This amp is planned for high
frequency use only (10kHz and up). Because of this the values of the
components used seem ridiculous. I mean; a 1H plate choke ?!?
Yes indeed, with a 5K+ plate impedance this gives a bottom end of around
800Hz at -3dB (3kHz at -1dB). The only concern is the
capacity of the output tube's grid (Cgp), slew rate comes into play and
has to be checked. Due to the low grid capacity of the 71(A) the grid
will only require about 1.5mA at 50kHz and 40V swing: this is actually
the most extreme circumstance. The 12A will have no problem with this,
but itself requires about half that at 50kHz and 13V swing. The
preamp/cross-over will have to be able to manage this.
Schematic of the signal section.
The B+ supply will be fed by the battery supply. Taps are made at
the appropriate batteries.
Iron is on order. They should be quite small due to the low inductance
that is required (3kHz and up). I am currently working on trying to
integrate x-over and power amp, more on that later. The output stage is a
UX171, a really old version of the 71A. The 171 looks exactly like a
normal 71A globe, until you light it up. The UX171 has a 5V/0.5A tungsten
filament, just like the UX210 and more popular 211. This version was made
just in the first few years of the 71, so they are really hard to find.
The nice thing about TT (besides the cool glow) is that they sound more
detailed and fresh in the highs. And that is a good thing for an HF amp of
Due to the results of the Oris 103C experiment I will no longer be
needing this HF amp. It will never see life beyond the breadboard. Still I
think it makes a very nice HF amp.
© Jim de Kort