A priori: You arrived here because I was (one of) the first world wide
to replace the original DX-393 tube with a GS-36B in a AM-6154.

I did this modification around 2005 and managed to rescue some
pictures I took
at that time.

Several requests for information urged me to made this page, not knowing that
my first ever YouTube ‘movie’
would be -as of today- my most popular one (! . . . : -)


Introduction.

Large amounts of FAA AM-6154 amplifiers appeared on the European and US surplus markets in the 90′s.
I bought one of my AM-6154′s in 1992 in Weinheim (Germany) due to parting US Forces in Darmstadt.
The amplifier served in several 144 MHz contests, but during one contest (2000 or so) it collapsed.

Here is a picture of the specific amplifier. Look at it, that’s why you’re here ; -)

For whatever reason it took several years to investigate the issue. Inspection revealed that
the tube (Amperex DX-393) arced when the anode voltage exceeded ca. 1400 V.
Attempts to (re)condition the tube failed. Here is a picture of this tube.

Because no original tube could be found on the surplus market at that time, the choice fell on a (at that
time) presumed Russian equivalent, the ГС-36Б or GS-36B.

The GS-36B is a good tube, in a sense that it has good IMD characteristics at relatively low anode
voltages and, unlike e.g. the 4CX250(B/R), has no secondary screen (g2) emission (‘-Ig2′, i.e. negative screen current).
So, the GS-36B is ‘more linear’ compared to e.g. a 4CX250(B/R).

The DX-393 and GS-36B have almost identical physical dimensions, albeit that the anode diameter of
the latter is somewhat smaller.

In order to revitalise the AM-6154 I chose a thorough approach. Some ingredients of this approach
have been published on this site, some ingredients were hosted elsewhere. Unfortunately the owner
of the latter machine died, and so did the contents of his remote host, unlike the Google search results.

Over the past years I received various questions concerning my AM-6154 modifications. It took me
quite some time to find some of the original material/photos. Recently I found some detailed pictures
of my modifications, which I share here for the benefit of AM-6154 owners who want to upgrade
and improve their AM-6154.

Anode compartiment.
Please open this new tab for reference. The URL in the picture unfortunately is not valid anymore.
C21 has been removed and copperfoil was wound around the GS-36B anode due to its somewhat smaller diameter.
Fiddling with the fingerstock may be an alternative to ensure that the anode has good RF- and electrical contact
with the anode resonator.

See pictures 1a & 1b below (PLEASE click on images to enlarge in a new tab). Picture 1a courtesy of OE6TZE.


Figure 1a C21 + anode of the original tube               Figure 1b. New anode compartiment (C21 removed).


Cathode compartment / input circuit.

Figure 2 (tnx OE6TZE) shows the original cathode compartment and figure 3a and 3b show what has to be done.
(click on images to enlarge in a new tab).


Figure 2. Orig AM-6154 cathode compartment.                       Figures 3a & 3b  RED dots to be removed and discarded.

Also remove the glass over voltage protector! My experience is these devices detoriate in time.
The cathode pins of the tube socket were connected to GND. After figure 3 had been
followed, my result was figure 4 (also, click on picture to enlarge in a new tab).


Figure 4. Follow up of figure 3.

The input circuit was changed to a 1/2λ type, using the existing ‘input tune’ variable capacitor on
one side of the 1/2λ and the G1-capacitance on the other side.

My implementation is depicted in figures 5 and 6 below (again, click on them to enlarge in new tabs).


Figure 5. Modified input circuit / cathode compartment.

As the input circuit is a hi-Z contraption I damped G1 (control grid) with around 1 kΩ (in fact 2 x 2k2 1W parallel = 1k1).
To prevent ‘surprises’ (like short cuts in the input tune cap) I inserted a large (ca. 1 nF) capacitor
between the input tune capacitor and the 1/2λ coil.

It may seem that the input coil might be somewhat low in inductance (only 1 turn Ø 8mm), but it works well because
the tube input Z is high ‘at the other side’ (1k1) and the coil is placed ‘in the middle’ (low Z).
Note: look how the windings are wound.

With some fiddling, i.e. moving the coupling coil in- and outwards, input VSWR = 1.000 can be realised.
Return loss of 40 dB is possible in conjunction with tweaking the input tune capacitor. This can be tested in
‘cold state’, i.e. no voltages applied to the tube. The VSWR50 = 1 dip should be sharp.

Bias circuit.
The GS-36B needs a higher (or lower) bias circuit compared to the original DX-393 tube.
If I remember correctly the GS-36B draws around 50 mA anode current when Vg1 = ca. -40V (and Vg2 = 390V).
The original DX-393 needs around -80V, so the bias (-Vg1) circuit needs some tweaking.

As I am convinced that the only relay in an amplifier should be the output coax relay, I made a solid state
bias / idling circuit. Here is a picture how I did this. I replaced the original zener with 2 x 39V/1W zeners parallel
and a NPN transistor (e.g. TIP50 or 2SC2502, see junkbox) in conjunction with an opto coupler (I used a CNX36).

Note: I always use +12V to key (‘PTT’) amplifiers. Thus NOT key to GND!

C1 was replaced by a >200 uF/400V elco (from a surplus computer power supply) and C2 was discarded.
The result is a more stable -Vg1 with SSB, hence a more linear behaviour of the amplifier.
Below a picture (fig. 5) of the actual grid bias circuit, click on it to enlarge in a new tab.

Figure 5. Modified grid bias circuit.

Screen current.
A GS-36B needs more Ig2 and has no secondary emission. In order to increase Ig2 I shortcutted R12.

Result.
After all this, my result is depicted below with 2.5 – 3W of drive (not more!). See figure 6 (click to enlarge in a new tab).


Figure 6. The result. Around 600W key down.

On my YouTube channel there are two movies where my modified AM-6154 appears.

The first movie is the original test in 2006.
In the second movie the AM-6154 served as West station during the Marconi CW 144 MHz contest in 2007
(which we won in The Netherlands :- )