of tdpri.com. I'm planning to build something like this, maybe without NFB. What do you think about this?
Thank you! 🙂
Hi,
just some comments at a first glance:
-if you have line level already, you will not require the additional 4s preamp if you consider without nfb,
-6v6GT SE in UL will give you pwr in the ball park of 5W,
-I would use some (local) nfb, but it's diy, so you may try out,
-nice help designing from scratch: LTSpice with tube models, just search the forum,
Marcus
just some comments at a first glance:
-if you have line level already, you will not require the additional 4s preamp if you consider without nfb,
-6v6GT SE in UL will give you pwr in the ball park of 5W,
-I would use some (local) nfb, but it's diy, so you may try out,
-nice help designing from scratch: LTSpice with tube models, just search the forum,
Marcus
I've seen in the datasheet that output power is 5W. However, I've talked with some designers and they told me to have a higher output transformer because it can drive higher than 5W. Does this have any drawbacks?
As already mentioned, you won't get 8-10w output from a single 6V6. If you really need that much power you'll need to use a different tube or build a PP amp. And a 6V6 in triode will cut your power output in half, at least.
Some people try to squeeze as much power out of a tube as possible. I often wonder what the point is. Remember, twice as much power only results in a 3db increase in sound output. It takes 10x the wattage to make something sound, subjectively, "twice as loud". If you want to hear what a 3db increase sounds like, just stand 39" (1m) closer to your speakers. It's not much. And that 3db increase is at full power output. Under normal listening conditions it's typically unnoticeable.
No, you don't need to use a choke. The majority of SE 6V6 and similar 6BQ5 amps didn't use one. These small SE amps were often used in various brands of lower end record players or console stereos and, if you don't want to build totally from scratch, a lot of people start with a chassis pulled from one of them.
You might get some ideas from this thread on AK: Magnavox Flea Power: Getting More Out Of The 8600 Series - A Lot More! | Audiokarma Home Audio Stereo Discussion Forums
The 6BQ5 was used more often than the 6V6 and those schematics can typically be used with 6V6s too. Also consider the 6AQ5 which is similar to the 6V6 in a 7 pin miniature bottle.
You can use either the 12BH7 or the 6CG7 in place of a 12AU7. The 12BH7 has the same pinout, the 6CG7's heater is not center tapped so it will need to be wired slightly differently.
Also, there is no reason why you need to stick with a 9 pin miniature on the input. There are also tubes with octal and loctal bases that will work. Or, if you want to save some money on tubes, the 7C5 is the loctal version of the 6V6 and there are also versions with 12.6v heaters, the 12V6 and 14C5.
Some people try to squeeze as much power out of a tube as possible. I often wonder what the point is. Remember, twice as much power only results in a 3db increase in sound output. It takes 10x the wattage to make something sound, subjectively, "twice as loud". If you want to hear what a 3db increase sounds like, just stand 39" (1m) closer to your speakers. It's not much. And that 3db increase is at full power output. Under normal listening conditions it's typically unnoticeable.
No, you don't need to use a choke. The majority of SE 6V6 and similar 6BQ5 amps didn't use one. These small SE amps were often used in various brands of lower end record players or console stereos and, if you don't want to build totally from scratch, a lot of people start with a chassis pulled from one of them.
You might get some ideas from this thread on AK: Magnavox Flea Power: Getting More Out Of The 8600 Series - A Lot More! | Audiokarma Home Audio Stereo Discussion Forums
The 6BQ5 was used more often than the 6V6 and those schematics can typically be used with 6V6s too. Also consider the 6AQ5 which is similar to the 6V6 in a 7 pin miniature bottle.
You can use either the 12BH7 or the 6CG7 in place of a 12AU7. The 12BH7 has the same pinout, the 6CG7's heater is not center tapped so it will need to be wired slightly differently.
Also, there is no reason why you need to stick with a 9 pin miniature on the input. There are also tubes with octal and loctal bases that will work. Or, if you want to save some money on tubes, the 7C5 is the loctal version of the 6V6 and there are also versions with 12.6v heaters, the 12V6 and 14C5.
@aerenyasar
As an EE, you already know the Laws of Physics don't change to accommodate your desires. A priori, a SE 6V6 must be Class "A". The plate dissipation limit for the 6V6 is 12 W. The maximum theoretical efficiency in Class "A" is 50%. You always get less, in the real world. That 5 W. number previously mentioned is absolutely correct.
An O/P transformer whose power handling capability is > than 5 W. gets selected to obtain magnetic headroom. Said headroom is needed to avoid core saturation from a NFB associated deep bass error correction signal, when the O/P "iron" is inside a global loop.
