New amplifier project

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cressy
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#1 New amplifier project

Post by cressy »

I'm about to start on a new amp project after doing little to nothing with amps since i built the current one back in 2013.
To be honest, it simply isn't powerful enough to drive my speakers the way I'd like it to and buggering around with the psu and various other bits over the last few years can't compensate.

As it is, the kt77 se amp puts out about 6w ish. However the good thing is that it has proven to be a stable design that has never blown up, got too hot, made any funny noises, and no magic smoke has come out of anything. infact it has never had a component failure. So before I pull it apart for bits I will draw up a schematic for it.

So to a new amp. I need more poke, and have access to loads of varying bits. I have here a set of sv811-10 triodes that Paul kindly lent me years ago, loads of kt valves and el34 and a good few sets of transformers for power and output.

Now I am capable of designing an amp that will work and sound OK but i am not really an amp designer. I do the turntables and leave the rest to others, i know my limits.

So i could build a schematic off the Web but am a little sceptical of some of them.

So ive decided based on the set of requirements I have, ive decided to build a version of mikes el84 push pull pentode amp.

It will be a little different to mikes in a few respects, e.g I'm going for a brute force power supply in place of mikes and the optx's will be 6k6 instead of the 8k in mikes.

Using psud I've done a few sims of the power supply I intend to use. The cct requires 300v and I've got 301.99v so spot on.

It consists of a 280-0 600ma mains into a 1n4007 bridge, into an 8uf 500v cap, 10h 500ma Danbury choke, into a 2200uf 500v epcos dustbin sized bomb err cap :shock:

The sim was done using the correct dcr for the mains and choke as measured using the constant current sink option set at 200ma. 40ma each for the el84s and another estimated 40 for the drivers and phase split.

I'll also have a think about a slow start mechanism.

I have built a chassis for it that matches the passive I use and will spray it the same green once it's had all the cutouts ect done

Pics will follow :D
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Mike H
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#2 Re: New amplifier project

Post by Mike H »

I used gold-plated valve sockets, if it helps. :D
 
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pre65
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#3 Re: New amplifier project

Post by pre65 »

Ant,I thought you might be more suited to a copy of Dads amp ?
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cressy
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#4 Re: New amplifier project

Post by cressy »

Far too complicated for me tbh
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#5 Re: New amplifier project

Post by cressy »

Got a little bit done on it today, got the chassis valve socket panel cut out and rebated and marked up the tx positions.

Image

Outputs, heater and choke on the top and the mains and
Bomb, scuse' me cap inside. The chassis will need blocks under the optx's to raise them up rather than dropping them through as the cap sticks into the area where they would drop through. It will be mounted in the rear corner opposite the mains tx running front to back. The 8uf cap will sit forward of the mains tx.

There will be a bulkhead running left to right inside the chassis about an inch back from the panel cutout to mount the cathode ccs boards on as I plan to build them as modules on veroboard

I'll also make a vented cover that will go over the top of the trannies and a vented base that will have a set of turned feet on it to get plenty of airflow through with vents in the top of the chassis. Should convection cool itself then

Sockets will be on the side again for the short signal path, at front right,although there won't be any controls this time, just the inputs.

And a blue neon lamp on the front left

A shot of the chassis, should have a nice clean line to it. The tx cover will be the same shape as the chassis and be silver the same as the 99 trim panel and the valve plate will also be silver.

Image

The aluminium silver and the British racing green should look nice and match this little passive

Image

The base will be matt black and will sit inside the chassis and have the same chamfer on its bottom edge. This will give a shadow line and make it look abit more low slung as with the cover and feet it will be quite tall
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#6 Re: New amplifier project

Post by Mike H »

cressy wrote: I'll also make a vented cover that will go over the top of the trannies and a vented base that will have a set of turned feet on it to get plenty of airflow through with vents in the top of the chassis. Should convection cool itself then
Mine can get quite hot, altho TBH I think it's mostly the mains transformer conducting thru the top plate. So really needs the ventilation, wouldn't like to guess how hot it'll be otherwise!
 
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#7 Re: New amplifier project

Post by cressy »

The tx on this one should run pretty cool as its specced at 600ma for the ht winding and 4a for the heater winding. In the 77 amp it barely even gets warm to the touch after a couple of hours use.

I'm not using the heater winding on it for the signal valves as the Danbury db776/vx92 heater tx I have has 3 6a windings on it so I'll run one channel pair of 84s off one, the other pair off another and the drivers and phase splitters off the other.

I'll use the mains tx heater winding to run a damper diode off. I'll insert it after the bridge before the first cap to slow start the ht although I still need to look further at that as i dont know what the v drop off it will be.

I'll need one of the short bottle ones mi dad uses on his amps so it will fit under the tx cover but I'm not sure what model it is
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#8 Re: New amplifier project

Post by cressy »

Got the chassis pieces cut and laid out.
I may need an additional shim on the left hand optx's to make it clear the cap inside although I won't know until I start bolting bits in
Image

As the terminals on the cap are along its centreline it might just clear it as it is

This shot shows the base dry fitted, shows the shadow line I should get and the matching chamfer on the bottom edge

Image

When i turn the feet for it im wondering now wether to do one at the front and 2 at the back and spray them black in with the base. They should be about 20mm tall so i want to try and hide the extra height of the base and feet as much as possible as i can't hide the chassis and cover height and I don't want it to look too tall and out of proportion.

The base will have sections removed for ventilation under the circuit section. Those areas will be rebated inside and mesh glued in from the back with hot melt and the mesh sprayed in with the base. It will also have a rebate routed on the inside face to clear the big cap

The 45mm gap between the output trannies will house the damper diode.

Hopefully it'll should all fit........
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#9 Re: New amplifier project

Post by cressy »

Got the tx spacers cut out for the tt's to drop in, they clear the cap with no problems and no need for extra spacing.

Also installed the internal bulkhead and cut out the ventilation hole in the base.

Image

The base has been routed out to clear the cap and the vent hole has had a section routed out for the mesh to drop onto and be glued in.

The cap has also been fitted.

I need to get some more 4mm studding and some cap nuts for the transformer bolts to mount them. I'll shrink some sleeving down around them where they go through the laminations
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#10 Re: New amplifier project

Post by Mike H »

Not too overly enamoured with the 3 feet idea, but just IMHO .. Image
 
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#11 Re: New amplifier project

Post by cressy »

Think you're right mike. Stood on some spacers I use for the jbe legs it looks like a bloody reliant Robin!

Image

We shan't be doing 3 feet......
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Mike H
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#12 Re: New amplifier project

Post by Mike H »

Oo no! Not the Reliant Robin look Image
 
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#13 Re: New amplifier project

Post by IslandPink »

Good work. Taking shape very quickly. Well-used router !
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#14 Re: New amplifier project

Post by cressy »

I was thinking about using a set of 6j32p pentodes in place of the ef86's.
They are supposedly the Soviet version of the ef86's and looking at their curves they look pretty much the same to me but I'm not up on pentodes tbh.
You reckon they will work in place of the ef86's mike?
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#15 Re: New amplifier project

Post by Mike H »

Never used them, but if graphs look similar, only one way to find out! :D

What they may sound like is something else of course.
 
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