Thursday, 25 February 2010

New Audio Stage

Those who have followed the story of the development of my experimental (not-so-)superhet (Blogs passim) will recall that I had too little overall gain to use the receiver with a loudspeaker - despite confirming operation with headphones.

In the quest for MORE GAIN, I decided the best option was to replace the existing pentode AF final stage with a triode-pentode tube, allowing a whole extra gain stage without adding another valve. In truth there are at least two other motives at play here...

1) I resented using the valuable EL84 valve in this application (I have a BTH stereo amplifier that uses EL84s in push-pull and I would rather hold the valve as a spare for that system)
2) I am interested in exploring the ECL80/82/84/86 devices, both for Tx applications a la Hans G0UPL et al and for AF amplifiers, particularly as I've got quite a few PCL86s in the collection.

So... I knocked up an ECL86-based amplifier for the SuperHet, which both worked and didn't!

It worked in the sense that I could now use the radio through the speaker - but didn't work in the sense that turning up the volume caused "motor-boating" instability. Despite some experiments with modifications to get rid of the low frequency instability (exploring the usual approaches of reducing the inter-stage coupling at LF, improving power supply de-coupling, etc) I couldn't sort it out in the cramped conditions of the receiver chassis. This prompted the construction of a single-ended triode/pentode amplifier "test-bed", seen here in close-up.

A scrap sheet of aluminium has 4mm sockets for all the important connection (except input via a RCA/phono) and a B9A base. The transformer is actually a reverb driver spare from my Fender Super Reverb amplifier, which I use for guitar and my Rhodes piano.

After a few minutes messing about with the circuit (which would have been a frustrating task on the receiver chassis) I had a nice-sounding, stable amplifier, seen here in operation on the bench...

"Plans" for the future of this prototype are twofold:
First, I'll copy the amplifier in the receiver chassis.
Second, I'll experiment some more with the circuit, to finalize a design for a stereo PCL86-based amplifier.

The first will be done soon (maybe at the weekend), but don't hold your breath waiting for the second - I've got a lot of other ideas competing for my attention!

...-.- de m0xpd

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