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I'm embarking on designing an amp for the first time (Bottleneck 6V6 Contest) and I'm trying to learn some of the basics that I can't seem to find. How do I know if a transformer has enough current for the heaters of the tubes I want to use? Assuming I will be using AC heaters, is it as simple as adding up the heater current of all the tubes and if the value is less than the transformer is rated then it will work?For example, will a transformer rated as 250V(125-0-125)@125mA CT & 6.3V@2A, be appropriate for an amp using two 6V6 and one 6SL7 tubes
0.45 + 0.45 + 0.3 = 1.2 amps
But the same transformer would not be appropriate for an amp using four 6V6 tubes and one 6SL7?
0.45 + 0.45 + 0.45 + 0.45 + 0.3 = 2.1 amps
Is there a rule of thumb for safety? That is, should the heater current of the tubes be a percentage of the rating (e.g., 75%). If I wanted to use the 5 tubes above, then I would need to have a transformer with a rating of at least 2.8 amps (0.75*2.8 = 2.1 amps), or is that unecessary - would a transformer rated for 2.5 amps be fine?
Edits: 05/04/09
What you propose works if you are wiring the filaments in parallel. If you wire them in series, you'll need 18.9VAC. Other than that, what the Pauls said.
Most transformers are rated into a resistive load. This means that yes, the filament is easy. Add up the currents with 20% margin for line and part variation and you are done.
The B+ is harder. The rms of a capacitive input filter can be many times the output current. Duncan Amps PSUD can help with this. An easy way to figure the RMS current from PSUD is to take square root of the peak current times the average output current and divide it by 1.5. This isn't 100% accurate, but it is good enough.
Example: 100mA average out with 2.5A peak = sqrt(0.1 * 2.5)/1.5 = 0.33A rms
If the layout is crowded or you want the transformer to run cool, you'll want to over size it quite a bit. If you've got more than an inch all the way around the transformer, oversize it just a little (that is if you don't mine the transformer being too hot to touch.)
Play safe and play longer! Don't be an "OUCH!" casualty.
Unplug it, discharge it and measure it (twice) before you touch it.. . .Oh!. . .Remember: Modifying things voids their warranty.
I'm taking this as an opportunity to bloviate on transformer design. If you want to skip the long part, yes you can meet the rating exactly with perfect safety, but should not exceed them.
Transformers are rated for the maximum current they can sustain continuously without compromising their lifespan and safety. There is a safety margin built-in for things like fluctuating power-line voltage or a reasonable amount of nearby hot components (tubes, resistors, etc.) and there is an assumption that reasonable cooling airflow is available, i.e. if in a box there are vent holes.
To put it crudely, if you don't exceed the ratings and something goes wrong, you can sue the manufacturer.
Since transformers die by excess heat, there is a balancing act done by power transformer designers, among core losses and copper losses. Core losses increase as the magnetic flux level increases, and copper losses are just the resistive heating of the copper wire. More expensive laminations such as M6 can take more flux, and more expensive bobbins, wire, etc. can take more heat, so there's an economic balancing act going on as well.
High-temperature materials are affordable these days - unlike the old days of 50 years ago. So modern, economically efficient designs will be smaller and run hotter than classic designs.
OK, that sets the stage. Now many DIYers prefer to keep the transformer temperature down. There are legitimate reasons like avoiding burns and keeping other components cool, or from a mistaken belief based on the lower safe temperatures of vintage parts. The easy way to do that is to reduce the current draw. This will not affect the core losses, only the copper losses, so it is no longer an optimal compromise - but it's the one that is easy to implement with commercially available components.
For the Bottlehead custom transformers, I have specified much lower flux levels than usual, which minimizes the external magnetic field and any transformer vibration. I also have kept the copper losses as much as possible in balance with the reduced flux, so as a net effect they do run cooler than modern designs run at their specified limits. We have retained the industry-standard high temperature parts so there is a temperature safety margin from both reduced core and copper losses.
Both of your examples are correct when using AC fillaments. For DC fillaments, your transformer fillament tap has to be twice as high.
Bernie
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