+ and - Power Supply Question

[dorrisant]

Is there any reason why I would not be able to use two different bridge rectifiers off off the same HT secondary tap? I want to use the HT secondary from a Fender Twin PT. It is a reissue Blackface. Unlike the original it was "cloned" from, it uses a full wave bridge rectifier... No problem, until I tried to derive a second negative HT supply. I want to incorporate the cathode follower driver found in the SSS. I tried to use one of the secondary taps to half-wave rectify into negative DC... No dice. No worries there, I'm always learning. Soldering the parts in and seeing things not work as planned is just another solid way of learning, right? Well no smoke was released, it just didn't put anything out as I had sort of hoped.

My biggest question is can I use the same HT secondaries to generate a negative DC supply from a second bridge rectifier? I would think I should ground the positive output from this second rectifier, but I don't want to cause any issues (or smoke) due to the original (normal) positive rectifier already having a ground reference. I understand that within any given house there are several rectifiers (comouters, TV's, etc.) hooked up and ground referenced, all tapped from the same 120vac line into the house. I just don't know if two rectifiers can be ground referenced to the same chassis with opposite polarities... Does anyone see an issue and if so can you explain. As always, I encourage this that answer to post reference material. I will read it all, I obviously have a deficiency in this department and would like to rectify that, pun intended. Ha!

[pompeiisneaks]

So long as you don't exceed the power draw the winding is designed for, I see no problem doing a second rectified - rail. Just ensure you've accounted for the draw of all devices, including the new rectifier/diodes.

~Phil

[Stevem]

Someone correct me if I am wrong, but a full wave bridge can not be used on PT that is already using a center tap as in the stock fender set up!

[pompeiisneaks]

I'd definitely like to know if this is bad too, superficially it seems fine to me, they're both sharing the center tap ground reference but have different paths. I did some googling and found a circuit someone shows how to do what I'm suggesting, I'd be happy to have someone point out if it wont' work though:

https://www.quora.com/Is-there-a-full-w ... ive-output

It looks like you decouple them from one another with some filter caps, but otherwise what I implied earlier would work.

Maybe someone more versed in power supply design could help. I'd seen power supply schematics before that did +15 and -15 on the same winding with full wave bridged but can't find those now. They also used voltage regulators to smooth it even more. LM317 and LM337 for + and -

~Phil

[pompeiisneaks]

ahh found one on a vox i worked on in the past Vox AC100cph uses a single full wave bridge rectifier and doesn't use the center tap to get both + and -. See here:

VoxPS15v.PNG

~Phil

[sluckey]

There ain't no center tap on the PT he's talking about.

[pdf64]

I think you may need dc blocking caps between the HT winding and the 2nd FWB that feeds the negative supply.

[dorrisant]

Sluckey is right, no CT on the PT...

Pete, do you have any examples of the blocking caps being used? Any references or links? I need to get some ballpark values so I can try that. I do see that there is some DC (235v form memory) at the bridge terminals for the AC input. There is just no DC whatsoever on the other side of the negatively polarized diode for the bias circuit.

I was just concerned that using a bridge rectifier would possibly give a ground reference if the negative terminal of the bridge is grounded. I don't know if grounding the positive terminal of a second bridge rectifier would cause any issue. I think I've seen random schematics with more than one bridge rectifier, but I could be mistaken. I wouldn't think there would be an issue there... Both references to ground are balanced between the AC primaries. It would be the same on the negative supply I need, but I don't know if there may be a violent interaction . I don't think so, I may just run it up a bit on the variac and see what happens... Of course only after I get some blocking caps in there.

Here is a link to the service manual of the reissue.
http://ampwares.com/schematics/65_twin_ ... manual.pdf

[sluckey]

If you have the negative terminal of one bridge connected to chassis ground in the normal way we see it, then you connect another bridge and connect it's positive terminal to chassis ground, you will destroy both bridges. If the bridges happen to fail shorted, then the PT will also be in danger.

How much negative voltage do you need? There is a separate bias winding on that PT but it will only give you about -60 or -70vdc.

[dorrisant]

How much negative voltage do you need? There is a separate bias winding on that PT but it will only give you about -60 or -70vdc.

Ok... two bridge rectifiers are out. I still need about -350 DC.

I did this here... https://tubeamparchive.com/viewtopic.ph ... 2&start=30

See the pic below. It wasn't tapped off of a bridge rectifier though. On the one I'm trying to do right now, I have the AC connected to the diode, then a 3K3 resistor. The one in the link above has the resistor between the AC and the reversed diode. just thinking out loud here...

[nworbetan]

I should apologize in advance that nothing I'm about to say will actually get the voltages you want from the transformer you're using. But maybe something will point in a direction that you haven't looked yet?

At least some of the earliest 100 watt Marshalls used two bridge rectifiers, but the only way that circuit works is if you have two secondary windings on the PT, a center tap won't do. I tried to find an actual Marshall schematic with that configuration, but this is the best I could come up with:

http://solodallas.com/the-evolution-of- ... superleads

If you google "split rail power supply bridge rectifier" or anything similar enough, you'll see a handful of examples that use two bridge rectifiers and they all have two secondaries too. Going by those examples you can either reference the "center" voltage to ground to end up with + and -, or you can reference the lowest voltage to ground and call it a voltage doubler. (Marshall used the voltage doubler design.)


You'll also see some examples that look like they're using a single bridge rectifier and a PT with a center tapped secondary and still managing to get + and - outputs. I'll refer you to Merlin's web site, he calls it a bipolar supply.

http://www.valvewizard.co.uk/bridge.html

You can maybe possibly reference the lowest voltage to ground in this design and call it a voltage doubler. It happens more often than you might realize, since it looks almost exactly like a bridge rectifier on paper unless you look closely. The reason I said "maybe possibly" is that I swear I've heard an anecdote or two about a transformer with a center tap that was internally connected to the transformer's shielding, or something to that effect. So that's something to look for if you scrounge an "unknown" transformer.

[JazzGuitarGimp]

The most logical (and possibly the only) way to do what you want is to use half wave rectification: tie one side of rhe transformer winding to chassis, then connect the other side of the winding to two 1N4007 diodes; the cathode on one diode and the anode on the other diode. Then use the free ends of the diodes as your bipolar supplies; connect the free cathode to the positive side of the plus supply filter cap (ground the caps' minus side) , and connect the free anode to the negitive side of the minus supply filter cap (ground the caps' plus side).

Using two FWB's will short out the transformer winding, quickly sending it to the trash heap (well, as quickly as it takes to raise the tranny's temperature to the point where the enamel on the windings melts).

Also note half wave rectification will require more reservoir filtering; about twice as much. So if your design call for 47uF reservoirs, use 100uF instead.

Cheers,
Lou

[martin manning]

Use the bias winding and a bipolar supply, I posted a design which has been used on a couple of SSS builds.
https://tubeamparchive.com/viewtopic.ph ... 411&hilit=

[dorrisant]

That is spot on! Thanks to all. I'm gonna build the bipolar supply but still have one question...

Is there any reason why I couldn't use another dropping resistor off of the V- to supply the negative for the normal Twin AB763 tremolo circuit? Or just tap on to the end of the 56k?

[dorrisant]

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