26 pre

pre65 · Post by **pre65** » Mon Mar 11, 2013 5:03 pm

I've made a start on the 26 pre.

I'm using Rod Coleman filament supplies, as they are a proven product, and they are now all soldered together.

The 26 are on a piece of Polytetrafluoroethylene sheet, and suspended by four wibbly wobbly (extra flexible) rubber mounts of unknown origin.

Grid bias, by 9v battery, will be used and I should have two Bulgin PP3 holders in a day or two.

IslandPink · Post by **IslandPink** » Mon Mar 11, 2013 5:52 pm

Oooh, Nice !
Those mounting things do look quite wibbly, too .
I've got the isolation parts I was looking at for the phono and they are usefully 'boingy' to make some difference, but those are better .

pre65 · Post by **pre65** » Thu Mar 14, 2013 3:01 pm

My PP3 battery holders came today, and they are nice.

They have pins for PCB connection or two lugs for soldering the connecting wires.

They can be screwed down to the mounting surface.

http://www.ebay.co.uk/itm/251067278312? ... 1497.l2649

pre65 · Post by **pre65** » Sun Mar 17, 2013 1:28 pm

On the raw DC supply before the filament module board Rod suggests a 100nf X2 capacitor across the mains transformer primary.

Looking in my spares box I only have 220nf in X2 type, will that be better ?

simon · Post by **simon** » Sun Mar 17, 2013 4:27 pm

Not sure it would be better - if it was Rod would no doubt recommend that in the first place. Perhaps the question to ask is will it make any difference? Probably best to ask Rod.

pre65 · Post by **pre65** » Sun Mar 17, 2013 4:29 pm

simon wrote:Not sure it would be better - if it was Rod would no doubt recommend that in the first place. Perhaps the question to ask is will it make any difference? Probably best to ask Rod.

It's in place now.

Mike H · Post by **Mike H** » Mon Mar 18, 2013 1:04 pm

100, 220, close enough

pre65 · Post by **pre65** » Mon Mar 18, 2013 1:36 pm

That was my thinking.

I'm putting the two 30va 6v toroids, the shottky bridges and the first 15,000uf cap on the PSU board. The second 15,000uf cap and dropping resistors will be on the pre board.

The plans are formulating in my bonce as I progress.

pre65 · Post by **pre65** » Mon Mar 18, 2013 2:56 pm

The PSU is finished now. So next up is the pre chassis.

Mike H · Post by **Mike H** » Tue Mar 19, 2013 2:46 pm

Am I getting de ja vu or have you tried a 26 pre before?

pre65 · Post by **pre65** » Tue Mar 19, 2013 2:58 pm

I have tried a 26 pre before, but it was ages ago.

This time I'm being more thorough.

Today I've been assembling things onto the main board, might have a picture later.

Mike H · Post by **Mike H** » Tue Mar 19, 2013 3:47 pm

Yes I thought so!

Hummed a lot as I remember.

pre65 · Post by **pre65** » Thu Mar 21, 2013 4:23 pm

Yep, hum was a problem on that iteration.

Managed to get a couple of hours on wiring up the main board. Funny how such a simple circuit takes up so much room. I've left space for a regulated supply for the Mk2 version.

pre65 · Post by **pre65** » Thu Mar 21, 2013 11:09 pm

Wiring up almost done.

Here is the valve holder PTFE plate ready wired before being fixed in position. The underside is inaccessibly when fitted.

pre65 · Post by **pre65** » Fri Mar 22, 2013 12:07 pm

Started testing the raw DC up to the filament modules. The voltage is a tad high so the dropper resistors need upping to R47
Just letting the caps discharge before changing them.

audio-talk

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