...SOLD...
Tubes selection made, now it's time to mix up all these good things.
Before publishing amplifier schematic let's have a look to the driver limitations.
High frequency: 12B4A & Miller's effect
The input capacitance of the tube C, in conjunction with the source impedance Rg of the stage, forms a simple low pass RC filter with an upper -3dB cutoff frequency equal to:
f = 1/(2π x Rg x C) where C = Cgk + Cga x (Gain12B4A +1)
The -3dB point with a 220K Rg resistor will be: f = 1/(2π x 220K x 39pF) = 18.8kHz
As I can neither reduce the load (90K _ See part 2) of the previous stage without lowering the E80CC gain nor increase Rg, some local NFB will help to keep a good overall bandwidth.
A 4 to 6 dB loop fine tuned with an FFT analyzer will give the desired bandwidth and distortion figure, definitely set by listening test.
Low frequency: inverted interstage transformer
Tamura IT is used with inverted primary / secondary coils.
In that way E140 grid current in the secondary cancels a portion of 12B4A current flowing in primary coil.
Unlike all Shishido's amplifiers working A2 all the time, this one can swing the E140 A1 where no grid current appears. This is why I have carefully chosen a 12B4A operating point where anode current do not exceed 10mA.
With such a primary current the transformer low frequency response is very good.
High frequencies are also very good without roll off with this Tamura transformer. I believe it is the way the transformer is wound that prevents attenuation when inverted connected.
Amplifier schematic
Power supplies
This amplifier requires both high and low voltage.
Low voltage is required to bias E140 grid through interstage transformer. Needs to be very steady and adjustable, a LM317 is the simplest way to achieve good regulation and very little ripple. It requires just a few parts and can be easily implemented using PCB's.
High voltage supply is a bit more complicated than usually mainly because the power stage high voltage (240V) is lower than the ones needed for previous stages (270/260V). It imposes to split the supply in two just behind the first choke keeping in mind the necessity of well calculated cells to insure good transient response. In that way I had to consider both ripple attenuation and time constant factor.
I choose choke input filter for low, almost perfect sine wave ripple and good regulation. Moreover it is less stressing for the power transformer than capacitor input filter.
Unlike capacitor input filter the ripple is not a function of current. It just depends upon voltage, choke and capacitor.
It can be calculated from formula:
Vripple = η VHV where η = 1,2/LC (L in Henries and C in µF).
In that case it will be 0,00048 x 260 = 0,125 Vcc or 0,044 Vrms and represent a 60 dB attenuation of the ripple behind rectifier (44 Vrms). Very efficient!
Amplifier is now complete. Sold to a German amateur.
