I’ve always wanted to build a massive linear amplifier since I was at school. In fact my old friend John Weston (G3LYW) gave me an 813 to use in an amp, I built up a power supply for it but never completed the linear so I’m going to have another shot at it!
I’m planning to build a linear based on two 813 valves in parallel and in the traditional grounded-grid configuration. The 813 is a good general purpose beam power tetrode and used to be popular with amateurs in the old days as a power amp valve. Not only does it do the job it looks the biz with it’s bright yellow/white filament.
Building a large power amplifier is really two projects in one, the power supply and the amp itself.
First up is the power supply. The 813′s need about 2.5kV for the anode and the filament needs 10v at 5 Amp so we are going to need a BIG transformer. So here it is…. this was bought of a fellow amateur via E-Bay.
The transformer’s secondary is 1000-0-1000v at 1 amp. I’m going to use a standard bridge rectifier circuit with capacitative filtering to get my 2kv. The rectifiers are standard 1n5408′s in series to increase the voltage tolerance. Here’s the rectifier board with the diodes plus equalisation resistors and caps.
The chassis is fashioned from aluminium and steel, ultimately it will be the size of a small cabinet.
The front panel has been primed and is ready to be spray painted.
I’ve finished all the metal-bashing, now onto the wiring. The front panel is relatively straightforward with a circuit breaker for the mains and a couple of switches for the HT and LT supply. The meter is an old vintage type from an aircraft test set that I will adapt to read 5Kv FSD with a string of multiplier resistors. I did try using standard 1/2 watt carbon film resistors but they were starting to fizzle with the HT on 1400v! not even full voltage! so I’ll have to find some with a higher rating.
The square in the middle is a SMD Arduino LOL shield that I have programmed to scroll DANGER!! HIGH VOLTAGE. It was sitting on my desk with nothing to do so I thought it would add an interesting” finishing touch” as they say.
You can see the main rectifier boards and the capacitor bank mounted in the upper section:
Here’s another view of the transformer in the lower deck. The small accessory transformer is in the foreground with the relays and switch-gear at the rear. The whole set-up looks complicated but is just a bigger version of an i-Phone charger.
I’ve built up the PSU and so far testing has shown that it all works, that is off load. The next step is onto the linear amplifier itself.
I’ve decided to go for a fairly standard circuit and layout. The 813′s will be paralleled in grounded grid configuration with an input pi network for each band, 160,80,40 and 20m.
Similarly, the layout will have all the main caps and tank coils on the RF deck with the rest underneath. In order to show the valves and the lovely tank circuitry I’m going for an aluminium chassis with a clear perspex front panel with large vintage meters. Here’s the main frame:
Next up is the RF deck. It’s taken a lot of drilling to get all the parts ready for mounting and I’m sure there will be a few adjustments needed. The two big holes at the bottom are for the valve bases.
The input pi network was made with a homemade PCB. The plan is to have each one switched in and out with relays.
Up next is to wind a few coils. This is the plate choke for the anodes of the valves. The idea is that it allows the HT to get to the anode plates but blocks RF getting into the PSU. I’m using a fibre-glass former of about 1 inch diameter, it was in actual fact an off cut from my vertical antenna. In the centre is an iron powder core to raise the inductance. I’ve checked it with my GDO and it does not have ant resonant frequencies in the HF bands. Total inductance about 900 uH, more than enough.
Now to put all the bits together. You’ll notice from the picture below that I’m using rather unconventional gas filled variable capacitors for the tank circuit. The plate cap is 500pF and the load cap is 1000 pF. These are high voltage capacitors and are rotated using the little motors. My thanks to Paul Bennett for donating them to the project.
The RF deck is dominated by the massive tank coil. This is a 3 inch diameter coil with about 28 turns of 4mm copper tubing. It has to be quite big in order to tune the 160m band. According to my GDO it will tune down to 1.8 Mhz with the current capacitor combination although I may need come capacitance to increase the Q, I guess we will know when it is tested.