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dropping resistors?

smolder

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I'm trying to repurpose a salvage PT. Until I got it hooked up I didn't realize that it's a 375-0-375... so I'm getting about 500vdc (without a load). What I'd really like is 425-430. Before I spend a bunch of time using resistors to try and drop this voltage... am I wasting my time, or is this a margin that I can reduce with reasonable effort?

details: half wave solid state rec.. 2xEL84 and 2x12ax7.
 

Prattacaster

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half wave solid state rec
I hope you mean full wave. It is pretty hard to tell how much it will drop with a load. Could drop 50-70 volts could drop 20v. It depends on the VA rating or current rating of the HT supply.

Lets speculate and say you need to drop 50v across the resistor. The idle current should be around 80mA. So the resistor value should be around 50/ .08mA = 620ohms. The wattage it would dissipate at idle is 50* .08 = 4W. I'd probably use a chassis mount 15w or 20W and mount away from filters.

Control the sag by using a PI filter arrangment. Recto- Cap- Dropping Resistor- Cap-OT

Even if you had to drop 100v you could still find a suitable resistor. Maybe a 25w or 50w chassis mount. Like this: http://www.ebay.com/itm/Chassis-Mou...721?pt=LH_DefaultDomain_0&hash=item4ac0dfb2f9
 

smolder

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Your correct of course... it is a full wave.

Ss this project started because I thought I could build a PA20 with the parts laying around (I had to buy pots)... at this point the thing is pretty much built, but I have the wrong PT. Might just be worth buying the right 290-0-290 unit and moving on. hmmm....

thnx Pratt
 

mark norwine

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calculating at this point is meaningless.....

We don't know the VA of the transformer. If it's not too big, chances are the B+ E will plummet on it's own once you pull some I.

Before you do anything, calculate the total B+ I that your amp design will have....then pull that I with a loadbank and re-measure your B+ E.

Once you know the loaded B+ E, you can calculate how much (if any) B+ E needs to be tossed. From there you can determine the resistors to use.
 

Prattacaster

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calculating at this point is meaningless.....

We don't know the VA of the transformer. If it's not too big, chances are the B+ E will plummet on it's own once you pull some I.

Before you do anything, calculate the total B+ I that your amp design will have....then pull that I with a loadbank and re-measure your B+ E.

Once you know the loaded B+ E, you can calculate how much (if any) B+ E needs to be tossed. From there you can determine the resistors to use.
Arbitrary calculation for this thread is not meaningless. He wasn't asking for an exact number on the resistor, but rather if it is worth the time and effortin matching a unknown PT to his amp design. My example was speculation as noted but shows how one could make use of that PT with a certain amp design.

Great info tho.

I think that if I were you I'd get the proper PT, you'll have a cooler running amp and peace of mind.
 

ked

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911
I may be wrong, but I think that 425- 430 volts on the plates is going to be way high for EL84 tubes.
 

smolder

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calculating at this point is meaningless.....

We don't know the VA of the transformer. If it's not too big, chances are the B+ E will plummet on it's own once you pull some I.

Before you do anything, calculate the total B+ I that your amp design will have....then pull that I with a loadbank and re-measure your B+ E.

Once you know the loaded B+ E, you can calculate how much (if any) B+ E needs to be tossed. From there you can determine the resistors to use.
Mark, I very much appreciate you commenting. But this is a bit beyond my abilities.

The PT is from an RCA 400 projector:

http://www.acofs.org.au/part_5_files/RCA (see also Viewflex)/Tech RCA 400 (1).PDF

But I can't find the full data you're asking for.

I'm tempted to put a 22k 10 watt resistor in between the rec and the first stage of the filter capcan, put in some old cheap tubes, and see what happens. I have a 50/50 500volt cap can, but the other caps are 450 volt so I'm trying to be cautious.
 

Prattacaster

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I'm tempted to put a 22k 10 watt resistor in between the rec and the first stage of the filter capcan,
Jeesh, Go Big or Go Home.

Don't do it. A 22k resistor @ 80mA idle current will dissipate about 140w and blow up. Try a 1K or less.
 

smolder

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I may be wrong, but I think that 425- 430 volts on the plates is going to be way high for EL84 tubes.
The PA20 starts with 390dvc in the filter section... but that's at 110ac... I was calculating some margin for the 120ac that come out of my walls.
 

smolder

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Jeesh, Go Big or Go Home.

Don't do it. A 22k resistor @ 80mA idle current will dissipate about 140w and blow up. Try a 1K or less.
Thanks for the heads up. I'll see watt I have under 1K but that is rated for 10 watts.
 

EFK

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1,038
Best option - get the right transformer.

