G4FPH's Amateur Radio Pages - Amplifiers
Below are pictures of a state of the art (at least for this station) desktop amplifier plus compact power supply.
The amplifier users 4 off QY4-400 tetrodes in passive grid and will run key down all day and all night with or without an aerial connected! The output tank is a Pi-L configuration, with fixed values of capacitance and two roller inductors. Click on the thumbnail below for a bigger picture.
A big amp needs a big PSU and, of course, the mandatory easy-carry adaptor (thanks to Malc, G4FQI for that one)! Click on the thumbnail below for a bigger picture.
The amplifier below has never let me down. It currently uses 2 off TY3-250 triode in grounded grid. These valves drive well but need a lot of cathode bias (tens of Volts) to keep the standing current down. The original QY4-250 tetrode valves were not too happy in this configuration, so I swapped them out for proper triodes at 20 Euro per pair (have you seen the price of 3-500ZG)! If you think the quality of construction is poor, you should have seen the Mark I that I built when I was still at school. Sadly, the Mark I - the original hamsterfryer - is no longer with us. Click on the thumbnail below for a bigger picture.
I guess one day I shall have to start using external anode valves. Trouble is they have about as much soul as central heating. I much prefer real log fires. Here's the valve equivalent! Click on the thumbnail below for a biggure picture and relax in front of the glow.
Well it finally happened! My first valve amp. with an external anode device. This amplifier is a real heap, but is on the air regularly on all bands up to and including 20 m band, and has not yet electrocuted any cats! In a fit of madness, I rebuilt the PSU using a 2.2 kVA toroidal transformer plus commercial voltage doubler rectifier/capacitor filter board. The transformer is OK-ish, the rectifier/filter board however is very poor. 10 x 220uF, 400 Volt electrolytics. The resultant 22 uFarad, 4 kVolt equivalent device has neither the 'C' or 'V' for the job!
The transformer primary is 230 Volt, no taps, with a 700 + 700 Volt secondary. I know the mains supply in the UK is now nominally 230 Volt but at least here the actual Volts never fall below 240. So the transformer gets more nearly 5% more than it really should and the output from the doubler typically sits at 4.1 kVolt DC. Hear those capacitors groan!
The 6.5 Ohm + 6.5 Ohm secondary resistance (!) of the toroidal transformer and the somewhat ripple cancelling nature of the full wave voltage doubler circuit cannot make up for the fact that 22 u Farad of smoothing on 50 Hz mains is just not enough. I think 15% hum mod is frankly not acceptable - if I had wanted to build an AM TX with low depth of modulation I would have done so! Now, were the capacitors 470 uFarad units, that might just do it! Suppliers of expensive commercial amps. built in the UK or for 60 Hz mains pay attention!
Click on the thumbnail below for a bigger picture of the RF deck front panel - it has more holes than Swiss cheese!
The photo of the inside shows a Russian GS-31B triode valve in place. I have now swapped this for the 1500 Watt dissipation, GS-35B triode. The big coil is necessary for proper low band performance. C1 and C2 in the tank is too small, so there are lots of padders too. Also, there is no tuned input on the cathode so the input VSWR is poor. The two redeeming features of this amp are the valve and the G3SEK triode board that works well, but needs a mod. to up the cathode bias on the valve to rduce the standing current. In short, I need to rebuild the entire RF deck on a new chassis. Click on the thumbnail below to confirm for yourself what a state this thing is in!
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Copyright ©2003 G4FPH - even the spelling mistakes.
Last modified: 4 June 2003