UK Vintage Radio Repair and Restoration Discussion Forum - View Single Post - Hum pickup on inter-stage coupling transformer

G6Tanuki · 29th May 2020, 7:12 pm

One thing to be aware of is that to minimise currents induced into the chassis you need to make sure that both 'sides' of any transformer-winding pass through the *same* hole in the chassis. This is particularly relevant to heater-windings - if you've got 2 or 3 Amps of heater-current going on, and you feed the transformer secondary-windings through different-though-adjacent holes in the chassis you can induce some horrible circulating currents in the chassis.

[My approach to this sort of issue - in a 'communications'-receiver - is to introduce aggressive audio roll-off below 300Hz. Things like not including cathode-decoupling capacitors, or if you do use them use low values like 0.25uF for the output-valve and 10nF for anything earlier in the audio-chain]

29th May 2020, 7:12 pm	#19
G6Tanuki Dekatron Join Date: Apr 2012 Location: Wiltshire, UK. Posts: 14,007	Re: Hum pickup on inter-stage coupling transformer One thing to be aware of is that to minimise currents induced into the chassis you need to make sure that both 'sides' of any transformer-winding pass through the same hole in the chassis. This is particularly relevant to heater-windings - if you've got 2 or 3 Amps of heater-current going on, and you feed the transformer secondary-windings through different-though-adjacent holes in the chassis you can induce some horrible circulating currents in the chassis. [My approach to this sort of issue - in a 'communications'-receiver - is to introduce aggressive audio roll-off below 300Hz. Things like not including cathode-decoupling capacitors, or if you do use them use low values like 0.25uF for the output-valve and 10nF for anything earlier in the audio-chain]