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Overdriving Power Transformers

5/3/2000 4:38 PM
Chris Harden
Overdriving Power Transformers
This has been mentioned briefly elsewhere here, but I'd like to ask for comments on whether you should spec OT's for the power you expect to run, over (to give headroom) or under (earlier saturation?). Would you get a more compressed sound running a OT harder than it's spec?  
5/3/2000 5:17 PM

Hi - I think, that it is best to keep the specs. As far as I know, the transformer might be given you more distortion but I do not believe, that it will be harmonic distortion to make the amp sounding better.  
5/3/2000 5:28 PM
Gil Ayan

What will happen is that if the PT is overloaded, it will not be able to sustain the B+ as the current demand exceeds the transformer specs. The sound, to me, is not pretty... It will not be "compressed," it will be more of a pronnounced sag...  
BTY, the sound of a cranked Fender, for example, is in part due to the PT not being able to handle the current demand. If you put say 6L6s in a Deluxe Reverb, the amp will still put out only about 30W.... If you put a scope, you will find that when the output is on the order of 30W, the B+ starts dropping dramatically.  
5/3/2000 5:53 PM
Chris Harden
Overdriving Output Transformers
Sorry! I've just been misleading you! In my title I wrote power transformer, but I meant Output Transformer. I know that power transformer needs to be well spec'ed, what I intended to ask was whether say a 30W OT transformer would work with a pair of EL34's that are capable of nearer 50W.  
Sorry to confuse you all.  
5/3/2000 6:20 PM
I have an email from Dr.Z around here somewhere -- can't find it. What I remember is him saying that the Maz-38 has a 40W OT, but the Prescription has an 80-100W OT. The two amps represent different approaches. The Prescription has a great deal of overhead as far as the OT goes, whereas the Maz-38 will generally drive it's OT harder. I think the prescription is more of a master volume amp.  
Hope that makes sense.
5/3/2000 6:20 PM
Don Symes
Re: Overdriving Power Transformers
Put it this way - whenever someone asks about a 'better' OT for a Princeton, the answer is 'use the OT from a Deluxe ... more iron, better bass response... ' and some other good things.  
For a given application, comparisons usually focus on the realtive superiority of the larger/heavier item.  
That said, if you have the budget, try both - in fact, try as many as you can get your hands on. The OT is one of those magic components where specs matter, but aren't anywhere near the whole story.
5/3/2000 8:14 PM
Peter S

I think it depends on the application, the tone you want to hear and how you will be using the amp. The early Marshall JTM 45 amps had 30 watt transformers and IMHO they sound glorious when cranked. I have been building amps for 20 years and I'll use the OT that I think is bets suited to the application......there is no one right answer. FWIW I have a homebuilt amp that is a cross between a Plexi50, a tweed Fender Bassman, with PI section that resembles a vox or Matchless and the output stage is Marshall.......when I first built it I had a Hoffman 50 OT in sounded great with HUGE low end and wseet highs.......later I installed a Mojo tweed Bassman rpelacement tranny. Now keep in mind that I always run the amp wide open and I get my clean sound by picking softer.....when I want grind I hit the strings harder. The mojo OT is alot smaller than the Hoffman but it still retains that monster bass tone yet the midrange and treble tones are alot smoother and much more complex sounding......also now the amp will sustain forever whereas before it sounded colder harder with much less singing quality. It's all a matter of what you prefer to hear.

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