voltage drop with regard to rectifier tubes?

[ALBATROS1234]

i was wondering as i learn about more different tube options some things are obvious others just slightly elusive. i found a chart on this site that compares rectifiers and all of their max plate voltage and ma supplied etc. voltage drop? some tubes say around 20v others 50v but then it mentions increse by 1.1 or 1.2 which i get and see the result of with my meter. what exactly is voltage drop with regard to a rectifier tube and what does it mean for circuit design choices. ie do some circuits or tube types prefer different voltage drops value  as each tube type has a unique output impedance ,grid impedance etc. i am just a bit foggy on this concept and what it means to me an amp builder?

[PRR]

There's no magic. It's just a loss you have to account for.

Your boss, a fussy eater, demands to be served exactly 1 pound of potato. If you buy the coarse thick-skinned potatoes you expect a 0.3 pound loss in peeling and bruise-trimming; the fine thin-skin spuds may only have 0.1 pound of peel and bruise. So to end up with 1 pound exact you have to start from 1.3 pound or 1.1 pound. Likewise when you *design* a power supply, and know your DC voltage, you have to allow for rectifier loss to pick your AC voltage.

All these rectifier losses vary with everything. In an exact "design" what usually happens is we prototype with any convenient PT, see where it comes out, and then custom-order the production PT accordingly. But as I understand _your_ Purchasing Policy, that won't happen. (You don't buy much new and certainly not custom-order.) So the loss-factors are more a guide of what to expect from available parts.

[jjasilli]

Short Answer:  After SS rectification, the output DC voltage will be 1.414 times the AC voltage put into the SS rectifier.  Rectifier Tubes have internal impedance.  Hence, they cause a voltage drop -- the DC voltage will be less than the 1.414X it "should be".  Different types of Rectifier Tubes have different impedances which are usually stated in their tube specs.  It's handy to have a chart showing their important characteristics.


Long Answer:  The AC power voltage is (ideally) a sine wave.  AC voltage can be stated in terms of peak-to-peak; peak; or RMS.  A voltage meter can only read RMS. (An oscilloscope can read all 3).  The first thing a voltage meter does with it's incoming voltage from the probe is to use a diode to lop off the bottom half of the AC wave.  Now all that's left to measure is the peak (not peak-to-peak) AC voltage.  But the meter isn't "fast" enough to fully measure the AC voltage which changes rapidly from -0- to Peak @ 60X per second.  The meter can only read & display about 70% of the Peak AC voltage.  This is the RMS voltage. 

It's the Peak AC voltage that's rectified in your amp.  Now that the voltage is converted to DC, the volt meter can read all of it.  It appears that a voltage boost of 1.414X has magically occurred.  But this is not magic, just an inherent shortcoming of the voltage meter.  The magic would vanish if a 'scope were used to read the AC & DC voltages.

For SS rectification ea diode will drop about 1 volt.  But this is so insignificant that it's ignored. 

[ALBATROS1234]

1st ,thank you gentlemen for taking time to respond. i am an electrician and used to be an electronics tech 20years combined experience. i understand the ac theory and the fact that a bridge rectifier gives you at output dc of approx 1.41 x ac rms. it seems from what i have read and observed/measured on my workdesk is a tube recti gives you approx 1.2 to 1.25 x input ac rms . so if i get this right this is the loss of what they speak of and one tube has a higher drop than another so the one with the higher drop is somehow less effcient in its power handling capabilities and the "voltage drop" is the amount the drop the voltage will be in relation to the quite efficient ss recti which gives ideally 1.41 x. that being said you want a tube with the least voltage drop in order to make the most of your tranny. unless you are seeking a saggy amp in which case you would want larger voltage drop.

 i knew basically how rectifiers in general work in relation to ss vs tube as well as how filter caps fill in the ripples in but for whatever reason when i looked at it the other day i was thinking if the observed meter reading is going up regardless why is there a voltage drop? and is that in relation to what exactly. thanks again for the layman explanations.

and yes PRR  i am a broke bastard  so i look for deals on used tube gear and get lots of spare parts and convert this from virtual junk but its also fun to learn about the items i am repurposing . there is a hit and miss factor and you never know what you are gonna get but thats kinda part of the fun to me. i assess what i have and build what seems fun from the ingredients in the pantry. 

[pompeiisneaks]

As was mentioned each rectifier varies in and of themselves AND each type has a different drop, look at this thread:

https://el34world.com/Forum/index.php?topic=20455.0

The second post is a link to a summary of different types and their voltage drop and approximate voltage increase.

~Phil

[Willabe]

....it seems from what i have read and observed/measured on my work desk is a tube recti gives you approx 1.2 to 1.25 x input ac rms . so if i get this right this is the loss of what they speak of and one tube has a higher drop than another so the one with the higher drop is somehow less effcient in its power handling capabilities and the "voltage drop" is the amount the drop the voltage.....

If you compare some of the rectifier tubes specs in the link pompeiisneaks posted you will see that they go from 1.1 to 1.3.

And the current a rectifier tube can handle does not always follow how much voltage drop it has.

[tubeswell]

Apart from different rectifier tubes having different plate resistances, the amount of forward voltage drop a rectifier tube produces is also related to the load on the power rail. If its working hard to pull more electrons out of the positive pole(s) of the filter caps - because there is a lot of current demanded, or because there is a lot of capacitance to charge up (as well as a lot of current being drawn off in the circuit) - the tube rectifier forward voltage drop will increase.