17-10-2016, 10:05 AM
(This post was last modified: 17-10-2016, 10:11 AM by Mike Watterson.)
I bought a €5 Argos clock radio (About £3.50 then, closer to £4.50 now) to get the mains transformer. The Ia might be 4mA to 8mA on a battery valve radio (less on DL96 and 67V sets and DL92 than DL94) and as long as it doesn't saturate the core, it's fine. You can always measure the secondary inductance with power on & off and no signal?
The "best" voltage (between 3V and 12V) depends on valve circuit and speaker impedance. Impedance ratios being voltage or turns ratio squared.
For single ended mains use I use a VA rating twice the audio and it seems fine. Push pull is obviously no issue at all as the currents cancel.
On a mains powered set you could in theory use DC in another winding or on secondary (Nx primary where N = Voltage ratio) with isolation capacitor to speaker) to cancel the HT bias current.
I've also used 100V line transformers that have 500, 1K, 2K 4K taps for push pull as one tap can be used as a centre tap.
Mains 120 & 120 to 24 + 24 (or 110/220 to 20 & 20) can be used as 5:1 interstage transformers for single ended or push pull outputs. Some 3VA ones are available (if no DC) or use 6VA if there is DC.
A shaver transformer will work as SE or push pull interstage, or as a "Line level" transformer. Good isolation and often built in power limit (something non-standard about core construction? Though some have an external thermistor.) Beware of embedded thermistors, though often the terminals are visible.
The "best" voltage (between 3V and 12V) depends on valve circuit and speaker impedance. Impedance ratios being voltage or turns ratio squared.
For single ended mains use I use a VA rating twice the audio and it seems fine. Push pull is obviously no issue at all as the currents cancel.
On a mains powered set you could in theory use DC in another winding or on secondary (Nx primary where N = Voltage ratio) with isolation capacitor to speaker) to cancel the HT bias current.
I've also used 100V line transformers that have 500, 1K, 2K 4K taps for push pull as one tap can be used as a centre tap.
Mains 120 & 120 to 24 + 24 (or 110/220 to 20 & 20) can be used as 5:1 interstage transformers for single ended or push pull outputs. Some 3VA ones are available (if no DC) or use 6VA if there is DC.
A shaver transformer will work as SE or push pull interstage, or as a "Line level" transformer. Good isolation and often built in power limit (something non-standard about core construction? Though some have an external thermistor.) Beware of embedded thermistors, though often the terminals are visible.
