Power Supply for 4 x EL84's in PPP

EDL · Post by **EDL** » Mon 04/12/10 2:57 am

Hi all, I'm after a schematic for a power supply with a bridge rectifier and without a choke for a 4 x EL84 PPP power amp and a 2 x 12AX7 pre-amp. Something similar to the attachment if possible. I've made and amp with 2 x EL84's using a similar power supply and it is very successful. I just wanted to go the whole hog.
Any ideas?
Cheers,
EDL

Post by **zaphod_phil** » Mon 04/12/10 11:48 am

If this is a "Marshall" 36W, you can follow the power supply design from any of the 36W amps in the Downloads section, but using SS diodes instead of the vacuum state rectifier. If it's some other type of 4X EL84 PP amp then it would need to be discussed elsewhere, such as PPwatt.com or AX84.com

krx · Post by **krx** » Mon 04/12/10 11:57 am

You won't have enough current for 4 EL84s with that design. A bridge rectifier cuts your current capacity in half (remember Ohm's law). You should have at least 200mA available (250 would be better), so you'd need 400mA with a bridge rectifier.

ZP -- this question is certainly relevant to all the 36W builders whether it's an exact Marshall circuit or not.

Post by **zaphod_phil** » Mon 04/12/10 12:04 pm

I'm not yet convinced that it's important to all 36W builders, when there's a good working 36W power supply design already there. If it ain't broke why fix it? Now if there are some good reasons to fix it, then they certainly would be worth discussing, hopefully with some experienced 36W builders involved....

krx · Post by **krx** » Mon 04/12/10 1:06 pm

Well, there's the GDS 36W PT. And what else? Options are good to have.

Here's a couple good ones around $60:

http://edcorusa.com/Products/ShowProduct.aspx?ID=638 (tube recto)

http://edcorusa.com/Products/ShowProduct.aspx?ID=646 (solid state)

Post by **zaphod_phil** » Mon 04/12/10 3:35 pm

OK, now we're talking about the transformers.

I agree, let's find some more options. Obviously, one could always use AC30 PTs. And I'm pretty sure Heyboer does 36W PTs other than for GDS. The prices of those Edcor ones look pretty good too.

breakfastbuddy · Post by **breakfastbuddy** » Tue 04/13/10 7:57 am

250 ma PT ? thats way to much . how much power are you planning to take out of that amp ? ,25x 325 = 81watt ,
remember the bridge the bridge also rise the voltage , even if the current capasity goes a little down

krx · Post by **krx** » Tue 04/13/10 10:14 am

breakfastbuddy wrote:250 ma PT ? thats way to much . how much power are you planning to take out of that amp ? ,25x 325 = 81watt ,
remember the bridge the bridge also rise the voltage , even if the current capasity goes a little down

No offense, but you don't know what you're talking about. The transformers I suggested were for use with a normal full-wave rectifier, not a bridge rectifier. And yes, you DO need 250mA of current capacity for 4 EL84s. Each EL84 can draw 50+mA, then you need some extra for your preamp. 200 would be okay, too. If you use a bridge rectifier you actually need to DOUBLE the current of your PT since it is only providing half the voltage (Ohm's law).

You can't simply divide your output power by the voltage to get the current. That would assume the circuit is 100% efficient, which is far from true.

57chevy · Post by **57chevy** » Tue 04/13/10 11:18 am

EDL's schematic specified a HT winding rated at 280VAC and 310ma and no CT. A bridge recto will put out 1.414 x 280 unloaded B+ and the PT can supply 310ma. That's more than enough juice for a 4XEL84 amp.

The only time you have to consider double voltage or half current is when you're comparing a FWB to a conventional FW recto and both are using the same PT.

EDL's schematic is fine for his purpose as it is.

krx · Post by **krx** » Tue 04/13/10 11:37 am

57chevy wrote:EDL's schematic specified a HT winding rated at 280VAC and 310ma and no CT. A bridge recto will put out 1.414 x 280 unloaded B+ and the PT can supply 310ma. That's more than enough juice for a 4XEL84 amp.

