Conrad Johnson ART preamp question.

[Zen Mod]

Thanks to share Your experience!

I know that my schematic is not same. I never did exact copy of any amplifier. I just like the simplicity, and I want to make my version.
So I really like the E88CC version, but I can build the dual 6H30 shorter signal path. The only question is the bias I should use. I don't want to try lot version because the high quality resistors costs lot of money.

Sajti

😉 then look at graphs , do not use Spice for that ;

try with ordinary resistors and - when you find your sweet spot - replace them with precious ones .

From what I've read, I believe the ART uses a separate coupling capacitor for each tube. That adds considerably to the cost of the preamp since the coupling caps are Teflons.

nope . I can tell you that coupling cap is 3x2uF , parallel , common for all anodes

say that all anodes share saame anode load resistor ( even if made of several ones - each connected to one anode ;Sajti's schematic is almost on spot with value . )

 

[sajti]

😉 then look at graphs , do not use Spice for that ;

try with ordinary resistors and - when you find your sweet spot - replace them with precious ones .

I used spice to find some technically good bias point.
I will do the listening. Just odered some resistors from Partsconnexion, for the E88CC version.

nope . I can tell you that coupling cap is 3x2uF , parallel , common for all anodes

say that all anodes share saame anode load resistor ( even if made of several ones - each connected to one anode ;Sajti's schematic is almost on spot with value . )

My version use only 4xE88CC, because I don't really need as low output impedance as the original design.
Nice to hear that my values are close, because I have never seen the schematic. Just read about the preamp.

Sajti

 

[sajti]

I will give a chance for the E88CC version first, because I have 24pcs. of Tungsram tubes.

Sajti

 

[E.T.]

Smoother warmups

Soft start and abrupt delayed start are totally different kinds of fish.

Here is my own soft start version: reference voltage for B+ is taken from bias shunt regulator. Since both regulators are in series, time constant depends on current drawn from B+, so the stabilizer literally waits for tubes to heat up to start drawing current, then slowly brings up B+ voltage.

I had an idea once to put a big power resistor in series right after the rectifier tube, so it and the filter caps would create a time constant of say 30 sec. After that a switch would bypass the resistor.

Dunno if it would actually work, tho, and the resistor would get unwieldly for any amp bigger than a headphone amp.

The idea was to eliminate the full plate voltage hitting the tubes all at once.

Your thoughts?

 

[Salectric]

I can't see any benefit to a soft-start with a preamp. However, if somebody is really concerned about this, just use a thermistor on the power transformer primary. The voltages will ramp up gradually. Easy and cheap. But also unnecessary in my opinion.