Ampdog
R.I.P. 23 June 2022
The following is not new, but hopefully somewhat of a solution for those who want to DIY valve pre-amplifiers but am stuck with the familiar 6V 250V power transformer availability. Short of having one wound (and rather cheaper), one can consider using two low voltage transformers back-to-back. Small low voltage transformers are freely available.
I have recently used the following ending under R120 for transformers. I used a Chinese LLB48* 230V to 12-0-12V 600mA transformer, feeding a #OM 1212* similar transformer rated at 500mA on the 12-0-12V side.
The centre-tap leads may be cut off, and a bridge rectifier at the 24V junction feeding a 6800?F capacitor, can provide a well-smoothed 24V heater voltage for 4 x ECC81 valves or similar. Another bridge at the 230V 'output' of the second transformer feeding a say 220?F 450V capacitor plus further filtering can provide some 230Vdc for the valves at about 12mA.
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* The Chinese can be quite free with their ratings - a smaller transformer than the LLB48 boasted 12-0-12V 1A! I thus give the following sizes as a rough indication of how big they were: LLB48 core: 50 x 42 mm, stack about 33mm. #OM1212: Same l x b dimensions, stack about 26mm.
As these are often wound on nylon forms with separate sections for primary and secondary, one can remove some 50 - 60 turns from the secondary of the second transformer to give about 290Vdc as h.t. This will saturate the core somewhat but give a higher dc h.t. output. The extra heat generated from the increased loss will get the second transformer slightly over luke-warm, but it still worked well after several hours, not reaching hand-burning heat.
# The '# ' in the second transformer's initials got torn off with a label; it should be easily recognisable though.
[For convenience, I bought these from Communica. I am not an agent for Comminica, nor do I have any ties with them.]
I have recently used the following ending under R120 for transformers. I used a Chinese LLB48* 230V to 12-0-12V 600mA transformer, feeding a #OM 1212* similar transformer rated at 500mA on the 12-0-12V side.
The centre-tap leads may be cut off, and a bridge rectifier at the 24V junction feeding a 6800?F capacitor, can provide a well-smoothed 24V heater voltage for 4 x ECC81 valves or similar. Another bridge at the 230V 'output' of the second transformer feeding a say 220?F 450V capacitor plus further filtering can provide some 230Vdc for the valves at about 12mA.
_________________________________________________________________________
* The Chinese can be quite free with their ratings - a smaller transformer than the LLB48 boasted 12-0-12V 1A! I thus give the following sizes as a rough indication of how big they were: LLB48 core: 50 x 42 mm, stack about 33mm. #OM1212: Same l x b dimensions, stack about 26mm.
As these are often wound on nylon forms with separate sections for primary and secondary, one can remove some 50 - 60 turns from the secondary of the second transformer to give about 290Vdc as h.t. This will saturate the core somewhat but give a higher dc h.t. output. The extra heat generated from the increased loss will get the second transformer slightly over luke-warm, but it still worked well after several hours, not reaching hand-burning heat.
# The '# ' in the second transformer's initials got torn off with a label; it should be easily recognisable though.
[For convenience, I bought these from Communica. I am not an agent for Comminica, nor do I have any ties with them.]
