Duncan's PSU - 12AX7/6V6/EL34 current?

[Normster]

I've been fooling around with Duncan's Power Supply Designer and I'm not too sure about the values for plate (and screen) load for each tube type. For example, if a 12AX7 tube chart shows 1.2mA, do I use 2.4mA to represent both triodes as a load? I've seen many existing models that show 1.5mA for a single 12AX7 so it doesn't make sense to me.

Also, when entering values for pentodes in a push/pull circuit, do I double the current draw for the tube or is it some other value based on PP operation? (i.e., 70mA for a 6V6 or 140mA for the pair.) Same question for the screen current.

The reason I ask is that I'm trying to design a PS using a 5AR4 rectifier that can be used with both 6V6 and EL34 output tubes. The simulation shows that I exceed the limits of the 5AR4 with EL34s but I know the circuit should handle it based on Marshall and Fender circuits I've seen in the past.

Sorry to be such a newbie, but this is the first time I've ever tried designing around a tube rectifier and I'd like to verify the operation in Duncan's simulator before I commit it to a circuit board.

Thanks,
Norm

[Ray Ivers]

Norm,

FWIW, I always use the max value (2.5mA) for each 12AX7 gain stage, but perhaps you're using a whole lot of them and need to be a bit more conservative, in which case 1.5mA should be fine.

The RCA RC-30 manual gives 225mA @ 450V for the 5AR4 max, and the 40W example amp for the 6CA7/EL34 indicates 143mA max-signal plate current @ 450V B+, so you should be OK with a pair of either EL34's or 6V6's.

Ray

[Normster]

Thanks, Ray!

[Satamax]

Hi guys!

Normster, for your plate requirements, there's TSDL on Duncan's site. That will lead you to he specs sheets, and you take the numbers at the max dissipation for the tubes you use. 2 power tubes = 2X max dissipation. for example, 25 watts for an EL34 at 425 volts is 25/425= 58,8 mA per tube or 117.6 for the pair.

Bye.

Max.

[Normster]

Hi guys!

Normster, for your plate requirements, there's TSDL on Duncan's site. That will lead you to he specs sheets, and you take the numbers at the max dissipation for the tubes you use. 2 power tubes = 2X max dissipation. for example, 25 watts for an EL34 at 425 volts is 25/425= 58,8 mA per tube or 117.6 for the pair.

Bye.

Max.

Thanks, Max! I spent so much time looking up tube charts that I didn't even think about the math.

That pretty much confirms my suspicions that I should stick to 6V6s for this build. With only 380V on the plates, I'd be pulling over 130mA (plus screens and preamp). However, now you've got me wondering...would I fry the 5AR4 or just introduce sag? SS is so much easier.

[Ray Ivers]

Max,

YMMV, but spec'ing PT's based on a rated-plate-dissipation rule-of-thumb (some output stages run the tubes at 2X+ rated P.D., peak) will pretty much guarantee at least some power-supply sag in the vast majority of guitar amps that run in any class other than A1. If that's what you want, cool, but IMO it would not be a good match to a high-power, high-current-draw design. As I mentioned in my earlier post, a 40W 450V B+ 2-EL34 P-P stage pulls 143mA max (EL34's are rated for 150mA Ik per tube), and the B+ voltage of a 118mA PS would drop to whatever voltage/current it could supply on each full-power waveform peak; sag city. A 118mA toroid PT under these conditions would be a truly unhappy camper.

I don't mean to get on your case at all, Max - I would not be surprised to find that many guitar-amp manufacturers use a similar (if not identical) procedure to yours to spec the B+ windings of their PT's, which will result in significant cost savings when purchasing in quantity.

Ray

[Satamax]

Hey Ray, nei bother. I'm not an EE, all i've learnt over the years is with you, and a few others. What i knew for sure at first, was ohms law and direction of electrons was oposite to the voltage, and that's about it! So, you know, it's always better to corect me when i'm trying to help, opening mi big mouth and talk shite! Normster, why wouldn't you try to double your rectifier, i mean 2 5AR4 in parallel for the double current capabilities.

Bye.

Max.

[Bruce / Mission Amps]

Norm,

FWIW, I always use the max value (2.5mA) for each 12AX7 gain stage, but perhaps you're using a whole lot of them and need to be a bit more conservative, in which case 1.5mA should be fine.

The RCA RC-30 manual gives 225mA @ 450V for the 5AR4 max, and the 40W example amp for the 6CA7/EL34 indicates 143mA max-signal plate current @ 450V B+, so you should be OK with a pair of either EL34's or 6V6's.

Ray

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The only problem with using expected current draw is, its usually less then what I expected.
So, when laying out a new B+ rail and the respective dropping resistors past the screen node, I always use 1.1ma per triode now.

[Rob S]

*****************************
The only problem with using expected current draw is, its usually less then what I expected.
So, when laying out a new B+ rail and the respective dropping resistors past the screen node, I always use 1.1ma per triode now.

Me too.