Sunday, 29 November 2009

AF Amplifier

OK - I've got the output transformer, I've got 250V, now its time to make the AF stage of my new valve receiver project.

Here is Harry sm0vpo's circuit...

Fortunately I am a kleptomaniac - I found an EL84 amidst my mountain of junk (along with the other tubes Harry used in his receiver) and some B9 valve bases. Talking of bases, here's the "pin-out" of the EL84...

At this point, things start to get tricky - you can't slap a valve circuit together quite as easily as a transistor equivalent - there is much more "mechanical engineering" involved.

The first stage of mechanical engineering yielded a working amplifier soon enough...

Here's a close up of the EL84 in action for all you tube fetishists out there...

Now for the second stage of mechanical engineering - which requires tedious chain drilling and filing [Dear Santa, please can I have a 3/4 inch chassis punch for Christmas? Your elves might like to know that a suitable model is available at CPC (order code ME18313) at the very reasonable price of £6.57. If you want to get me a 25mm one too (order code ME18317) I would be very pleased. p.s. I have been a very good boy all year.].

Here's the result...

The AF amplifier looks lonely at the far end of the Aluminium Chassis (Maplin code: XB68Y) with those empty valve bases waiting for more development on the project...

The amplifier works well enough - I'm using it to listen to the Advent Carol Service from St John's Cambridge on Radio 3 as I write this.

Next step will be the AF pre-amplifier and BFO - watch this space!

...-.- de m0xpd

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