Transformers - Sizing?

[gearhead]

Time again for one of my overarching, potentially embarassing, I-have-no-clue questions. LOL.

How do you go about deciding what parameters you want for a transformer, both PT and OT?

Some are obvious, such as whether the OT has the impedence taps you want (4, 8, 16 ohms). Or if you have a rectifier, you want a PT with power, heater, -and- rectifier outputs.

Especially, for:

PT- Voltage and current. I imagine the voltage is driven by what power tubes you use and attendant max voltage needed, and the current by how much you're driving (power and preamp)? Is it crucial to have center tap on heater?

OT - Watts and Primary impedence? For watts, imagine it's based on Power tube type (and number). Impedence, no clue

Edited to add: Also looking for rough numeric parameters, if possible. Ex - range of PT voltages based on driving EL-34s, 6L6, etc.

Thanks all.

[d2camero]

PT

1. determine your B+
2. determine if you want tube or SS rectification (for tube add 20-50V)
3. divide by 0.9 for losses
4. divide by 0.7 for conversion to AC
5. divide by two for centre tap

e.g.

B+ 378V
before tube rect: 428V
before losses: 475V
before rectification: 679V
centre tap voltage: 339V

So I need a 340-0-340 V PT



From the tube's datasheets:
- for current on HT add up all your preamp and power amp tubes
- for heater currernt, simply add up all the heaters current
- and if you use tube rect, get a 5V winding

OT

Much harder to decide, because the OT impedance depends on the power stage topology, class rating and tone. If you are designing a power stage from scratch, look at the tone lizard SE and PP pages, this is how to design the output stage and determine all the components sizes. If you are taking from an existing design just use what the design asks for.

Look at the output tube data sheet (our friend). For example for a 6L6GC in PP class A, with a B+ of 250V and cathode bias, we use a 5k OT.



d1

[mchauck]

Wow d2camero! I really thank you for that!!!

That just gotcopied and put on my wall. Hope you don't mind

BH

[dehughes]

I saved that as well. Thanks d2camaro!

[gearhead]

Thanks, a Lot of Great info!

How do you determine the B+ you want? Have been reading the data sheets, and they give max and "typical" values for power tubes (in varous modes of operation). So you kind of determine how hard you want to push them and go from there?

Using your calcs, the TS Trainwreck PTs (300-0-300) would be putting out 378V at B1+, which is just a few volts shy of the 395V target/zone. As a guide, that's pretty dang close

[d2camero]

Choosing the B+ voltage for the output stage is a good question. Usually this is determined first before jumping into the power supply.

Fender uses high B+ to get lots of clean headroom in the power section. Lower the B+ and the power section will break up earlier.

[d2camero]

About the datasheets:

They are great for a few things:

1. knowing the maximums that you can apply to a tube
2. getting a good starting point on designing an amp
3. using the charts for changing the characteristics of the amp

However, data sheets don't tell us anything about how to get different tones out of the amps. For this we need experimentation by either yourself or others who have done the work and have documented it in books or on the net.

[David Root]

One little wrinkle: d2's method is excellent and gives the PT sec. at idle. Some suppliers specify their PTs at rated DC mA, eg Allen Amps PTs. The difference is about 6%, ie 340VAC @200mA is 360V@ idle, in that range. Will likely vary depending on sec. VAC and/or DC mA spec.

Above assumes the specs are based on 120VAC wall power supply into a 120VAC primary. A lot of PT's primaries are specd for 117 or even 115 VAC primaries, so if your wall supply is 120VAC, which it will be in US & Canada, you will get a little higher sec voltage from that too. So it pays to read the fine print if you want a very closely defined idle B+. This has a real potential effect on filter capacitors if you are cutting it close to begin with on their VDC rating