G4DCV GS31b 144MHz Amplifier/gs31-5.jpg

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If you do not understand high-voltage safety procedures do not attempt to build a tube amplifier.

The HT voltage will kill

The side panel of the power supply compartment.

On the left is the capacitor stack which is 8 x 400uF, 450V computer-grade electrolytics.

The black block at the top is the glitch resistor in the HT feed which is 100R, 250W thick film. I had several surplus high power 6kV rated thick film resistors and at first it seemed attractive to use one. But now I am not at all sure about using thick-films in this application, in the past I have had one go open circuit during a flash-over which makes a very expensive HT fuse. I found several surplus thick-films very cheaply but these are expensive components if bought new. A big wirewound would be much better and more likely to survive severe overload during a flash-over. HT is fed to the RF deck using RG213 coax and I'm using a C-Type connector to get the HT into the RF deck.

Below the glitch resistor is the rectifier board which is a conventional full-wave voltage doubler using 1N5408s. I know many people argue that equalising capacitors and resistors are no longer required and may do more harm than good. But old habits die hard and I haven't yet lost a rectifier in an amplifier doing it this way.

Below the recitifier board is a second 100R, 250W thick-film resistor which I have used for the HT transformer soft-start. Conventional soft-start circuits use a low voltage relay, often 24V, fed through a dropper from the mains and then rectified. See the GM3SEK Triode Board Manual page 28. Because I use a seperate heater transformer (I want to run the blowers and heater for 2 minutes before applying HT), I have used the redundant 12V heater winding on the HT transformer secondary, rectified and smoothed it and used it to power the soft start relay directly. It works fine and saves about 12W and makes the amplifier slightly greener :-)

To the right of the glitch resistor is the HT voltmeter chain which is 10 x 1M0, 350V metal film resistors in series with 2 x 100k in parallel at the bottom of the chain giving me about 12V for the HT voltmeter. Note the two seperate grounds for safety. Please see the GM3SEK Triode Board Manual pages 17 and 18.

The HT transformer was supplied by Linear Amp UK although I bought it secondhand from Peter G3ZSS- thanks Peter. It has a 450-0-450V secondary which is voltage doubled. The power supply gives me 2.5kV off-load and 2.3kV at 1kW output. The secondary windings are fused with two UK type domestic 1" fuses. Although not specified in this type of application in practice they usually work fine, the AC waveform from the transformer secondary allowing them to interupt the current OK. I have not used a fuse in the HT line. Fusing a 2.5kV supply is fraught with problems. Fuses rated at several kV are difficult to find. Any fuse used here needs to be self-quenching (sand filled) otherwise it is likely to arc over and not interupt the current. Ordinary glass fuses may simply explode. A soft start in the HT transformer allows the use of a smaller value mains fuse in the HT transformer primary.