Amp building question, Power Transformer related.

trey85stang

Member
Messages
261
I have a power transformer that is rated at 350-0-350. But I have a problem... When I power this thing up my two 350 taps are actually putting out 430v's... Is this transfomer bad or do I have something wired incorrectly?

PT in question is a hammond 273bx.

I have a few other question as well about this build I am doing... but I need to sort this out before moving along :)
 

trey85stang

Member
Messages
261
Can you describe exactly how you're measuring 430 volts? Tell us exactly where you're putting the meter probes.

Also, is this pre or post rectifier (thus an AC or DC measurement), and what kind of rectifier is in the amp?
Actually, on another forum I think the problem was identified. My CT is not grounded.

Im putting the probe directly on the tap from the PT, pre rectifier (AC measurement). The rectifier i believe is full wave. There are two rectifers inline after each tap and then they are connected. Im an amp building newb so Im not 100% sure, my first build actually :)

Here is the circuit in question though:

 

phsyconoodler

Member
Messages
4,315
Ok,I see only one full wave rectifier with centre tap.You have four diodes that make up the rectifier.
This is the way it works:350v on each tap gets rectified and you measure the DC voltage after the rectifier.The basic formula is Voltage x 1.4(diodes) = rectified DC voltage.So 350 x 1.4= 490v dc.
I am assuming you are measuring with tubes in,in that case it will be lower because the tubes draw current and suck the DC voltage down a bit.That is normal.
It would help if you told us what amp you are building so we can help you further.You need to get the basics down before continuing.

What is R67-varies,in the upper left hand corner?
 

trey85stang

Member
Messages
261
Ok,I see only one full wave rectifier with centre tap.You have four diodes that make up the rectifier.
This is the way it works:350v on each tap gets rectified and you measure the DC voltage after the rectifier.The basic formula is Voltage x 1.4(diodes) = rectified DC voltage.So 350 x 1.4= 490v dc.
I am assuming you are measuring with tubes in,in that case it will be lower because the tubes draw current and suck the DC voltage down a bit.That is normal.
It would help if you told us what amp you are building so we can help you further.You need to get the basics down before continuing.

What is R67-varies,in the upper left hand corner?
Thanks for the info about the rectifiers. the amp in question is a 50w SLO. R67 goes to the bias circuit, since the schematic leaves room for different transformers R67 can be adjusted to achieve the bias idle. In which case, since you explained the formula for rectification, Ill be able to calculate what I need a little bit better :D
 

WaltC

Member
Messages
2,129
Plus (as an aside) Hammond transformers used to be (still are?) rated at an input voltage of 115V AC. Most house voltage is pushing (or over) 120V AC at this point, so the 350-0-350 rating is conservative (some say grossly misleading <G>) and that can throw off you calculations too.

FWIW
 

trey85stang

Member
Messages
261
Plus (as an aside) Hammond transformers used to be (still are?) rated at an input voltage of 115V AC. Most house voltage is pushing (or over) 120V AC at this point, so the 350-0-350 rating is conservative (some say grossly misleading <G>) and that can throw off you calculations too.

FWIW
Thanks for the info, My home appears to get 115v exactly. Thats what I read right off the IEC. I thought perhaps I was getting a slight over voltage as well so I checked it.

BTW, after I grounded the CT the voltages dropped to 350v to the two taps :)
 




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