# Reducing secondary voltage from Hammond 270AX?



## VHTO (Feb 19, 2016)

In the parts bin, I have an older Hammond 270AX power transformer.

Specs are: primary - 115V
Secondary voltage: 480VCT at 50mA

and there's a 6.3V output for the filaments.

I'd like to use this for a low-wattage type build -- perhaps a Tweed Deluxe.

What's the best way to reduce the secondary voltage (and ideally, without adding much/any noise to the circuitry?)

Thanks in advance


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## Lincoln (Jun 2, 2008)

Are you looking to build something with a EL84 tube? That 270AX is already quite a low voltage. The recommended power transformer for a tweed deluxe is the 290-BBX and it's rated 710Vct @ 115mA. But when it's operating in the amp, it is no where near that voltage.


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## VHTO (Feb 19, 2016)

I certainly could go with an EL84 and do some sort of 5W variant.

The other question has to do with heater wires for the rectifier -- all of my other PTs have separate feeds for the rectifier, this one doesn't. Can I simply run everything in parallel (as long as amperage works out) or will that create other issues?

Or just go with a solid-state replacement for a 5Y3 or similar?

Tks


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## Lincoln (Jun 2, 2008)

You'll have to use solid state (diode) rectification for sure because you don't have a 5 volt winding for the rectifier tube. 
I'm facing a similar situation in the AX84 high octane build I'm working on. The open circuit voltage of the transformer I'm using is 460 volts. EL84's like about 380 volts. I'm hoping voltage comes down when in the circuit. If it doesn't, I'll be looking for ways to drop some voltage too.


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## dtsaudio (Apr 15, 2009)

Unfortunately your are stuck with sold state rectifiers unless you add a 5V power transformer.
If you use a full wave choke input rectifier arrangement your DC voltage should be around 215VDC. So pretty low. However if you use a bridge rectifier you will get upwards of 600V peak, and about 430V fully loaded. So too high. That's the theory. Real life may be slightly different.


Lincoln said:


> I'm using is 460 volts. EL84's like about 380 volts. I'm hoping voltage comes down when in the circuit.


It might. Depends on whether it has a choke. You might be able to bring it down with a resistor as well. The problem is going to be at idle when the amp is drawing minimal current.
The numbers are based on the calculations found in this document.


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## Lincoln (Jun 2, 2008)

Nice little chart @dtsaudio ! That's for sure a keeper.


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## _Knighthawk_ (Mar 2, 2014)

Indeed! Thanks @dtsaudio . Very handy. No idea what it means but it seems like it will come in handy once I figure it out!


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## jb welder (Sep 14, 2010)

Another thing to consider is knocking down the primary voltage with a bucking transformer.
If that transformer is old and really spec'd for 115 (or 117) volts, modern line voltage will cause higher HV winding _and_ heater voltage. You don't want a heater voltage that is too high. 
So the bucking transformer will drop the primary voltage resulting in all secondary voltages being lower. But you probably don't want to go much lower than 6V on the heaters. In some cases old transformers will end up with heater voltages around 7V which is hard on tubes. Using a bucking transformer on the primary is a good option in cases like that.
There's lots of info on the net regarding the issue. It's like a built in version of this, but doesn't need to be adjustable: Vintage Voltage Adapter
Once you get the heater voltage to a reasonable level, if the HV winding is still too high, there are ways to reduce it, such as zener diodes.


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## Hammerhands (Dec 19, 2016)

How about using a half-wave rectifier?


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## VHTO (Feb 19, 2016)

The transformer is older and is spec'd for 115V and 44VA


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## dtsaudio (Apr 15, 2009)

Just realized the Hammond chart I posted earlier is cut off at the bottom. I've reloaded it so the whole document is there now.


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## _Knighthawk_ (Mar 2, 2014)

dtsaudio said:


> Just realized the Hammond chart I posted earlier is cut off at the bottom. I've reloaded it so the whole document is there now.


Thanks redownloaded.


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## greco (Jul 15, 2007)

dtsaudio said:


> Just realized the Hammond chart I posted earlier is cut off at the bottom. I've reloaded it so the whole document is there now.


@Lincoln Did you see this post?


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## Lincoln (Jun 2, 2008)

greco said:


> @Lincoln Did you see this post?


I sure did, thank you.


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