Here are 2 schematics I am thinking of building.
Please let me know witch one will be the best or if you can improve
on this http://s949.photobucket.com/albums/ad33 ... v2.jpgease let me know. [/url][/list]

Hey, this is a great idea to build a little 2-tube amp without the need for either a power transformer or a huge power resistor for series filament tubes (and power loss in that type circuit, as well). It's clever how you get the filmanet voltage for the 1U4 from the cathode circuit of the 117N7. Both schematics look good and seem quite similar except that you use .01 coupling caps in one, and .047 in the other. I would say I would build the version that I had the most caps on hand with! I wonder if the one with the .047 would give more bass response, though. A great idea to expand on this would be to build it doubled for stereo for use with a CD or MP3 player or FM tuner. Of course the mono version is great for a mono FM tube tuner. Another interesting question would be could you run TWO 1U4 filaments off the cathode of a 117N7? I would imagine so, if you modified the cathode resistors appropriately, and probably add value to the cathode bypass cap. Then, you could add a preamp statge for a magnetic phono, or a tone amp stage with bass and treble controls. Since triode tube "sound" is often sought after, I imagine you could use a 1.4V filament triode tube (such as 1H5GT, 1G4GT) in place of the 1U4. Then, you also would not have to worry about the screen grid circuit, simpifying the overall wiring a little bit. Let me know what you think of these ideas!

I think I would wire the input attenuator (volume control) differently. The way they've got it set up you'd have a dead short across the input at min volume. That could be a real problem to the input device, depending on what it was.

The coupling capacitor and the grid resistor form a voltage divider. The equation for the corner frequency (-3dB) is F=1/(2ΠRC). The commonly used corner frequency is 5 Hz so you would want about .068 μF for coupling to the power amplifier. .047 μF is close enough but with .01 you would be 3dB down at 34 Hz.

I also believe it to be bad for the plate load of a tube to be larger than the grid resistor of the following tube.

Might consider a higher resistor value between two filter caps. Using 47 or 68 ohms will leave a lot of ripple. Probably need about 20 times this resistance?

Other than difference in frequency response circuits will work about the same.

Not a fan of any AC/DC transformerless circuits but it will no doubt work, be sure to keep AC line polarity correct or someone may be in for a surprise... Norm is correct about increasing the resistor between the filter caps, the P/S will have ripple(hum)if it's too low... Most similar circuits use 1000-1500 ohms...


Tom

Here's one I found in an old magazine. It's only one tube but another tube could be added for more gain. I originally built it for a regen amp and something to do on a Sunday.

I have this schematic but it uses a input transformer which are hard to find and are expensive.(from my research.)

This is the original schematic from a 1949 mag.
http://books.google.com/books?id=I9kDAA ... l=en&ei=RH ...

Hi

The input transformer can be another type with good turns ratio. Could even use a small output transformer in reverse.

Nicely drawn schematic but notice it's still connected directly to the AC line.

In my schematic I specify Isolated 120 VAC

Here are the group of schematics I have compiled (without the dynakit stuff.)

http://s949.photobucket.com/albums/ad33 ... 71Kmod.jpg

I have included a schematic with the 1K mod in the power supply and a volume control mod.

OK I found it. Radiotron Designer's Handbook 4th Edition Section 12-3(i):
"The load resistance should never be greater than the following grid resistance (Rg2) and should preferably be not more than one quarter of Rg2."

The data sheet for the 1U4 has suggested values. http://frank.pocnet.net/sheets/093/1/1U4.pdf

I think the suggested values of .25 Meg load resistor, .33 Meg for the screen grid, and 1 Meg grid resistor for the following tube would be best. That will give the lowest distortion, best high-frequency response, and an ample gain of 62.

why the volume control in the front. would it be best after the pre-amp

I suspect that in this simple two-stage amp, the volume control is in front of the first stage to prevent overloading it with too much signal input voltage. This is a common practice with most amplifier designs including those used in AA5 radios. However, with pre-amps designed for low-level signal inputs such from microphones or magnetic phono cartridges, the control is usually placed after the high-gain stages to reduce circuit noise as the volume is turned down. In this case, the pre-amp is specifically designed for inputs that will not exceed a certain maximum level.

Dave

This looks intriguing enough. Good luck with this project and keep us posted. I've always been fond of the battery tubes.



Larry

You are a lot like me. Building stuff with tubes and parts you have on hand. A great way to learn more about vacuum tubes.

I once built a small amp using a 6DE7 and 6C4 with a 600 ohm to 8 ohm transformer. Amp worked but IIRC required plenty of filter capacitance to get rid of the hum.

I had originally built it to drive a 600 ohm antique radio speaker which it did coupling the primary through a capacitor and connecting the other speaker lead to ground.

I am thinking about building that circuit again, but this time doing it right since I know more about tubes.

I have an amplifier that has been 'in the works' for over a year because the more I learn about tubes and how they did things the more that stupid schematic changes. If I could get it to stay the same for a few it'd be great!

Post a topic about the amp in this section of the forums and the people here can tell you if you're going in the right direction.

Looking forward to seeing the evolving schematic, Kyle .

Larry