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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: 15 Jun 2011, 18:58 
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Cheap-Jack: you asked about the bias voltage for the 12AU7. It is -4.5V.

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PostPosted: 16 Jun 2011, 16:22 
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Hi.
mwhouston wrote:
the bias voltage for the 12AU7. It is -4.5V

IMO, I think -4.5V cathode bias is a bit too high as per its Vp-Ip-Vg transfer curves.

I'd try to move the -4.5V operation point further down to -2.4V, a more linear curve line area which can accommodate larger voltage swings than -4.5V. This will result higher Ip from 4.09mA (-4.5V bias voltage across the 1.1KR cathode bias resistor) to 7.4mA. But still only 1.8W, well under the tube's plate rated 3W max dissipation. So no sweats.

With -2.4V bias operation point set, now you can replace the 'ugly' cathode resistor/bypass cap with 2 NMHi rechargeble batteries in series, +ve polarity pointing the cathode, needless to say. These 2 batteries (2x1.2V) will provide the required -2.4V fixed bias for the tube.

I can bet NMHi battery bias would sounds better than any resistor cathode bias.

Also I am not happy with the huge plate-grid loop NFB in yr linestage. So it is there to reduce the overall gain of the linestage, correct?

c-J

PS: The 2.3V NMHi battery works like a chime for over a year now in my 1-stage 1-tube
phonostage - carefree!!!! :thumbsup:

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PostPosted: 16 Jun 2011, 16:43 
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I thought it high too. I'll re-check the measurement.

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PostPosted: 17 Jun 2011, 09:55 
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Mistake Number 4

When installing the cct. brd. into a metal case I never thought that the jack carrying the AC power would short the 12V DC and 260V HT. But it did. This cost me two AC 24V Wall warts before I realized what was causing the problem. So use a plastic plug to the chassis to the wall wart or drop a tail out with connector on the end but don't make part of the DC network part of the AC network.

It's a little rough inside but THIS IS a prototy. Sonically nothing missing the preamp just has an ugly hart. Alps blue velvet is used for the attenuator with a 4oz brass knob.

Cheap-Jack: Double checked the bias. There is 4.8V dropped across the cathode resistor.

I originally had silver cubboard knobs for the top plate with a 4oz black chrome knob but one of the cubboard knobs had a bad thread. So I swapped to gold knobs. And thus the volume knob was changed to match. Plenty of room under the bonnette.And the attenuator is on the output because this retains the 0.5M input impedance.

Enjoy the snaps. I have made this baby an art piece. Well, I think so.
Attachment:
SSTAmpInside.jpg
Attachment:
SSTAmpFinished.jpg
Attachment:
SSTAmpBack.jpg
Attachment:
SSTAmpTop.jpg


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PostPosted: 17 Jun 2011, 18:39 
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Very nice build. Well done. :thumbsup:

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PostPosted: 17 Jun 2011, 18:46 
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Suncalc wrote:
Very nice build. Well done. :thumbsup:

Matt: I e-mailed you the other day about some design work for 6N1P where the plates are paralleled and this preamp as the front end to that. Did you received the e-mail?

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PostPosted: 18 Jun 2011, 09:53 
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Hi Mark, I love simple circuits. The reason for the larger cap is so that you might want to drive long cable runs and feed relatively low input impedances at the other end. Most gear now has relatively high input Z of say 10K and up, but some older stuff can go as low as 1K. I like to have the Z out in my projects to be in the 200 ohm range and an F3 of about 1-2 HZ. This guarantees it will work with anything. A second and somewhat related concern is the drive capability. For something with a nominal 1 volt output I like to have the capability to do about +/-20 volts to allow for headroom. (part of the reason I like SRPPs).

Hi again, just saw the bit about the bias voltage, I'm with C-J on this. My thinking is that 4.5 is a bit much for the application. I find that 12AU7s seem to run best with about 1.5 to 2.5 volts bias. It is a more linear operational point. For small signals (under about 0.1 volts output) it might not matter, for larger ones it probably will, but might not be audible. It would show on my distortion analyzer though. Possibly on a DSO FFT. I have some vintage Sylvania 12AU7s that sound super in this type of circuit. The new JJ ECC802S are quite nice as well. The gold pin ECC83s are IMHO not as nice and the Black Sable ones rather nice but not worth the price difference over the standard 802S. BTW Wima makes polys in the size and voltage range that make good output couplers for this type of circuit. All in all I love simplicity.

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PostPosted: 18 Jun 2011, 12:22 
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Hi.
gofar99 wrote:
For small signals (under about 0.1 volts output) it might not matter, for larger ones it probably will, but might not be audible.

Most CD/DVD players are rated max 2Vrms at 600R O/P Z. So pushing the 12AU7 operation point to lower than 2.5V is not adversable. That's why I suggested take -2.5V as cathode bias voltage to ensure no clipping the I/P signal swing in the most linear curve area.

c-J

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PostPosted: 18 Jun 2011, 20:05 
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gofar99 wrote:
For small signals (under about 0.1 volts output) it might not matter, for larger ones it probably will, but might not be audible.

cheap-Jack wrote:
Most CD/DVD players are rated max 2Vrms at 600R O/P Z. So pushing the 12AU7 operation point to lower than 2.5V is not adversable. That's why I suggested take -2.5V as cathode bias voltage to ensure no clipping the I/P signal swing in the most linear curve area.

What is the best way to get the bias voltage down. How about a 30K plate resistor instead of the 20K one??

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PostPosted: 18 Jun 2011, 20:48 
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Mark, roughly how much did you charge for the completed preamp in case?
I am getting a small 6F3 tube amp (US $120) which outputs 2.5W into 8Ohms.
Judging from the specs, do you think a preamp is necessary?

Technical Specification of 6F3 mini tube amp:

Output power: 2.5W+2.5W@ 8 ohms
Frequency Response: 30Hz-40KHz (+/-1dB)
Input Sensitivity: 800mV
Residual Noise < 0.6mVac
Input AC Voltage: 100V-240V
Input Impedance: 10K ohms
Output Impedance (speaker output): 4/6/8/16 ohms
Output Transformer: Japan Z11 EI48*24
Power Consumption: 33W
Dimension: 132mmW x 122mmD (with RCA & knob) x 170mmH (with 6F3 tubes)
Weight: 1.8 KG
Input: RCA jacks
Output: Banana jacks
AC: Standard IEC with power switch and fuse (2A).
Attachment:
6F3-03 silver.jpg


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