This is my first post on your forum, I hope I am not breaking too many rules 
OK, I want to build a tube amp. I really just want to do it for the heck of it, and to get the learning experience out of it. I will admit I am a t00b n00b, although I do have a general idea as to how they work and I have used them before (in a vacuum tube Tesla coil ). I also am knowledgeable in electronics in general and am experienced in working with high voltage, please do not try to oversimplify things for me.
That out of the way, the main goal of this project is to produce a working stereo tube amp (it can use semiconductors, but I want it to be tube based) that can use a normal line (1v p-p) level input, and can fill a 20'x20' room with sound at a decent volume (about what 50w of transistor power would give). I want to keep the price decently low (would $50 be reasonable with a well stocked junk box and some scrounging?), that sounds ok (as good as a decent transistor amp).
I have on hand that I would like to use...
-some car speakers I would like to use for the speakers, but realise they are probably not the ideal speaker.
-some tubes; a pair of used 6080 dual triodes and a pair of 6146 single triodes I would like to use (to keep cost down), would they be reasonable for the power out I want?
-assorted caps/resistors. I think I have most everything I will need short of the magnetics and possibly the tube
As far as what I need to get, I was thinking this for the output transformer. Then as far as the b+/filament transformer I was thinking of ebaying one of the right voltage (need to decide on a tube first).
Thanks!
OK, I want to build a tube amp. I really just want to do it for the heck of it, and to get the learning experience out of it. I will admit I am a t00b n00b, although I do have a general idea as to how they work and I have used them before (in a vacuum tube Tesla coil ). I also am knowledgeable in electronics in general and am experienced in working with high voltage, please do not try to oversimplify things for me.
That out of the way, the main goal of this project is to produce a working stereo tube amp (it can use semiconductors, but I want it to be tube based) that can use a normal line (1v p-p) level input, and can fill a 20'x20' room with sound at a decent volume (about what 50w of transistor power would give). I want to keep the price decently low (would $50 be reasonable with a well stocked junk box and some scrounging?), that sounds ok (as good as a decent transistor amp).
I have on hand that I would like to use...
-some car speakers I would like to use for the speakers, but realise they are probably not the ideal speaker.
-some tubes; a pair of used 6080 dual triodes and a pair of 6146 single triodes I would like to use (to keep cost down), would they be reasonable for the power out I want?
-assorted caps/resistors. I think I have most everything I will need short of the magnetics and possibly the tube
As far as what I need to get, I was thinking this for the output transformer. Then as far as the b+/filament transformer I was thinking of ebaying one of the right voltage (need to decide on a tube first).
Thanks!
Car driver in TWQT pipes can sound amazingly good. They do have a tendency to brightness. I used them at the start. They have the advantage of been very sensitive and fairly robust. Consider having the drivers mounted firing at the ceiling or at 45 degrees to the front. Also dammaring the cones can help a lot. Not all car drivers are made equal though. Older Pioneers can sound excellent. I have a pair of coxials mounted in the wall behind my head as I write.
You will need to consider paralleling your 6080's to even get 4W of SE power. You might be better considering PP for a little more power. If going SE consider going parafeed. You could get by with a +B of only 150V if you used simple mosfet CCS's. The advantage of parafeed is that you could use toroidal mains transformers as output iron. These will perform much better than your suggested airgapped iron (designed for guitar means designed to distort). Output blocking caps shouldn't be an issue at these voltages, you could even use bypassed electroytics initially. Doing it this way also cuts down on power supply costs as CCS loading has excellent PSRR. You might just bring your project in at you suggested budget.
Shoog
You will need to consider paralleling your 6080's to even get 4W of SE power. You might be better considering PP for a little more power. If going SE consider going parafeed. You could get by with a +B of only 150V if you used simple mosfet CCS's. The advantage of parafeed is that you could use toroidal mains transformers as output iron. These will perform much better than your suggested airgapped iron (designed for guitar means designed to distort). Output blocking caps shouldn't be an issue at these voltages, you could even use bypassed electroytics initially. Doing it this way also cuts down on power supply costs as CCS loading has excellent PSRR. You might just bring your project in at you suggested budget.
