understanding the mA on my PT HT center tap

[Curranm]

OK. I have been cobbling neat-o amps together for a few years. lots of success. BUT a lot of things still elude me. what does the mA reading from a PT HT do for me. and what does it relate to in tube usages/how many tubes I can use with a certain PT? I have been looking at heater mA to determine how many 12AX7's I can use AND what type of power tubes the PT can supply. SO example. my favorite PT has 345-0-345 off of the HT @150mA. the heaters give me 6.3v@5A. Soooooooooooooo can I use (2)KT88's(1.6ma each for the heaters) and (5)12AX7's(.300mA each) for a total heater combination of 4.7A's............ AND IF SO how can I learn to calculate what is good/safe for a PT to handle?

[Curranm]

I have asked this question in another thread BTU I am hoping for clarification on the mA for the center tap

[PRR]

> (2)KT88's(1.6ma each for the heaters) and (5)12AX7's(.300mA each) for a total heater combination of 4.7A's

Both numbers are written wrong. 1.6 AMPS. 0.3 Amps or 300 mA. However you got the answer right.

The hi-volt plates also suck current. It can be figured. For most sane amplifiers, it is easier to look in the datasheet. Find a configuration which is *similar* to yours. Lets say two KT88 in push-pull aiming for large output. Over 500V supply to get close to 100 Watts output.
http://www.mif.pg.gda.pl/homepages/frank/sheets/086/k/KT88.pdf

It sucks 320mA at full roar.

[jjasilli]

Ditto.  For HT current, preamp tubes can be disregarded. Because they draw only about 1 or 2mA ea. their total current draw is insignificant.  Look @ the at the current draw for screen(s) + plate(s) for all the power tubes, under max signal conditions.  This info is on the tube charts, fort various ways of using the power tube(s). 


Note that for musical instrument amps, signal tends to come in pulses or beats.  As a practical matter, max signal condition is not continuous for 100% of the time.  Hence there is little to no benefit to "over-spec" PT's for current handling. Quality PT's can be under-spec'd up to 50%.  This can cause voltage sag.  So choice of PT can design for sag if you want it.

[kagliostro]

Some time ago there was a thread discussing about PT and available current ....

a rule of thumb may be to consider the available DC current around 60% of the disposable AC current (when there is a capacitor input filter)

Franco