It is about time I built a push pull amplifier

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Paul Barker
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#166

Post by Paul Barker »

The reason I put the source SW in the picture is to show you the RF showing up is in the source. you could argue that back EMF from the amp is producing it in the source, but take it from me, remove the amp you get the same RF on the SW. This is a very crockety old valve sig gen, it has never put out a very nice SW.

The sloap (indicating LF response issues) is not bad at all any longer. The little peak at the start we had with the 1619 version is vertually gone. PRobably all to do with better loading of the output valves.
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Paul Barker
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#167

Post by Paul Barker »

Was googling information on the Quad II output transformer in order to find out the relational size of the feedback winding. It appears to be 10%.

But I also picked up that the Mackintosh uses unity voltage gain with 100% feedback. A fascinating concept.

So the 1/3rd feedback on the primary of these interstages is not necessarily unusual. Anything seems to go.

The trick is arranging the windings so that the feedback is effective at HF (i.e. the same rules about HF coupling exist as they do in the signal windings. <which is that HF coupling is lost with two much capacitance between sections. [inter-winding capacitance]>).

I was expecting the 100% feedback model to apply for me when I connected the interstage as such, but as you know it tipped me into something I didn't want, a valve phase splitter. I chose transformer phase splitter deliberately, and I have to say I am glad I did.
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Paul Barker
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#168

Post by Paul Barker »

OK so we arrived on the journey to the 6b4G just before Owsten, with sufficient power output, and just about enough bass.

the transformers are no supposed to go beyond 25 watts.

Anyway, remember how great Steve's KT120's sounded?

No I am not ordering KT120's, I already have GU50's.

So I did a quick measure up of the triode characteristics and I recon that into my 8k load with the 336v power supply and a -48v fixed bias, each valve of the push pull will see about 100mA give or take.

So in class A triode push pull I would expect 40 watts power output. I am not going to calculate the distortion, I'll build it.

We are now entering overdrive of the output transformers aswell as the driver. So we could say that up to 20 watts we have audio and between 20 watts and 40 watts we have guitar.

worth a go.
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#169

Post by Cressy Snr »

Sounds like a good plan Paul.

With 40 Watts on tap, it should drive just about anything and if the GU50 is anything like as good as the KT120 valve it should be a cracker of an an amplifier.

Only you will of course know whether eventually, it will be worth some better output transformers.
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Paul Barker
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#170

Post by Paul Barker »

SteveTheShadow wrote:
Only you will of course know whether eventually, it will be worth some better output transformers.
these output transformers could break up just right for guitar amplification at 40 watts, and my input valve should overdrive beautifully, so maybe I can get a credible guitar amp out of it.

Here is the overdrive just at it's start: I can make it worse :o

Image
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Nick
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#171

Post by Nick »

But thats single ended clipping, so not likely to be the output transformer, more likely unbalanced output stage or driver stage. I would have thought.
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Paul Barker
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#172

Post by Paul Barker »

I thought it was the input valve . That is what I was picturing not the output transformer distortion. This actually sounds fine, something like 2% distortion. The bias is -1.5v and when this asymetrical clipping occurs it is well into positive grid voltages.

The way to fix it is bypass the cathode resistor so that the rest of the amp doesn't need the input valve to be over driven. Unless one is making a guitar amp.

I shall be able to push the OPT's to distortion when I have 40 watt output stage. Can't do it right now.
"Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." – Albert Einstein
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