# Always 2 transformers on a tube amp ?



## bagpipe (Sep 19, 2006)

I've noticed that most of the tube amps I've seen, when the amp is removed from the cabinet, have 2 transformers. From reading a bit, I believe these to be the Power Transformer and the Output Transformer. I'm guessing the Output transformer is to "match up" with the impedence of the speaker cabinet. Is the power transformer used to step up the voltage to the large voltages that the tubes require? Could someone provide an "Idiots Guide" to the transformers in a tube amp ?


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## Milothicus (May 22, 2007)

you're pretty much right. the power transformer changes 120V source to (usually) 2 other levels. one is the lower heater voltage (6.3V) used to heat the tubes to start electron flow, and the other is to supply high voltage to the tubes and output transformer. the HV can be over 300V.

the primary side of the output transformer sees the high voltage levels. since you don't want 300 volts going into your speaker, the output transformer brings that back down to a level that a speaker can handle.

i hope this is accurate. i apologize if i've missed something.


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## Adicted to Tubes (Mar 5, 2006)

Elements of what you said is true,the output transformer does match impedance,but it also does one other major thing.
A tube amp has AC voltage coming into it from the wall socket.The power transformer isolates that and converts the 115 or so AC volts and bumps it up to whatever your particular amp needs to operate.Lets say 350v.It has a few different windings that give you high voltage,filament voltage and sometimes bias voltage.Then it goes to a rectifier which turns that AC(alternating current) into pulsing DC current.Either diodes or a tube does this.Then that DC voltage is what your amp uses to amplify the signal coming into it(which is also AC from your guitar pickups).Then the output transformer takes that signal and converts it back to AC voltage that the speaker can use.A speaker cannot have DC voltage or it will melt the voice coil immediately.
A solid state amp functions quite differently and does'nt require an output transformer.


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## RIFF WRATH (Jan 22, 2007)

thanks for sharing the info. coincidently I have been wondering about OT's.
further question please: what type of voltage and/or amperage is a typical speaker receiving? 
thanks
RIFF


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## Michelle (Aug 21, 2006)

This might not be totally accurate Riff where speakers have an impedance rather than straight resistance.

Using the formula P=E2/R, re-arrange it to E= square root[PXR], so, with an 8ohm load at 20W, the voltage would be.....(getting calculator)... 12.7V

And using the formula P=IXE, I=P/E, so 20W/12.7 = 1.6A

Another is P=I2R

Unless of course I have remembered the formulas wrong but they seem to check out. :wave:


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## RIFF WRATH (Jan 22, 2007)

Thanks Michell. I was going to pick up a switch for adding an external speaker in to , and in this 1 horse town (actually there are lots of horses in town, Elmira Ont.) (and 2 at home) the local hdwre carries very little in electronics (really miss the Radio Shack junk) and i was going to buy a switch at the auto parts but was concerned about amperage and voltage.
cheers
RIFF


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## Wild Bill (May 3, 2006)

RIFF WRATH said:


> Thanks Michell. I was going to pick up a switch for adding an external speaker in to , and in this 1 horse town (actually there are lots of horses in town, Elmira Ont.) (and 2 at home) the local hdwre carries very little in electronics (really miss the Radio Shack junk) and i was going to buy a switch at the auto parts but was concerned about amperage and voltage.
> cheers
> RIFF


Michelle's given you some good math, Mr. Riff. One thing about the situation that works in your favour is that all switches will carry far more current than they'll break.

What this means is that you could have a switch rated at 5 amps and it will break the circuit on/off no problem. If you just set it and leave it alone it will handle far more! So for a speaker load if you turn the volume down before hitting the switch you'll never have a problem.

Also, remember that audio is not a continuous steady current but rather varies up and down with the sonic wave. So you worry more about current as an average. Most techs rate the average current as about a third or so of peak.

Home Depot has some nice big toggles in their electrical dept. From Elmira you could also visit Neutron Electronics in Guelph. They're on the #7, just at the 1st light west of the #6 "tee". Great bunch of folks!

:food-smiley-004:


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## RIFF WRATH (Jan 22, 2007)

thanks for the info. actually after doing some googling am looking into using a mono jack switch so that there is always a speaker load. can't figure out how to wire yet, was messing with my multimeter last night.
cheers
RIFF


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