Hoffman Plexi 100W Power Supply Diagram

[basschops1528]

Hey guys, me again.

I apologize for posting so rapidly after my first lengthy post but I just have important questions that need to be answered by experts.

Looking at the Hoffman PS diagram (attached below), there's two things I'd like some advice on.

First, there's only one fuse and I'm worried that if something happens, the bottom wire (blue on PT secondaries) will still conduct through the bridge and back through the center tap. I propose adding an additional fuse just opposite the first one at the top, and need feedback on that. (see bottom)

Also, I've been doing a lot of reading about inrush current limiting and getting soft starts. Does this type of setup, right off the bat, seem like inrush would be an issue?
If so, I thought about NTC thermistors but heard they can blow up. I've proposed adding resistors in parallel with the standby switch that can be shorted like asap after the amp is turned on. Good or bad idea? (see bottom)

As a side note, once I know what my PT puts out for the HT and CT (Gray, Blue and Yellow wires on the diagram), how can I calculate what the voltages will be after the bridge, the first caps, second, third, etc. Would be good to know right? I want to make sure that I get the proper voltage rated caps, bridge, and resistors (watts). I'm unsure of the center tap use here as well.

Thanks again guys as I really appreciate the help.

[EL34]

Please stop reporting your post
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You are posting this is schematics
please post in the board that I move this post to

[shooter]

I believe if the bridge fuse goes, the DC side is fine, break a dc circuit anywhere and you get no volts - after the break, your bridge will still have dc, if the fuse did it's job, but won't hurt having a 2nd fuse.   look at a Marshall 100W schematic that has voltage readings and that should give you a pretty good idea of what to expect at each tap.  Each tap will change with the load you have applied.

[sluckey]

If you want two fuses then a better way would be to put them in line with the DPDT STBY switch.

[basschops1528]

Thanks guys,

I think I have a better idea now.

How about those soft start resistors?

[shooter]

soft start resistors

I've used a variation of that PS in 2 builds without them, without issue. build it without, gator-clip them in and out and see what you think, fwiw

[basschops1528]

Sounds like a good idea.

I had another idea of building the power supply as-is with all the caps, resistors, fuses, and bridge. Would it be safe to fire it up and take measurements or do I need some sort of load? I'm hoping to not need one because my knowledge on getting the proper load will be little to none.

[sluckey]

You don't need a load to test that the PS works. But without a load all the voltages will read high and they will be about the same at every B+ node.

I have three 1.5K 25W resistors connected in series that I use to put a load on a B+ line. I use it to put a load on the PT, rectifier, and filter caps in old amps (such as Hammond organ amps) that I'm messing with. A 4500Ω resistor on a 450V power supply will put a 100mA load on the PT.

On a new build with new components I will fire the PS up without a load just to check for any surprises.

[basschops1528]

So if I use the three 1.5k 25W resistors in series will they be able to handle the power put out by 450 volts? I calculated 1.5k x 3 =6k ohms---> 450V/ 6k ohms= 0.075 A x 450V = 33.75 W and if i used one 4500 ohm resistor,  450V/ 4500ohms= 0.1A x 450V= 45W   Sounds like a gamble to me. I want to add that I'm very careful to have my information in proper order before I do anything because I'm always scared of breaking stuff or getting hurt. Forgive me if I ask too many questions. 

[sluckey]

I calculated 1.5k x 3 =6k ohms

Better check the batteries in your calculator!   

Three 25W resistors are capable of handling 75W. 450x450/4500=45W. So there is plenty of safety margin. They do get hot, but hey, that's the point. Just used these a couple weeks ago to test the PS in a Hammond AO-63 amp.

I only mentioned 3 x 1.5K resistors because I had them. I have the choice of a 1.5K, 3K, or 4.5K load, depending on where I connect my gator clips. I can also put a 100mA load on a 300V PS. I have plenty of other size power resistors also but the 3 x 1.5K gets used mostly.

The resistors you use may be totally different. Just don't try to pull 200mA out of a PT that is rated for 100mA. Once again, you don't need a load just to see if the PS puts out voltage.

[basschops1528]

Oops! My math was bad haha. So you are right. I feel like I'm getting something mix up though. Whether or not resistors are in series or parallel they share the power dissipation, it's just a matter of how much they dissipate depending on the setup?  I always thought you used resistors in parallel to share the power.
ex.  Two 200ohm resistors 1/2 watt in parallel would equate to 100ohm 1 watt, while two 200 ohm resistors in series would be 400 ohm still 1/2 watt....but does that matter... at that point the ohms are doubled anyway and half the current of the first setup will flow... so... six-or-one-half?

[shooter]

Two 200ohm resistors 1/2 watt in parallel would equate to 100ohm 1 watt, while two 200 ohm resistors in series would be 400 ohm still 1/2 watt

That's correct, but I think the point here is finding a suitable "load" that can limit current to the suggested 100mA and handle the power that 100ma and 450vdc will produce.  Like Sluckey said, you can "get there" many different ways.  The 1st way is know EXACTLY what you need in the 1st place.

[basschops1528]

Thanks a lot guys, I appreciate all the help.

[terminalgs]

Your fuses will open if there is a short in your circuit and protect the rectifier and PT,  but if there is a problem with your rectifier, nothing will protect your PT.   You can move the two fuses to the AC windings of the PT, between the PT and the rectifier and protect everything.


Also, the second fuse on the ground side isn't needed. if there is a short, or heavy draw somewhere to the left in the circuit, as soon as one fuse opens, the current draw will no longer draw. 


As for the resistor across the standby switch,  I think a big resistor like 100K 2W will do the trick.

