EL509 OTL Amplifier

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Cressy Snr
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#61

Post by Cressy Snr »

Hi Nick

Couldn't leave it alone as it was nagging at me.
So I had a nice cup of tea, said my relaxation mantra
and armed with the circuit diagram and a very strong pair of reading glasses
I found this

Image


What is this going to do to the feedback?
I ask this because as a single ended kind of guy I have never done an amp with feedback before.

The simple resistance check I did when I built the driver stage a few weeks ago would not have revealed this. Indeed it verified that the correct resistors were fitted and that they went to the right places, but that is all.
There were no voltages published either for this bit of the circuit so I didn't check the cathode of the input valve.

I'm off to bed now. It can wait 'til tomorrow. No sense in playing with lethal voltages at this time of night.

I'll PM the circuit anyway Nick because there could be other faults.

Steve
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Nick
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#62

Post by Nick »

Its late (so I expect I am wrong) but how you had it, I think will remove AC feedback (the cap, will provide a short for AC), you will still have any DC feedback, but I think the way it should be will give both AC and DC feedback.

But how you have it will have a corner that may be the cause if the instability.
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Cressy Snr
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#63

Post by Cressy Snr »

Something else as well now I've had a sleep.

The way I have the cap connected will bypass the cathode resistors to ground, which will substantially increase the gain of the first stage. The first stage is a 12AX7 high gain valve which makes the relative increase the worst case scenario as there aren't that many valves with higher gain than this one.

So we have far too much gain at the front end being cascaded through the system to the output stage. In this situation there only needs to be the slightest bit of feedback around the loop to send the whole thing crashing down.

I had hit upon the cap problem before I posted last night after disconnecting the feedback. There was a great deal of hum but at least it was continuous noise, not motorboating. Investigating the feedback loop further revealed what you see in the pic.

Now I just need to put it right and test it out.

I'm not holding my breath.
Steve

PS that's the problem with this higher power game. When these amps fail they make a much more scary noise and generate much bigger bangs.
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#64

Post by simon »

SteveTheShadow wrote:However as soon as the HT is applied to the output tubes BLAM BLAM BLAM BLAM we have a stunningly good low frequency oscillator.
and
SteveTheShadow wrote:I know you managed to sort Simon's Blatting 2A3
In my case the BLAM BLAM BLAM BLAM was due to lack of gridstoppers on the C3gs and it made the 0A2 VR tubes click like mad. I concur that it sounds horrid! Thank heavens for Nick and someone who knows what they're doing. :D
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Nick
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#65

Post by Nick »

Thank heavens for Nick and someone who knows what they're doing.
I am glad you pointed out it took two of us.

Myself AND someone who knew what they were doing :-)
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Cressy Snr
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#66

Post by Cressy Snr »

SteveTheShadow wrote:I'm not holding my breath.
Steve

PS that's the problem with this higher power game. When these amps fail they make a much more scary noise and generate much bigger bangs.
Told you I wasn't holding my breath. It still motorboats but without the accompanying blatting noise on top of it. That's one fault cleared at least.

I think now I need to look at the HT decoupling capacitors on the B+ rail of the driver section. There is definitely nothing wrong with the driver stage circuit now.

The power stage works perfectly in isolation and can be biased up to within tolerance by tweaking the pots.

Time to start on the perhiphery and pay attention to the supporting gubbins.

Steve.
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Nick
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#67

Post by Nick »

Just a thought, doesn;t BR use torroids on his kits, it may be your TX has a higher source impedance which is making the decoupling more critical.
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Cressy Snr
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#68

Post by Cressy Snr »

Nick:

You have a PM
Steve
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#69

Post by simon »

Nick wrote:Myself AND someone who knew what they were doing :-)
:lol:
Andrew
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#70

Post by Andrew »

I see in Bruce's patent.

http://patft.uspto.gov/netacgi/nph-Pars ... PN/5604461

He's talk's about using a 6C33C as an output device, hmmmmm

-- Andrew
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#71

Post by Cressy Snr »

Right.

Inserting the HT fuses one at a time.
The negative rail on its own has just the normal hum that would cancel out if the other side was on.

The positive rail on its own however sounds like a buzz saw rattling around in a bag of spanners. So It is now isolated to something in the positive side of the supply B+ (I think).

Now if the power stage on its own with the HT on both sides working is quiet then the noise must still be coming from the driver stage itself or its associated supply. I think I'm being logical here. Tell me if I am not chaps.

We are slowly homing in on it.
Steve
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#72

Post by Cressy Snr »

Well I cured it but I have lost two valves in the process. One shorted grid to cathode and destroyed itself. Its partner on the bank now has an open heater.

It was a bad B+ decoupling cap in the end which I found by removing them until the noise stopped, plus of course the feedback miswire. The driver stage used 500V B+ to give the swing needed to drive the output, so both faults were in the worst possible place they could have been, the 200V peak to peak swing eventually driving two of the output valves to destruction.

I have certainly dragged up all my old equipment fault finding skills, from where in my head they have lain dormant for fifteen years.

Have to get a new set of valves now so it will probably be a couple of months before I have saved up the readies.

Steve.
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#73

Post by Andrew »

Oh that's a shame, to loose a pair of valves having got so far.

Can you run it with two pair of the best survivors and change the heaters?

cheers,

-- Andrew
Cressy Snr
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#74

Post by Cressy Snr »

Well I've had it up and running on four valves and it sounds very nice indeed. liquid and smooth like you wouldn't believe. The top end is simply beautiful. There is not much power with the four valves but it is extremely neutral sounding, totally different to single ended with output transformers.

I had a pair of meters monitoring the HT and bias whilst it was playing.
Suddenly the bias disappeared on number three valve, a soft plop and a reduction in volume. Then number three started glowing red hot and that was the end of that.

I know now why however. It was the position I had the amp in when playing it under test. To get access to the underside of course the amp has to stand vertical, which means the valves are horizontal.

In my case the orientation put the grid assembly facing downwards with its broadest axis looking at the bench. Grid sags due to expansion, touches the cathode and that's it; the valve shorts grid to cathode and turns full on destroying itself. I actually saw the spark as the grid touched down and the bias went out on that valve. but it was too late to save the poor thing. I've only got three left so that's all for now. But you have to look on the bright side. At least it works and sounds good. :D

A bloody expensive lesson to learn though :shock:

Steve.
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Nick
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#75

Post by Nick »

Upside down is much safer (to late I know :-()

I have a collection of paperbacks in the music room to create piles to support amps.
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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