A "2 stage" SE amp whose O/P power is in the desired range can be constructed around a 12AX7 and 2X 7591s. "Golden Age" manufacturers took advantage of the fact the small signal circuitry appropriate to "12" W. tubes, like the 6V6, works with 7591s, essentially unchanged. You can walk in their footprints. A key area of concern is 7591 grid to ground resistance limits. Current production 7591s are intolerant of taking liberties with the grid to ground resistance limit and the high plate resistance 12AX7 triode must work into a substantial load. You resolve the impedance mismatch by inserting a ZVN0545A source follower that's DC coupled to the triode's plate.
Large valued electrolytic capacitors are less costly than large inductances. Very large valued 1st filter caps. are a source of noise. Scan the archives for my posts on "hash" filtration.
As an EE, you already know the Laws of Physics don't change to accommodate your desires. A priori, a SE 6V6 must be Class "A". The plate dissipation limit for the 6V6 is 12 W. The maximum theoretical efficiency in Class "A" is 50%. You always get less, in the real world. That 5 W. number previously mentioned is absolutely correct.
An O/P transformer whose power handling capability is > than 5 W. gets selected to obtain magnetic headroom. Said headroom is needed to avoid core saturation from a NFB associated deep bass error correction signal, when the O/P "iron" is inside a global loop.
A "2 stage" SE amp whose O/P power is in the desired range can be constructed around a 12AX7 and 2X 7591s. "Golden Age" manufacturers took advantage of the fact the small signal circuitry appropriate to "12" W. tubes, like the 6V6, works with 7591s, essentially unchanged. You can walk in their footprints. A key area of concern is 7591 grid to ground resistance limits. Current production 7591s are intolerant of taking liberties with the grid to ground resistance limit and the high plate resistance 12AX7 triode must work into a substantial load. You resolve the impedance mismatch by inserting a ZVN0545A source follower that's DC coupled to the triode's plate.
Large valued electrolytic capacitors are less costly than large inductances. Very large valued 1st filter caps. are a source of noise. Scan the archives for my posts on "hash" filtration.
I'm going to second Eli here, getting anything north of 5W out of a 6V6 in SE-UL is a tall order.
The self biased schematic with the 392Ω bias resistor will give you about 4.3W at a B+ of ≈300v. If you increase the B+ to around 330v you'll add another watt to the output but you likely won't like the harmonic response. Now the difference between 4.3W and your 8W requirement is only 2.7dB. If you switch to a slightly higher efficiency speaker, you'll be set to go.
I would suggest you might want to rethink your requirement list a little bit. Think about what's important to you (like sound level and tone) verses what you think the solution should be (8W to 10W or a 12AX7 preamp).
I am not telling you what you should or shouldn't build. I just think that your requirements seem to be a mixture of function (i.e. "Pure-natural sound", "Mid-level volume operation", etc.) and physical dictate (i.e. "8-10 Watts", "12AX7 preamp", etc.). You need to decide what's really important to you. Maybe ranking your requirements in order of priority to you would be a good start.
Just a thought.
The self biased schematic with the 392Ω bias resistor will give you about 4.3W at a B+ of ≈300v. If you increase the B+ to around 330v you'll add another watt to the output but you likely won't like the harmonic response. Now the difference between 4.3W and your 8W requirement is only 2.7dB. If you switch to a slightly higher efficiency speaker, you'll be set to go.
I would suggest you might want to rethink your requirement list a little bit. Think about what's important to you (like sound level and tone) verses what you think the solution should be (8W to 10W or a 12AX7 preamp).
I am not telling you what you should or shouldn't build. I just think that your requirements seem to be a mixture of function (i.e. "Pure-natural sound", "Mid-level volume operation", etc.) and physical dictate (i.e. "8-10 Watts", "12AX7 preamp", etc.). You need to decide what's really important to you. Maybe ranking your requirements in order of priority to you would be a good start.
Just a thought.
Eli is absolutely right here - use a 7591. You get more power and you also get more gain, so you have more choice of input tube.
But if you want something more generic, just use an EL34. Sounds good in triode as well, in which case you don't need to bother with global feedback.
But if you want something more generic, just use an EL34. Sounds good in triode as well, in which case you don't need to bother with global feedback.
Here's a specimen schematic off DIY Audio - haven't analysed it in any detail
EL34 SE ultralinear schematic
EL34 SE ultralinear schematic
Attachments
I'm dubious about UL designs that do not incorporate loop NFB of some kind. Damping factor is not adequate in combination with many (most?) speakers. 🙁
A few dB. of global NFB added to UL mode "finals" improves damping factor and corrects minor aberrations of the O/P "iron".
