12B4A Tube Pre-amplifier DIY

If you do a quick search though the diyaudio.com forum you will come across an old thread that was once very active. It is based around building a 12B4A based pre-amplifier. The circuit given uses a regulator in the cathode for a constant current design but a resistor can be substituted in its place. The main circuit is very simple, easy to make which I always like. It was also getting quite favourable reviews from those who had built it so I decided to have a go myself. I wanted a pre-amp at the time to test with a 20W Mr Liang 845 SET, which although is an integrated, sometime needs a good pre-amp for a little more gain. I also later tested this pre-amp with the large Korsun (Dussun) V6i transistor power amp from China and a DIY 25W Nelson Pass F5 Class A power amp. The 12B4 is a small power tube with a Noval base and a plate dissipation of 5.5W.



The pdf link below contains the original build guide, original schematic and psu. This pre was built using what-ever parts I had already or could find free and has been tweaked and upgraded over the last year or so as my system changed. This project started as a cheap build but I was impressed by the sound and continued to work on it adding better parts as time went on and altering the original circuit. A good design with basic parts will always better a poor design with boutique parts IMHO.

I originally tested it with a PEC pot which is musical if a little warm sounding in my system at the time, ML electrostatics, which are not exactly very forward sounding but quite laid back, pure and natural. The PEC is better on regular dynamic Hi-Fi speakers. I later tested a Alps Step Attenuator which uses small surface mount resistors instead of the usual plastic track and increased the output cap to 10uF to prevent any roll off of low frequencies and phase shift in the mid band. Anything 1uF or higher should be ok. Cathode by-pass caps can also be tuned to suit your own system and ears. I used a 100uF in the end after trying a 220uF and a 47uF, the 220uF sounding a bit thick and the 47uF a bit lightweight.

Thanks to Val for the aluminium knob and the Russian Silver Mica Caps used as bypass on the 10uF SCR Output caps. The SCR caps are nothing expensive but those 0.01uF 500V Silver Mica caps make such a difference on this amp and make the unit sound far more refined and 'expensive'. Thanks to Dave H for the case, I think it was originally some instrument box with a card reader slot in the front.


Circuit Tweaks - Modified Circuit

Further checking of the circuit via TubeCAD software sometime later showed that the 12B4 tube in the original circuit was being driven very hard which can stress and shorten tube life. I had a set of tubes fail; going into grid current after extended use of this pre-amp as a buffer at 3/4 full volume. This resulted in a small amount of DC appearing at the grid of the tube and hence appearing on the volume pot were the input signal comes from. This manifests as a scratching / dirty sound from the speakers as the volume is adjusted.

To fix this I used TubeCAD to select more suitable resistor values and set the tube at 70% maximum rating rather than the high 90's as the original circuit shown in the pdf file. The anode resistor was lowered from 6K8 to 5K3 (anything 5K1 to 5K6 is OK). The cathode resistor was increased from 470R to 1K. This dropped the anode current from 27mA to around 20mA and a lower plate dissipation as a result whilst keeping everything else happy in the TubeCAD simulation.

It is also possible to add an input capacitor between the pot and grid of the tube. This isolates the grid from the pot such that if a tube fails DC will not appear on the POT and back up to the source. This is optional (not shown in the circuit) and is really just a belt and braces approach.

** Original and Modified Circuits attached as a Zip file below for Full DIY build **


12B4A Tube Pre-amplifier
12B4A Tube Pre-amplifier
12B4A Tube Pre-amplifier
12B4A Tube Pre-amplifier