Help with powering down a trafo

[jmillerdoc]

Hi there, I just bought a tube pre-amp kit off eBay, called the Wendy. It requires a trafo that has the following outputs 180v-0-180v @ (0.06A), 6V @ 0.5A, and 6V @ 1A.
I have a vintage trafo that is 180-0-180v @ 150mA with a 6.3v at 3A. How can I use this trafo for this tube preamp? Can I run some resistors in series to the leads? Obviously I have to split the 6.3v into two to obtain x2 6.3v secondaries. What resistor values would you put in series to the leads to bring the amps down to the levels needed on the tube preamp power inputs? Also what wattage rating would you use on the resistors?
Thanks. I know this is probably a simple formula to figure this out but I don't know it and I am not willing to risk finding it and getting it wrong.
Much appreciation,
Jeff
Lawton, USA

[billr]

Hello Jeff.

You don't need to do anything to the transformer. In fact as the capability of the transformer is much higher that the load, it will run cooler, which is no bad thing.

I note that two six volt supplies are mentioned. Is one of them for a rectifier? if so, which one?

If the cathode of the rectifier is not connected to the heater, you can run all the heaters off the one supply.

you don't need any resistors at all.

hope that this help.s

kind regards

bill

[boywonder]

As already mentioned, having a power transformer that can supply more current than you need is no problem. The current is determined by the resistance of the load, so the preamp tubes will only draw around 60ma or so (.06A) from the transformer even though it can supply up to 150ma.

If your transformer could only supply say 40ma or so (.04A), then you would run the risk of overheating it and/or shortening it's life since the tubes would be pulling 60 ma out of it, exceeding the current capability.

Running the transformer under it's rated current can result in a B+ (supply voltage) a little higher than the design voltage since the transformer is not loaded down to it's design current rating. The no-load to full load difference in output voltage is typically around 5%-10% depending on the transformer.

[jmillerdoc]

Hmmmm....OK then, I wonder what is wrong than. I hook the thing up properly and when I did It smoked two of the resistors on the PSU. here is a link to the exact unit I bought:

http://cgi.ebay.com/Pre-AMP-Amplifie...lenotsupported

If you look at the third pic down you will see the PSU up front, there are two resistors between the tube and the 2 big black caps, these are the ones that go up in smoke.

There are 2 6.3-0v power inputs on the left. One goes to the filament on the PSU tube, the other goes directly into a chip style rectifier the that voltage goes across to the preamp unit. The resistors that burn, one goes across the large caps + and - poles, the other connects directly to one of the pins on the tube, both are rated at 2w 100R. I thought maybe the amps were too high thus......sizzle. Do you think the thing is just defective? Pointers would be great. I have never messed with tubes before, I am a chipamp guy. I wanted to build a LM3886 amp with this as the preamp, a kinda hybrid of sorts. I really dont even understand how a tube rectifier works to be honest.....I obviously need help!
Thanks alot y'all
Jeff M.
Lawton, USA

[Arnulf]

If you look at the third pic down you will see the PSU up front, there are two resistors between the tube and the 2 big black caps, these are the ones that go up in smoke.

I can barely make out two tiny things that could be resistors, one immediately to the right of the tube socket and one further up above first one. Is that correct ?

The resistors that burn, one goes across the large caps + and - poles, the other connects directly to one of the pins on the tube, both are rated at 2w 100R.

They appear more like 0.25W to me Note the layout on PCB (the big rectangle) underneath them - that's about how large 2W resistors should be, they are approximately the length of diameter of noval socket. Oh and resistors dissipating significant power should not be soldered flat onto the PCB - instead there should be some room underneath them for better air circulation (= longer pins).

100R is an unusually low choice for bleeder resistor. Are you absolutely certain that the first resistor is connected across large capacitors' + and - ? I would expect 100R resistor to be used in CRC filter (between two large capacitors' + terminals) rather than the setup you described.

Please note where the second 100R resistor is connected on both ends and let us know. We cannot help you unless you either describe the circuit well enough or draw the schematics (= preferrable).

[billr]

I concur, the bleeder resistors across the caps are usually 100k.

If you have a 250v supply and put a 100 ohm load across them, you are trying to get a lot of current out of the rectifier, hence the smoke.

[Wavebourn]

Oh eh, you need one more 6.3V transformer for your rectifier! One 6.3V is for rectifier, another one is for the amp, and that pair of 100 Ohm mimic a centertap of that 6.3V to ground, or to a positive bias created by a voltage divider.

[billr]

hi.

surely that should make no difference? the pin out diagram that I saw for the 6x4 showed that it was an indirectly heated rectifier with no connection from the heater to the cathode. so having both the rectifier and the other valve heaters taken from one source should make no difference.

unless of course the cathode is connected to the heater. and the diagram i've got is wrong.

you are probably right that the 100R resistors are an artificial ground reference for the heater cct.

So i am guessing that there is ht on the heater supply from the rectifier somewhere.

I would ask that the heater supply to the two valves is disconnected, and the PSU is connected only.

Turn it on. Measure to see if there is any DC on the rectiifer heater lines. If there is, you can't share the heater power source.

just buy another 6.3v transformer, they are dirt cheap.

hope this helps.

Bill.

Originally posted by Wavebourn
Oh eh, you need one more 6.3V transformer for your rectifier! One 6.3V is for rectifier, another one is for the amp, and that pair of 100 Ohm mimic a centertap of that 6.3V to ground, or to a positive bias created by a voltage divider.

[jmillerdoc]

Is it possible that b/c I am trying to share the 6.3v for the heater supply that this is causing my problem? I am sorry, I meant 1/2watt not 2watt (oversight, but means alot!)

As far as the schematics, here is a rough attempt, also the two resistors.
First, the 2 resistors in question, I was incorrect. One of them connects the + side of the large caps, the other goes from the + side of the large caps to a pin on the tube, the exact pin....if while looking from the top of the tube and orienting the pinholes as if you are looking down at a "U" would be the third one going clockwise starting from the right arm of the "U". There are 7 pinholes.

One of the 6.3v inputs follows the following path...both leads hook to either pin #4 and #5 per the above description I just gave, I presume the filament of the tube on the PSU.

The other 6.3v goes as such...through a chiplike rectifier then onto the 8200uF 200v medium sized cap seen on the board then onto what looks like a transistor (inside the black heatsink), the negative lead then tracks along to connect to both the negative, or grounds, of the 6.3v power out strip and the high voltage power out strip, as well as both negative arms of the large caps. The + leg goes only to the 6.3v + power out strip.

I find it odd that the chip rectified 6.3v negative arm shares with the center tap on the 180-0-180 at the neg arms of the large caps? Why share neg (DC) 6.3v with AC CT of 380v trafo? Why not keep them seperate?

I cannot tell much of what is going on on the preamp board as the tracings are on both sides of the board and are obscured by the components on the top of the board, I can tell you that one of the 6.3v(DC) (by the way, there is only one 6.3v power input on the preamp board) goes to pin #1 (as labled on the board, which is the first pin on the left arm of the "U" as I described above on the PSU tube, they count counterclockwise from the left arm of the "U" on the preamp tubes. The other lead goes to pins #4. So both filament leads on the preamp board are supplied by DC.

I hope that description helps.

Looking forward to your replies!

Jeff

[SY]

The circuit board layout, with electrolytic caps right next to a hot rectifier, is really poor engineering. Electrolytics are very heat-sensitive.

Once you get it up and running, start thinking about building the power supply on another layout, otherwise expect spectacular failure after a few months.