I just built a 2 tube 3.5 watt amp using the project info at Fred Nachbauers page, the Miniblock II
http://www.dogstar.dantimax.dk/tubestuf/mnibl2-1.htm
Mine came out like this, and I got it to work without too much
excessive fuss, but since I'm not a pro it took a little time (2 days):
This circuit uses 24 volt bell transformers, diodes for the power
supply, and a diode volatge doubler as well for the HV, as well
as a diode bias circuit which comes from the filament supply.
The amp is excellent, low hum, nice clean sound. Running the output of a computer into it, or a tiny transistor radio's earphone jack into it and then its output into a big speaker, you'd feel you have a full blown stereo. Nice glassy bell-clear tube sound too.
My problem is that a mic or guitar isn't powerful enough to really
drive it, you get some sound but not pronounced. According to the
specs of this amp, you need 1/2 volt of input to really take advantage of it. I did some searching here & there, didn't find "it" yet,
but what I'm looking for exactly is a 1 tube pre-amp, that can do from a mic to maybe close to 1/2 volt, and hopefully have some of the "easy" perks that this amp has, since I'm on a budget.
A "One tube pre-amp" easy to build like this:
No tranformer needed, can get the filament power from 120AC using a big voltage drop resistor.
The power supply can be made using diodes (like 1N4007's) and the high voltage using a voltage doubler diode circuit like the above project had.
Triode or pentode based, all I need is 1/2 volt output (for mic, guitar or turntable to be able to drive the second amp)
Perhaps using a common old radio tube:
50C5 12AV6 12BA6 (or even a 12AX7 since that seems to
be super popular in many audio circuits.
Has anyone seen a project/scematic like this. I'm sure it's a shoe-in
type of design if anyone gave though to it, but I'm not an engineer. 🙂
Thanks in advance if you have a link or idea on this... 🙂
http://www.dogstar.dantimax.dk/tubestuf/mnibl2-1.htm
Mine came out like this, and I got it to work without too much
excessive fuss, but since I'm not a pro it took a little time (2 days):
This circuit uses 24 volt bell transformers, diodes for the power
supply, and a diode volatge doubler as well for the HV, as well
as a diode bias circuit which comes from the filament supply.
The amp is excellent, low hum, nice clean sound. Running the output of a computer into it, or a tiny transistor radio's earphone jack into it and then its output into a big speaker, you'd feel you have a full blown stereo. Nice glassy bell-clear tube sound too.
My problem is that a mic or guitar isn't powerful enough to really
drive it, you get some sound but not pronounced. According to the
specs of this amp, you need 1/2 volt of input to really take advantage of it. I did some searching here & there, didn't find "it" yet,
but what I'm looking for exactly is a 1 tube pre-amp, that can do from a mic to maybe close to 1/2 volt, and hopefully have some of the "easy" perks that this amp has, since I'm on a budget.
A "One tube pre-amp" easy to build like this:
No tranformer needed, can get the filament power from 120AC using a big voltage drop resistor.
The power supply can be made using diodes (like 1N4007's) and the high voltage using a voltage doubler diode circuit like the above project had.
Triode or pentode based, all I need is 1/2 volt output (for mic, guitar or turntable to be able to drive the second amp)
Perhaps using a common old radio tube:
50C5 12AV6 12BA6 (or even a 12AX7 since that seems to
be super popular in many audio circuits.
Has anyone seen a project/scematic like this. I'm sure it's a shoe-in
type of design if anyone gave though to it, but I'm not an engineer. 🙂
Thanks in advance if you have a link or idea on this... 🙂
frank754 said:
And that voltage drop resistor would have to be big indeed as you would be burning away about 35 watts for a single 9pin tube.
Sheldon
frank754 said:
Transformers are good use lots of em. Here's a single tube design that's transformer coupled. You can change the gain by choosing different ratios. http://boozhoundlabs.com/transformer-output-pre/
Sheldon
Isolation transformers can be had for ten or twelve bucks. Others might not agree 😀 but I think my life is worth a bit more than that.
OK, I'm game on the transformer for the filament, as I can get
a 12 v transformer at Radio Shack (I think), and that's OK.
BTW, a ramshackle voltage drop filament scheme can be had perhaps
by putting a night light bulb in series with the tube filament?
My power here is 125v not 117, so maybe it would be enough
for a 6.3? Just a thought.
What I have a problem with is that all the sophisticated parts I have to order on the net, and it takes a week to get them, plus huge shipping.
It's a shame that Radio Shack doesn't have more stuff anymore, like they have no electrolytic caps in the tube voltage range.
I did read the huge 50 page thread about the 12B4 circuit,
is this one that's highly recommended?
It's confusing, they revise the schematic about 3-4 times, etc.
