A while ago browsing the web to enrich my knowledge, I stumbled on a phono schematic around the C3g. Well known in the hobbyist world and a must try for many, this tube just average in triode mode is a real champion as a pentode.
I discovered these great qualities while seeking a first stage tube for my LCR phono stage. Sound was really involving, but gain was too high for comfortable listening in my setup and I did not persevere in that direction until today.
The scheme I found from Intact audio forum, is very simple, but I noticed a few design mistakes that will surely affect the final result. A weird RIAA network and a rather high output impedance. Both easily fixed.
Network is passive and when properly calculated can give a very accurate correction. In that way the best is to follow Lipschitz 1979 publication. To make it short we have to keep in mind that network is dependent upon first stage Zout, second stage input resistance Zin and Miller’s input capacitance Cin. This last point is often neglected and a reason why you cannot change tubes without modifying network time constants.
Now, components calculation is easier. We just have to write down all we need.
1 – Time constants
R’1C1 = 2187 µS
R’1C’2 = 750 µS
R2C1 = 318 µS
C1 // C’2 = 2,916
2 – Components value
R’1 = ( Zout + R1 ) // Zin
Zin ~ 50 Zout
C’2 = C2 + Cin
First stage gain, Zout and bandwidth
A pentode gain is merely equal to tube Gm by load. If we want to keep second stage input resistance in a reasonable value (~ 500K), C3g load have to be something like 10/15K to respect Lipschitz recommendations.
A graphical study will help find the best operating point and I finally stopped on 12K@10mA. Gain will be very close to 150 and Zout 11,6K assuming a 400K internal resistance. Rg2 (Zin) can either be 510K or 560K depending upon RIAA resistors availability. These have to be 1% or closer tolerance.
To keep gain high G2 is fully decoupled to the lowest frequencies. As a bonus, decoupling dramatically reduces the grid-anode capacitance. Effect on bandwidth is a huge improvement, with high frequencies well above 10Mhz. To prevent any unwanted oscillation it is necessary to use a grid stopper close to G1 pin.
Second stage gain and Zout
I did not go with a 5687 like in the original design. Amplification factor is too low and this tube needs a lot of mA to sound right.
Because gain of a standard phono preamp is about 300/350 or +50dB, I have to find a tube with at least a gain of 20 to compensate RIAA network insertion loss (-20dB @ 1KHz). After much thought to decide if I should use a high µ triode with degenerative feedback or a classic medium µ anode follower, I stopped on the great sounding E182CC fully bypassed.
With a µ of 24, a low internal resistance and quite a high Gm (15mA/V), it is a good G3g companion. Moreover, the second triode will make a nice shared current coupled stage. I used this topology in the E88C/E86CC LCR phono preamp with great satisfaction.
Output impedance will be about ρ/µ+1 (~65Ω). This is far better than the 4K of the original design and will allow good frequencies transmission even with long cables.
preamp schematic
more to come...
