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Old 28th Mar 2019, 9:25 am   #41
wd40addict
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Default Re: Hum after installing an isolation transformer

One side of the secondary should be earthed and connected to the chassis. The other side of the secondary obviously connects to the dropper.

If this is the setup you have, and it still hums, how close is the transformer to the amp and what happens if you move them further apart?

Finally what rating is the transformer? The Ampro has half wave rectification so the current drawn will be asymmetric. This may cause the transformer to physically hum. A larger transformer should be better.
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Old 28th Mar 2019, 9:45 am   #42
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Default Re: Hum after installing an isolation transformer

The hum turned out to be the photo-electric cell. It was not in complete darkness after I removed the projector part.
I unsoldered it, now the hum is gone.

I now need to fix another problem though. One that has been there from the beginning. It sounds like I am frying bacon all the time.

I cleaned the pots, didn't help.
Changed the preamp tube, didn't help.
Changed the phase inverter tube, didn't help.
Changed the preamp plate resistor, didn't help.

I don't have extra power tubes at hand to test if it is them.
And I can't start the amp with just one power tube to see if one of them is the cause. Heaters being in serial and all.

Do you have any ideas?
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Old 28th Mar 2019, 10:50 am   #43
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Default Re: Ampro Stylist projector

I suspect the problem here is that of common usage vs literal meaning.

A transformer sold as an Isolation Transformer will have particular care taken over the electrical separation of its mains primary and isolated secondary whereas any transformer with separate primary and secondary windings will be an isolating transformer insofar as there is no direct connection between the windings. An example of the first type is the familiar yellow building site transformer and of the second, the usual mains transformer(s) in a radio.

I say usual- there are radios with auto (non isolating) transformers which save a few pence compared with isolating ones but can reduce the power wasted in dropper resistors.

So, literally a mains transformer is any type of transformer with mains on a winding. It may be auto wound (non isolating), separate windings (isolating) or separate windings with reinforced insulation (safety isolating).

In the end, it comes down to the degree of protection required for the use to which equipment is put. When covers are off (for servicing perhaps) all bets are off too and the appropriate safe working practices must be employed to mitigate the risks to anyone involved.
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Old 28th Mar 2019, 11:43 am   #44
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Default Re: Ampro Stylist projector

Thanks for that Herald. we've gone back to the question in post 12*. I'm glad that my attempt to answer it [in post 20*] doesn't seem so far off and things can get complicated, as I suggested. Using the Isolating Transformer might have seemed to be a quick solution but safety is paramount as you say.

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Old 28th Mar 2019, 1:53 pm   #45
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Default Re: Ampro Stylist projector

Frying bacon is capacitors or anode resistors...
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Old 28th Mar 2019, 2:18 pm   #46
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Default Re: Ampro Stylist projector

Does 'frying bacon' vary with the volume control?

Also are you running the amp with a source connected? I am surprised a low impedance source feeding the mic socket didn't override any hum output from the PEC.
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Old 28th Mar 2019, 2:57 pm   #47
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Default Re: Ampro Stylist projector

It goes up and down with volume yeah.
And, there is bacon sound with a guitar plugged in.
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Old 28th Mar 2019, 6:08 pm   #48
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Default Re: Ampro Stylist projector

Have you replaced all paper capacitors? This type of noise is often anode resistors, R7 or less likely R9/R11.
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Old 29th Mar 2019, 12:22 am   #49
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Default Re: Ampro Stylist projector

Remove C3 and replace R4 with a wire link. Have another listen, should be quieter.

To take it further change R6 & R7 for metal film types. Replace C5 & C6 if you haven't already.
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