Shoog
This is looking promising...
As far as el cheapo, I noticed here there is a kit available for a little over $100 (I could handle that) but it seems to be lacking the output iron (another $50)... What exactly would I have to obtain to build el cheapo?
I could probably handle $150 if I knew I was going to have all of the right parts, and it would work when I was done...
But back to building something with my tubes....
First, how much 'tube' power will it take? I believe that the sound output for a tube amp at a given o/p power is higher than a transistor amp because they only amplify the audible range (or something like that) Second, will the tubes I have now be able to do the job? I would like to know early on if they will work so I know if I should be trying to build a design around them, or if I am starting from scratch...
Also, what do se, psrr, and parafeed mean?
Thanks!
As far as el cheapo, I noticed here there is a kit available for a little over $100 (I could handle that) but it seems to be lacking the output iron (another $50)... What exactly would I have to obtain to build el cheapo?
I could probably handle $150 if I knew I was going to have all of the right parts, and it would work when I was done...
But back to building something with my tubes....
First, how much 'tube' power will it take? I believe that the sound output for a tube amp at a given o/p power is higher than a transistor amp because they only amplify the audible range (or something like that) Second, will the tubes I have now be able to do the job? I would like to know early on if they will work so I know if I should be trying to build a design around them, or if I am starting from scratch...
Also, what do se, psrr, and parafeed mean?
Thanks!
PP 6080 might give you up to 10Wrms with careful design, you would be better off selling the 6146's on eBay and adding whatever money you get for them to the pot.
IMHO You might be better off finding a clapped out scott, fisher, radio shack, lafayette or dyna amplifier using 6BQ5/7189 and salvaging them for the iron, and perhaps even the chassis. If you are lucky you will end up with a set of usable tubes as well. This will also put you in the range of 15w pc which is reasonable for the speakers you proposed.
Kevin
IMHO You might be better off finding a clapped out scott, fisher, radio shack, lafayette or dyna amplifier using 6BQ5/7189 and salvaging them for the iron, and perhaps even the chassis. If you are lucky you will end up with a set of usable tubes as well. This will also put you in the range of 15w pc which is reasonable for the speakers you proposed.
Kevin
Do a search on parafeed, there are a few designs on the net - it will explain the basic principle. PSRR means Power Supply Rejection Ratio and referes to the ability of the circuit to reject power supply hum from the output. SE(single ended) designs have poor PSRR and so need more power supply filtering and hence expense, Push Pull designs have good PSRR and Parafeed amps have the best PSRR and so cheap power supplies.
Your not going to achieve you objective of designing a usable power amp without knowing the basic principles and terminology. A bit of time spent researching the field of tubes would help a lot. The are plenty of basic resources out on the net.
Shoog
Your not going to achieve you objective of designing a usable power amp without knowing the basic principles and terminology. A bit of time spent researching the field of tubes would help a lot. The are plenty of basic resources out on the net.
Shoog
sorry about the acronyms, now that they have been defied they seem stupidly obvious, the forum I usually live on just uses different ones.
Hmm, so in a parafeed design you are using the voltage drop across a separate inductor (which is stepped down by an xformer to match the independence of the speaker) as opposed to using the inductance of the o/p xformer? I an see how this would be beneficial...
Hmm, so in a parafeed design you are using the voltage drop across a separate inductor (which is stepped down by an xformer to match the independence of the speaker) as opposed to using the inductance of the o/p xformer? I an see how this would be beneficial...
Suitable inductors are hidiously expensive - so forget about that as an option. A simple solid state CCS does a better job and eliminates possable resonante issues. The inductor or the CCS provides a constant DC current - which is usually done by the output transformer in an SE design. It also acts as the output load in parallel with the output transformer. Since a CCS has a very high output impedance at AC, it actually contributes very little to the actual AC load which the anode see's. The anode load is dominated by the much lower impedance of the output transformer.