OK, so I use the final more accepted schematic from fdegrove:
Mic jack, 100k pot to ground, center lug to 100 ohm resistor into grid.
470 Ohms 2w resistor from cathode to ground
100 ohms in series with a 3.3 uF/450v from plate to output jack,
with 100k resistor to ground at that jack.
One guy says that it runs fine, and he puts 255v 25ma onto the
plate.
PLEASE inform me if there is a better set of values up to this point.
What I propose to do for the plate voltage, is hack it together from
the AC using diodes, similar to the Miniblock project, doubling my line current:
create a voltage doubler circuit using two 1n4007's and two
470uF 200v caps (each for just 1/2 the phase), then run that
for the B+ without regulation, should that be ok for starters?
See the power supply on the amp I built, but for this I would
make the B+ from the line, making sure the chassis & line were
grounded properly and the chassis was grounded,
and not use the 125-->24-->125 xfmrs, just the line AC
with diodes for the B+
Should this work?
Not trying to be a real cheapo, but the semantics would be
great if in fact a HV xfmr is not used in the equation.
A lot of old radios were powered the same way, plus with filaments
in series.
a 12 v transformer at Radio Shack (I think), and that's OK.
BTW, a ramshackle voltage drop filament scheme can be had perhaps
by putting a night light bulb in series with the tube filament?
My power here is 125v not 117, so maybe it would be enough
for a 6.3? Just a thought.
What I have a problem with is that all the sophisticated parts I have to order on the net, and it takes a week to get them, plus huge shipping.
It's a shame that Radio Shack doesn't have more stuff anymore, like they have no electrolytic caps in the tube voltage range.
I did read the huge 50 page thread about the 12B4 circuit,
is this one that's highly recommended?
It's confusing, they revise the schematic about 3-4 times, etc.
OK, so I use the final more accepted schematic from fdegrove:
Mic jack, 100k pot to ground, center lug to 100 ohm resistor into grid.
470 Ohms 2w resistor from cathode to ground
100 ohms in series with a 3.3 uF/450v from plate to output jack,
with 100k resistor to ground at that jack.
One guy says that it runs fine, and he puts 255v 25ma onto the
plate.
PLEASE inform me if there is a better set of values up to this point.
What I propose to do for the plate voltage, is hack it together from
the AC using diodes, similar to the Miniblock project, doubling my line current:
create a voltage doubler circuit using two 1n4007's and two
470uF 200v caps (each for just 1/2 the phase), then run that
for the B+ without regulation, should that be ok for starters?
See the power supply on the amp I built, but for this I would
make the B+ from the line, making sure the chassis & line were
grounded properly and the chassis was grounded,
and not use the 125-->24-->125 xfmrs, just the line AC
with diodes for the B+
Should this work?
Not trying to be a real cheapo, but the semantics would be
great if in fact a HV xfmr is not used in the equation.
A lot of old radios were powered the same way, plus with filaments
in series.
frank754 said:
Indeed they were. We also used to believe that radiation was good for you, and you could cure just about any disease with the judicious application of leaches. Is your, or your family's lives really not worth $10 to you?
Strangely, new research suggests that low doses of radiation may actually reduce the incidence of cancer.
The findings obviously require corroboration, but apparently negative correlations have been found between natural background radiation levels and rates of cancer.
pm
Maybe I'm clueless here, but I've been a Ham plus electrician off & on for over 30 years. (I'm 52, and my dad was an engineer for DuMont TV). Built a SW receiver in 1970 from just a schematic.
If there's some kind of radiation, it's in the AC in the house and surrounds us. I'm sort of envronmental too, but it's just me, my wife and our cats, and we all need discipline from time to time.. 🙂
If, by not using an isolation xfmr, making sure the plug & polarity of the chassis is grounded, and if not, by someone bypassing it, causes it to blow the fuse on the unit, I don't see the problem here. Safety can be built in. I see more ops for a mishap with 400-800 volts on a high Mfd
capacitor, even with the unit unplugged for any random amount of time.
My question was about AC powered HV supplies without an xfmr,
has anyone here done this to some extent, especially where you
can use a diode/high uFd circuit to provide filtered DC, and use a voltage doubler circuit as I mentoned above (in the pic link too but without the xfmrs), doubling the AC provides 250+ volts filtered DC maybe not prime for a "driven to the max" HV scenario. but enough for something like a pre-amp where you need only to get something decent going to provided a gain & a good feed, not even nearly max anything out.
So I was just hoping someone could confirm the 12B4 circuit
was fairly well tweaked and good-to-go in the version that
I mentioned, and that perhaps I could do the 1 tube preamp
with this and the power setup I proposed.
Also still taking suggestions on 1 tube preamps using the other tubes I mentioned 50C5 12AV6 12BA6 (or even a 12AX7)
but hopefully a cheap plentiful tube that has a lot of NOS stock.