The only time you have to consider double voltage or half current is when you're comparing a FWB to a conventional FW recto and both are using the same PT.

EDL's schematic is fine for his purpose as it is.

Wrong. You will only get 0.62 times the given current capacity when using a full-wave bridge with capacitor input. You cannot increase voltage without decreasing current (ok, it's not exactly half in this case -- that was an approximation). Anyways, 310ma * 0.62 = 192mA. That might be okay but it's borderline unless you know your PT specs are understated.

Design guide here: http://www.hammondmfg.com/pdf/5c007.pdf

EDL · Post by **EDL** » Sat 04/24/10 1:52 am

krx wrote:
57chevy wrote:EDL's schematic specified a HT winding rated at 280VAC and 310ma and no CT. A bridge recto will put out 1.414 x 280 unloaded B+ and the PT can supply 310ma. That's more than enough juice for a 4XEL84 amp.

The only time you have to consider double voltage or half current is when you're comparing a FWB to a conventional FW recto and both are using the same PT.

EDL's schematic is fine for his purpose as it is.
Wrong. You will only get 0.62 times the given current capacity when using a full-wave bridge with capacitor input. You cannot increase voltage without decreasing current (ok, it's not exactly half in this case -- that was an approximation). Anyways, 310ma * 0.62 = 192mA. That might be okay but it's borderline unless you know your PT specs are understated.

Design guide here: http://www.hammondmfg.com/pdf/5c007.pdf

Thanks for the banter over my intended 4 x EL84 PPP PS design.

I've attached the design of the PS currently running in my 2 x EL84 Amp - it works fine. I just thought I'd throw in an extra 65mA each for the 2 extra EL84's - the pre-amp will remain the same - 1 x 12AU7 & 1 x 12AX7.

Would I be better off going for a center-tapped PT? I have my PT's custom made anyway. If so, please indicate the Secondary V and mA I should be looking to achieve for 4 x EL84 plus about 50mA for the pre-amp.

You may even like to suggest a better PS configuration for my 2 x EL84 Amp - maybe I'm doing it all wrong power supply-wise.

Thanks guys.

EDL · Post by **EDL** » Sat 04/24/10 1:55 am

57chevy wrote:EDL's schematic specified a HT winding rated at 280VAC and 310ma and no CT. A bridge recto will put out 1.414 x 280 unloaded B+ and the PT can supply 310ma. That's more than enough juice for a 4XEL84 amp.

The only time you have to consider double voltage or half current is when you're comparing a FWB to a conventional FW recto and both are using the same PT.

EDL's schematic is fine for his purpose as it is.

Hi 57chevy, please check out my last post - cheers - EDL

stevesuk · Post by **stevesuk** » Sat 04/24/10 4:04 am

I have a parts customer who intends to build an 18/36W with an EZ81 valve rectifier, switching over to diodes for 36W operation.
Now there is an interesting concept

Steve UK
www.valvepower.co.uk

Post by **rock_mumbles** » Sat 04/24/10 12:19 pm

EDL wrote:...Would I be better off going for a center-tapped PT? I have my PT's custom made anyway. If so, please indicate the Secondary V and mA I should be looking to achieve for 4 x EL84 plus about 50mA for the pre-amp.

You may even like to suggest a better PS configuration for my 2 x EL84 Amp - maybe I'm doing it all wrong power supply-wise.

Thanks guys.

You would be better off with a CT PT. A 290-0-290 volt @ 250-300mA transformer with a full wave rectifier will put 580V to the rectifier and the rectified no load voltage is 0.7 X HV so you would have about 400VDC at full current 250-300mA (good EL84 voltages for using a tube rectifier). Since you are using diodes (ss) you would probably be better off use a lower voltage transformer like the Edcor krx referred to above...

