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-   -   ECL82 monoblock 'spud' and a question (http://www.diyaudio.com/forums/tubes-valves/174239-ecl82-monoblock-spud-question.html)

Arnulf 25th September 2010 10:31 AM

ECL82 monoblock 'spud' and a question
 
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I've finished the changed boards for my ECL82 today, going from triode mode to pentode. It is a very straightforward circuit so I enclosed the schematic in case anybody else wanted to give it a try.

PCL82 tubes will work in place of ECL82 with 16V heater voltage but the two 100R resistors (R14 and R15) must be changed for higher value (330R will do nicely).

Oops, forgot the change the name of the tube :rolleyes: :p

Bias is adjusted via R10. After amplifier warms up (a couple of minutes) it should be tweaked so that voltage across resistor R5 reads 12V or thereabouts for cool operation (Ik = 38 mA). Anything above 12V across R5 will push the tube above maximum suggested anode dissipation and therefore shorten its life somewhat.

Feedback ratio is adjusted via R9. I ran the amplifier open loop during testing.

-=-=-=-=-=-=-=-=-

And now for something completely different :D

How do you guys deal with noise pickup when amplifier is operating with no input source ? Do you simply try to avoid such situations, do you use extremely low input Z resistor (how low is it safe to go - I use computer as sound source, I imagine it's capable of driving 1K or less ?), do you have some technical solution (perhaps a rectifier and leaky integrator to control input-shunting relay) ?

pedroskova 25th September 2010 11:36 AM

Try a pair of male RCA shorting plugs (signal tied to ground).

M Gregg 25th September 2010 11:38 AM

Arnulf,

Are we talking input cables disconnected or just no signal?

I use relay selector input and one input is mute "input to Ground" because the input of the amp is Capacitor coupled. So at power on input is auto muted and and then connects to the source! This is case with my pre amp also!


Regards
M. Gregg

M Gregg 25th September 2010 11:47 AM

You could just use a switch that disconnects the input from source and connects amp input to Gnd. I would not drive the source input into a short!

As long as the inputs have a resistor to Gnd there sould be no "pop" when switching. If you are asking what value I use the total value of the volume control 50K at each input to Gnd.

Arnulf 25th September 2010 11:54 AM

I'm afraid 'no signal output' equals 'disconnected signal cable' in my case, the signal output goes to virtually infinite Z once shut off so when signal is removed (= MP3 player turned off) the amplifier starts buzzing at mains frequency which it apparently picks up which is slightly annoying, if still harmless to the speakers. Once the signal appears (even if it's put on 'mute') the amplifier becomes dead silent.

Of course the cabling is still an utter mess as I'm testing the boards on ny desk so this problem might be even less severe once I get the enclosure ready and wire everything up neatly, using shielded (and earthed) wire for signal.

DF96 25th September 2010 02:18 PM

Capacitive pickup on C1? Is it a physically large component? This is often a disadvantage of decent capacitors. Make sure no mains or heater wiring gets near the input. Ideally, ensure that C1 can't 'see' any AC - put a metallic grounded screen between if necessary. Keep C1 near the metal chassis if possible - it may seem counter-intuitive but a good electrostatic screen behind something can provide a limited degree of protection from things in front of it.

When your input is plugged in the low output impedance of the source shorts any buzz to ground.

Arnulf 25th September 2010 02:34 PM

It is 15 mm raster, "box" type.

I wonder how low I can go with R1 (grid leak of the input section) before my signal source would object and whether going down to 1K or even lower would reduce the noise enough to be worth the effort. If it's capable of driving low-Z headphones ...

M Gregg 25th September 2010 03:15 PM

Quote:

Originally Posted by Arnulf (Post 2313971)
It is 15 mm raster, "box" type.

I wonder how low I can go with R1 (grid leak of the input section) before my signal source would object and whether going down to 1K or even lower would reduce the noise enough to be worth the effort. If it's capable of driving low-Z headphones ...

Why don't you just use a cheap Tx on the input?

Regards
M. Gregg

Arnulf 25th September 2010 03:38 PM

I don't see how this could prevent environmental noise from mains wiring making it to the grid.

M Gregg 25th September 2010 03:52 PM

I assume if you mute " input to Gnd" the hum stops?

I put foil around the input capacitors and wrap a drain wire around this, tape into place connect to Gnd.

You can tell the pick up by moving your hand close to components! If it is the caps then you will know!

You could try a mains Tx across the input and see if it changes noise level. This would answer the input Tx question!


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