Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: octal on February 28, 2015, 02:08:41 pm

Title: Power transformer deration / sizing
Post by: octal on February 28, 2015, 02:08:41 pm

Any tips on sizing power transformers? If my PT can provide enough B+ current to meet the tube data sheet "max output" numbers for the output tubes plate+ screen draw + whatever current the preamp needs, is that good enough? 


My concern is that the datasheet current draw max output numbers are rated at a specific wattage and THD, and in a guitar amp application, you might be pushing the output (and the current draw) quite a bit past that. Any "rules of thumb" for this?

Title: Re: Power transformer deration / sizing
Post by: eleventeen on February 28, 2015, 03:08:06 pm

The transformer is typically rated for "steady state" ampacity. Usually it is not the B+ demand as much as it is the filament demand. Obviously, you cannot turn the heaters off; but you *can* play quietly, eg; using much less than full-rated power.


The question is, if you "overdraw" the power transformer, how much is too much? An extra 10 or even 20% shouldn't be a problem for short periods of time, but inevitably the tranny will start to run hotter and the voltage output will sag. Over longer periods of time, the insulation and and innards of the PT will start to "cook" out and probably bring on failure earlier. Maybe if the tranny only lasts 20 years instead of 35 years, that isn't such a big deal.



 

Title: Re: Power transformer deration / sizing
Post by: octal on February 28, 2015, 08:44:12 pm

Thanks! I've also heard that if you "underdraw" one of the secondaries, you can maybe push the ratings a little bit more on the other secondaries. (I guess in terms of the overall VA capacity of the transformer and the primary winding(s).   Hoping that's true... I've got quite a bit of extra capacity on my filament windings but I'm at about 105% on the B+ winding at the datasheet max output. I'm just going to build it, measure the sag at full blast and see how hot the transformer gets.

Title: Re: Power transformer deration / sizing
Post by: PRR on February 28, 2015, 10:19:09 pm

For amps under 50 Watts output, it is not an issue. The variable plate load is only part of the total load. The datasheet numbers are for a fairly high output, you can only beat so-much more out, and probably not non-stop. And "small" power transformers will sag badly before they over-heat enough to matter.

Over 100 Watts output you should be more generous.