VALVE AUDIO TRANSFORMERS, An alternative approach, by G0CWA
I, like most of you, find probably the hardest components to find at a reasonable price are the input/output transformers for a valve circuit. Short of winding your own there are very few alternatives. I have found by playing with transformers most power supply transformers can be used with very little loss in efficiency although frequency response is better with larger transformers (2A and above output)
Although designed for maximum efficiency at 50 or 60 Hz, i.e. least losses, most will provide an acceptable performance at audio frequencies.
Dual Primary transformers can even be used in push pull circuits, the two primaries (or secondaries) being connected in series the start of one winding being joined to the end of the other, so the winding direction stays the same. Instant centre tapped primary.
This text and an excel spread sheet are on makearadio.com at the following link.
Output valve transformers using power transformers spreadsheet nick-transformers.xlxs and the explanations file nick-valvetransformers.docx, by Nick Strong G0CWA
Hope you find this a useful idea and equally importantly it saves you money, all the best and 73 Nick G0CWA
Yes, it has been a well known application to use, what is commonly called, a filament transformer as a audio output transformer for the common audio amplifier circuits that use the 6V6, 6AQ5 or the higher powered 6L6. They are not ideal but they work well. The distortion is higher but most of us would never notice.
A lot of the newer amateurs do not know why the transformers they can buy are rated at 6.3 or 12.6VAC. These were the common voltages for the filaments in the tube rigs of years past.
Thanks for comment, this is one of those things that is shrouded in folk lore and just thought it was worth mentioning.
If you are interested some of my projects are on makearadio.com at
they include radios, carbon amplifier, a psu etc.
I am new to this forum so please treat me gently !!!
Only joking 73de G0CWA Nick
Nick, exceptionally interesting projects you have completed. I had heard of using a carbon rod as an amplifing element but I had never seen one until today. That was fun to read.
There are many other projects you have done and they all seem interesting. Will have to read some more.
Welcome to QRZ. Hope you post often.
A quick question do you think it would be a good idea to list my projects with seperate links ? Is it a good practice on this forum?
Glad you liked projects.
Sorry about file problems, I saved origional files in excel 2007 and word 2007 format, it is a simple matter for me to save in earlier versions if you are still having problems. If you need in any other format contact me directly via e-mail with required format and I will try to convert and send back to you directly.
To err is but human but to louse things up comletely takes a computer!
73 and bye for now Nick G0CWA
Hi Nick, it would be a good idea to post all your projects but not all at the same time. Every week or two post another one of your projects and you should be getting people interested in your next posting by doing that.
It will draw more folks to your postings and they will have question and hints on how to do the same things with other parts. This is supposed to be a place to exchange ideas and seek assistance for technical problems.
If your projects get even one person interested in replicating your efforts then it's a good thing.
Another source are the power transformers used for aircraft. They are invariably 400 cycle (Hertz) and many surplus places get rid of them really cheap.
doesn't take much to get the Z you need for general use in audio.
I know of at least one 1000W AM BCB station that used a spare plate transformer (from a BC-610 I believe) to replace a shorted modulation transformer back in the day when Chief Engineers were employees of every station and often hams. Reportedly listeners couldnt tell any difference in the audio.
For a modern version of can-do listen to WBCQ on SW and the Radio Timtron show on 5110. Tim is the well known WA1HLR of AM ham radio and often gets into the tech stuff in between his usual bouncing off the wall show. He is also a frequent contributor on AM ham forums and has helped many to revive an ailing rig with rather unorthodox solutions.