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Old 22nd November 2008, 07:08 AM   #1
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boy I love it in here! This weekend''s project was converting an old secondary amp from a 1940's console into a cathode follwer preamp.

I couldn't have done it without you guys...

Anyhoo, the EL41 and 6AV6 came out of the little chassis, to make room for a couple of 6j5 metal cans. Fugly beasts the 'j5s, but I have a small stash, and I'm told they sound fine.

THe power trannie stayed on along with the old 40/40/20 can, and a 6x4. A quick change of dropping resistors and a poor mans hand wound choke added made up a supply giving around 275V @10mA - spot on!

THe heaters were floated to cope with the elevated cathode voltage, and thats the power supply sussed out.

At hand was a 250k log dual with power switch attached - only one hole to drill!!! Colour me happy. An el-cheapo RCA terminal plate fits existing old terminal holes in the chassis, so all good there.

Between the pot and the grids, 220nf caps keep the grid potential from blowing out a source device... No grid stoppers. Bias set at 3.75V, and we are about there. 440nF caps separate the output DC from the power amp.

Switched on, a little curl of smoke (a wispette really - no flames...) curls out of the underspecced power supply bleed resistor. It gets replaced by a more suitable item.

Plugged into the obligatory el-cheapo DVD / Cd player on one side and a Maclaren power amp on the other, I poke Ricky Lee Jones into the tray and await magic.

Oh, the speakers are demolition OBs - removed in total from another old console they remain on the old baffle and are seriously under-rated for the amp (4 ohm, 10W speakers meet an effortless 80WPC MOSFET monster).

Hummmmmmmmmmmmmmmmmmmm.

Ah well, the delights of an E core trannie mounted directly to a folded steel chassis.

BUt, turn Ricky up a bit and as signal overcomes noise, the little lady sings! Numbers tomorrow, but it does sound fine and is pretty much what I'm looking for to front this powerhouse amp and bring it a little valve tubbyness.

Having proven the idea, its now onward to build a more suitable chassis and smooth out hte power supply a bit.

Here's the thanks to those who have contributed without even knowing.

Eli - various posts all over the place.
Joel T - its almost a clone of joel's buffer/pre here http://www.joeltunnah.com/images/12AZ7buffer.JPG
The Valve Wizard - for teaching the theory and maths at www.valvewizard.co.uk
George - nothing particular apart from reducing my fear of failure and increasing my delight at smoke

Many others - cheers all. Diagram and images soon. To those of you itching to get into this - just do it!!!
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Old 23rd November 2008, 08:47 AM   #2
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Default schematic and some numbers...

hum aside, its still filling me with the delight of the eternally surprised. Very low WAF, although She has come in a couple of times and cooed appreciatively in that condecending tone they also use for the kids after the first successful toilet-trained event...
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