max acceptable dropping resistance in a power supply?

[ALBATROS1234]

a week or so ago i posted about getting my hands on some rca 6973 types and wanted to do a model 24 as per sluckeys version. my on hand transformer was too high voltage wise so, i ordered a vintage hammond in great shape with a 315-0-315(advertised spec) hv winding. when it got here testing revealed it is actually 340-0-340 which when i knocked up a quick ps as per the schematics and hooked it up was aproaching 500vdc at the 1st node. sluckeys node 1 ps voltage is stated to be 350vdc. i have been trying to see what it will take to add a dropping resistor in series with the power feed and found that even 220k 3watt only gets me just above 410vdc. this is of course no load whatsoever just a transformer into a 5y3 thru a 220k followed by a standard 3cap/2resistor 3 node ps as per the schematic. i assume that with a load it will pull it down some. how much resistance can i add to the head of a ps to drop overall voltage?

also sluckey are your voltage readings stated on your schematic with the 305-0-305 classictone or the 320-0-320 hammond? also are these readings under load of the amp circuit? i strongly suspect they are so is 412vdc at node 1 acceptable unloaded with the 220k dropping resistor leading the way?

i am a newbie to tubes and designing/building amps but not new to electronic repair and electricity i know theorhetically the 6973 can handle 440max as per the rca receiving tube manual but i would surmise this could and would change the tone and whole dynamic of the amp with the voltage higher than expected let alone having to adjust further down so as to not cook the 12ax7s. i really hate to get yet another transformer , how can i make this work? or in fact is 412vdc unloaded going to be close enough to 350vdc loaded assuming that that is what sluckeys schematic is requiring which i am fairly sure it is. thank you guys i hope i am not being to much of a nuisance . i really need the guidance of those with real world hands on experience rather than reading what should be in an ideal world as per my school books.

[jjasilli]

The best thing to do is to read some reference books on tube amps.


315-0-315 is incomplete without stating current draw.  IOW, the complete spec is 315-0-315 @ xxx mA.  If your circuit draw is less than spec mA, the AC secondary voltage will be higher, than spec and vice-versa.  Unloaded, or a -0- mA, expect the AC secondary voltage to be 10 - 15% higher than spec.  "Electric Guitar Amplifier Handbook", Jack Daar.  Free web download.  Loaded means with tubes In.  You should clearly state if you are measuring loaded or unloaded voltages.  The tube specs will tell you the draw of the tubes you intend to use, to closely estimate loaded voltage per tranny specs.

You can drop up to  50VDC with a dropping resistor in the B+ line.  "Inside Tube Amps", Dan Torres.


W = V x I.  A 50V drop X say 100mA = .1A = 5W across the resistor.  A 10W resistor should be used.  3W seems low.

[sluckey]

You must have a load on the power supply to evaluate voltages. Build the amp. Those cathode biased 6973s will pull the B+ down quite a bit. Once built you can get a meaningful assessment of B+ voltages.

For example, I used the same PT in my cathode biased Supro S-6424 and my fixed bias Plexi6V6. Supro B+ is 350V. Plexi6V6 B+ is 400V.

[ALBATROS1234]

thanks for the replies....sorry, the ma of the xfrmer is160ma for the hv winding. so this is sufficient for the amp i am building in fact 10ma more than the amp requires on paper.

 i did mention in my post that this was just a power supply without a load .just the transformer feeding a 5y3 into a ps with 3 nodes(3 caps 2 resistors) .i also stated that i was sure that this was running higher than it would with a circuit load(thats kinda what i was asking, i guess poorly).i also know that loaded means "tubes in" and that the tube spec mentions required current draw i am not that much of an idiot. i did mention the rca recieving manual above so i saw the tube requirements.

 the 10 to 15% unloaded and approx 50volts max drop answers are in the ballpark of what i was looking for.therefore... 500vdc x 15%=75volts so that tells me it would be approx 425 loaded. this is with no dropping resistor. when i put the 220k after the 5y3 output it got me to 412vdc which x15%=62volts which makes it right at approx 350 loaded.as per what sluckey stated(thanks sir)

so the math actually works with a 220k(i can get a 10watt) for getting my voltage to where i need it for the circuit .....but i would have to drop 62volts.is that ok or "close enough" to 50volts. to me it seems so. my question as clearly stated as possible is......