If there's some kind of radiation, it's in the AC in the house and surrounds us. I'm sort of envronmental too, but it's just me, my wife and our cats, and we all need discipline from time to time.. 🙂
If, by not using an isolation xfmr, making sure the plug & polarity of the chassis is grounded, and if not, by someone bypassing it, causes it to blow the fuse on the unit, I don't see the problem here. Safety can be built in. I see more ops for a mishap with 400-800 volts on a high Mfd
capacitor, even with the unit unplugged for any random amount of time.
My question was about AC powered HV supplies without an xfmr,
has anyone here done this to some extent, especially where you
can use a diode/high uFd circuit to provide filtered DC, and use a voltage doubler circuit as I mentoned above (in the pic link too but without the xfmrs), doubling the AC provides 250+ volts filtered DC maybe not prime for a "driven to the max" HV scenario. but enough for something like a pre-amp where you need only to get something decent going to provided a gain & a good feed, not even nearly max anything out.
So I was just hoping someone could confirm the 12B4 circuit
was fairly well tweaked and good-to-go in the version that
I mentioned, and that perhaps I could do the 1 tube preamp
with this and the power setup I proposed.
Also still taking suggestions on 1 tube preamps using the other tubes I mentioned 50C5 12AV6 12BA6 (or even a 12AX7)
but hopefully a cheap plentiful tube that has a lot of NOS stock.
Firstly, I agree with the others who point out the danger of not using an isolation transformer. I'm sure you can understand that it would be irresponsible for this forum to take any other stance.
Secondly, for your preamp purposes you could consider using a 6AU6/EF94 pentode, which was commonly used in microphone preamps by some well-known companies. It is widely available and cheap to buy. You might need to acquire a few examples, in order to find one that is not too microphonic or noisy.
Secondly, for your preamp purposes you could consider using a 6AU6/EF94 pentode, which was commonly used in microphone preamps by some well-known companies. It is widely available and cheap to buy. You might need to acquire a few examples, in order to find one that is not too microphonic or noisy.
well not any disease but for many diseases involving contamination of the blood, leaches are good for that... anyone see the new "above the influnce" ad campaigns where kids put leaches on their gums? not to derail or anything...pinkmouse said:
frank754 said:
I'm no expert, so feel free to ignore my thoughts. If you are working with voltages around 240 or less, you aren't exposed to voltages higher than you would be with many appliances. However in those appliances it's pretty easy to isolate the user from the live elements. And the user will have some idea when they are in danger or not (inside the cover). In audio components you always have an input and output. Without isolation a fault can cause those to become live without a user expecting it. Easy to complete a circuit, since earth is the reference - Zap! With a high voltage cap, you at least have to complete a circuit with the other leg, doubles the odds in your favor. Plus, you gotta be digging inside and should know to be careful. With a transformer isolated circuit you have to try harder to get into trouble. Your call for your own use.
But, the other issue is that this forum addresses a wide range of experience. You may know how to properly deal with the issues. The same cannot be assumed of the rest of us. So it pays to be safe and insist on specific safety standards for circuits published or discussed here. Political correctness, maybe. But there's really not much reward for taking the risk of not insisting on the simple expedient of input transformer isolation for audio components in this mileiu.
Sheldon
I agree, for the most part, and see the gist of the thread, basically I was hoping to help integrate a nice power supply design, just maybe a bit above line voltage, not a big hazard, into a small chassis with one tube and maybe 6-8 resistors/caps/ 1 pot, 2 jacks etc.
As I see it, there's no flaw in design if it has a 3 prong grouded plug.
So as not to kill the thread (and I'd hate to get a bad rep here,as I have some killer ideas), please forget about the power supply and maybe shed some info on some nice 1 tube circuits along these lines.
OK, 6AU6, my dad had quite a few of them in his stash they seem like a good choice, remember I'm looking for a pre-amp from just a mic or guitar to provide just 1/2 volt, or like the energy from a tiny transistor radio or walkman.
See this link from the post I quoted earlier,
is this a valid circuit ( fdegrove was involved )?
http://www.diyaudio.com/forums/attachment.php?s=&postid=820976&stamp=1137683454
As I see it, there's no flaw in design if it has a 3 prong grouded plug.
So as not to kill the thread (and I'd hate to get a bad rep here,as I have some killer ideas), please forget about the power supply and maybe shed some info on some nice 1 tube circuits along these lines.
OK, 6AU6, my dad had quite a few of them in his stash they seem like a good choice, remember I'm looking for a pre-amp from just a mic or guitar to provide just 1/2 volt, or like the energy from a tiny transistor radio or walkman.
