EL509 OTL Amplifier

What people are working on at the moment
Cressy Snr
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#46

Post by Cressy Snr »

OK
The chassis is about done.
Here is what it will look like with the valves in.
I added some nice little tapered legs, made from some oak I found in the cellar. The result is pure Festival of Britain :D

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Here is a close up of a veritable forest of valves. Badge is off the inside of a door on an old 1950s sideboard to reinforce the period look.

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Steve
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colin.hepburn
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#47

Post by colin.hepburn »

That should look brilliant when it all lit-up Steve
Good work
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Cressy Snr
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#48

Post by Cressy Snr »

Here is the progress so far today.

All the hardware has been mounted on the chassis apart from the heater, bias and preamp transformers. These need their looms prewiring and all their the tags double insulating before they are fastened in their final positions and the cover is fitted.
A few pics below.



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It's those little details that count. Bakelite knobs had their red dots restored after the polishing session removed the paint. Red Fender jewel indicator light adds to the retro feel



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Here's what the underside looks like with the pre/driver and output stage modules in place.



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Here is the PSU detail before all the wiring and caps go in. Mains inlet, on/off switch and fuse holder are mounted next to the mains TX. The two pots to the right are the bias adjusters for each channel.
The two bridge rectifiers mounted on the heat sink will power the output stage and the heater supply for the amp. Heatsink vents through slots on the top plate. The indicator bulb will be powered from a spare 6.3V heater winding on the mains TX.

I can see the finish line approaching and I am getting a bit nervous.
Steve.
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Paul Barker
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#49

Post by Paul Barker »

That's very retro looking. Nice.
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Toppsy
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#50

Post by Toppsy »

Steve,

it's looking mighty fine. You are making faster progress with your build than me on mine. But then I have two of the beasts to construct to your one! Well that's my excuse anyway. Both my copper sheet tops are now at the chrome platers so these should be back early next week. I should imagine yours will be up and running by then.

Colin
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#51

Post by Cressy Snr »

The job is nearing completion now.

Here are a few pics of the progress made over the past couple of days.

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On go the four transformers. The Sowter at the rear has two HT windings for the driver and output stages. Next size down is the heater transformer. Front left is the bias supply transformer which is vastly oversized but it was what I had in my box. Small OEP transformer supplies the 12B4a preamp stage I added. That makes a total of five separate power supplies that need to be built!



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As I said there are five power supplies inside this amp. I've finished three of them. Two to go. The two solid state bridge rectifiers supply the output stage and the heaters.
Four black caps at the rear feed +/-170V HT at 1A to the eight EL509 power valves.
Blue 22000uF cap in the centre is associated with the 25V series DC heater chains.
The bias supply for the EL509s has been built around the pair of adjustment pots on the small board, at the upper left, below the mains TX.
Four small black caps in a line decouple the negative bias supply to ground.


One thing to remember when dealing with positive and negative supplies in one amp is to........
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Every step of the way I have marked each component and its associated wiring in red as they have been installed. We always used to do this at BT whenever we modded or built new equipment. The argument was that you could leave it at any point and come back knowing where you were. Also if you went off sick then someone else could pick up the job and know what you had done and how far you had got.

Bruce Rozenblit argues that it is this amp, among all the circuits in his book, that is the most challenging to build, in terms of the skills required for success. I know exactly where he is coming from and IMO it is nothing to do with the compexity of the circuit as it is really quite simple. There is just a lot of it!

The skill I think Rozenblit is talking about is all to do with the decisions you make concerning the internal layout and the sequencing of the build. You have to have laid out and built plenty of valve amps before and made all your mistakes on those.

With this project it is oh so easy to end up with a complete rat's nest if you are not constantly thinking about where you are going to put components and in what sequence you are going to do things. Get a rat's nest and you've had it, because I cannot see any hope of successful fault tracing, especially in the drivers and PSU unless you think, think and think again every step of the way to avoid a mess.
Jeez, I'm sounding like a schoolteacher here. Clearly I still have yet not switched off for the summer holidays :roll:

Steve
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#52

Post by Cressy Snr »

That's it!

Apart from wiring in the input cables to the selector switch the OTL is completed. It has been powered up and tested without the valves, just to check that the right voltages are appearing in the right places, and with the right polarities.

One fault was found with the left channel bias voltages, which were far too high. The problem was quickly traced to an incorrect resistor value in the left channel bias supply. That was that. No smoke, sparks or funny smells.

