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Questions on transformers

Started by Solder U, August 19, 2013, 08:04:13 PM

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Solder U

Hey guy's, came across a non-working fender ss amp for dirt cheap and was thining about rebuilding/modding the amp. So I just wanted to start looking at parts and came across several different types of transformers. I understand the basic idea of what a transformer is and what makes it tick. But, if its cool with you guys I just have a quick question on the different types of transformers, their uses, and when different transformers would be required. Also, I am not at all opposed to being referred to reading material either.

What are the differences between power transformers, output transformers, chokes and reverb drivers. When is it necessary for an amp to have more then one transformer? My assumptions on reverb drivers is that you need it to either power the reverb tank or the tank's output. let me know if I am off base. Thanks in advance guys!

Enzo

You need a particular transformer type when you need the job they do to be done.

Everything that plugs into the wall needs a power transformer (or a switching power supply which takes its place).  It transforms the 120 volts of AC from the wall outlet - or 240v in other places - into the voltages the circuit needs.  In a solid state amp, it might need 35v to run on, so the power transformer turns 120v into 35v.


Output transformers are something only tube amplifiers need.  Oh there will be a few odd exceptions, but mostly.   Vacuum tubes run on high voltage and low current.  Speakers run on high current and low voltage.  A transformer does exaclty that sort of transformation.  SOlid state amps do not use an output transformer because they already work with low voltage and high current.

The spring reverb unit needs to have energy put into it to shake the springs.   That requires a tiny little power amp.  The reverb transformer is nothing more than a tiny little output transformer.  If you disconnected it from the reverb pan input jack, you could connect it instead to a small speaker and hear the music.  The 12AT7 reverb drive tube uses high voltage and low current.  The reverb transformer steps that down to the lower voltage the reverb spring unit wants.  You will generally only see reverb transformers on tube amps with a tube driving the pan.  (Some tube amps have solid state reverb drivers)

A choke is an inductor.   it is made like a transformer, and looks like one, but it is not one.  It is used in the power supply as part of the smoothing filter for your high voltage.   It does a similar job as a filter cap, just works differently.  You will only see them in tube amps.  Well, they also are used in the crossovers of large speaker systems.

Solder U

Sweet! Thanks a ton! That clears up a lot for me. But that brings me to another question. If most SS amps do not need an output transformer, is there cheaper way to Mod one to have multiple ohm outputs, such as 4, 8, and 16 outputs?

Enzo

It already has that.  All a solid state amp has is a lower limit.  usually the impedance it has a rated output for.  So if an amp says 100 watts at 4 ohms, it will also be happy as a clam with 8 ohm or 16 ohm loads.  it just will not like a 2 ohm load.

teemuk

^ True. But if you want to get such scheme to work effectively (about same output power to each load) then you need to scale up (or down) the power amp's supply rail voltage.

Which is in fact what many manufacturers used as impedance matching scheme for solid-state amps some years ago.

Alternatively one could just fit in a basic impedance matching transformer or autoformer to the output and call it a day. Here's one example of a product formerly made by Peavey but I'm sure that several other manufacturers offer similar products:
http://www.peavey.com/assets/literature/manuals/80370223.pdf

Solder U

#5
So, as I understand the concept of changing the transformer to a tappable one more then messing with a "power rail", (not sure what that means) if I changed the transformer of say, a Fender Frontman 100 head, which has 100 watts at 4 ohms, to a 50 watt 2, 4, 8, and 16 ohm output transformer, what kind of issues might I suspect from changing the wattage?

BTW, I started reading the Solid State book posted in this forum. Really interesting read so far.

J M Fahey

#6
QuoteI changed the transformer of say, a Fender Frontman 100 head, which has 100 watts at 4 ohms, to a 50 watt 2, 4, 8, and 16 ohm output transformer,

You can't, Fender Frontman (a nice amp, by the way), does not have an OT.
As mentioned above, it's optimized for 4 ohms, yet will run happily with 8 or 16 ohms, only at some reduced power.
Which may still be more than what you need ... or your neighbours tolerate.

Solder U

Hey all, thanks again for the replies. I think I got a handle on this particular subject. I really appreciate your time and patience!

Enzo

Teemu, you are exactly correct, but I think the original poster here does not yet understand the basic output transformer need.   I think he is confusing different technologies.

SOlder, as Juan said, there is no output transformer there, so there is no "changing" it.


However, on an amp that did have an output transformer, changing a 100 watt transformer into a 50 watt one is only going to do one thing - burn up.   The amp makes 100 watts, all the transformer does is convert it from one form to another.   Putting a 50 watt output transformer into the amp doesn;t make the amp a 50 watt amp.   it is just a 100 watt amp with an undersized transformer. now.

Solder U

Oh wow. thanks for that. I didn't think about it that way, though now that I know it seems as if that should have been obvious... thanks again!

phatt

Hi SolderU,

Maybe if you tell us the model for this non working fendud amp we can all work on the same page and you can then focus on getting it to work again. Well I assume that is the aim? 8|

Meanwhile keep reading teemu's book :tu:
Cheers Phil.

Solder U

#11
It is a fender Deluxe 85, and it needs to still be available when I arrive in FL. the guy says that one of the caps is blown and needs a new transformer. I did some reading on the issues with these amps and was planning on replacing all the caps, pots, and jacks, then seeing if it worked before replacing the transformer. i am aiming to get it work again, or course, but I also want to learn something from the experience as well as try to alleviate some of my anxiety on build/repairing solid state amps.

phatt

OK then what happens when you turn it on?
Do any lights come on?
Does it make any hum or hiss through the speaker?
Do any knob turns make any hum hiss louder?

If some kind of life exists then try inserting a guitar cord into the efx loop send return sockets which are a common failure point.

The worst thing you can do right now is replace everything  :trouble
as that will almost certainly end in tears. You need to establish you have power
(Check mains fuse).
Then check you have secondary voltage at the transformer.
(If the Led comes on then you likely have secondary voltage)

The main DCV will be split supply rails of around 30 to 40 VDC so that's +30 and -30 with reference to ground.

Then the preamp section runs maybe 15-0-15 VDC.
All this supply stuff is in the bottom left corner of the schematic I'm viewing.
Not much use as I can hardly read any values on this poor quality Jpeg. :grr

Anyway try a few things and get back with what you find.

Phil.

Roly

If you say theory and practice don't agree you haven't applied enough theory.

Solder U

Sweet, I should be in Fl next week. I'll be able to take a look at it then. If not I'll be on the prowl for another project amp like this and i'll keep yawl posted on what happens!