Shoog
Shoog
Your not going to learn jack about tube amps by buying one. I still suggest that you at least have a go at building something with what you have got.
I was thinking that if you did decide to go parafeed you could use an isolation transformer to get your powersupply. Think of using salvaged caps from a PC power supply, they are usually 200V or 250V.
Because the 6080 runs at a low 100V with a high 100mA current (200mA if you go parallel), then the output transformers could be easy to find 10V:240V or even 12V:240V, or you could even use PA line transformers which can be had as salvage. At a pinch you could even use some EI mains transformers until you come across some toroidals. The EI's would perform badly in the top end but it would get you going for next to nothing.
Your not going to rock the house with a max of just 4watts but it will be a good introduction, and if you like the results you can always come back and make a more serious amp when you have more money to throw at it. Car drivers are usually in the 100db per watt range so you will be surprised at how far your low wattage will go.
Another possability would be to build an output transformer less headphone amp with the 6080 as a Single Ended Push Pull arrangement. Someones probably already done it and there might be a circuit to hand.
Don't get sucked into the idea that spending small will automatically produce rubbish results. Spending big should only ever be attempted when you know what to expect at the end.
You will kick yourself if you turn down this opportunity to design and build something of your own.
Shoog
I was thinking that if you did decide to go parafeed you could use an isolation transformer to get your powersupply. Think of using salvaged caps from a PC power supply, they are usually 200V or 250V.
Because the 6080 runs at a low 100V with a high 100mA current (200mA if you go parallel), then the output transformers could be easy to find 10V:240V or even 12V:240V, or you could even use PA line transformers which can be had as salvage. At a pinch you could even use some EI mains transformers until you come across some toroidals. The EI's would perform badly in the top end but it would get you going for next to nothing.
Your not going to rock the house with a max of just 4watts but it will be a good introduction, and if you like the results you can always come back and make a more serious amp when you have more money to throw at it. Car drivers are usually in the 100db per watt range so you will be surprised at how far your low wattage will go.
Another possability would be to build an output transformer less headphone amp with the 6080 as a Single Ended Push Pull arrangement. Someones probably already done it and there might be a circuit to hand.
Don't get sucked into the idea that spending small will automatically produce rubbish results. Spending big should only ever be attempted when you know what to expect at the end.
You will kick yourself if you turn down this opportunity to design and build something of your own.
Shoog
I don't want to buy an amp (that is why I am here), but I don't have a problem buying one then scavenging the parts...
You say a normal mains transformer would work as an output transformer? Would like a normal 120v->12v 1a transformer work? Perhaps if I put 2 speakers in series so that the whole thing would have the same independence of a 240-12v one?
In that case could I just use a variac + filter as the power supply?
For a quick lashup could I just use a full wave rectifier of diodes and insane rc filtering (I have a few 350v/3600uf caps I could use for a lashup), or would I need to have an inductor to get the hum to a reasonable level?
Thanks!
You say a normal mains transformer would work as an output transformer? Would like a normal 120v->12v 1a transformer work? Perhaps if I put 2 speakers in series so that the whole thing would have the same independence of a 240-12v one?
In that case could I just use a variac + filter as the power supply?
For a quick lashup could I just use a full wave rectifier of diodes and insane rc filtering (I have a few 350v/3600uf caps I could use for a lashup), or would I need to have an inductor to get the hum to a reasonable level?
Thanks!
Variac is not going to work. You need a suitable mains transformer. If you use silicon (which is OK) then your caps would be fine. Think CRCRC for the power supply. Chokes would be nice but are not necissary.
You will have to get a datasheet to decide on your exact operating point (ie current and plate volttge) for the tubes, you will only know the correct output transformer when you know your operating point. The datasheet usually suggets a suitable operating point. You will need to be thinking of at least 50VA (ie about 5amps) for your output transformers. This is because they are only designed to work down to 50/60hz and you will be expecting them to operate below this, if they are undersized they will saturate and distort. The reason that mains transformers will work is because with parafeed you are preventing DC current from entering the transformer.
