• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

First foray into tubes!!

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So I figured I'd start simple.

Excuse the sideways-ness of the schematic :).
I'll be building the two sides, so good for about 10w, ample through 91dB speakers.

Just curious - exactly how interchangeable would a 12AU7 be with the AX - I've known them to be swapped over in guitar amps with no problem and producing favourable results...

Should be commencing at the end of the year (college project), and some interesting addons, like a remote controlled motorised volume pot etc.


L

se6v6gtschem.gif
 
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Editing posts works OK, providing you don'r leave it too long before you do so. A useful tool is the preview button, which helps you get it right before posting.

A bit more about the 12AU7: this is not a very linear tube and there are better 9-pin alternatives, like the 6CG7 and 6FQ7. They're not quite so easy to find and may cost a bit more, but they are quite linear - true miniaturizations of the 6SN7. They're not able to sub successfully for a 12AX7, though, anymore than the 12AU7 is, and their pin-outs are different anyway.
 
That design has several weaknesses.

The B+ rail in SE amps should be well filtered, as the common mode rejection found in PP circuitry is absent. That PSU is anything but well filtered. Also, 6V6 performance will improve significantly, if the screen grid supply is regulated.

The 500 KOhm volume control interacts adversely with both cable capacitance and the Miller capacitance of the triode to roll HF info. off.

22 KOhms as a grid stopper for a 'X7 section is OVERKILL. A low gm type, like the 'X7, requires no more than 100 Ohms.

Designs (like the 1 being discussed) that employ loop NFB need to roll infrasonic noise off at the amp's I/P. The O/P trafo core must be protected against saturation from a large LF error correction signal.

BTW, a high gm type in the voltage gain block is protection against slew limiting. Remember, the driver has to be able to swing the HF error correction signal associated with loop NFB.
 
The filtering MAY be adequate for pentode mode (as drawn) but certainly wouldn't be enough for triode... I'll run the numbers later.

100K pot would be much better with a 12AX7 - and the high value grid stopper is unnecessary unless you have serious RF interference in your neighborhood.

I'd use a 100K plate load for the 'X7, not 220K. The cathode resistor would drop to 1.2K or so.

I wouldn't worry about limiting input frequency response unless you're driving this with a phono preamp
 
The first thing I did was to run PSU Designer to see how much power supply ripple there is - it's about 10 Vp-p. The plate resistance of the 6V6 is about 80K, so that 10V will be divided 10:1 - about 1 Vp-p appearing across the 8.5K transformer primary. The transformer is 1000:1 for impedance, or about 32:1 for voltage, so there will be about 31 mVp-p to the speaker, about 12 mV RMS. Whether that's audible will depend on your speakers' placement and efficiency. I would try for better than that.

6V6 data sheet

Duncan Amps PSU Designer
 
The picture below is a screen shot from PSU Designer - adding 20 uF and 220 Ohms gave a ripple reduction of about 4:1.

An externally hosted image should be here but it was not working when we last tested it.


The 70 mA current is the output tubes, 10 mA is the screen current plus a bleeder resistor (68K / 5W to ground) which will improve regulation, and the 2 mA is the two input stages.
 
Psychobiker said:
How to correct that, then?

L


L,

Fixing the PSU is not especially difficult. Switch to a 5V4 rectifier, as that yields a higher voltage than the 5Y3. Those extra Volts come in handy when additional filtration is added. Use a 1st filter cap. of 15 muF. immediately after the 5V4. Follow the 1st filter cap. with a choke of at least 5 H. Increase the B+ "tank" cap. to 100 muF. from the 40 muF. shown. Betcha the ripple is a lot lower.

BTW, the 5V4 has a cathode sleeve and it's connected to pin 8. Take the "raw" B+ from pin 8. OTOH, the 5Y3 is directly heated. So, there is no difference between pin 2 and pin 8 as a "raw" B+ takeoff point.
 
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