New KT120 PP In The Style of the Leben CS1000P

What people are working on at the moment
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Cressy Snr
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#1 New KT120 PP In The Style of the Leben CS1000P

Post by Cressy Snr »

OK chaps!

I got the steel this morning :)

Using the tools shown, a kitchen worktop edge as a former, a quarter-inch steel angle iron straightedge clamped to the work,
a bit careful measuring, marking and some judicious bashing with the rubber mallet, we got this nice steel chassis.
Image

Image


And here is the chassis with a set of valves atop for scale.
Image

It'll look really handsome with some hardwood end cheeks.
I'm pleased with how the metal bending went considering the lack of a full sheet metal workshop :wink:

I'll start punching the holes tomorrow.

For me, caressing the metal into shape, marking, spraying and sanding wood is the best part of building valve amps.
Getting right down to metal bending in order to fabricate something unique, controlling the whole process from start to finish is so satisfying.
Last edited by Cressy Snr on Sun Aug 18, 2013 4:27 pm, edited 2 times in total.
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Cressy Snr
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#2

Post by Cressy Snr »

Our Ant came up trumps this afternoon with some nice walnut blanks, 13 x 4 x 3/4 inch. These will make good end cheeks for the chassis, and once sanded and oiled, should look rather nice.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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#3

Post by JamesD »

Very nice days work on it! Its going to be lovely!

We'll expect well shoddy :D
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Dave the bass
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#4

Post by Dave the bass »

+1 Thumbs Up Shoddy!

We're so 'now'! :-)

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Cressy Snr
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#5

Post by Cressy Snr »

Notice I've changed the title :)

Sod it, I'm going for it!

Can't see any point in tinkering 'round the edges, somebody has got to build one and it might as well be me, seeing as I have the experience with this valve. :twisted:
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Cressy Snr
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#6

Post by Cressy Snr »

Now then, this is more like it.

Image

The mains transformer is the large, custom-made 3x300V@300mA per winding unit I purchased from Mike ages ago.

Parallel that little lot up and you have a 300V winding capable of nearly one amp of current.
Let's say I go for the lower power end of the KT120 scale, with a 100mA idle current, per tube and stay in Class A.
I will be using barely half the power capability of the mains TX.
I might get 400V HT with a CLC solid state rectified output stage supply with one of those massive 500V 2200uF EPCOS caps to keep things in order.

A small toroid and valve rectifier for the driver power supply and you're away.

I have a 25V, 300VA toroid so I can run series heaters to the power stages and a use a little 6V 20VA transformer for the driver heaters.

I'm aiming at around 25 - 30WPC.

The more I think about it, the more I like it.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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#7

Post by Cressy Snr »

I have a nice pair of 6A6 RCA. These are the same valve as the 6N7 but with a UX7 base. These common cathode, twin triodes were specifically designed for push pull drive will drive the output stage nicely.
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#8

Post by Andrew »

Too shoddy!
Analogue, the lost world that lies between 0 and 1.
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#9

Post by JamesD »

Now you are talking! Bring on the Wellie! :lol: :lol: :lol:
Cressy Snr
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#10

Post by Cressy Snr »

Here's the layout:

Image

Mains transformer has been stripped down so the end bells can be sprayed up.
Large maroon cover fits over the open frame, 10H, 500mA output stage PSU choke.
Needless to say, the cover will be sprayed up to match the mains transformer and chassis (after punching)

The output transformers will be suspended inside the rear of the chassis. Nick's OPTs are on loan and are a drop-through design.
I don't want to end up with a pair of whacking great holes in the top plate, when I give them back (unless he lets me buy them).
Eight bolt-holes will be much easier to deal with and will be less troublesome when fitting new OPTs.
Of course all this depends on whether it sounds any good.

I've worked out the power in triode mode at my chosen op-points and it will give a good solid 25W of class A power, for 85W total plate dissipation per channel.
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#11

Post by Cressy Snr »

Progress is slow.

Image

Top surface has most of the holes drilled and punched.
0.22uF Sprague orange drops (courtesy of Hotrox) are well-made, substantial, low cost and have a good reputation for tone.

The 6A6s have had to be shelved due to a lack of suitable heater transformers.
Instead I will be using ECC81s as voltage amp/drivers. They have proved their worth driving PP EL84s and I have upped their idle current for KT120s.
I have fished out two 20VA, twin 6V/2.5A transformers, and a twin 12V toroid.
This arrangement means that each valve has its own dedicated heater supply, 4 x 6V for the KT120s and 2 x 12V for the ECC81s

The mu of 60 from the ECC81 will give plenty of headroom to drive KT120s in triode mode
and should I want to go pentode with the output stage at some point, there will be enough gain for plate-to-grid feedback.
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#12

Post by Cressy Snr »

Case, mains transformer end-bells and choke pot have been sprayed.

Image

Transformer parts - satin black.
Case - British racing green.
That's a full week's worth of bending, marking, drilling de-burring, punching, filing and painting.
No bleeding! :) A first for me.

Walnut end-cheeks need to be cut and fitted after the paint has cured.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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#13

Post by IslandPink »

Jeepers :!:
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#14

Post by Paul Barker »

Shoddy.
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Cressy Snr
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#15

Post by Cressy Snr »

Walnut end cheeks have been cut, sanded and had their first coat of finishing oil.

Here is an idea of what the finished case will look like.
The parts have been offered up to check for trueness and fit.
Everything looks fine.

Image

The end cheeks will only have a quarter-inch reveal along the top and front edges, once the inside fixing brackets are in place and the case gets screwed together.
Sgt. Baker started talkin’ with a Bullhorn in his hand.
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