Find and look at a datasheet and then we can go from there.
Shoog
You will have to get a datasheet to decide on your exact operating point (ie current and plate volttge) for the tubes, you will only know the correct output transformer when you know your operating point. The datasheet usually suggets a suitable operating point. You will need to be thinking of at least 50VA (ie about 5amps) for your output transformers. This is because they are only designed to work down to 50/60hz and you will be expecting them to operate below this, if they are undersized they will saturate and distort. The reason that mains transformers will work is because with parafeed you are preventing DC current from entering the transformer.
Find and look at a datasheet and then we can go from there.
Shoog
Heres a link to a good datasheet
http://www.pmillett.com/tubedata/HB-3/Receiving-Type Industrial Tubes/6080.PDF
and another
http://tdsl.duncanamps.com/pdf/vm308.pdf
The datsheet curves suggest;
anode voltage = 100V
anode current = 100mA (running in parallel should give 200mA)
dissapation = 100 x 0.1 = 10watts (max allowed = 13W) per triode
Running in parallel gives 100V / 0.2A = 500R plate resistance.
Assuming a mains transformer of 240V primary and a 10V secondary
240/10= 24
24 x 24 = 576
assuming a 4ohm speaker, reflected impedence will be 576 x 4 = 2304R
So our original calculation is way to high an impedence.
try again;
Assuming a mains transformer of 110V > 10V
110/10 = 11
11 x 11 = 121
assuming a 4ohm speaker, reflected impdeance will be 121 x 4 = 484R
Which is about spot on. So you will need two 110V:10V transformers of +50VA
Or Assuming a mains transformer of 240V > 24V
240 / 24 = 10
10 x 10 = 100
Assuming a 4ohm speaker, reflected impedance will be 100 x 4 = 400R which would just about do, but with slightly higher distortion.
And again a mains transformer of 240V > 18V
240 /18 = 13.3
13.3 x 13.3 = 176
Assuming a 4ohm speaker, reflected impdedance will be 176 x 4 = 704 which would give lower output power but also lower distortion.
The transformers can be rearranged in the future to produce a different plate load with a 8ohm speaker.
Heres the bad news, with a realistic top efficiency of 20% we will only be getting 4Watts per channel.
Still any of the three output transformers would be easy and cheap to get hold of.
Assuming that the 6080 sounds good in itself, this amp should sound as good as any "normal" SE amp, and possably even better.
With this output impedance you would need a parafeed DC blocking cap in the region of 100uf which will mean a good electrolytic. The real issue you will face is finding a suitable solid state CCS, though there are plenty of option out there, most are fairly complex. There is a single chip that would do a good job which "Tubelabs" has used a lot to good effect.
Shoog
http://www.pmillett.com/tubedata/HB-3/Receiving-Type Industrial Tubes/6080.PDF
and another
http://tdsl.duncanamps.com/pdf/vm308.pdf
The datsheet curves suggest;
anode voltage = 100V
anode current = 100mA (running in parallel should give 200mA)
dissapation = 100 x 0.1 = 10watts (max allowed = 13W) per triode
Running in parallel gives 100V / 0.2A = 500R plate resistance.
Assuming a mains transformer of 240V primary and a 10V secondary
240/10= 24
24 x 24 = 576
assuming a 4ohm speaker, reflected impedence will be 576 x 4 = 2304R
So our original calculation is way to high an impedence.
try again;
Assuming a mains transformer of 110V > 10V
110/10 = 11
11 x 11 = 121
assuming a 4ohm speaker, reflected impdeance will be 121 x 4 = 484R
Which is about spot on. So you will need two 110V:10V transformers of +50VA
Or Assuming a mains transformer of 240V > 24V
240 / 24 = 10
10 x 10 = 100
Assuming a 4ohm speaker, reflected impedance will be 100 x 4 = 400R which would just about do, but with slightly higher distortion.
