power transformer wiring

[black_labb]

hi guys,

this may seem like a silly question, but would i be able to wire the ground on a power transformer secondary to, lets say the 300 v tap, then the + to the 600v and the -ve to the 0v, giving me a 300-0-300 v transformer? my first thought is that the fact that the chasis/ground is connected to earth, so it wont be an issue, but i wanted to check so that i didnt electricute myself adjusting the volume.

the reason i ask is i bought this power transformer to use for a 5e3 type amp, but using a 5u4 rectifier. i thought that the voltage needed was only the ground and the +ve nessisary. i started wiring it up tonight and noticed that i was wrong.

if doing this is alright then would having mismatched +ve and -ve be ok? the 370 tap centre, with the 690 positive (2x 370 is 740) and 0 being the negative, making a 370-0-320 transformer(assuming my rationalysing is correct).

thanks for the help.

[Kramusha]

This is no good idea, because the 370VAC tap gives you 521VDC and the 320VAC tap only 451V. If you rectify both voltages, the 320VAC tap does nothing. Then you have a one way rectifier.

Here I have a picture for you.

You can use a resistor instead of the rectifier tube. Therefore you must look in the datasheet of the rectifier tube.

Stefan

[Steve Conner]

Well, I guess that is totally the wrong transformer! My approach to salvaging this would be to use a hybrid bridge rectifier made of the tube rectifier and two silicon diodes.

[Kramusha]

That is a very good idea. I never heard something like this..

I draw/drew/drawed a picture of the circuit. You can find it here.

Of course you have to connect the kathode of the capacitor with ground.

Use 1N4007 or something similar instead of 4004.

Stefan

[black_labb]

wow, thanks alot guys, what kind of voltages would you expect to come from that circuit kramusha? I need to really look at it closely, i dont know much about electronics (i do need to learn a bit about diodes for an exam thats not to far away though) ill figure out the circuit so i can understand it before i use it. dont quite understand how it is supposed to work atm.

i cant believe i overlooked something as simple as this when ordering the pt.

and thanks again, you saved me a big headache

[Kramusha]

I think the voltage after the rectifier is 490V.

My calculation:
370V * 1,41 (Squareroot of 2) = 521V
521V - 0,7 (Uf of the diode) - 30V (Voltage drop over the 5u4) = 490V

What type of amp do you want to build? (Is this right? I'm so bad in english and need to learn, please correct me if I'm wrong )

Stefan

[black_labb]

I think the voltage after the rectifier is 490V.

My calculation:
370V * 1,41 (Squareroot of 2) = 521V
521V - 0,7 (Uf of the diode) - 30V (Voltage drop over the 5u4) = 490V

What type of amp do you want to build? (Is this right? I'm so bad in english and need to learn, please correct me if I'm wrong )

Stefan

i was making a 5e3 (tweed deluxe), so the voltage may be a bit high, but im sure i can use a dropping resistor to get it where i want it.

[CarlZ]

I look at that drawing and I think this is some sort of universal transformer that's intended to be used with a bridge rectifier. As a bridge you don't use a center tap to ground as per typical Fender designs. I say this because I'm not seeing a balanced center tap as you've already noted. However in a bridge configuration the B+ voltages will be sky high! So for the smaller lower voltage amps you'd use the 370v tap and bury the rest, which should work ok.

Now if what I've just said is accurate, then what's the point of having a 5v rectifer winding on this thing? For a bridge you'd need a pair of 5AR4's and you don't have the current capacity for that on this winding.

Unless I'm missing something, WTH were they thinking when they spec'd this tranny for the winder?!!

-Carl

[Steve Conner]

There are some other very strange transformers in Weber's range that have been discussed here in the past. I think his transformer winding contractor may have got dropped on his head as a kid.

A bridge of tube rectifiers would need three isolated filament windings and three tubes (two single diodes as well as the regular dual diode common cathode rectifier) This is why I suggested the arrangement with two silicon diodes.

Connecting to the 70 and 370v taps will give you 300V AC, which should get you closer to the 5e3 ballpark.

[black_labb]

There are some other very strange transformers in Weber's range that have been discussed here in the past. I think his transformer winding contractor may have got dropped on his head as a kid.

A bridge of tube rectifiers would need three isolated filament windings and three tubes (two single diodes as well as the regular dual diode common cathode rectifier) This is why I suggested the arrangement with two silicon diodes.

Connecting to the 70 and 370v taps will give you 300V AC, which should get you closer to the 5e3 ballpark.

now that you bring that up, i could also use the 370 as the grounded reference and the 620 for the +ve. that should get me to the right value. 250 rms is 353.5 peak, which could work well for bridge rectification, seems to be around the correct voltage from memory

maybe ill do that and eventually get the right PT and then put a 5y3, might use the 5u4 for a bassman style (with some changes for more versatility). ill figure something to do with that transformer if i end up changing it. maybe make an amp with bridge rectification if i end up changing the 5e3 to its proper format.

[black_labb]

just found what voltages to expect, and 250vac is too low, 300vac seems better as you say. using the 690 and the 410 tap gets 280vac, which comes to 396 peak, which would be suitable to use for bridge rectification (only about 5v too high which can be fixed if nessisary).

im thinking ill use solidstate rectification until i find something to do with the pt in there now, then ill get a proper centre tapped transformer and take the ss rectfication out and replace it with a 5y3 or similar.

maybe ill make a external preamp using that transformer, including tremolo, maybe reverb, some gain stages to get some dirt ect, and then on the next amp i work on ill make it so that i can bypass the preamp i put in there (or even just make a power amp). ill see how things go.

[CarlZ]

Unless you absolutely positively have to have a tube for some reason, diodes are smaller, more efficient, significantly cheaper, and far more reliable. If you can get er running with diodes, I'd consider just leaving them in there. And if you feel the need for that voltage drop you get from tubes, just stick a few resistors in each leg and voilla instant tube emulation! Ok, I'll be ducking for cover now having suggested such heresy. LOL

-Carl

[Steve Conner]

I'm not about to complain, I've never once built an amp with a tube rectifier. I once made one with all the B+ and bias rails regulated by MOSFETs and it still sounded fine. :P

http://scopeboy.com/toastpix.html

The design I'm working on just now is in a 5e3 combo cabinet but has a diode bridge rectifier and puts out 50w.

[black_labb]

well, it seems i have found a solution. wether its just a temporary one im not sure, but ill do it.