EL34 current requirements from PT

[VacuumVoodoo]

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.

AFAIK Hammond has a strange way of specifying their transformers.the Assuming they adhere to standard notation, it says 150mA DC when only the end taps i.e. 690V is rectified. But I could be wrong so you should ask them directly.
IMO&E a CT two phase rectified 2x345V can't deliver 50W with only 150mA from a class AB PP amp.

[Ian444]

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.

According to the Hammond guide a centre tap PT delivers 1 x rated current, compared to 0.62 x rated current for a full wave bridge rectifier. The required current depends on type of rectifier according to this datasheet. The OP did not specify this. Geeez, why does such a simple question turn out so complicated?

And how does one get a clean 50W from a pair of current production EL34, I would like to know the answer to that one...
35W is as much as I can muster and the screens are just over the limit, nowhere to go....

[VacuumVoodoo]

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.

According to the Hammond guide a centre tap PT delivers 1 x rated current, compared to 0.62 x rated current for a full wave bridge rectifier. The required current depends on type of rectifier according to this datasheet. The OP did not specify this. Geeez, why does such a simple question turn out so complicated?

Because it's not so simple,especially when you mix average rectified DC values with RMS and the current you try determine is not sine shaped but pulsed. It's where it gets confusing....like in that Hammond sheet. Then you get a power supply that sags like an old witches tits.

And how does one get a clean 50W from a pair of current production EL34, I would like to know the answer to that one...

The short answer: it's pretty easy when you know how.
Longer answer: If you don't know: follow the data sheet recommendations and get someone to design the power supply correctly. I have no problems pulling over 50+ clean going to 80W cranked from 2xEL34. I wouldn't recommend to run current production EL34 above around 475V on the plates.
It has more to do with electrically safe amp assembly methods at higher voltages than with tubes themselves.

35W is as much as I can muster and the screens are just over the limit, nowhere to go....

mismatch between OT impedance ratios and PSU capability.

[Ian444]

Interesting that the data sheet for Mullard gives a -33 to -36V Vg1, yet specifies a Vin(g1-g1)rms of around 48V, (48V rms is around 65V peak) are they driving the grid 1 positive to get their 50W? 110mA max per tube hardly looks like 50W on a load line? Or is that rms? Max signal on the screen 23.5mA, how about 80mA peak on my 35W amp. Does not make sense to me, if someone can explain I would be very appreciative.

[Ian444]

Some more info - at 35W there is B+ 440V and screen supply is 400V which is fed to the screens through 1K resistors. At heavy clipping, 50W, B+ 430V and screen supply 365V. The screens are already dissipating more than max, doesn't make sense compared to the Mullard data. I have tried 5K2 and 3K6 OPT's with different loads and the output power changes little. Just curious but also frustrating. 35W? Output power is the square of V RMS divided by load resistance is it not?

[Ian444]

I have no problems pulling over 50+ clean going to 80W cranked from 2xEL34.

80W output at 50% max efficiency would require 160W input power. 80W output and 80W dissipated in heat through the tubes themselves. Thats 40W per tube, but EL34's are rated at 25W, so what happens to the other 15W?

[VacuumVoodoo]

Isn't this a mystery? I would never claim to have bypassed the laws of physics, at least not when I'm sober...
Cranked PP produces something closer to a square or trapezoid wave than a sine even when input is a sine wave.
Then we have each tube conducting every other half cycle and in full cut-off the other half time...and a PSU capable of only so much current... or I would have expected 100W cranked...net result is that average over time plate dissipation in each tube is lower then it is when idling biased to 70% of max.
Careful construction needed here, with a tube in cut-off it's corresponding OT half primary winding is working into almost open circuit...beware of plate to heater arcing on the tube base or its socket!

It's 13:35hrs here and I need to grab some lunch............

PS. the numbers in that Mullard data sheet don't compute, i think they switched digits on output power should be 45W not 54W...compare to TFK data sheet before you call me a blasphemer...

[Firestorm]

Interesting that the data sheet for Mullard gives a -33 to -36V Vg1, yet specifies a Vin(g1-g1)rms of around 48V, (48V rms is around 65V peak) are they driving the grid 1 positive to get their 50W?

This is a peculiarity of some data sheets (especially Mullard and Philips). They mean the RMS value of the peak-to-peak voltage (the grids are 180 degrees out of phase) rather than the peak voltage, so it's really 24V * 1.414 = 33.936, so they don't intend you to drive the grid positive (in Class AB1). Compare most RCA datasheets, where they give the input voltage as "Peak AF Grid No. 1-to-Grid No. 1 Voltage," by which they also mean peak-to-peak, but without the added wrinkle of stating it as RMS.

[guitarmike2107]

There is a big difference when current is given as "mA" vs. "ma"

WTF?

Current is named after André-Marie Ampère , generally you will find units named after people are denoted with a capital letter, hence why mA is correct. other examples are Pascals (Pa), Newtons (N) Ohms Farads (uF)etc

Never seen ma to mean something different to mA.. your source please?

[VacuumVoodoo]

mA - DC component
ma - ac component
mV - DC component
mv - ac component, such as low level signal riding on DC offset

by same convention "I" is used to denote DC current and "i" denotes ac signal. Same goes for "U" and "u" for voltages.
Perhaps this notation is not in use any more....in that case you'll have to forgive this vintage carbon based entity aware of its own existence for being stuck in the day the music died.

[surfsup]

You guys are confusing the %*#^ outta me! haha but the sustain on that "zagray" brought me back to reality, damn!

[vibratoking]

mA - DC component
ma - ac component
mV - DC component
mv - ac component, such as low level signal riding on DC offset

by same convention "I" is used to denote DC current and "i" denotes ac signal. Same goes for "U" and "u" for voltages.

I agree that "I" can be used to denote DC current and that "i" can be used to denote AC current. This analogy also applies to "V" and "v" for voltages. I have never seen it applied to the units of voltage or current. In my experience, mA/mV denotes current/voltage whether it is DC or AC. "ma" or "mv" is just a mispelling.

[VacuumVoodoo]

It makes more sense to only capitals for designating units as these are names. But I'll stick with "I" vs "i" and "U" vs "u". I have to unlearn an old habit.

[rp]

Got nothing tech to add you're all well beyond me and I learned a ton and saving this post. But I must challenge this:

The amp has headroom like Eiffel tower and a Van Damme kick in the gut.

Van Damme's a wus, Walker Texas Ranger could so kick his butt!

[rdjones]

Got nothing tech to add you're all well beyond me and I learned a ton and saving this post. But I must challenge this:

The amp has headroom like Eiffel tower and a Van Damme kick in the gut.

Van Damme's a wus, Walker Texas Ranger could so kick his butt!

You might need to refer back to the "kickboxer vs Marine" thread. ;->

rd