12B4 Preamp

[David McGown]

I needed a low-gain preamp to try my recently completed Pass ACA amps that I have in my near-field system in the basement up in the main system. All I have are passive preamps, and the ACA's need a few dB of gain. Anyway, over the past few weekends I thru together a little 12B4 preamp. It is a pretty basic grounded cathode/capacitor output taking advantage of the low impedance of the 12B4, with Gary Pimm Mosfet CCSs as plate loads at 25mA, Mills 5W 510 ohm WW cathode resistor with 330uF/25V Silmic II bypass caps. 3.3uF output caps. I am using a CCS loaded shunt regulated power supply (Morgan Jones Statistical Regulator) at 225V for each channel. Voltage at the plate is 135V. Outboard power supply so I can keep the transformer at a distance for low noise. All built in pretty little Hammond extruded aluminum enclosures. I had most of the parts, but had to wait for Mouser for two weeks for the enclosure (Texas freeze and power outage).

Breaking in the near-field system, but sounding pretty nice (new "working" projects always do, don't they... )

A few glamour shots

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[David McGown]

Pic No. 2

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[David McGown]

Pic 3 all closed up

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[ratbagp]

Lovely work and I like the blue enclosure.

What is the function of the circuit board thingie next to the electrolytic cap at the lower left?

Could you post a schematic?

ray

[David McGown]

Ray,

There are a couple of circuit board near the electrolytic caps. One is a dual DN2540 cascode CCS (one B+ input, split to two CCSs). The second, and more prominent board (1" wide x 3" long) is the shunt regulator, which is a string of 5.6V Zener diodes. I have two regulators stacked via standoffs, one for each channel. Morgan Jones talks extensively about this regulator in the 4th edition of Valve Amplifiers (no tube amp builder should be without it). He uses a tag board, I went with a circuit board, with the string zig-zagging on both sides to keep it compact. It does put out a bit of heat (around 4.5W per regulator), but radiates via the diode leads. Next time I build one, I will need to stand the zeners off the board for better circulation.

I do not have a schematic drawn up, but do a search on 12B4 preamps and you will find circuits nearly identical to mine. I used a CCS in the plate supply, others use a bypassed LM317CCS in the cathode with a resistor load. You can do choke load or even an output transformer (5K:600 from Edcor should work well).

David

[ratbagp]

Thanks for the info, especially the shunt regulator.

I actually built a SE amp years ago with 5 or 6 12B4A per channel. It sounded pretty decent but I moved on to other things and disassembled it. I was influenced by some commercial line stage that used multiple 9 pin tubes per channel and supposedly produced more detailed sound. I've still got the tubes kicking around.

ray

[David McGown]

12B4 Preamp Schematic.pdf

(17.11 KiB) Downloaded 444 times

Ray,

Just sketched up my schematic for reference.

David

[ratbagp]

Thank you.

ray

[Roscoe Primrose]

I tried to use 12B4s as drivers years ago, but couldn't find any that weren't VERY microphonic.... How are yours?

Roscoe

[David McGown]

I tried to use 12B4s as drivers years ago, but couldn't find any that weren't VERY microphonic.... How are yours?

Roscoe

Quiet as a church mouse. I have a speaker within 1 foot of the preamp. I have heard the trick is to use JAN 12B4A (GE), they are screened better for microphonics. But that is only what I heard on the internet, that vast font of disinformation.