Couple question on power rail voltages

[dude]

Been reading and experimenting on the power rail, just want to confirm my thinking is correct.


If B+, highest voltage in an amp from the rectifier is say 400vdc, the OT center tap to this point can produce different voltage at the power tubes plates, usually a few volts less than the B+ but some OT (I assume depending on their windings) can cut more voltage to the plates, like maybe up 10v or 15v. Is this correct?


Second, with 6V6s or any other tube it's a good idea to have the screens voltage a little less than that tube's plate voltage, maybe 3v to 10v?


So, if one has an OT that drops say 15 volts, the power rail resistors may have to be adjusted to keep the screens less than the plates? Keep that 3 to 10 volt range?


And then adjust down the rail to PI and Preamp according to what the builder prefers.


I'm asking these questions as I have a 1K Resistor after the B+ (usually where a choke would go) 400V B+, after 1K drops to 393v which goes to the power tubes screens, but the OT gives the tube plates less than that 393v, like 390v, so my screens are getting more voltage than the plates...? Not good, correct? My OT is dropping about 10v or so volts.


So, I assume I would need to raise that 1K to say 1.5K to keep the screens voltage slightly lower than the plates. Correct?


I'm also assuming nothing is wrong with the OT, just different OT windings can cause more or less voltage drop..?


Thanks in advance,
al
 


 

[shooter]

just want to confirm my thinking is correct.

I'm playing in the woods n painting on the beach so I'll agree easily.  I've found the G2 volts at idle may be a couple volts eitherside of plate, I try and make sure based on that voltage I don't hit Max I for G2 by using a screen resistor, other than that I don't get to excited that at idle it's some value less than plate.

[Tony Bones]

1. Don't obsess about a few volts one way or the other. It's like #85 on the list of things to worry about. Especially at this point in your budding career as a boutique amp designer.

2. Nothing interesting happens when the screen potential goes a few volts above or below the plate voltage at idle. As soon as you start playing music the plate will start swinging a lot more than a few volts above and below the screen. Whether the screen is at 393V or 403V while all of this is happening makes virtually no difference at all.

If you're just starting out then I suggest you copy a proven design, build with the iron you have on hand, and let the volts fall where they may. Once it's up and running you can experiment with replacing that 1k resistor with something else and see what happens.

[PRR]

> cut more voltage to the plates, like maybe up 10v or 15v. Is this correct?

Yeah, so??

> it's a good idea to have the screens voltage a little less than that tube's plate voltage, maybe 3v to 10v?

NO!!

What happens when you PLAY? {As Tony says...} Plate voltage goes up and DOWN. To like 300V -below- Vg2. Does anything blow up? NO. We have Pentodes, not True Tetrodes. ("Beam Tetrode" is a pentode in practice though was a "funny tetrode" on the patent sheet.)

We usually pick power tubes which will work-out with Gg2 "near" Vp. G2 needs more filtering than Plate, so we often (not always) find a few V drop in filtering; but if not, no worries.

> raise that 1K to say 1.5K

No.

In other electronics it becomes practical to use huge load impedance, high Vp, and then a low Vg2 to reduce G1 drive demands. This does add the complication of a lower G2 supply with wildly varying G2 current, so we audio dudes usually do not do this. (But see 6550/KT88 amps with 600Vp and 300Vg2.)

[drgonzonm]

Been reading and experimenting on the power rail, just want to confirm my thinking is correct.


If B+, highest voltage in an amp from the rectifier is say 400vdc, the OT center tap to this point can produce different voltage at the power tubes plates, usually a few volts less than the B+ but some OT (I assume depending on their windings) can cut more voltage to the plates, like maybe up 10v or 15v. Is this correct?


Second, with 6V6s or any other tube it's a good idea to have the screens voltage a little less than that tube's plate voltage, maybe 3v to 10v?


So, if one has an OT that drops say 15 volts, the power rail resistors may have to be adjusted to keep the screens less than the plates? Keep that 3 to 10 volt range?


And then adjust down the rail to PI and Preamp according to what the builder prefers.


I'm asking these questions as I have a 1K Resistor after the B+ (usually where a choke would go) 400V B+, after 1K drops to 393v which goes to the power tubes screens, but the OT gives the tube plates less than that 393v, like 390v, so my screens are getting more voltage than the plates...? Not good, correct? My OT is dropping about 10v or so volts.


So, I assume I would need to raise that 1K to say 1.5K to keep the screens voltage slightly lower than the plates. Correct?


I'm also assuming nothing is wrong with the OT, just different OT windings can cause more or less voltage drop..?


Thanks in advance,
al
 


 

1.  OT Windings drop voltage to plates on power tubes (summarized).  Two factors, here, the DCR (Direct current resistance) and L value of OT, typically found on transformer spec sheets.  In theory the L (stated in Henrys) is independent of frequency.  But the L value affects the impedance of the OT because impedance is frequency related. (One of the reasons, typical voltage readings are stated at 1kHz, other amps, may be stated at 400Hz)  (The Henry value is stated usually stated at a low frequency, and low voltage relative to that which would power the power amp.  )
    a.  look at UL OT' applications (ultralinear) the plate has lower voltage than the screen. 
Look at UL OT's, the screen voltage is typically higher than the plate. 
2. Adjust the B+ rail voltages for PI and preamp.  No law states that B rail voltages to the PI and preamp need be lower than Power tubes.  From a practical point, it just works out because of filtering to reduce ripples.  At least one circuit (DC powered), was posted on this forum where the preamp had higher voltages than power amp.

[jjasilli]

Dude, you have asked more than one question:  (i) re B+ voltage drop through the OT windings; and (ii) screen voltage vs. plate voltage.  (i) has already been fully addressed.  Tony Bones has addressed (ii). 


Screen voltage vs. plate voltage has been addressed in detail over the years, with a number of very recent posts now on the Board.  Typically, the screen and plates are drawing B+ in parallel from the same source, with the screens separated form the plate supply by a dropping resistor.  In this circuit, at idle the screens will have a voltage near the plate voltage, maybe even a few volts higher.  Why? Ohms Law.  At idle the plates are drawing a fair amount of current, but the screens draw nearly -0- current.  Hence the screen voltage drop is near -0-; i.e., it stays at the supply voltage But the plate voltage drop, at idle, will be 10 -15% of the supply voltage.  So, this can result in the screen voltage exceeding the plate voltage at idle.  The screen B+ dropping resistor and stopper resistors (if any) have no effect at idle, because if current is not drawn then there's no voltage drop across these resistors to the screens.  Under signal conditions the screens do draw current which now causes a voltage drop.  Under signal conditions the screens will be at a proper lower voltage than the plates.  Screen stopper resistors are used to limit screen current especially in case of overdrive, to save the delicate screens from damage.

To get lower screen voltage at idle, some other type of circuit must be used for the screen supply.  E.g.'s: separate PT or PT winding; UL; voltage divider; zeners or mosfet.