Hunter Bandit 2000B conversion from four 572B tubes to two GI-6B tubes.

By Dave KD5FX Jan. 14, 2015

 

Here’s a shot of the amp with the cover off and the tubes pulled out. The nice thing about

this amp is the power supply, rated for 2500 volts at a full 1 amp.

 

Here’s a shot inside the amp showing the four 572B tubes, you can’t find these any more.

Well, they have a few available but they are JUNK. You can’t find quality 572Bs any more.

What to do? Why, convert it over to Russian GI-6B tubes of course!

There are a few problems to surmount (as usual). First off, the 572B tubes have a 6 VAC

heater while the GI-6B tubes are 12 VAC. The amps internal transformer has only the

6 VAC secondary so we’ll need to add another small 12 VAC transformer.

 

 

It’s rather tight inside the 2000B but looks like there is plenty of room for a 12 VAC

transformer here where this useless Low/Med/High voltage switch is located. So we’ll

wire it for High voltage all the time which is how I run it all the time anyway and put

the transformer here.

 

 

Here’s a shot inside with the four 572Bs removed. Plenty of room for the new tubes

but they must be mounted vertically, they would touch the sides if mounted horizontally.

 

I found an old chassis that is about 4 X 6 inches which will fit in there just fine and

mounted the two tubes on it. It was originally 2” tall so I cut it down to 1” and cut the

two holes for the tube sockets.

 

It will mount very well right here where the old tubes were. I also took the small metal

plate out that normally sits under this to allow the wires to hook up to the tubes heater and grid.

Here’s a shot with the new filament transformer installed. 120 or 240 VAC primary and

12 VAC output. Note the large 10 watt 20 ohm resistor on the input side. This drops the

secondary voltage down to a reasonable 12 VAC or so, without it the voltage is too high

for the tubes (> 13.5!)

 

Without the load from the filaments the HV jumped up to 2700 volts which was just too

much for the original 6 HV caps. So I added two more for a total of 8. I also changed the

original 41K bleed resistors to 100K models. This reduces the load on the HV circuit but

it also takes MUCH longer to bleed down the HV when you turn it off so be very careful!

As you can see, all eight fit in the space of the old six but just barely.

 

 

I also added a Bias Board from K4POZ. It’s very easy to set the bias current to about

140 to 150 ma using this board.

 

Here’s a shot of the underside of the tubes. I mounted the tube sockets on the bottom

of the small chassis to lower them just a bit more. The two heater wires are right on

top of each other so it looks like there’s only one wire in this picture. The Bias Board

connects via the RF Choke on the top and the input which goes through the .02 cap on

the bottom of the picture. You can also see the end of the filament choke labeled ‘out’,

I reused the same choke.

 

I re-used the same parasitic resistors to feed the Plates of the tubes. Also used some small

paper clips to connect the B+.

                                                                                                                                          

Here’s the schematic of the changes.

 

Parts list:

2 GI-6B tubes from the Ukraine (via Ebay)   $35.00

2 tube sockets from Rasim Hirkic in Canada $94.50

1 Allied Electronics 12 VAC transformer      $38.50

1 Bias Board from Ron K4POZ                      $27.00

2 extra filter caps and bleed resistors             $10.00

                                                            Total  $205.00

 

The other misc. parts came from my junk

drawer (which is really a garage full of junk!).

 

It works quite well! Next project is a tuned input!

73, Dave KD5FX