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Old 10th Jul 2019, 8:14 pm   #124
Herald1360
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Join Date: Feb 2007
Location: Leominster, Herefordshire, UK.
Posts: 16,528
Default Re: Mullard 5-10 amplifier

Well the hiss is clearly coming from the grid resistor so the only other "problem" could be the gain of the amplifier as set by the feedback.

Different components are mentioned for different speaker impedances but they should be such that they give the same degree of feedback for the different output voltages from the amp depending on the speaker impedance. The design article only shows one secondary on the output TX so one would assume that the TX ratio was chosen for one particular load and the appropriate feedback network fitted. Possibly someone may have thought that to change the speaker load, all that was needed was to change the feedback network which would increase the gain if the 15R network was connected to a 3R output TX.

You could try measuring the actual amplifier gain and comparing it with what it should be which is quoted as 40mV input for full output.

Further digging may be required to measure output TX ratio if it's not multi-tapped and there's no helpful markings!

Also bear in mind that the hum and noise spec limit for the 5-10 is "only" -75dB relative to full output. Typical modern amps will manage about 20dB or more better than this!
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Last edited by Herald1360; 10th Jul 2019 at 8:27 pm.
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