The transformers are virtually silent. I had thought that mechanical hum might have been a problem due to the sheer number of EI core transformers on the chassis, but nothing. A bit anti climactic really:)


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Here is a view down the length of the chassis

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And here is the engine room all fettled and sorted. Every stage has its own power supply, which is why the PSU takes up almost half of the amp.

Tomorrow the inputs get wired, more testing with the valves in then we are up and running.
Steve
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#53

Post by Cressy Snr »

Oh dear,

Here we bloody go again!
I'm giving up with this neatness lark. It's clip leads and rat's nests from hereon in.

So what seems to be the trouble then?
Right.

The HT to the output stage is isolated by fuses which are removed whilst the checkout is in progress. So far so good. Everything checks out, all the voltages are just about spot on, polarities are correct. Bias is perfect at -40V on the positive tube banks and -210V on the negative side. Bruce Rozenblit reckons that if you have got this far you are virtually guaranteed a working amp, so naturally I was full of it by then Yessss!

However as soon as the HT is applied to the output tubes BLAM BLAM BLAM BLAM we have a stunningly good low frequency oscillator.

Isolate the output stage by removing input stage tube. Silence, just the gentle rush of the output stage through the cheap pair of speakers. But power up with all the tubes in and the output stage HT switched on and we get a hum that starts very quietly, builds up, then a sharp CRACK! followed by BLAM BLAM BLAM until the switch is thrown. When the power is removed we get several rising warble tones at different frequencies, one on top of the other in rapid succession as the sound fades out.

Now we either have positive feedback when it should be negative. But you can't simply reverse the output TXs as there aren't any. Or we have feedback through the power supply (more likely) from somewhere as there are five of the things.

I've been at it all day and am out of ideas.

I stepped out of my comfort zone with this amp, thought I knew more than I actually do, got a little cocky and pleased with myself and am now paying for my cocksure arrogance big style. It is such a shame as I have really enjoyed the challenge and have worked so hard on both the looks and the layout but I've had it with the damn thing for one day.

Has anyone got any suggestions.......please. I'm very 'umble and creeping now.
Last edited by Cressy Snr on Mon Jul 30, 2007 10:12 pm, edited 1 time in total.
Andrew
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#54

Post by Andrew »

Hi Steve,

Just ideas nothing concrete, I'm afraid. Can you post the cct?

I had a phono that did this, I'd missed off a grid stopper on an ECC88, whoops, some cables it was OK and others it wasn't, it went whomf whumf whoomf etc. Doctor Who theme tune, BBC Radiophonic workshop kind of whoomf. I suspect the different capacitance of the cables that made the nice amp a nice oscillator.

Also might be worth beeping ground continuity.

cheers,

-- Andrew
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#55

Post by Darren »

Steve, I think we all step out of our comfort zone when building these things. The quest to learn drives us to do it !!

I agree with Andrew, i'd first look at the grid leaks. Not that I know much about fault finding amps... :lol:
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Paul Barker
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#56

Post by Paul Barker »

I'm not familiar with the circuit, can you post an image of it?
Cressy Snr
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#57

Post by Cressy Snr »

Paul
you have two PMs
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Nick
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#58

Post by Nick »

I guess from what you say both chans are the same?

I am sure you will sort it, but if not, as ever you are welcome to bring it over here, if only for another head to be scratched at it, and maybe a scope might help.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
Cressy Snr
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#59

Post by Cressy Snr »

Thanks Nick
I might just do that. I know you managed to sort Simon's Blatting 2A3. after he got completely stumped as I seem to be.

I don't yet know why and I'm leaving it for tonight but there seems to be
-6v of feedback coming along the left channel but only 1.5V coming back down the right. Could this difference be setting up a resonance in the PSU? The PSU is common to both channels and of course with this going on the gain on the right will be more than the left. Also the Zouts will be different on both sides. I can't see the driver stage liking this at all especially as it is being asked to swing 200V peak to peak to drive the output stage.

I think it it might be the 6V that is wrong as the voltages around the driver stage valve cathode followers on that side now seem to be about 70V short of where they should be. Given the fact that the HT rail for the driver stage is also now down by 50V I may be onto something. There might be more than one fault going on here

When the oscillation starts the Ht in the power stage goes out of control, fluctuating wildly between 170V and about 35 as the PSU caps struggle to keep up. It is a testament to the toughness of Rozenblit's design that I don't have a burned out mess with that lot going on.

Steve
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Nick
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#60

Post by Nick »

Steve PM me the circuit, I do have the PDF somewhere (I bought it off the web to think about building the OTL myself, but never got round to it)

It can't hurt to have more heads peering at it.
Whenever an honest man discovers that he's mistaken, he will either cease to be mistaken or he will cease to be honest.
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