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Old 30th August 2013, 06:47 PM   #1
robsan is offline robsan  Switzerland
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Default Mullard ECL86 3W Economical Amplifier

Greetings DIY people! First post here...

I got bitten by the bug not too long ago and for my first amplifier build I decided to use the Mullard 3W single ended design found at this location: Three Watt Economcal Stereophonic Amplifier

It seemed simple enough and I decided to build it with the following small assumptions/modifications:
- I would leave out the tone controls from the original design and only use an alps pot that I had in stock for the volume control
- The ECL86 would become a PCL86 (because I have several pairs around for another amplifier that I wanted to put to use) - so I had to find some 12-13v for the heaters.

After having to solve some issues with the feedback loop getting the whole thing into oscillation (thanks Thomas for helping ;-) I managed to make it work and have to say I'm quite happy with the result for my first project. It's been playing since a couple of days with no issues.

So here's where I'm looking for some advice: looking at the square wave response with my scope, I'm finding that it's far from perfect (basically it sags at lower frequencies) - I've attached the scope output for 500Hz and 4kHz.

Is there any way to improve this? After reading around I've discovered that this is usually a sign of bad low frequency response that could be due to the coupling cap between the triode and pentode sections (by the CR filter formed between the 0.1uF cap and 820k resistor).

Any suggestions for improvement? Is my understanding above correct?

Thanks in advance!
Robert
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Old 30th August 2013, 07:06 PM   #2
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No, it is a loose of high frequency components, mainly by the transformer's interwinding capacitance. For a simple circuit, the response is satisfactory. Don't worry and enjoy it‼
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Old 30th August 2013, 08:18 PM   #3
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I agree on the output transformer. It's pretty well spec'd in the design, what kind of OPT did you use?
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Old 30th August 2013, 08:22 PM   #4
DF96 is offline DF96  England
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Square waves will always sag at lower frequencies, unless the amplifier goes down to DC. The OPT prevents this, so some sag will be seen. How much sag depends OPT quality, NFB design etc.
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Old 30th August 2013, 09:30 PM   #5
robsan is offline robsan  Switzerland
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Quote:
Originally Posted by Regenpak View Post
I agree on the output transformer. It's pretty well spec'd in the design, what kind of OPT did you use?
I used a Lundahl LL1682/50mA (http://www.lundahl.se/pdf/1682.pdf) and since the recommendation from the original circuit was 7k impedance, inductance >10H and primary resistance <350 ohms, the 1682 seems not too far from specs on an 8 ohm speaker.
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Old 30th August 2013, 09:34 PM   #6
robsan is offline robsan  Switzerland
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I didn't even post a picture of the little critter, here it is :-)
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Old 31st August 2013, 06:16 PM   #7
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How did you end up simplifying the feedback section in the end since you eliminated the bass control? I assume feedback was positive so you got a howl?
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Old 31st August 2013, 11:05 PM   #8
robsan is offline robsan  Switzerland
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Quote:
Originally Posted by carbondated View Post
How did you end up simplifying the feedback section in the end since you eliminated the bass control? I assume feedback was positive so you got a howl?
It was a bit of experimental debugging...

I assumed that C4 and RV7 were there for the bass adjustment and only left R9 and C7 in the circuit. This didn't work very well and was making the whole thing oscillate: B+ on the scope was starting to oscillate at a low frequency after 12-13s of power up. (see attached traces of B+ before I switched the amp off)

I disconnected the feedback loop and the problem went away so I tried increasing the 1.8k resistor to 10k (because I had a pair available) and reconnected the feedback: that fixed it. I'm not sure why that had any influence on the B+ stability - I suppose that the oscillating output valves were pulling too much current from the cap reservoirs.

However, I would like to understand how to calculate the amount of feedback applied so that I can choose the right resistor and capacitor (10k + 560uF right now)- I'm unsure it's working correctly right now even though the amp sounds right.
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