2nd choice, for a make-do parts amp: go choke input. It will run cooler than a resistor, drop the voltage (depending on choke chosen) and the voltage on the output side of the choke will not fluctuate nearly as much as a resistor between the power supply and plates. If you're not after excessive sag, look for something that can handle around 150 mA, which will be oversized for a 2XEL84 amp. You can find big chokes on ebay for real cheap $$$, pulled from organ, pa or projector amps.

Also - since you're trying to drop voltage, use a 5Y3 or a 5R4 rectifier if you have a 5V winding on the PT.
 

smolder

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@ELK... yah... it does have 5 volt, but when you see below, there isn't room with the cap cans, and I want a true PA20 tone. I thought I had a 5 henry choke laying around, but can't seem to find it. Must have used it.

I disconnected the (weird) series 30 ohm R setting under/over the PT and put a 1K 6.5 watt (pretty much all I had that was close) in between the rec and the first filter stage. I also dropped the ac by 12 volts... ended up getting 350dvc once it settled down. I neglected to check the AC from the PT with the wall voltage dropped. The amp works and sounds great... but I could tell the resistor was getting hot so I shut it down.

Here's what I'm doing:





It's a left over hammond AO-35 chassis. I'm using the 50/50 capcan and two stages of the original cap can. Dual pots for the two channels. A single input... the first switch near the input jack is hi/lo (1Mr), the second is jumping the two channels, and the third is lead bass. The only thing it won't be able to alter tone and volume between the channels... but I only have so much room on the front (and thought my solution was kind of clever).
 

MikeMurray

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Gotta agree - dealt with trying to drop B+, it never really works nicely. Killed a MM choke trying to use it as an input - not sure what the deal is, but you need to get a specific input choke for that if you want.

If you want to drop volts, go 5Y3 or EZ81 paired with an extra cap and resistor in front to drop volts - something like a 16uF and a high wattage 470ohm resistor might make things useable. You could also try the old zener trick, but for some reason I could never get that working right.
 

Prattacaster

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Real estate is tight there. A chassis mount resistor on the side wall above the PT would be the best spot/option, if you dont mind screw heads on the outside of the amp over there.

If it is cathode bias you are going to want the B+ at 350 max, even lower would be preferable.

but I could tell the resistor was getting hot so I shut it down.
It will get hot no matter how big it is, measure the voltage across the resistor next time and calculate disspation. V/R=I once you have I or current then multiply current by the voltage it dropped. That will give you idle dissipation. Remember dissipation will go up when you play. You have room for a 50w resistor on the side wall, that will keep things cooler.

I think even a 290-0-290 pt with a SS recto would give you too much voltage right? 290*1.41=408vdc.

If you want 350vdc then 350/1.41= 248. You'd want 250-0-250@ about 120mA

After seeing your project I'm still thinking different PT, but I'd probably explore the resistor option as well, can't blame you for wanting to hear it.

I also dropped the ac by 12 volts
Remember the filament voltage should be at least 6.00volts.
 

smolder

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Real estate is tight there.
Yah... I had a buddy who was recently out of work make me a handful of head cabs for these. The lead dressing is critical because of the small box... but this is a pretty good learning ground for efficiency and keeping the noise floor low.

I think even a 290-0-290 pt with a SS recto would give you too much voltage right? 290*1.41=408vdc. If you want 350vdc then 350/1.41= 248. You'd want 250-0-250@ about 120mA
I wondered about that as well. What I'm really after with this amp is as close to a true PA20 as I can get. The spec is 290-0-290.

Remember the filament voltage should be at least 6.00volts.
I was using the 'Amp Preserver' in -12 volt mode... mostly as a safety precaution.

I think I'll order the right PT and start there working from the original schematic. I'll set this 375-0-375 on the shelf for the right project down the road.

Thanks for all the help guys... I learned a lot from this exercise.
 

Kyle B

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Jeesh, Go Big or Go Home.

Don't do it. A 22k resistor @ 80mA idle current will dissipate about 140w and blow up. Try a 1K or less.
No it won't. Adding a 22k resistor in series to the PT outputs will end up limiting the current to something far far less than 80mA. With 400V coming out, a worst case situation would be 22k right across the PT, and that would only be 400/22k=18mA

Which ends up only 7.2W if my math is correct......


Either that or I'm extremely confused about how electricity works.
 

Prattacaster

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Adding a 22k resistor in series to the PT outputs will end up limiting the current to something far far less than 80mA.
The resistor is being inserted after the recto. Your point is taken, without a change in the cathode resistor the current will be limited and so dissipation would not be that high. 22k is excessive tho.

Mosfet circuit would be great, if you had the space.
 

donnyjaguar

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4,201
I agree on the choke input filter. The choke itself stores energy and gives a nice smooth and stable output. I don't generally consider voltage sag as being a worthy attribute to any decent amplifier.
 




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