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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Audio Mirror 6AS7

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Hello guys, first I wanna sorry for my poor english language skills... Need some toughts about áudio mirror 6as7 amplifier (DIY Loudspeaker Projects Troels Gravesen › AudioMirror). I bought 100 6as7 tubes and don't know what to build with them.. I have built a lot of solid state amps and just kits of tube amplifiers. Lookin at áudio mirror 20w with 2 6as7 per channel.
Any toughts on using on the same circuit 16(sections) of 6as7 per channel is viable? Just add more tubes and increase the current from PSU? And about the driver stage , how should I exite all this tubes in parallel? Any changes neessary? Any ideas on the output transformer?
Sorry for the amout of quetions but I have no experience on calculating tube amplifiers.
Best regards, Odion Carvalho - Brazil.
Some ideas ? :D


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6AS7 maximum DC current is 250mA per tube (0.25 Amps).

20 Watts rms into an 8 Ohm load requires 1.58 Amps rms.
For a sine wave, Peak amps = Root (2) times rms Amps. Root (2) ~ 1.414
1.58 Amps * 1.414 = 2.235 Amps peak.
A 20 Watt rms amplifier into 8 Ohms requires 2.235 Amps peak output.

If you want to use only 2 6AS7 tubes to get 20 Watts rms, you will need an output transformer.
An output transformer can reduce the output impedance, and more importantly in this case increases current.

6AS7 tubes have a mu (u) of 2. The driver will have to swing lots of volts.
The driver will also have to drive the capacitance of parallel triodes.

Probably the most efficient 6AS7 amplifier will be push pull. Two 6AS7 could produce 20 Watts
with the plates driving the push pull transformer.
When you parallel the 2 triodes in a 6AS7, you should use individual self bias resistors, with individual bypass caps. Thats 4 Rs and 4 Cs.

I am certain there are examples in this forum using 6AS7 in push pull with an output transformer. Perhaps one of them produces 10 Watts.
You are half way there.

There may not be any examples of Parallel push pull with an output transformer, but you need that.
So . . . to go from a single 6AS7 to two 6AS7 will most simply require:
Doubling the B+ current, paralleling the two triodes in each 6AS7s (one 6AS7 for Push, and one 6AS7 for Pull); and using an output transformer that is rated for 2X power (more current and laminations) and also rated at 0.707 of the primary impedance of the 10
Watt single 6AS7 amplifier.
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If you really want to build an output transformer-less tube amplifier, you are going to need a lot more current from the power supply. Think of a + 150V and - 150V volt B supplies, that can put out 2.35 Amps. Wow!
And the many many 6SA7 tubes will put out lots of heat.

Per tube, as much as:
Filaments 6.3V @ 2.5 Amps, 15.75 Watts
Plates up to 26 Watts

8 tubes
Filaments 126 Watts
Plates 208 Watts
334 Watts total

That is 668 Watts for Stereo
It is great for Winter, turn off the heater; but you need an Air Conditioner for Summer.
Don't look at the power bill in the Summer time.

If you do such an ambitious project, make sure that you use a current balancing resistors in each and every cathode. Failure to do that, will most likely result in a failure of one or even several tubes.
Antek makes a very reasonably priced 1,000W 115V dual secondary transformer that works great to get the 150VDC required. I think I bucked the voltage down a little by using one or both of the heater windings. Filament duty is a 24V SMPS series/paralleled to get 12V. I used 12SN7's because they're pretty cheap.
And yes, as 6A3Summer says the heat produced is immense. Something to consider living in Brazil.
Thanks guys, take a look on this one... I think is the simpliest that I found .... with 16 tube sections I will have a good space heater (in Brazil , 35º C)
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Audio Mirror 20 wpc 6AS7 SET monoblocks.
My main problem is: What should I do to drive all this tubes.... I don't think a OTL is as option, since I wanna use a pair of B&W Nautilus (4ohms)... very low impedance and afraid of a amp failure that can damage 20k+ speakers.....
"And the many many 6SA7 tubes will put out lots of heat." Meaning the chassis will get very hot too. I've an amp that uses 6 x EL34's, even with a fan blowing air onto the chassis and components below the chassis gets nasty hot, something to think about especially if you have kids.

For the sort of amp your thinking about I would recommend some sort of protection circuit to cut power to the amp should any one valve start to run away. This isn't too difficult to do, a sense resistor in every valves cathode so rising voltage trips an SCR which engages a relay to cut the mains supply. This really needs a separate transformer, but only a little one, 12v 20VA say.

Good luck with your build, Andy.
One more thing to consider. I've also been using 4 ohm speakers but use Zero autoformers.

They will more closely match the speaker impedance to the amp and shunt any possible DC to ground.

My amps have been running flawlessly since built. Any offset is in the millivolt range.

Good luck with whatever you decide!

Audio Mirror 20 wpc 6AS7 SET monoblocks
Good call, that is:
Just as I previously said, use a push pull output transformer, and 4 individual self bias resistors and 4 individual bypass caps.
Much more efficient, and much more reliable (and a lot less trouble).

And . . . you could have transformers made with 4, 6, 8, or 16 Ohm output taps according to your needs.
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