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In Reply to: I wanna tube! Please advise posted by ck42 on October 31, 2001 at 19:00:58:
You may want to consider a headphone amp.
1)It is chaeper.
2)It is easier to make one
3)Unless you already have nice speakers, you will be able to achieve a much better sound for money with headphones.When I was getting into tubes, I got my feet wet with Foreplay and Assemblege 300B SET. After putting together these kits I came to a conclusion that I will never do kits again. Don't get me wrong, I had fun doing that, but not as much as I would have by putting an amp from the scratch. Besides it was quite obviouse, especially with Foreplay, that I can do better with less money.
Now, overall I've spent quite a lot of time and money to get to where I am, but the final version of an amp is fairly inexpensive and provides great results.
Just a thought.
jenya
Follow Ups:
Jenya:Thanks for the response. Speaking of headphones, my primary listening is in fact done with cans. This is also the reason I've expressed interest is building a tube preamp vs. a tube poweramp...I can use the preamp as a headphone amp ;) ...well, that and thinking maybe it would be cheaper for whatever reason.
What you say about designing your own for less than kits...this is kinda the reason I was asking some of my questions; I don't want to buy a kit that sux.
I've been pouring over everything that I can find online regarding tube amp operation (and tube operation itself!) and have come to an early conclusion that I want to start off with a SET amp. The weird thing I seem to be seeing is that they all are basically the same circuit. It's doesn't appear that there are too many ways of designing an SET. And if this is indeed the case, then it would be a no brainer to just make one and then tweak it via better componetry and minor circuit mods. I mention this because it seems to contradict the idea of 'designing a better one' vs. the kits. If there are few ways to design it, then improving on the kits would be hard to do.........I feel like I'm just repeating myself at this poin t so I'll stop with that idea.
The one thing I haven't been able to figure out yet is the multiple tube stages that I see in most schematics. I understand the input stage, but then the plate goes right into the grid of the next stage on so on. What exactly is the purpose of this?
Oh, I've got a copy of "Beginners Guide To Tube Audio Design" from OCSL on its way to me now ;) Hoping it will shed some more light on things.
"early conclusion that I want to start off with a SET amp. The weird thing I seem to be seeing is that they all are basically the same circuit. It's doesn't appear that there are too many ways of designing an SET. "I think it is good that you decided on SET. It will give you a very good reference baseline. As far as number of different topologies for SET, I think in a crude way you are right. There are few tricks that you can play with though. Ultrapath for example. Some people would also tell you that there are 11 ways of implementing parafeed, which is another variation of SE. Power supply designes can vary from two $0.05 diodes, couple caps and resistor to a heavily regulated one. CCS loading is another trick worth of playing with. I am sure there are more.
"And if this is indeed the case, then it would be a no brainer to just make one and then tweak it via better componetry and minor circuit mods. I mention this because it seems to contradict the idea of 'designing a better one' vs. the kits."
I don't know if 'designing' a better amp would be an objective. Afterall, as far as I understand, all tube circuitry theory was pretty much complete by 1950-60. Not much you can invent there. However, ending up with a better amp than you would get by assembling a kit is a very achievable goal. As I mentioned above, there are few variable that you can play with to end up with a better sounding design.
"The one thing I haven't been able to figure out yet is the multiple tube stages that I see in most schematics. I understand the input stage, but then the plate goes right into the grid of the next stage on so on. What exactly is the purpose of this?"
After you finish reading your book, you will get a better idea how this works. In the nutshell, tube is a voltage amplifying device. When current passes through a first stage tube there will be a voltage drop across it. This voltage is applied to the grid of the next stage just like the one that was applied to the grid of the firts tube from your CD player. You are describing a dirrect coupling. Some circuits use a decoupling capacitor between stages.
By the way, if you decide on making headphone amp, you may not need multiple stages. My current version consists of only only stage that serves both for amplification and output, all thanks to a remarkable qualities of 6C45 tube. The simplest SE disign that can exist.
Regards,
Jenya
Jenya:"I don't know if 'designing' a better amp would be an objective. Afterall, as far as I understand, all tube circuitry theory was pretty much complete by 1950-60. Not much you can invent there. However, ending up with a better amp than you would get by assembling a kit is a very achievable goal. As I mentioned above, there are few variable that you can play with to end up with a better sounding design."
Hehe....this is really was I was trying to say :)
"When current passes through a first stage tube there will be a voltage drop across it. This voltage is applied to the grid of the next stage just like the one that was applied to the grid of the firts tube from your CD player."
This still seems weird to me, though I'm sure it will all make sense very soon. For right now, it seems like I'm amplifying a signal, and then using an AMPed signal just to run a grid on another amp...it doesn't seem doing this would server much purpose...that book better get here soon! This is driving me nuts.
As for the headphone amp, my original goal was indeed to build an amp for the cans....but, I started to think maybe I should just go ahead and do a preamp since there didn't seem like there'd be much more to it. The preamp would give me the added ability to hear the 'tube amp' through some speakers. Only thing is, pretty much all the preamp circuits I see don't accomodate a headphone output. I *think* it should simply be a matter of finding the right OPT with a secondary to run the cans. Or is there another (better) way of doing it?
BTW, your answers to my other questions were terrific...thank you VERY much.