[sluckey]

Two 200ohm resistors 1/2 watt in parallel would equate to 100ohm 1 watt, while two 200 ohm resistors in series would be 400 ohm still 1/2 watt

That's half right. Two resistors in series would be 400Ω and 1W. Each resistor would still be rated for 1/2 watt but if you pump enough current thru them to dissipate 1/2w in one resistor, the other will be dissipating 1/2w also. Total dissipation would be the sum of the individual dissipations.

[basschops1528]

So guys, my project is coming along great, but I have a few more questions before I start really soldering everything.

 1. Should I use just one or both soft start resistors where they are in the schematic I posted above.
2. Do I need to worry about arc suppression or will my sb switch handle it (rated 10A 120Vac 6A 250Vac)
Can I add caps in series across the switch instead as a snubber?
3. I noticed on the layout that two 100ohm resistors create an artificial CT on the board for the heaters. The real center tap is said to be grounded as well. Is this safe? I have attached a picture of the layout

[sluckey]

1. Should I use just one or both soft start resistors where they are in the schematic I posted above.

What are you calling a "soft start" resistor? If you're talking about the resistors across the standby switch then don't use any.

2. Do I need to worry about arc suppression or will my sb switch handle it (rated 10A 120Vac 6A 250Vac)

Don't worry.

3. I noticed on the layout that two 100ohm resistors create an artificial CT on the board for the heaters. The real center tap is said to be grounded as well. Is this safe?

Yes, it's safe. But it is redundant. Use one or the other, not both.

[basschops1528]

OK so switch is fine and I will go with 100 ohm resistors as CT because valve wizards article says its good for him canceling.    So why would you recommend not using the resistors across the SB switch? My goal is basically to protect the amp from inrush current damage.

[sluckey]

There were none on the original amps. The slow warm up of tubes provides some inrush current protection.

[basschops1528]

Even with a bridge rectifier instead of tube rec? What would be so bad about the resistors anyway? Just picking your brain, and I used 18 gauge stranded wire for the heaters though it seems thin compared to the heater secondaries on the PT. My calculations said that 7amp draw from all tubes on avg was about 50% of that gauges max current. Sound good? BTW I've been learning a lot from you guys thanks

[sluckey]

I never said it was bad.

[basschops1528]

Let me rephrase that.... Would they cause some problems with the power supply you may know about?

[sluckey]

Let me rephrase... It's kinda like teats on a boar hog. Please take that in the lighthearted manner I mean it.  It's just an opinion.

[basschops1528]

How about the heater wire. Do you think it will hold up?

[sluckey]

Yes, although 18awg will be hard to work on the little tube sockets.

[basschops1528]

It was ha-ha but it worked well though. I ran both wires right over each other and back out toward the edge of the chassis. All other wires should be at right angles.  I promise this will be the last question. Is 20 gauge buss wire OK for the connections between dropping resistors and capacitors on the turret board? (From B+ to screens and V3... Etc..

[sluckey]

I use 22awg.

[basschops1528]

Oh wow OK!  I bought it off tubedepot so I'm assuming it should hold up to 400v+ thanks again sluckey

[sluckey]

Voltage rating is kinda irrelevant when talking about buss wire. Voltage rating is more about insulation.

[basschops1528]

I also noticed that the balancing resistors I bought for the 100uF caps were 1 watt. I think they seem a little too small. Would you agree with at least  3 watt metal oxide?

[PRR]

250V across 56K is 1.116 Watts.

[basschops1528]

The secondaries put out about 360 (180 per lead) after the bridge wouldn't it be more like 360 X 1.3= 486 loaded? Unloaded 360 X 1.4 = 504. Then that divided by 56k. I calculated ( at least the B+) to be 4.5 W at 504v  and 4.21W at 486v. I also think that because the resistors are in series that it plays a role as well. So then these wattages would be halved.

[sluckey]

The secondaries put out about 360 (180 per lead) after the bridge wouldn't it be more like 360 X 1.3= 486 loaded? Unloaded 360 X 1.4 = 504. Then that divided by 56k. I calculated ( at least the B+) to be 4.5 W at 504v  and 4.21W at 486v. I also think that because the resistors are in series that it plays a role as well. So then these wattages would be halved.

Each resistor will only see half the total B+. Your math is wrong. This is the correct formula...

      P = E²/R

[basschops1528]

Yes! That's what I had figured because like I said, they are in series. What would be an expected voltage at the screens supply? I want to be really accurate before I buy more resistors.

[sluckey]

Yes! That's what I had figured because like I said, they are in series. What would be an expected voltage at the screens supply? I want to be really accurate before I buy more resistors.

Each resistor would see half the B+. Just call it 250 to make the math easy. So, P = 250x250/56000 = 1.1 watts.

Screen voltage will only be a couple volts lower than main B+. Just call it 500V. Which other resistors do you need to buy? Why not just use the values on the schematic?

[basschops1528]

Well, I bought 1W carbon film resistors and I want to be conservative and have MO just in case. So basically the B+ and screens will have the same wattage resistors because the filters are the same besides a separating choke.

[sluckey]

Well, I bought 1W carbon film resistors and I want to be conservative and have MO just in case. So basically the B+ and screens will have the same wattage resistors because the filters are the same besides a separating choke.

You need larger wattage for those resistors. A good rule of thumb is to calculate actual dissipation (in this case 1.1W) then double that number and buy the next higher commonly available wattage rating. It's also good practice to use metal oxide resistors in the power supply because they are flame proof.

I would recommend using 3 watt metal oxide resistors for every resistor shown on that power supply schematic. Hoffman sells every resistor you need at 50 cents each.

[basschops1528]

Perfect  thanks!