If you are using a non center-tapped PT you need 400mA... 400mA x .62 = 248mA

breakfastbuddy · Post by **breakfastbuddy** » Mon 04/26/10 3:36 pm

you power output is the same what ever you chose , goes voltage up the current will decrease , i think KRX mixes things together . every el 84 tube has a max output of 12watt , so max output is 48watt at most . with 48ma each that is 192 ma and the voltage is 250vdc this is 177vac , so thats not for real . can some one else with more knowledge pitch in here ,

krx · Post by **krx** » Mon 04/26/10 5:00 pm

breakfastbuddy wrote:you power output is the same what ever you chose , goes voltage up the current will decrease , i think KRX mixes things together . every el 84 tube has a max output of 12watt , so max output is 48watt at most . with 48ma each that is 192 ma and the voltage is 250vdc this is 177vac , so thats not for real . can some one else with more knowledge pitch in here ,

I'm not mixing anything up. Also, the 12 Watts is the dissipation of the plates. It's related to the power output but is not the same thing.

kleuck · Post by **kleuck** » Mon 04/26/10 7:39 pm

I'm not sure to understand the Hammond PDF, but i'm sure that two diodes/tapped PT or Graez's bridge are strictly equivalent electronically speaking.
They provide the same current and the same voltage (neglecting the doubled voltage drop in the diodes), so the "V (Avg) D.C. = 0.45 X Sec. V A.C." is as weird as is the "I D.C. = 0.62 X Sec. I A.C." in the PDF.

krx · Post by **krx** » Mon 04/26/10 8:57 pm

kleuck wrote:I'm not sure to understand the Hammond PDF, but i'm sure that two diodes/tapped PT or Graez's bridge are strictly equivalent electronically speaking.
They provide the same current and the same voltage (neglecting the doubled voltage drop in the diodes), so the "V (Avg) D.C. = 0.45 X Sec. V A.C." is as weird as is the "I D.C. = 0.62 X Sec. I A.C." in the PDF.

The PDF is correct.

it more carefully -- look at the labels on the diagrams.

kleuck · Post by **kleuck** » Mon 04/26/10 9:19 pm

I suppose i don't understand the PDF ; but i'm sure that a 2*280 volts / 80VA pt will give exactly the same output voltage and current than a 1*280 volts 80VA one with a bridge.

Post by **rock_mumbles** » Mon 04/26/10 9:38 pm

Assume we have a capacitor input load...
from the Hammond pdf:

FULLWAVE BRIDGE Capacitor Input Load (non-center tapped PT)
V (Peak) D.C. = 1.41 X Sec. V A.C.
V (Avg) D.C. = 0.90 X Sec. V A.C.
I D.C. = 0.62 X Sec. I A.C.

FULLWAVE Capacitor Input Load (center tapped PT)
V (Peak) D.C. = 0.71 X Sec. V A.C.
V (Avg) D.C. = 0.45 X Sec. V A.C.
I D.C. = 1.00 X Sec. I A.C.

So for a non center tapped PT with 290V and 300mA and a bridge rectifier let's do the math:
V (Peak) DC = 290V X 1.41 = ~409V DC
V (Avg) DC = 290V X 0.90 = 261V DC
I DC = 300mA X 0.62 = 186mA
(since the voltage is increased the current is decreased)

and for the center tapped 290V - 0 - 290V @ 300mA PT
V (Peak) DC = (290V X 2) X 0.71 = ~409V DC
V (Avg) DC = (290V X 2) X 0.45 = 261V DC
I DC = 300mA X 1.00 = 300mA
(since there is a voltage drop there is no current drop)

So the voltages are for all practical purposes the same, but the HV current available with the CT'd PT and full wave rectifier is 38% greater that with the non-center tapped PT and bridge rectifier.

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Power Supply for 4 x EL84's in PPP

Power Supply for 4 x EL84's in PPP

250 ma PT ?

Re: 250 ma PT ?

You're raising a few more questions for me guys . . . . .

Re: You're raising a few more questions for me guys . . . .

bridge rectifier vs full wave rectifier

Re: bridge rectifier vs full wave rectifier