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 NEW  Matt presents bias and operation data for the 6V6 tube in SE operation - 6V6 Single-Ended (SE) Ultra Linear (UL) Bias Optimization.

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PostPosted: 05 Apr 2011, 16:59 
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rasty wrote:
The left channel is in out of phase with the right. The right one is out of phase with the input, but the left one is in phase.

Hi rasty, the tube inverts the signal, so both channels should be 180 degrees out of phase relative to the input signal. Double check the pinouts of the tube as you may have a wiring error here. Post a photo if you are unable to track it down.

Cheers

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PostPosted: 15 Apr 2011, 09:57 
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Joined: 06 Jul 2009, 09:22
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Hi, i wanna build this hybrid head amp, as in layout i no see connection of TUBE PIN9 ? where it goes?
one more, the multiturn 50k can be replace by resistor 4K7 like in schematic?
thanks for the answer.

warm regards,

fery


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PostPosted: 15 Apr 2011, 19:48 
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Joined: 08 May 2009, 08:20
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Location: Duesseldorf, Germany
Hi and welcome ;-)

Pin 9 of the tube is the centre connection for the filaments. Pin 9 can be used to supply the filaments
with 6,3 Volts. Then, pin 4 + 5 are connected to 6,3V in parallel, with pin 9 as ground (for DC) or pin 9 as the 2nd AC pin.
On this layout, the filaments are powered with 12VDC thus only the pins 4 and 5 are used.

The 4k7 resistor is optional (R3) and can be used instead of the potentiometer.
The project page wrote:
R3 - A fixed resistor (R3) may be used in place of the 50k potentiometer (P1). 4k7 worked for many 12AU7 tube types, but you should pick a value to get half the supply voltage (see setting the bias Photograph 14).

The resistor R3 or the potentiometer P1 acts as plate load resistance for the tube and it is also used to BIAS the mosfet (one channel).
A potentiometer is a better choice as you can adjust the load resistance and the bias very accurate. The circuit works with a low
voltage and low currents and there is not much headroom to set a proper operating point at the loadline. If you would choose
a fixed resistor with 4700 0hm with 5% tolerance, which is not unusual, the resistance would vary from 4400 Ohm to 4900 Ohm.
Now - if the plate current varies, the tube bias also does... This causes a non equal operating point and mosfet bias on both channels.
On the other hand, if you match your resistors closely, you should get the same operating points on both channels,
but the needed resistance of R3 depends on the fact which 12AU7 you may want to install. The plate resistance (Ri)
and the plate current (Ip) may vary from tube to tube... It is a bit of try and measure. I guess, Rogers did a lot of this...
For a better understanding, take a look onto the GE datasheet (page 3). You can see some relations between voltage and resistance... GE 12AU7

I would not recommend to use fixed resistors instead of P1. And i would recommend to match the resistors R2 for both sections.

I hope this was a little help and also a bit understandable ;-).

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Tom.

Some of my projects: TDA2050 Chip Amp, the LM3886 Gainclone Thread and the Szekeres Headamp Thread.


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PostPosted: 16 Apr 2011, 03:04 
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Thanks Tom for help, i will try to make pcb first...


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PostPosted: 23 Apr 2011, 17:27 
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Joined: 23 Apr 2011, 15:07
Posts: 3
Great project, its been a pleasure o build, no problems at all and the sound is great, I even did the printed circuit.
I have a few questions.

1) I want to add a pair of VU meters from anders electronics, C360 VU, 0-200uA and 650ohms. I want to know if they will interfere with the quality of the sound, and if there is a simple circuit to drive them.?

2) If I connect the amp to an iPod using the jack i guess I take the amplified signal so the iPod amp will modify the signal, is it posible to get a line signal with no distortion from the iPod or it doesn't matter?

3) I know there is a delay circuit to avoid the noise while switching on, but is there something to avoid the same noise while shutting down?
Thanks agin for the project.


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PostPosted: 02 May 2011, 12:44 
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Joined: 02 May 2011, 12:19
Posts: 1
Hi,

Thanks for the instructions, it's been fun building this and it works perfectly fine! In fact there are two of these, me and my friend both built one but they're almost indentical so I'll be the only one posting the results here. We did the electronics pretty much as in the instructions, the board layout is the same etc. We used RCA input and found a neat regulated power supply that doesn't produce much AC noise.

Image

The heating problem is solved like this:
The tape between the metal and the transistors conducts heat but not electricity and the screws are plastic. The metal thing is pushed against the case and there's thermal grease in between those.

Image

The case is made from aluminium. Pieces of 3mm and 10mm thick aluminium, some bending, drilling, machining, bead blasting and here's the result:

Image
Image
Image
Image


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PostPosted: 02 May 2011, 15:45 
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Joined: 26 Feb 2011, 15:02
Posts: 30
My contribution.

The red pack are 4x 100µF Wima capacitors for down to 32 ohm headphones (only 1x 100µF for Sennheiser HD600 for example): the Eagle3D symbol needs a better definition!

One sided PCB, reduced cabling.

I dont know, Neutrik Jack on board?

Im working on the power supplies. The best choice ist SLA battery, for a very nice a much greater sound, but the tube needs a lot of current! Charger ist not a problem, but the capacity will be big for longer listen.

more to come...


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PostPosted: 03 May 2011, 12:33 
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Joined: 22 Apr 2011, 20:26
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Thanks for the instructions Roger and everyone. I was asked for a 'conversation piece' so here is what I came up with. I added the turn on relay linked from the 1st page in this thread and made a LED board which let me easily add orange UFO lighting. I made LM317 regulator with 2, 1000 uF and filtering and it wasn't enough for the printer power supply I used so you can see a big 4700 uF cap added after the fact which made a very big difference and dropped hiss to very low levels.

Image

Image


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PostPosted: 04 May 2011, 12:50 
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Joined: 26 Feb 2011, 15:02
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...progress.

A good preamplifier also?

Simply, details to finish...


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PostPosted: 08 May 2011, 22:56 
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Location: Winnipeg, CANADA
Excellent builds KrHn and civilmonkey! Congrats and enjoy the headphone amp.
Cheers

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