How much current is enuff...

[sliberty]

On voltages, my rule of thumb is 1.4 * PT voltage, minus the rectifier loss.

Ex:

300 * 1.4 = 420 - 50 = 370

Usually puts me in the ball park. Some rectifiers lose more voltage than others. SS hardly loses anything.

[ampfab]

What type amps are you considering building?
are you cloning something?
are you going to build an amp that is similar to another classic amp?
I often look at specs and schematics of other amps and their transformers to get a good ballpark.
go to triode's website, look at the transformer specs for an amp that is similar to what you are considering building.
with a bit of studying, you'll get a good sense of what you'll need.

[rp]

On voltages, my rule of thumb is 1.4 * PT voltage, minus the rectifier loss.
Ex: 300 * 1.4 = 420 - 50 = 370
Usually puts me in the ball park. Some rectifiers lose more voltage than others. SS hardly loses anything.

Maybe. Sometimes the problem is when you need to hit an exact pV eg cloning a mid '50s 6L6G tweed. You don't want above 400V cause then you're in late '50s early '60s amps and it rules out metals and coke bottle 6L6s, but below 370V you're into the smaller 6V6 amps and who wants to run 5881s at 340V? So you really really want 380V. I've found it's impossible to control all the variables and figure it out. The above formula can easily be off by 30V in either direction IME. I do the formula but use the experience I've gained and then factor in my cosmological constant. Last few have been spot on so guess I'm getting a good feel for this stuff. For my 5C8 I just knew I needed 330Vac nothing less or more. I had to have it wound, but I got the voltage of the schematic 370V, actually I got 380V, even better. If I'd followed the above formula, which applies perfectly here, I'd have been left with like 340V real life pV - wimpy 6V6 land for a Twin. I'd have been bummed.

[randalp3000]

Ok, I think I have it figured out. Please correct me if I'm wrong.

A Rocket for example has 4 EL84's and 3 12AX7's

Add the plate and screen dissipation from the data sheet for an EL84. 12 watts plate + 2 watts screen = 14 watts total divided by the projected B+( I'll say 300 VDC).

so 14 watts/300VDC = .047amps x 4 tubes = .187amps or 187mA

12AX7 (Sylvania data) 2watts

2watts/300VDC = .007amps x 3 tubes = .02amps or 20mA

add the 2 and you get 207mA

Probably over shoot a bit to be safe.

again, If I'm way off base please let me know and I'll delete this so I don't misinform anyone.

thanks, RP

[matt h]

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