• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

heathkit SA2

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Recently rebuilt a heathkit SA2 using there detailed manual( they even tell you how long to cut and where to bend every wire). New caps and resitors were installed. I used the hum trim pots but there still remains some hum how do I trace this down? I own a digital meter and soldering iron


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Localising the source/cause is important - as many red herrings can be suggested as a solution.

Try and check if it is just from the input stage (short the signal or turn down a pot after the input stage). Try and check if it isn't the input stage input wiring (short the input valve grid to 0V). Run the input stage heater from a battery (this will indicate if it is heater related).
ALL electrolytic caps. must replaced. The orange "thingy" a previous poster pointed at looks like a 'lytic to me. Jim McShane is a good source for tubes, parts, and advice. The can cap. previously mentioned is (IMO) best replaced by a clamp mounted part occupying that opening. Mr. McShane stocks that sort of stuff. ;)

What, if any, source was used, when the hum was detected? You need shorting plugs in the I/P jacks to test, without a signal source being attached.

Take a close look at the schematic. Notice AC heating in the phono section. Only a 7025 equivalent, with its spiral wound, hum bucking, heater, has a ghost of a chance of being quiet enough. Buy a pair of phono grade, current production, Sovtek 12AX7LPS tubes from McShane. That Saratov, Russia, made variant IS a genuine 7025 equivalent, whose price is reasonable too.

Heath's gain structure is wretched! They attenuate line level signals and then amplify them with the 2nd gain block of the phono section. That wreaks havoc with the S/N ratio. That can be "cured" by replacing the source selector switch with an inexpensive Lorlin brand part from Mouser and a bit of rewiring.

The "laundry list" goes on. :yes: Install a proper, 3 wire, safety grounded, power cord. Get "120" VAC away from the front panel by disconnecting the on off switch on the treble control. Install either a slide or rocker switch in an opening currently occupied by a convenience outlet, to control the on/off function. Add the uploaded SS diode tweak to the 5AR4 socket, with UF4007s, not noisy 1N4007s. The tweak may allow you to squeeze some extra time out of the old stock tube and is essential to reliable operation of the current production Sovtek 5AR4, which is the best of the affordable stock. NOS 5AR4s are outrageously expensive. :(

BTW, if you plan on recording LPs to another medium, a much more capable phono section, whose O/P impedance is low, is needed. Yes, I have a candidate for that job, should it be needed. :D


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