 " is 50vdc as per the book the absolute max(is 62vdc out of the question) or should i get a smaller resistor for instance a 200k 10watt  and live with a somewhat(12 to 15vdc) higher than expected voltage output?"

so far i have built 5 small single ended amps only 1 used a tube rectifier the other 4 used a bridge and were no particular amp design using control xfrmr with separate filament xfmr on the 230vac leg the voltage wasnt too high at all for anything i had done.the most recent build with tube recti was also intended to be a clone of a supro spectator. the transformer i had was also too high so i adapted the circuit for a 6L6/5881 rather than the 6v6. ultimately i ended up with a much different amp although i am happy with the result. this one i would like to make it as close to stock as reasonably possible.

i supposed another option would to be to build this 6424 with a bridge as i have done in the past. this will get my voltages fine(albeit a tad lower even) but i am sure it will change the character of the amp. in my limited experience the amps with bridge recti were far punchier and crisp. and it seems sometimes a lower voltage can make an amp sound better as on one built where i used the 208vac tap instead of the 230vac to give me around 293vdc. i also noticed that preamp triodes(6SL7 and 12AX7) tubes sound really good at lower voltages than i expected. i found plate voltages between 80vdc to 125vdc sound better to my ear than higher around 150 to 175ish. but that may be personal preference. thank you for the help

[sluckey]

All these numbers are meaningless. They will scatter like leaves in the wind when you actually build the amp. Only then will you have numbers that actually have meaningful value.

It's insane to think you can use a 220K resistor right after the 5Y3 to drop B+ to a usable value. Those 6973s will want to draw 60mA or more to give you the power they are capable of producing. 60mA through a 220K resistor will drop 13,200V!!! See what I mean?

Just build the amp with no resistor between the rectifier and the plate node. Your PT is close enough. Then if you need to drop a few volts at least you'll have some realistic numbers to work with.

[ALBATROS1234]

ok man,i just want to get this one right. i wanted to check with you guys when i got the xfrmr which was supposed to be 315 reading 340 then after rectification its even further off...... i truly do appreciate your input sluckey you have always given it to me straight. and believe it or not i dont come running here to ask questions as soon as something isnt perfect. and i do have and have read the literature (not just my electronics books but i have downloaded at least half dozen tube amp books in the last year) but sometimes the answers are not readily appaarent in a book and its sometimes its better to ask the pros and get a direct andswer to a direct question rather than filter through hundreds of pages and multiple books to get a vague answer that many times doesnt help. thats why theres a teacher at school. they dont pass out algebra books and expect you to just flip thru it and do algebra. plus math cant stop your heart. thanks again sir.

[PRR]

> when i put the 220k

If this is a 150mA amplifier.....

150mA times 220K is a 33,000 Volt drop. Which is nonsense in a ~~400V supply.

I can't follow your narrative. Say you have 450V which drops to 400V with 220K dropper. 50V/220K is a 0.23mA load. Not 150mA as you plan. Or worked another way, 450V to 400 drop with 220K tells me your *meter* is about 2Megs, which is a very typical value for a DMM.

Nevermind amplifier theory. You want basic DC Circuits. Resistor networks. NEATS!

[John]

max acceptable dropping resistance in a power supply?


Answering only that question, the max is what it takes to get to your desired voltage. Within reason of course: you don't start with a PT that gives you 800 vdc and use resistors to drop 400 volts. You start with the right PT. But anyway... in one amp, I have an 82K resistor between node C (that feeds the PI) and node D (feeding a 5879 because I wanted to drop the voltage that much. That little tube don't draw much current, so I needed that big of a resistor. Then, between node C and node E I have a 22K because I wanted a higher voltage on my 6SL7. Between node A & B I have a 1K 5W that only drops 15 volts, and it gets right warm. It's feeding the power tube screens, which are higher current.

