Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Heavenamps on December 07, 2017, 03:56:17 am
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I'm building a stereo HiFi amplifier for my senior project for my bachelor's degree. We're using a full bridge rectifier in the circuit. We wired everything up and triple checked all of the connections. When we turned on the amp to test the circuit, the fuse blew. We pulled the tubes and tried again, and once again blew a fuse. We disconnected the pre and power amp stages successively and tried once again, and once again blew the fuse. So we are pretty certain by now that the problem exists within the power section. We disconnected the load from the output of the rectifier circuit and still blew a fuse. We disconnected the high voltage lines of the PT secondary from the diode rectifier and tested their output. This time there was no blown fuse and the taps measured correctly at around 600VAC. So by now we're pretty certain the problem is with the full bridge rectifier. We checked and rechecked our configuration and everything seems ok. I'm wondering if there a trick to bridge rectifiers that I haven't considered. Any suggestions are welcome. I would love to get some advice from more experienced people. Thanks!
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It would be helpful to have the information of what kind of power transformer you are using and a link that shows the wiring of that PT.
I know at least one Hammond power transformer has an internal ground for the HV wiring, so if you use a full wave bridge rectifier, you will definitely have a problem and maybe ruin the PT.
With respect, Tubenit
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The most likely suspect... your PT high voltage winding has a center tap and you incorrectly connected that center tap to ground? And you also correctly connected the negative terminal of the bridge to ground. If this is the case it would explain blowing fuses whenever the bridge is connected. The solution is to disconnect the PT high voltage center tap from ground. Unfortunately, the bridge is now ruined and you must replace it.
***Bridge101... Never connect the PT center tap to the same point as the bridge negative terminal (or any other bridge terminal)
If the above scenario is incorrect, then your bridge was defective or it's connected wrong. If it's connected wrong, it is now defective. Bridge diodes are unforgiving. If you connect it wrong it will self destruct in a few milliseconds.
Probably no damage to the transformer.
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The PT is a classictone 40-18086. I did in fact ground the center tap only because it seemed like all of the schematics I referenced seemed to do the same. So you're saying just let the center tap float? I'll try it and see what happens. Thanks all!
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That PT is not meant to be used with a bridge! Doing so will yield an unloaded B+ voltage of 848vdc. Use it with a tube rectifier or a conventional full wave diode rectifier and you will connect the center tap to ground. BUT DON'T USE WITH A BRIDGE! Actually you can use a bridge if you don't connect the negative terminal to anything. But don't use that bridge because it is burned up.
And be sure to connect the primary IAW this drawing...
http://www.classictone.net/40-18086.pdf
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Oh wow I wasn't aware that the PT was unsuitable for a bridge. What is it that makes it unsuitable(just so we don't make the same mistake again.)?
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Oh wow I wasn't aware that the PT was unsuitable for a bridge. What is it that makes it unsuitable(just so we don't make the same mistake again.)?
The fact that it has 600VAC on the secondary. When you connect a bridge to 600VAC you will get 848VDC out of the bridge. Also, the bias tap on that high voltage secondary indicates the PT is meant for a conventional rectifier. A PT meant for a bridge would need a separate 50v winding rather than a 50v tap. And, a PT meant for a bridge would not need a 5VAC filament winding.
The mistake is not the choice of PT. The mistake is using a bridge with it. But as I said earlier, you can use a bridge if you don't connect the negative terminal to anything. But don't use that bridge because it is burned up.
Can we see your schematic?
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"But as I said earlier, you can use a bridge if you don't connect the negative terminal to anything... "
So do you mean don't ground the negative terminal, or don't connect the other end of the high voltage tap to it? Do you have a digram I can reference?
I will post our schematic here shortly
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what I said
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Is the FWB rated for high voltage? If the school lab stocked-up on 50V bridges (because "all" student projects are low-Volt and 50V is a penny cheaper in bulk), it aint gonna live. (Actually it may, since I think the base process yields 1KV 99% of the time, and they mark parts as lower-grade to fill specific orders.)
If it is a 1,000V FWB, you "can" use it where you should be using two diodes. You *don't connect* the "-" leg of the FWB.
And the first time you blow a fuse in ignorance, STOP and build a lamp-limiter. Ideally the lab already has one (reduces fires in school buildings). It is better to bask in a 100W incandescent glow than to burn-up every fuse in the lab in a try-and-fail effort to narrow the problem.
(An extreme smoke-test safety-measure would be a 12VAC transformer with a 10r 10W series resistor (or a 1156 tail-light lamp). Plug your "120V" side to this and hope for 1/10th voltages. Line isolation, near-safe secondary voltages, Short-Proof. If your build is a dead-short, you get a hot resistor and can leave it that way for many seconds while you take readings.)
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The High Tension winding with a centre tap is meant for a 2-phase rectifier (which can be configured in various ways see link http://www.valvewizard.co.uk/bridge.html (http://www.valvewizard.co.uk/bridge.html)).
As Steve says, yielding 800-ish VDC will be bad news for most audio tubes (despite what some datasheets may claim)
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Transformers provide AC voltage. If you ground any part of the transformer's winding, the other taps will alternate-- both positive then negative relative to that voltage.
You CAN connect a bridge rectifier to that, but you'll get +400v and -400v, not +400v and 0v. Your mistake was connecting the negative output (which isn't 0v) to ground.
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Why don't you just use a full wave rectifier as in Tubenit's diagram. Then you'll have the correct voltages you most likely want!