EL34 current requirements from PT

[JamesHealey]

I want to run two EL34 in push pull with a B+ of around 350v

Biased at about 65% to 70%

Can I get away with a 200mA PT?

Or am I going to require higher?

[selloutrr]

http://www.dreamtone.org/Calculate_Current_Form.htm

[Alexo]

Yep, I'd feel comfortable with a 200'er.

[surfsup]

(25W *0.7) / 350 = 50mA each
Screens 10mA *2 = 120mA total
with the preamp tubes, etc, you are at a 50% safety range at your output level.

At max output each tube is 71mA+10ma*2=162mA
With preamps not drawing much, you're at 170mA draw from the PT

[VacuumVoodoo]

When PP power stage draws 100mA DC from the supply the PT GV winding must deliver 200mA rms. Add another 25mA for preamp tubes.
Class AB1 PP has at best 50% efficiency, do the power transfer math: 50W output needs at least 100W input to plates, now add power eaten up by screens... you end up with PT HV winding rating of at least 120-130VA

[tubeswell]

See page 2 of tubenit & co's cheat sheet (attached)

[VacuumVoodoo]

It's a real cheat sheet with EL34 heater current given as 1A and HT current draw figure is for one tube not for two
Guys, use real data sheets, not some tables put together by someone who apparently doesn't know how to read a data sheet.

[tubeswell]

Well thankfully I was not the author

[VacuumVoodoo]

I know you're innocent

[surfsup]

Alexs, i used 50% based on merlins suggestion. Using max out math in post above of 170mA, at 50 pct that is roughly 250mA. what safety factor would you suggest if one was 'doing the math'?

[VacuumVoodoo]

Monday morning edit: inserted figures for James' design assumption in paranthesis & blue text


Alright, lets take a standard configuration:
2xEL34 50W output (or per original question 35W)

I'm using Telefunken data sheet from Franks data base where I find:
B+= 425V and a 3.4k Raa(B+=350V Raa=3.8k)
will let me overshoot my target by 10% with 55W output.
DC current draw at full 55W (35W) output is listed as
2x120mA (95mA) for plates
2x25mA (2x25mA) for screen grids

That's 290mA at 425V DC (240mA at 350V DC) so the power drawn from B+ supply is
.29Ax425V= 130W ! (0.24Ax350V=104W)
Did I or did I not write that class AB efficiency is at best 50%?
Check how much goes to useful audio power i.e. plates only: 425Vx0.24A=102W. (350Vx.19A=66.5W)
The rest is waste, screen grids don't contribute to output power.

The 130W (104W) must be supplied by the HT winding of the PT. Here's where it get's a little more difficult and wherer many misses are made.
Let's see :
425V (350V) is average DC i.e. it's half way between peaks and troughs of ripple riding on the rectified DC, allowing for 10% ripple implies peak rectified voltage is 5% above average so 425Vx1.05=446V (350Vx1.05=367.5V)
446V (367.5V) is what peak rectified voltage from HT winding will have to be
so HT rms voltage has to be 446V/1.41= 316V Let's make it 320V (367.5V/1.41=260Vrms)

Next, what current rating do we need? Look above, that power amp eats 130W (104W) and it's the minimum that the HT winding must supply at 320V AC (260V AC), so

130W/320V=0.4A (104W/260V=0.4A) and taking into account what the phase inverter and preamp will need + resistive losses in rectifier and filter caps I'll ask for HT winding capable of 0.5A rms at 320V. (as well as at 260V)

The above assumed silicon diode bridge rectification. For vacuum tube you will need to compensate for higher voltage drop.
It is also necessary to recalculate current rating to account for center tapped HT winding, as each half of the winding will be supplying current to charge the filter cap only every 2nd half cycle.
To not confuse you any further I'll ask you to trust me when I say that the current rating of the center tapped HT will be 0.5A/1.4=.35A

Now, WTF? Center tapped means we 640V (520V) end to end and with .35A it makes 224VA (182VA) !?!? Yes, but remember that each half of the winding delivers current (and thus power) every 2nd half cycle. There's the magic trick and the reason why center tapped HT winding will account for a more expensive transformer. More copper & a little more iron.

Another spec your transformer supplier must be given is called "Load Regulation", in other words how much voltage sag you will accept on the HT winding between no load and full load i.e. when current draw goes from 0.0A to 0.5A in our case. Industry standard is 10% or 5%, better than 5% is expensive.
If you don't spec it you will get 10% or worse. Better load regulation means thicker magnet wire = more expensive. Don't expect 5% regulation from a cheapo PT.

So James' amp will need 260V HT winding rated 0.5A

A certain very well known manufacturer told me I was crazy to specify 160VA HT winding for a 50W amp. Needles to say the order went elsewhere. The amp has headroom like Eiffel tower and a Van Damme kick in the gut.

[guitarmike2107]

If you have a 200mA centre tapped trannie you could use it,

You can always bias it colder or lower the screen voltage to reduce the power output, HT current requirements and power transformer sag.

Look at the Hammond datasheets for their guitar range replacement trannies.
50watt JCM800 PT is only rated at 150mA!
http://www.hammondmfg.com/pdf/EDB290GX.pdf

Some one can correct me if I am wrong but AFAIK the 6550 JCM800 use the same PT’s, just biased cool enough.

But if you want a stiff PT that laughs in the face of your seven string guitar, gives you maximum output power, and cost is not a concern then you may want to up the current spec.

[Alexo]

DC current draw at full 55W (35W) output is listed as
2x120mA (95mA) for plates
2x25mA (2x25mA) for screen grids

That's 290mA at 425V DC

It's only drawing 120 (95) ma from one plate at a time, not both simultaneously.

[RJ Guitars]

http://www.dreamtone.org/Calculate_Current_Form.htm

That is an excellent resource... anybody we know?

rj

[VacuumVoodoo]

DC current draw at full 55W (35W) output is listed as
2x120mA (95mA) for plates
2x25mA (2x25mA) for screen grids

That's 290mA at 425V DC

It's only drawing 120 (95) ma from one plate at a time, not both simultaneously.

And thus current going into OT center tap is?
120mA is what? DC, rectified average? rms?
Output power given is for sine wave input.
There is a big difference when current is given as "mA" vs. "ma"

No prizes for correct answers....