[plumcrazyfx]

This is good info.  I just assumed you wanted your ma rating higher than what you expected from the tubes - the bigger the better, right?  But this back and forth has given some really good information as to why I just don't want to do an early Pete Traynor and over-spec everything.  Or at least not if I want the plate voltages to be anywhere close to what the spec sheet says.  I now know there is Kentucky windage in selecting a power transformer.

[ginger]

I wonder ... why aren't more amp builders dead ?

[sluckey]

All that deadly talk is just a myth that we perpetuate to scare the faint hearted and make us feel more manly.

[jjasilli]

I wonder ... why aren't more amp builders dead ?

  All the great ones already are! (Though from old age.) We're just copy cats.

[92Volts]

Ohm's law: V = I * R

No matter how big the resistance, it will drop no voltage if there is no current flowing through it!

That dropping resistor is too large to pull much current through (as PRR points out). Ohm's law is the reason for that, and also the reason the resistor has little/no effect in a power supply which is not actively supplying power to tubes.

Without adding any power supply resistance, the transformer will give less than 340v when loaded. The voltage spec is when it's loaded. Unloaded voltage rise is "allowed" to be as high as 15% for some transformers, but is less for larger/better ones.

340v is only 8% higher than 315v. That sounds about right-- as you approach the transformer's capacity I think it will drop very close to 315v.

[ginger]

I was working on a VibroChamp  a few years ago...I was just doing something " quick"... didn't discharge the filter caps... next thing i knew , i was lying on the floor , with the amp on top of me... was as if someone had hit me in the chest with a baseball bat.... I probably discharge filter caps more than any "living" person now....doesn't pertain to this topic.. just a " lightning "memory

[shooter]

the way is get started, and close;
I know all the PA tube parameters I want, like 100mA @ 400 vdc plate.
ohms says R = 400 / .1 = 4000 ohms
ohms says W = 400 * .1 = 40W, so I buy a 5k 50w R and use as a "substitute" for my PA tube(s)
fire up the PS with the DL R, measure how close I got, I expect a little high because it's loaded less with my 5K vs the calc'd 4K

Doing this does 2 things, avoids tube fail because I wired things wrong, or validates that my correctly wired PS is close to the paper design.

next thing i knew

did something similar with the B+ for a 5Kw transmitter, 3.5Kvdc, came to with tirets (?)sp
the nuero guys checked me out and it went away after an hour or so, I went home! 

[92Volts]

I was working on a VibroChamp  a few years ago...I was just doing something " quick"... didn't discharge the filter caps... next thing i knew , i was lying on the floor , with the amp on top of me... was as if someone had hit me in the chest with a baseball bat.... I probably discharge filter caps more than any "living" person now....doesn't pertain to this topic.. just a " lightning "memory

I've learned to religiously follow the "1 hand behind your back" rule, until double-checking caps at least. I've gotten zapped twice, by live circuits but I know the same voltage could be present in anything I haven't double-checked!

Both hurt like hell but only traveled from my fingers to the palm of the same hand, and neither felt like a near-death experience. Still a very painful and effective reminder of why I was only working with 1 hand, and how much worse it could have been!

[ginger]

Yeah... i didn't fall out of my chair.. i was knocked to the floor... I've been shocked , we all have... but THIS was another thing .

[ALBATROS1234]

Great stuff guys. I actually calculated the resistance I would need to drop the voltage I wanted but the problem is I had no load on anything.i just kept putting in larger resistors and expecting a larger voltage change when I got to 220k it seemed I was getting somewhere so I posted to see if it was as wrong as it seemed to be going.repairing broken electronic  things is much different from building something from scratch. Repairing was much simpler like uh why is this not doing that. Many times it was obvious and easy. Balancing a tube circuit is something I have only been playing with for a year. And half that was reading and dreaming .I am really enjoying it though.

My plan is as sluckey suggests build the amp turn it on and see where I am with the voltage then figure if and what I need in the dropping resistor dept. I guess I was just eager to start and knocked up the ps to see if the 340 was workable she. The circuit requests 305 to 320ct. Perhaps under load the pt will drop to 315 and all will be grand.