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    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Transformers Back to back vs. Voltage Doubler

rcollege

Member
2010-12-02 5:06 pm
Hi all,

I am designing a preamp and have a few options to achieve B+ of over 140v.
This will supply B+ power only...a separate supply will feed the heaters.
I'm planning to use either 1h4g, 6c5 or a 6y6 triode strapped for the tubes in a se circuit. I know the 6y6 is overkill for power but it sounds pretty impressive as a preamp in my system.

Now comes my options.
I have a self regulated Transformer (by SOLA) with 118V -128mA secondary- but buzzes slightly when under a load. Rectified it gives me about B+ of 146 with a 5k load... Voltage doubler circuit yields 275V
Though -I don't like the buzzing and I'm not sure if it will last long.

The other is Back to back transformers. 115V - 12 secondary -12Vsecondary to 115V primary...this yields the same as above with a very faint buzz coming from the transformer. I have had one heat up quickly depending on the load.

The third option is one I haven't used in the circuit...but I wired it up successfully. A voltage doubler for a 115primary - 59Vsecondary. This yields 150 under a 5k load. the transformer has higher amperage and no sound coming from the transformer when under load.

What advantage would the Back to back have over the Voltage double and vice versa?

Any advice or comments are welcome
Thanks

Roger
 
The only advantage of back-to-back transformers is using parts you already have. Two transformers will have twice the size and weight for a given output, twice the heat and poorer regulation. For a preamp, you may be using any of these transformers well below maximum ratings, so efficiency may not be a concern. Tripler or quadrupler on the 60V transformer would also work, but you'd be adding a number of large caps to the power supply.
 
@nigelwright7557 when you sa 9vac transformer backwards, does it mean that the transformer is a 220v primary 9v secondary and when you connected it you've connected the secondary to the other transformer (6.3v) and the supposedly primary becomes the secondary? is that correct?

Thanks,
John
 
@nigelwright7557 when you sa 9vac transformer backwards, does it mean that the transformer is a 220v primary 9v secondary and when you connected it you've connected the secondary to the other transformer (6.3v) and the supposedly primary becomes the secondary? is that correct?

Thanks,
John

Yes it works very well.
 

rcollege

Member
2010-12-02 5:06 pm
I found a industrial transformer that is 440, 220v primaries to 120v secondaries. I wired it backwards as it is made to run in reverse as well

Now I have 115 primary to 220, 115 secondary big isolation transformer that I'm considering using for my power amp. I'm thinking of wiring it as a doubler to get a higher B+
 

Shoog

Member
2002-08-15 10:16 pm
Eire
Back to back transformers work quite well and if you choose your secondary voltage right you can use the spare one (assuming twin secondaries) to run your heaters. They tend to run quite hot if not overrated for the job, and they will sag significantly under variable load - but this can easily be addressed by been generous with your supply caps.
If you have suitable transformers go for it, you can always change out later when a good transformer comes along. I would never consider a voltage doubler.

Shoog
 
Has any one here tried the transformers used in bathrooms?
They give 120vac and are cheap or America converters which I am using in a preamp ,which are also 120vac.

If they are true isolating transformers, then they should be safe. If they are autotransformers ( and I suspect many of them are ) they must not be used, as they do not isolate your amplifier from the mains supply.
 
Ones used in bathrooms are all isolating transformers .Some American ones are auto types ,mine is a proper transformer

Here in Australia we see a lot of attractively cheap, Asian sourced stepdown transformers, and invariably these are autotransformers. They're probably not lethal if you're using them with a double-insulated appliance, but they pose significant risks if used in audio.

A small isolating transformer I've found very useful for B+ supplies is the Triad N-68X: http://www.mouser.com/ds/2/410/N-68X-223861.pdf