EL509 OTL Amplifier

What people are working on at the moment
Cressy Snr
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#16

Post by Cressy Snr »

Changed my mind now.

Image

This looks better and I reckon the 12ax7 is still far enough from the mains transformer.

6C33s like those Joule Electras?

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Nick
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#17

Post by Nick »

I should be able to start playing with the 6c33c's soon. Hopefully I will just have heat from the valves, as I am using fixed bias, so no cathode resistor. The 7A of heater current per valve will be fun though.

Thats a mighty big TX you have there steve for the bias supply. What current are you expecting to provide?
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#18

Post by Cressy Snr »

Nick wrote: Thats a mighty big TX you have there steve for the bias supply. What current are you expecting to provide?
Exactly Nick.:wink:
It was the only one I could find in my box with the right output voltage and cheapskate that I am I thought I might as well make use of it. It is a bit big but it'll do for now.

Here is the layout with a 12B4A preamp stage built in.
Anyone for 15 pin bowling?

Image

The board is 2 feet by 18in so building it as an integrated will be a suitable solution to the size problem. When my missus saw the size of the thing she said something I can't repeat for young ears :D

Steve
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Nick
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#19

Post by Nick »

and cheapskate that I am I thought I might as well make use of it.
I know the feeling. I am hoping that the custom torroid I am waiting for is ok, as I guess that sort of thing would be good for the sort of amp you are building, and in theory, it could go inside the box, given that you have no OP TX's to worry about.
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#20

Post by Cressy Snr »

Well I've gotten started on the OTL.
I've decided to take a modular approach to the build. This means that each bit can be built and tested before putting it into the finished circuit. I don't want any spectacular stuff when I fire it up.

Image

The first module underway is the power stage. This comprises eight EL509s; four per channel.
The positve HT is fed by the red cables and the negative HT fed with the black cables. The heater wiring loom will be fed in on the opposite side of the output board. The bus bar running up the centre of each tube bank is the output rail which in the finished amp will be connected direct to the positive terminal of each speaker output pair.

Steve
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#21

Post by Cressy Snr »

The output stage module is completed

Image

Input/driver stage is next on the list.
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Dave the bass
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#22

Post by Dave the bass »

Nice job Steve,

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#23

Post by Cressy Snr »

So now we have heaters!

Image

The driver stage heaters have been wired up and the whole thing tested with raw AC from the 20V heater transformer.

Image

The rig has been on soak test for the past three hours with no mishaps.
Voltages are stable and read correctly with just the unrectified output from the heater TX.
Heater transformer gets very warm but that is only to be expected with that lot to power.

Well something works anyway.

Steve
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Nick
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#24

Post by Nick »

I like the use of the checkerplate. Good idea that, I may have to steal it :-)
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#25

Post by Andrew »

I thgink I preferred the back painted acrylic, myself, but it looks smashing.

-- Andrew
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Greg
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#26

Post by Greg »

And if it doesn't work, you've always got something left over to wipe your your feet on! :lol:
Cressy Snr
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#27

Post by Cressy Snr »

Aww come on lads; you know me better than that. :D

The aluminium plates are actually subchassis sections which facilitate the modular construction technique.

They will be stood off the underside of a varnished birch top plate and the valves will poke up through the wooden surface with sufficient space around the bases to avoid discolouration.

Steve
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#28

Post by Darren »

That sounds better Steve.... :D
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Dave the bass
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#29

Post by Dave the bass »

SteveTheShadow wrote:
They will be stood off the underside of a varnished birch top plate and the valves will poke up through the wooden surface with sufficient space around the bases to avoid discolouration.

Steve
Thats just slack :D Aincha never heard of 2nd hand MDF and ply :wink:

DTB
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Cressy Snr
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#30

Post by Cressy Snr »

Here is the driver stage about a third of the way done.
This needed some serious thinking about what to put where, particularly as I decided to stick a 12B4a preamp stage in there as well.

Image

Image

I've got it all to do again at the other end of the board now :roll:
How Andrew shoehorned a similar thing plus a PSU into that little copper box I'd love to know.

Come back Loftin & White - all is forgiven :wink:

Steve
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