As the OP is in Turkey, I have no idea as to what parts availability is. If relatively modest in cost Hammond magnetics are available, Hammond's 1630SEA looks quite good for a build using 7591 "finals". Plenty of the requisite magnetic headroom is present.
A few dB. of global NFB added to UL mode "finals" improves damping factor and corrects minor aberrations of the O/P "iron".
As the OP is in Turkey, I have no idea as to what parts availability is. If relatively modest in cost Hammond magnetics are available, Hammond's 1630SEA looks quite good for a build using 7591 "finals". Plenty of the requisite magnetic headroom is present.
Thank you everyone for your responses! I've considered more on output power that I want.
I'm considering all of your suggestions on 7591 and EL34, but I'm also thinking on going for lower output powers as you've suggested. @EliDuttman you're definitely right with the %50 efficiency for class A. That was also what was bothering me.
Considering 6v6s case with a theoretical 8k 5W output transformer, I've worked on a load line graph to exercise more on designing output stage. I'd be glad if you could check and tell if there are any mistakes I'm making. There are many different methods on the internet which is confusing.
Here's what I've done:
1) I've decided to set my B+ as 275V. If the load was resistive, this would give 34.37 mA, lets say roughly 35 mA.
2) I've drawn a line between 275V and 35mA.
3) Without changing the gradient, I've shifted this line up to a level close to max dissipation.
4) I've seen that my grid voltage is -19V. I've checked where my line intersects 0V and 38V. For a 0<->38V input, my output is around 140<->410V
5) I've calculated the output RMS power: ((410-140)*0.707)^2 / 8000 = 4.55W
You can see my graph in attachment
Before going into calculation of cathode resistance and capacitor, I wanted to ask if I'm on the right path. I'd be glad if you can give me a feedback.
I'm considering all of your suggestions on 7591 and EL34, but I'm also thinking on going for lower output powers as you've suggested. @EliDuttman you're definitely right with the %50 efficiency for class A. That was also what was bothering me.
Considering 6v6s case with a theoretical 8k 5W output transformer, I've worked on a load line graph to exercise more on designing output stage. I'd be glad if you could check and tell if there are any mistakes I'm making. There are many different methods on the internet which is confusing.
Here's what I've done:
1) I've decided to set my B+ as 275V. If the load was resistive, this would give 34.37 mA, lets say roughly 35 mA.
2) I've drawn a line between 275V and 35mA.
3) Without changing the gradient, I've shifted this line up to a level close to max dissipation.
4) I've seen that my grid voltage is -19V. I've checked where my line intersects 0V and 38V. For a 0<->38V input, my output is around 140<->410V
5) I've calculated the output RMS power: ((410-140)*0.707)^2 / 8000 = 4.55W
You can see my graph in attachment
Before going into calculation of cathode resistance and capacitor, I wanted to ask if I'm on the right path. I'd be glad if you can give me a feedback.
Attachments
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I see, thank you @indra1. Is this normal, or what should I do to increase the output power? I'm not asking changing the circuit or tube wise, I'm just asking for load-line analysis. For a 5W output transformer, this value should be around 5W right? Am I missing something? I'm just trying to understand how 5W SE 6V6 amps in everywhere work.
Just over a watt is all you'll get with a single 6V6 in SE with those parameters. It's a very, very nice watt, but you will need very efficient speakers. 🙂
You need to drop the load impedance, which will give higher power and higher distortion, or go to a UL or Tetrode mode if you want more. 4 watts or so in tetrode.
Or, go to a parallel 6V6 output stage at half the load impedance for an easy way to double the power. Or Triode connected push pull would be sweet for sure!
The 6V6 triode connected is one of my personal favorite applications, but it has very limited usefulness with most speakers.
You need to drop the load impedance, which will give higher power and higher distortion, or go to a UL or Tetrode mode if you want more. 4 watts or so in tetrode.
Or, go to a parallel 6V6 output stage at half the load impedance for an easy way to double the power. Or Triode connected push pull would be sweet for sure!
The 6V6 triode connected is one of my personal favorite applications, but it has very limited usefulness with most speakers.
@Lingwendil
Actually I'm planning to build an UL-SE with as less distortion I can get. In that case, is that 1W power level enough? How does load line analysis change for UL mode?
Actually I'm planning to build an UL-SE with as less distortion I can get. In that case, is that 1W power level enough? How does load line analysis change for UL mode?
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You should get a couple watts in UL, but UL almost requires the use of some negative feedback to get the output impedance down and consistent. If you do that then it will work well. I really don't have a ton of personal experience with UL but each time I've used it it was not as good as triode without some sort of feedback to iron things out. I prefer to run pentode/tetrode or triode (on occasion) for my output stages with a bit of feedback to take the edge off.