And again a mains transformer of 240V > 18V
240 /18 = 13.3
13.3 x 13.3 = 176
Assuming a 4ohm speaker, reflected impdedance will be 176 x 4 = 704 which would give lower output power but also lower distortion.
The transformers can be rearranged in the future to produce a different plate load with a 8ohm speaker.
Heres the bad news, with a realistic top efficiency of 20% we will only be getting 4Watts per channel.
Still any of the three output transformers would be easy and cheap to get hold of.
Assuming that the 6080 sounds good in itself, this amp should sound as good as any "normal" SE amp, and possably even better.
With this output impedance you would need a parafeed DC blocking cap in the region of 100uf which will mean a good electrolytic. The real issue you will face is finding a suitable solid state CCS, though there are plenty of option out there, most are fairly complex. There is a single chip that would do a good job which "Tubelabs" has used a lot to good effect.
Shoog
Ok, sounds good! (no pun intended)
Ok, for the o/p xformer, I have a few 115v->12.6vct 50va filament xformers (bought them for the tesla coil powered y 811a'a I linked in the first post), and a 115v->11.5v 150va transformer. I assume the bigger (thus better bass due to less saturation?) one would be better, but would there be any noticeable difference between the two?
For the power supply, why wouldn't a variac work? Seems like ac is ac, especially once rectified and filtered, but I have very little experience designing power supplies... I don't have a real isolation transformer, but if I don't need it to be more than 50va then I could make if using 2 of the above transformers back to back. I also can make a ss ccs no problem, do you guys have a favorite design?
Also, if I understand you correctly to use a mains xformer I need to be running parafeed, so I am going to need a large inductor... How large are we talking, and does it need to have any special attributes?
Thanks!
Ok, for the o/p xformer, I have a few 115v->12.6vct 50va filament xformers (bought them for the tesla coil powered y 811a'a I linked in the first post), and a 115v->11.5v 150va transformer. I assume the bigger (thus better bass due to less saturation?) one would be better, but would there be any noticeable difference between the two?
For the power supply, why wouldn't a variac work? Seems like ac is ac, especially once rectified and filtered, but I have very little experience designing power supplies... I don't have a real isolation transformer, but if I don't need it to be more than 50va then I could make if using 2 of the above transformers back to back. I also can make a ss ccs no problem, do you guys have a favorite design?
Also, if I understand you correctly to use a mains xformer I need to be running parafeed, so I am going to need a large inductor... How large are we talking, and does it need to have any special attributes?
Thanks!
I'am not certain, but I think that a variac is an autotransformer.I tried it once and blew a 5amp fuse.Don't go there.
Back to back transformers work and I have used them in headphone amps and preamps. Because you are going class A the +B should remain fairly stable.However losses would be significant in this application and I wouldn't recommend it. You really are going to have to bite the bullet and spend part of your budget on these.
I use 120VA toroidals in my SET and they work well.I would say that the 50VA should work fine, and may present a few advantages in the top end.Are yours toroidals or EI.
The CCS is used to load the output tube and takes the place of an inductor. A SS CCS will be massively cheaper than a parafeed inductor-which cost more than output transformers !!! Also a CCS will out perform a choke by a huge margin. It also makes design of the amp easier because the choke and the ouput transformer tend to resonate.
Shoog
Back to back transformers work and I have used them in headphone amps and preamps. Because you are going class A the +B should remain fairly stable.However losses would be significant in this application and I wouldn't recommend it. You really are going to have to bite the bullet and spend part of your budget on these.
I use 120VA toroidals in my SET and they work well.I would say that the 50VA should work fine, and may present a few advantages in the top end.Are yours toroidals or EI.
The CCS is used to load the output tube and takes the place of an inductor. A SS CCS will be massively cheaper than a parafeed inductor-which cost more than output transformers !!! Also a CCS will out perform a choke by a huge margin. It also makes design of the amp easier because the choke and the ouput transformer tend to resonate.
Shoog
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