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Old 8th Sep 2018, 7:41 pm   #1
mpegjohn
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Join Date: Aug 2013
Location: Chichester, West Sussex, UK.
Posts: 265
Default Mullard 3-3 amplifier quiescent voltages

Hi,
I am building a Mullard 3-3 amplifier, and I am a little concerned about the quiescent currents and voltages I am getting.

With the amplifier as published in the Mullard audio handbook, I get 33.6V on the EL84 cathode. So I reckon I have 60mA cathode current, when it should be 50mA.
Any ideas why that might be?

I changed the cathode resistor to 660Ohms, and now that gives me 53mA. better, but all the DC voltages are of course askew:

C9 306V
C6 286V
C3 209V
EL84 A 299V
SG 286V
C 35V
EF86 A 28.3V
SG 33,8V

Can someone help me please to understand what is amiss. Due to the DC feedback I have got a little confused, so some description of how that works as well would help.

The amplifier does work, and very well. But I can't leave it like this.

Regards,
John.
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Old 8th Sep 2018, 8:43 pm   #2
Sideband
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Location: Croydon, Surrey, UK.
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Default Re: Mullard 3-3 amplifier quiescent voltages

According to the Mullard test voltages, the cathode should be around 28V. You can probably go 10% plus or minus in real life so although a little high it isn't drastically so. Have you tried another EL84....a new valve these days does not necessarily mean a good valve especially if it's a Russian or Chinese copy. The Mullard description explains why the cathode voltage is higher than in a normal RC coupled circuit. This is directly coupled so the operating conditions are different. Also remember that the voltages measured were not recorded on a digital meter which will give higher readings anyway.

Circuit here http://www.r-type.org/articles/art-003h.htm

Another thought....have you checked for any instability? If it's oscillating at supersonic frequencies for some reason, that will cause the output valve to draw more current.
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