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Old 9th May 2013, 08:39 PM   #21
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Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
It's hiss, not hum, right?

If it has only one pot in the first stage, it is going to be difficult to make it work optimally at all volume levels without adding a master volume or a power soak or inter-stage or output pad of some kind. Such amps just expect the background noise to be tolerable at low settings. First you should establish whether the noise is normal for the model, and if not fix it. If it is normal, then you need to figure out whether the noise is introduced in the preamp, in which case you could add your own master volume to reduce the noise before you start getting preamp distortion. If the noise is from the power amp even at idle and is normal for the model but unacceptable for your uses, add a power soak or switch in/out a resistor network in series and parallel with the speaker. Cutting output power to 1/4 will be noticeable but won't make a huge difference in the noise if you're really seeking near-inaudible silence.

If you put the 8 ohm speaker in series with a 2.5 watt 8 ohm resistor,
then put those both in patallel with a 16 ohm 5 watt resistor,
then all that in series with an 8 ohm 10 watt resistor,
and all that in series with an 8 ohm 10 watt resistor,
and all that in parallel with a 16 ohm 20 watt resistor
it will handle 40 watts and the speaker (and the hiss noise) will be 1/16 the power and everything (including the hiss) will be noticeably quieter, less than half the volume. Of course, you should add a high-current DPDT toggel switch to connect / disconnect the 'hot' side of this resistor network to/from the amp with one pole and to/from the speaker with the other throw. Or if the speaker has a plug, you could wire this into a box that you can plug between or not. Probably $12 for the resistors.

If you like deep bass you could also consider switching in the low-pass portion of a crossover to the speaker wiring, to cut the treble hiss. How effective this is depends on the frequency of the hiss. Instead of adding tone controls to my little champ, I'm considering adding some filters at the output...but perhaps on the primary side of the output transformer 'cause that one's tube...
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Old 10th May 2013, 07:45 PM   #22
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Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
Correction, somehow a line got repeated:
put the 8 ohm speaker in series with a 2.5 watt 8 ohm resistor,
then put those both in patallel with a 16 ohm 5 watt resistor,
then all that in series with an 8 ohm 10 watt resistor,
and all that in parallel with a 16 ohm 20 watt resistor
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Old 10th May 2013, 07:48 PM   #23
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Join Date: May 2010
Location: was Chicago IL, now Long Beach CA
If you use a power soak or pad resistors, remember that the amp is working really hard though it's not makng much sound, so don't abuse the amp.
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