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Old 11th January 2007, 10:16 PM   #1
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Default One tube preamp

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):

Click the image to open in full size.

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...
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Old 11th January 2007, 10:19 PM   #2
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Default Re: One tube preamp

Quote:
Originally posted by frank754
No tranformer needed, can get the filament power from 120AC using a big voltage drop resistor.
Highly dangerous, and not recommended at all.
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Old 11th January 2007, 10:50 PM   #3
Sheldon is online now Sheldon  United States
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Default Re: One tube preamp

Quote:
Originally posted by frank754
no tranformer needed, can get the filament power from 120AC using a big voltage drop resistor.
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
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Old 11th January 2007, 10:56 PM   #4
Sheldon is online now Sheldon  United States
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Default Re: One tube preamp

Quote:
Originally posted by frank754
no tranformer needed, can get the filament power from 120AC using a big voltage drop resistor.
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
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Old 11th January 2007, 11:37 PM   #5
Sherman is offline Sherman  United States
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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.
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Old 12th January 2007, 12:35 AM   #6
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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?


Click the image to open in full size.

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.
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Old 12th January 2007, 12:41 AM   #7
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Quote:
Originally posted by frank754
A lot of old radios were powered the same way...
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?
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Old 12th January 2007, 12:55 AM   #8
mach1 is offline mach1  Australia
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Quote:
We also used to believe that radiation was good for 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
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Old 12th January 2007, 02:08 AM   #9
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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.
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Old 12th January 2007, 02:34 AM   #10
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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.
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