UL with some global negative feedback would be a good choice, as would a pentode driver tube with plate-to-plate feedback, but that's a bit out of my realm so I'll let someone more experienced with pentodes as drivers get into all that if you are interested there.
UL with some global negative feedback would be a good choice, as would a pentode driver tube with plate-to-plate feedback, but that's a bit out of my realm so I'll let someone more experienced with pentodes as drivers get into all that if you are interested there.
A 8 Kohm end to end primary impedance is what you use with push/pull 6V6s. Don't go above 5 Kohms in a SE setup and (FWIW) 4 Kohms seems good to me.
Power O/P tubes have short service lives, when pushed to their dissipation limits. The 12 W. plate dissipation shown on the previously linked 6V6 data sheet is a conservative Design Center system number. An extra W. would roughly be the Design Maximum system value. Yet higher is the Absolute Maximum value, which (most definitely) should never be reached.
Unlike SS devices which are mechanically robust and electrically fragile, tubes are mechanically fragile and electrically robust.
Power O/P tubes have short service lives, when pushed to their dissipation limits. The 12 W. plate dissipation shown on the previously linked 6V6 data sheet is a conservative Design Center system number. An extra W. would roughly be the Design Maximum system value. Yet higher is the Absolute Maximum value, which (most definitely) should never be reached.
Unlike SS devices which are mechanically robust and electrically fragile, tubes are mechanically fragile and electrically robust.
Ok so according to your feedbacks, here we go again:
This time I've chosen 4k transformer and 300V B+. I've done the steps before. My grid voltage is around -24V. This allows me input 0<->48V and output 160<->410. My output power is then (410-160)^2/(8*4000) = 1.95 W.
Also, I'm right now operating at 300V and 37mA, which is 11.1/14 = 79% of max dissipation.
I think it is now better. But still not sure what I should aim for to have a UL setup. I saw on the web that there are different curves for UL design, but 6v6 has not have one. Is it important to use one of them, if yes in what terms?
This time I've chosen 4k transformer and 300V B+. I've done the steps before. My grid voltage is around -24V. This allows me input 0<->48V and output 160<->410. My output power is then (410-160)^2/(8*4000) = 1.95 W.
Also, I'm right now operating at 300V and 37mA, which is 11.1/14 = 79% of max dissipation.
I think it is now better. But still not sure what I should aim for to have a UL setup. I saw on the web that there are different curves for UL design, but 6v6 has not have one. Is it important to use one of them, if yes in what terms?
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Can you give us the whole schematic, or at least the output stage - are you thinking pentode or UL or what exactly? How about the global feedback?
Any particular reason you are attached to the 6V6? I agree with Lingwendil - use an 18-25 watt output tube you can use in triode if you like it better. No global feedback needed, and should sound better than pentode or UL.
6V6 is a nice choice in push-pull. In SE it won't give you much power, as has been pointed out.
Any particular reason you are attached to the 6V6? I agree with Lingwendil - use an 18-25 watt output tube you can use in triode if you like it better. No global feedback needed, and should sound better than pentode or UL.
6V6 is a nice choice in push-pull. In SE it won't give you much power, as has been pointed out.
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@andyjevans I'm thinking of building an single ended UL. I've seen many applications of 6v6 tubes for single ended UL with moderately low output powers (5W OPT), that's why I'm sticking to them. If you have any other suggesiton that suits my needs I'd be happy to hear it.
I don't have a schematic. I've seen lots of examples for 6v6-SE-UL, but I wanted to build my own. That's why I'm stuck in trying to learn how I should move on step by step.
What I have on my mind is an SE UL output stage with cathode resistor and bypass capacitor, with an output transformer of %46 tap (or similar). I want to experiment the NFB as I build the system, checking how I like the sound.
The question in my mind right now is how much power do I need at the output (not talking about pushing to distortion limits).
I don't have a schematic. I've seen lots of examples for 6v6-SE-UL, but I wanted to build my own. That's why I'm stuck in trying to learn how I should move on step by step.
What I have on my mind is an SE UL output stage with cathode resistor and bypass capacitor, with an output transformer of %46 tap (or similar). I want to experiment the NFB as I build the system, checking how I like the sound.
The question in my mind right now is how much power do I need at the output (not talking about pushing to distortion limits).
Paul Joppa has provided us with a useful rule of thumb for mating amps and speakers. Joppa's Rule states that for "typical" listening spaces an amp/speaker combination should be capable of producing 102 dB. SPL peaks at a 1 M. distance. Given a 5 WPC SE UL 6V6 amp, speakers that are at least 95 dB. sensitive are appropriate.
Remember, more speakers are destroyed by combination with an amp driven frequently into clipping than being over driven by lots of power.