A Linear amplifier for the 20A, 2x813 G2DAF

 

2x813 G2DAF linear amplifier

With the 20A in good working order, I started to think about getting a bit more useful power output. I was thinking about building a quad 807 amplifier when I remembered about an old 813 amp in the shed. I bought that amp for parts years ago, but could not find myself to part it out due to it being so well built.

Upon closer inspection after I dragged it inside, I could see that it was built after G2DAF's idea 

The amp is built exactly like this, except 813´s and EY81's has been used. A portion of the RF drive is rectified and doubled by the tube diodes, and fed to the screen grids of the 813´s. This saves a screen grid power supply, and also a bias supply. The voltages follow the modulation envelope, so there is no real way to determine in which class of operation the tubes are in. G2DAF amps have a reputation of being splatterboxes. My experience using this on air with the 20A has been good. I have asked people with spectrum scope receivers to look for artefacts of my transmission on quiet bands, with nothing to report. That being said, I´m drivng this very gently with 20W. Could possibly be a different story, with higher drive and 800W out, rather than 300.

Dusty and dirty, but looking good under all that grime.

Spot the surplus parts. BC375E tuning unit band switch and plate tuning cap - the coil form is probably also from BC375 parts.

Look at that beautiful National R175A plate choke. I replaced that with a ferrite loaded choke when extending the amp for 160 meters, thinking that the R175A would not be sufficent. Turns out the ebay bought, ferrite loaded plate choke could not take the heat, it also lost it´s inductance after about a minute of RF exposure and current flowing through the choke.

Did some measurments on the R175A, and it measured 147uH, which gives a reactance of around 1750ohms, at 1.9MHz, so I reainstalled, and got close to 300W output on 160 as well.

 

Filament transformer, with added 6.3v winding for the EY81 tube diodes that rectify and doubles part of the RF drive, and feeds it to the sceen grids of the 813´s

Neatly laid out bottom chassis. Step start relay , electrolytics that died, and the screened RF drive compartment is visible.

Inside the screened compartment is the rf drive load resistors. I increased the value from 300ohms to 470ohms, to get a bit more screen voltage from the rf drive.

I cleaned the spiderwebs out of the amp, installed a couple of Philips QB2-250 tubes, and with a variac 

slowly let the electrolytics reform over a couple of hours. First tests showed close to 300W output with 20W drive on 80 meters. I also tried 20 meters, but that did not go as well getting only 80-90W. I found that the amp needed more drive, or a tuned input, so I borrowed the tuned input idea from PA0FRI´s grid driven linears, using a single variable capacitor, and an autotransformer wound on a toroid.

I had to really cram it in there, but I eventually found a good place for it in the seemingly spacois box. 

This really did the trick, and I was seeing around 275W out on 20M as well. Around that moment, the electrolytics in the hv psu let the smoke out, so I decided to make a completely new rectifier/capacitor bank, and also to redo the output tank, which only covered 80-40-20, and had a lot of bad splices in the tank coil. It also got very hot, meltig the solder from the taps, when operating on 20M. 

Squeeze, electrolytics, move please!

PA0FRI multiband tuned input. I only used L3 and a 2 section variable capacitor. The input circuit  was not needed on 160, so a relay is used to bypass it when on 160.

New rectifier unit. Way smaller than the old one. I enjoy etching PCB´s at home when I´m doing one-offs, or when I´m not patient enough to wait for China to deliver my designs. 

This rectifier board, when populated with 450v capacitors should be capable of handling peak 3600 volts.

Testing stage. That plate choke was later replaced by the original National R175A.

The pictured band switch was also replaced by the original BC375 part, since I decided to use a vacuum relay to handle the padding capacitor for 160 meters.

Used a T200 toroid as 160 tank coil.

Final design sans cleanup of tank coil wiring. B1B vacuum relay to handle 160m plate tuning.

With a new hv psu, repaired step start, tuned input, new output tank, the amp is now usable from 160 thru 10m.

on 160, it delivers close to 350W, and on 10 meters, around 150W, which is fine due to the relative low power from the 20A on 10.

I´m happy I resurrected this fine amplifier from the dust in my shed, as is now sits beautiful over the 20A.

Further enhancements would be to add a medium voltage transformer and go full AB1 with a fixed 400v screen voltage, and also a way of supplying a negative bias.

//CU DE SA2CLC