See this link from the post I quoted earlier,
is this a valid circuit ( fdegrove was involved )?
http://www.diyaudio.com/forums/attachment.php?s=&postid=820976&stamp=1137683454
I know that you said drop the PSu issue, but
What if you plug the amp into a different outlet one day. An outlet that has been wired the wrong way? Suddenly, your amplifier's chassis, alsong with all of your sources and speaker terminals, are at DC line voltage.
2 transformers, as in your existing amp, can be employed very cheaply to eliminate this potential problem.
James
p.s. Even if you are sure that a socket is wired correctly, in the UK I saw a TV programme where it transpired that the input to the house was wired the wrong way- the only reason that there was not a problem was that all of the appliances were properly designed.
What if you plug the amp into a different outlet one day. An outlet that has been wired the wrong way? Suddenly, your amplifier's chassis, alsong with all of your sources and speaker terminals, are at DC line voltage.
2 transformers, as in your existing amp, can be employed very cheaply to eliminate this potential problem.
James
p.s. Even if you are sure that a socket is wired correctly, in the UK I saw a TV programme where it transpired that the input to the house was wired the wrong way- the only reason that there was not a problem was that all of the appliances were properly designed.
frank754 said:
I think that one is very similar to a design published by fourm member Brett (a quick search would find it and a search on 12B4 would probably find several built examples). I think quite a few builders have made just that, with maybe slightly different operating points. Here's a headphone amp that uses a 6AU6 on the front and a 12B4 as the output tube. Don't see why you couldn't just use the front end as a stand alone pre. http://www.tubecad.com/2004/blog0013.htm (near the bottom of the page).
Sheldon
Thanks Sheldon, that tubecad site has given me some great ideas,
I will study that and come up with something nice, and that will take a bit of time.
In the meantime I bought a tiny solid state amp at radio shack, for 12.99, and it worka great as a preamp to get my mic, turntable or guitar to drive the miniblock II:
http://www.radioshack.com/product/i...igkw=amplifier&kw=amplifier&parentPage=search
but I'm sure going to build a tube model soon instead, but this works in a pinch.
jrevillug: I wired my entire house myself, so I didn't take error into consideration, guess there may be some poorly wired homes.
I remember working on a house once, years ago, where the more power you used in one area would make the lights dim in the other side of the house. Turned out it was a partially blown cardboard slot fuse that was acting like a resistor to the other side of the 240.
I will study that and come up with something nice, and that will take a bit of time.
In the meantime I bought a tiny solid state amp at radio shack, for 12.99, and it worka great as a preamp to get my mic, turntable or guitar to drive the miniblock II:
http://www.radioshack.com/product/i...igkw=amplifier&kw=amplifier&parentPage=search
but I'm sure going to build a tube model soon instead, but this works in a pinch.
jrevillug: I wired my entire house myself, so I didn't take error into consideration, guess there may be some poorly wired homes.
I remember working on a house once, years ago, where the more power you used in one area would make the lights dim in the other side of the house. Turned out it was a partially blown cardboard slot fuse that was acting like a resistor to the other side of the 240.
frank754 said:
Funny! And I remember a house in Sweden where a blown fuse caused the stairway light to come on dimly when you used the stove. Somehow the light made a circuit thru the stove elements.
And I ran an EL84 amp for years that had no HT transformer - straight from the 220V mains. (My bad idea). Always got a tingle off it. No ground.... Then one day I manged to route the B+ to ground via my CD player. The player did not survive. From then on I always used a transfo.
If you don't want to use a transformer, why not build a preamp using a battery tube? Current can be very light and you could get probably still get enough gain if you used a directly heated pentode intended for battery receivers. The 1T4/DF91 is a possibility, which has a 1.4v filament and can run well with a 90v B+ battery.
Think about it this way for a moment. The poster mentioned using the preamp with a guitar. This means that you are making a direct connection from circuit ground to the guitar which is in the players (often sweaty) hands. Are you willing to bet your life that EVERY outlet that you are EVER going to plug the preamp into is wired correctly.
Many musicians have been killed by something like this. There was a thread here some time ago about a guitar player who was killed at a public performance when he touched the guitar and the mic at the same time. What do you do for an encore?
50 VA isolation transformers (Triad N68X) are $14 USD at Mouser and they have a 6 volt filament transformer for $8. The isolation transformer can be wired to give 240 VAC also. This is the absolute lowest cost SAFE power supply available.
Many musicians have been killed by something like this. There was a thread here some time ago about a guitar player who was killed at a public performance when he touched the guitar and the mic at the same time. What do you do for an encore?
50 VA isolation transformers (Triad N68X) are $14 USD at Mouser and they have a 6 volt filament transformer for $8. The isolation transformer can be wired to give 240 VAC also. This is the absolute lowest cost SAFE power supply available.
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