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Friday, March 2, 2018

Importance of power transformer

Importance of power transformer, or importance of transformer to match your usage requirements!

(Editor: wow, it has been a while since we posted. We must make it a point (or a new year resolution), to post more frequently. Let's start with the target of at least 2 - 4 post a month, on any topics at all, related to audio, of course.)

We have stressed many times, the quality of the power transformer is directly proportionate to the quality of your audio amplifier. Taking the quality factor aside, there's another factor that is equally as important - the matching of the transformer primary and secondary voltages towards your incoming mains, and the amplifier voltage requirements. 

Why? Let's look further (with some remedies later): 

Primary voltage
Transformer is a fix ratio voltage converter. The step-up, or step-down ratio is fixed. What is present at the input, will be converted according to the designed ratio. If the input is higher, the output will be higher too, and vice versa. And that my friend, will be an issue where the voltage can be too high or too low for your design, be it regulated, or unregulated power supplies. 

Take mainland China equipment to be used in Bolehland for example. Often, mainland China equipment is built for 220Vac, where as Bolehland mains is 240Vac. The ~10% extra at input of the equipment will produce ~10% output at the secondary, and that will be very damaging, especially for tube amplifiers. 

Take the B+ as an example: 350Vac gets you about 470Vdc after rectification. 470Vdc is a very common voltage used by cathode biased 300B amplifiers, where Vplate is 400Vdc, and Vcathode is 70Vdc, adding them up to get B+ of 470Vdc. B+ PSU caps are usually rated 500Vdc, which is a very common values and economically available to be used in tube amplifiers. 

Simple B+ PSU, just for demo purposes:

Now say 220Vac mainland China amplifier is connected to 240Vac mains in Bolehland. (By the way, in my area, the usual voltage is 250Vac, mind you!) 

Look at what happened to the B+? It is whopping 520Vdc! What are the consequences to this? Or to say, what are the consequences of operating 220Vac input 300B amplifier in 240Vac Bolehland?
  1. 300B B+ may potentially exceeded max voltage, or very close to max value
    • Consequence: shorter 300B lifespan, or burnt 300B. 
    • 450Vdc is max. With above example, Vplate is at 450Vdc after minus the Vcathode.
  2. 300B max plate dissipation may potentially be exceeded
    • Consequence: shorter 300B lifespan, or burnt 300B, red plating.
    • Max plate dissipation is only 40W max for 300B. Some designer may run it close to max to produce max power. If it is biased conservatively, following old school design (~75-80%), then it may still be OK.
  3. 300B filament is 10% higher
    • Consequence: filament will burn out faster with 10% higher filament voltage, or worse, burn out  immediately (open circuit), causing tube to fail. 
    • Emission may be higher with hotter cathode, causing higher current, and issue #2 above may occur.
  4. PSU capacitors max Vdc rating exceeded
    • Consequence: shorter PSU capacitor lifespan, or even fireworks immediately, kaboom!!! 
    • Electrolytic capacitors of 500Vdc working at 520Vdc... go figure. Needs no further explanation. 
    • Some cheaper ones may even use 450Vdc caps and overwork that all the time (we've seen some)!
  5. Overworked power transformer
    • The transformer core and wire gauge may be rated for a certain values only. If it is higher than rated, it may be hotter, melted, degrade, rattle, and such. Of course, properly designed and overspec'ed transformer may not have such issues, which is rarely the case for low end on-the-shelf products. 

There may be more issues, but let's just stop here. 

Some references for your reading pleasure:

300B datasheet states that max Vp is 450Vdc. Do not exceed that!

Filament rating caution - need to be operated as near rated voltage as possible! 

Remedies - Alright, you may say that "I'm scared! How do I rectify this problem?" 

Here are some methods that you can consider:
  1. Use a step down transformer (240Vac -> 220Vac)
  2. Use an isolation transformer, or ultra isolation transformer with step down (even better, kill 2 birds with one stone!)
  3. Use a mains regulator that can regulate down (duh, some may not like regulation in amplifiers, me too!)
  4. Replace the power transformer (complicated, but may be worthwhile if space is a constraint to add additional gears)
  5. Connect a small transformer in reverse to drop the mains voltage slightly (not recommended due to several reasons) 
  6. Don't buy equipment with mismatch mains voltage (Just kidding!)

Secondary voltage - Now, what about the impact on the secondary voltages, after we discussed about primary?

Let's leave this for another follow up article. There's a lot more to cover here than the primary, trust me! After reading that, standard on-the-shelf power transformers will not be your cuppa anymore!

Till then, we've written too many words for this page, and have to stop!

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J&K Audio Design

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