Transformer questions

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Bonsaimaster
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Transformer questions

Post by Bonsaimaster » Thu Feb 23, 2017 1:05 am

I would appreciate if someone can help me understand some things about transformers.

1. Different amp designs use different transformers. Usually this is based on the number of tubes used. More tubes more amps needed on heater windings. However, what is the difference in high voltage on the amp sound. Keeping the heater supply constant what would either increasing the high voltage or decreasing the high voltage do to the amps sound?

2. Different power supplies use different filtering. Capacitors and resistors. Voltages are take off various points. Some schematics list voltages. My understanding is that the voltages are based on the tube draw. Why are there different areas that the current is tapped from. Early in the chain it is the OT and finally the preamp tubes. How are the resistor and capacitors calculated?

3. What is the effect of different OT resistance on sound. Like going from a 6.6k to 4K to 3.6K?

I hope that was clear

Thanks in advance

danman
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Re: Transformer questions

Post by danman » Thu Feb 23, 2017 5:12 pm

This is a really broad topic that would take alot of writing to cover. Here are two links to two excellent websites that go into the details of designing a power supply....http://www.valvewizard.co.uk/smoothing.html and...http://www.aikenamps.com/index.php/tech-info Check out the white papers and tech Q and A on Aikens sight. Every old tube company published tube manuals back in the day that contained all of the important tech info of every tube they produced. Once a designer chooses the circuit and tube compliment that they want to use, he can consult the manuals for most of the pertinent info such as max voltages and load lines for the power tubes. Do a Google search for "RCA Tube Manual" and you will get an idea of what they contain. Historically, many guitar amp manufacturers have run the tubes well over the specs that are listed in these manuals. This was most likely done to squeeze a little more power from the circuit and to achieve a particular sound. Typically, running at higher voltages will make the amp punchier and add some headroom. Running a lower B+ will cause the amp to distort a little earlier and compress more. B+ for the OT is usually taken off just after the first main filter cap. You will normally see the full B+ voltage at the power tube plates, minus a little because of the OT's resistance. As you go further down the power rail you will notice dropping resistors and smoothing caps. The resistors drop the voltages to each stage and the cap smooths the voltage further and decouples one stage from the next. The preamp is usually the last item in the chain and you will notice that the voltage has dropped consistently as you get closer to V1. The B+ voltage has also been smoothed by the series of caps as you get further down the line. This is important as the preamp is very sensitive and needs a clean power supply. This is a very broad and general explanation and of course, every circuit is a bit different.

Haze13
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Re: Transformer questions

Post by Haze13 » Fri Feb 24, 2017 5:38 am

There are different classes of amplifiers - Class A, AB, B and A2 with AB2 (positive grid) . All of them needs different current. Class A needs more current then the others.

Guitars Amp running their tubes harder, but no current-wise (AC30 and Ampeg SVT are an exception), actually "screens" are suffer the most.
Most if the amps are - Class B, since current per tube less than 40mA. If you will look at the tube datasheet, current for AB class is always higher then what you'll see in guitar amps for the same amount of High Voltage.

There are Single Ended and Push-Pull types of amplifiers too...

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