6V6 line preamp

[harlee]

Originally Posted by Salas

What is your B+ PSU DC right after the rectifier diodes across the first smoothing capacitor? Also using a 7812 for heaters means you strung them in series?

395VDC,
Yes, 7812 series reg, heater of 6V6 in series, about 5.8-5.9 across each tube heater.

[Salas]

Originally Posted by harlee

B+ is 292V, 18mA through the circuit.
Thanks Salas!

Originally Posted by harlee

Regarding the shunt reg sim, I would like to know, thanks.

Originally Posted by harlee

395VDC,

Based on your schematic and data I constructed a sim. Things of notice are:
1. Substantial voltage has to be be dropped across some series ballast to protect the tube cascoded chip for max current. As you did with your RC cells.
2. Ripple eating is so so but it works. Fed 2Vpp 100Hz dc source ripple and 10mApp 1kHz load play in the sim. 140-180mVpp on output. 12.5 times rejection on average (22dB).
3. The output capacitor looks better placed across the high value resistor and trimmer leg for filtering the TL431 ref node. Its a noisy enough chip by itself and that resistor combo is big for Johnson noise. Don't know about stability in that case, it takes much more elaborate modeling and feedback nodes breakage of some full TL431 sub circuit. A practical test would tell much faster. Zo goes higher the opposite way for cap full output shunt or resistor leg shunt, has to do with its bootstrapping the ref or not also. Thus it eats 100Hz ripple stronger due to AC feedback (pic3).

[6L6]

Originally Posted by Salas

No problem, bit better isolation even. Its the same for the original in post #81. The 6V6s are just 0.45A Ih each though.

Good, I didn't think there would be an issue, but it's always good to ask.

Yes, the heaters are only .45A, but the voltage doubler is a current thief, according to Broskie, you only get a bit more than 25% of the winding's AC current rating. (AC current/3.8 = DC current)

[Salas]

True you win some you lose some when transforming tension and current abilities, still 1.5A would cater if you had to.

[harlee]

Hi Salas,
Thanks for the update, as it is, didn't hear any instability. Have been using for a week now, seems ok, did change the 10uF across to the resistor and TL431 ref point this morning though, still run as good.
Thanks again.
Keep it coming if there is any more recommendations.

[Salas]

Was talking about the change mainly, so it proved stable too. Did it make any subjective change?

[harlee]

Didn't notice any changes.
I am now looking at the Tungsol cryo, what is the difference in soundwise?

[Salas]

Don't know, I only have the regular set. Not so expensive though. I hope they do not compromise them mechanically during the freeze and thaw cycle at least.

[yagoolar]

Today my NOS 6π6c's have arrived. So next pre stage will be made of them.

[Salas]

They are melodic tubes, I have them. A couple went noisy in a batch of ten but they were cheap back then, so no big deal.