"This still seems weird to me, though I'm sure it will all make sense very soon. For right now, it seems like I'm amplifying a signal, and then using an AMPed signal just to run a grid on another amp...it doesn't seem doing this would server much purpose..."It seems that you are understanding perfectly what is going on but not seeing the purpose in doing that. You can imagine that if you drive a second tube with an amplified signal, you will even further amplify that signal. Another thing is that not all tubes are designed for amplification. For example, output tubes usually don't have great amplification factor but are capable of conducting fairly high current that can be routed through an OPT. Therefore, all amplification has to be done before that stage. I am sorry, if I keep confusing you.
"I *think* it should simply be a matter of finding the right OPT with a secondary to run the cans. Or is there another (better) way of doing it?"
You could put together an SRPP preamp and use fairly big size decoupling capacitor for driving headphones. Personally, I don't like OTL (Output Transformerless) amps as much as the ones with OPTs. Nevertheless, it can sound quite well. Take a look at www.headwize.com
They have a fairly big library of hadphone amp projects. I may be wrong, but the problem that you may encounter with using preamp for driving cans is an output impedance of an amp. Generally, you want to use output tubes with fairly low plate resistance. However, it is not necceserely the case with preamp, since you are just trying to raise the voltage. To be honest, I am in the guess land right now and hopefully somebody else would be able to comment on this issue."BTW, your answers to my other questions were terrific...thank you VERY much."
Anytime.Jenya
"You can imagine that if you drive a second tube with an amplified signal, you will even further amplify that signal."I can see this being the case. It just seems that the first stage APM'd signal should go to the cathode and not the grid....the grid input just seems to be a modulation signal (thereby not needing to be a strong signal). This is what confuses me.
"Another thing is that not all tubes are designed for amplification. For example, output tubes usually don't have great amplification factor but are capable of conducting fairly high current that can be routed through an OPT. Therefore, all amplification has to be done before that stage."Now this sheds a LOT of light on things! This is something I can wrap my mind around. Is this what is happening with so many of the SET designs where there are only two tubes in the circuit?
"I am sorry, if I keep confusing you."
NO! hehe...you're doing great! I seem to be the one confusing myself, if anyone :)
"I can see this being the case. It just seems that the first stage APM'd signal should go to the cathode and not the grid....the grid input just seems to be a modulation signal (thereby not needing to be a strong signal). This is what confuses me."I think I understand the way you think about it. Amplifier doesn't produces power, it takes a power from a power supply and, as you point that out, modulates it with the signal. Try to think of a tube as a kitchen faucet valve. One end of it is connected to a high pressure water line (B+), another points out to the sink (ground). Now, you are putting your hand on the twisty part of the faucet (grid) and while listening to your favorite music, keep opening and closing faucet. There you got it, an amplified signal in the shape of the water stream. The same happens with electron stream. Valve is sitting between high potential (B+) and the ground and by adjusting the grid voltage more or less current goes through it. All the power, however, comes from the power supply. In multistage circuit, the last valve is the one that controls the output power of an amplifier. For it to conduct a lot of power, its grid voltage has to be swung over a very wide range. In order to achieve that range, the previous stages are used.
"Now this sheds a LOT of light on things! This is something I can wrap my mind around. Is this what is happening with so many of the SET designs where there are only two tubes in the circuit?"
I'd say yes.
Jenya
"In multistage circuit, the last valve is the one that controls the output power of an amplifier. For it to conduct a lot of power, its grid voltage has to be swung over a very wide range. In order to achieve that range, the previous stages are used."Ok...so would I be correct in saying: The higher B+ is (greater Voltage), the higher the modulating voltage MUST be on the grid?
If this is the case, then it would seem that it would be VERY near impossible to have a line level input modulating a SINGLE high(er) power amp (no in between stages)...you kinda have build up the modulated signal until it's big enuf to actually properly 'feed' the main power amp tube.
Groovy...I think I'm finally getting this!
"Ok...so would I be correct in saying: The higher B+ is (greater Voltage), the higher the modulating voltage MUST be on the grid?"Even though I understand your logic, I wouldn't call that assumption a rule. It depends on the bias voltage and limiting factors of the valve. So, let's say your tube's bias is set to -70V. Roughly speaking, you can swing grid plus-minus 70volts. You see, you can't allow grid to go positive, otherwise it will start sucking electrons on it and you'll get grid current. Anyways, I am pretty sure you can find two setups at which one of them may have lower B+, but use wider grid swing. The important thing is that you are getting the idea how this work. If you think of the kitchen faucet example, you can imagine that if you have a faucet open for a certain idle current, the amplitude of the swing around that point is determined by either the distance from idle current to a complete shutdown or the maximum available current. Beyond any of these points you will get what's called 'clipping'. This is a fairly loose analogy, but similar boundaries exist for valve. One is grid going positive, another is complete current shutdown with grid being very negative.
"...you kinda have build up the modulated signal until it's big enuf to actually properly 'feed' the main power amp tube."
Exactly!Jenya
I see where my logic was too generalized now.One thing that I've been pondering over recently is power supplies....
Am I mistaken or is NOT the purpose of a PS, in this case, to supply a clean DC signal? Some of the PS's I've been seeing are SO esoteric and EXPENSIVE to build! more than the amp itself many times!
I almost get the impression that different circuit topologies actually require a special type of PS. Please tell me this isn't the case ;)
I don't think you need to worry about esoteric power supplies right now. A power transformer, tube rectifier, a filter choke or two, few capacitors will do fine. Of course, you could go with SS rectifiers. Cheap, easy to make. Personally I have a strong preference towards tube rectifiers.Jenya
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