Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: crashtm1 on April 13, 2010, 07:35:54 am
-
I bought this new about 3 years ago... I do remember it was a PT for 100 watts.. The problem is that I lost the schem for it...Anyone have a copy or know what the wire color ID is?
-
Can you give a little more info? Maybe a pic?
When working with a complete unknown PT, I like to use my ohmeter to identify leads that have continuity to each other. Then I group those leads together with tape, cable tie, or just twist. Just grouping common leads often helps to identify their purpose.
-
When you identified common leads
and which windings has the larger resistance
you can try to verify it with a low voltage in the winding you suppose to be the primary winding
(high resistance higher voltage - low resistance low voltage windings)
and control the voltage you obtain in the other (supposed) side of the transformer (secondary windings)
if you use a 10v tension all will be easier but you can also use a 6.3v or a 12v tension and go on with a little math
Kagliostro
-
I've no clue what ya got there but most transformers are color coded as follows:
Black pair - line
Red Pair - HT (if center tapped then the 3rd red wire with tracer stripe is the CT)
Green pair - Filaments (if center tapped then the 3rd green wire with tracer stripe is the CT)
Yellow - 5V
Sometimes there's a bias tap off the HT. I've seen them all sorts of colors. Sometimes there's a shield lead. I have a transformer I'm playing with right now that has brown 5V and yellow 6.3V, so nothing listed above is etched in stone.
If you can determine for certain which pair is the primary, you can wire off all the secondary leads to a barrier strip & light up the transformer so as to measure the secondary voltages. When I do this, I use a lamp limiter. With no load you will get accurate voltages. Next you can disconnect the power and measure the DC resistance of the HT leads. From here you can use Duncan's PSU Designer to estimate the mA of the PT. If you don't just go hog wild with tubes galore, more than likely your filament current is sufficient.
-
I think crash wanted to attach a picture but it didn't come through.He'll be back.
-
I ohm'd out the xfrmr and have the following pairs: Blue - Grey (CT) - Yellow; Green & Green; Red & Red. The problem is the Blue & Yellow pair B+ is too high. Your filter caps are rated at 450V and I get 450V with only 90VAC input. It looks like the amp needs around 485V to operate like JCM800. We may need a different transformer. Also, there is no CT for the filament voltage which seems weird. Any thoughts?
-
Also, there is no CT for the filament voltage which seems weird. Any thoughts?
Thats normal, lots of thransformers have no center tap on the filliment winding.
This info has helped me with identifying transformer windings http://www.radioremembered.org/xfmr.htm read thru it and you should be able to figure it out. I usually attach all the leads to some type of terminal block to keep things from shorting out. .. Joe
-
Handwound transformers???
I bought this new about 3 years ago...
This finally rang a bell. Guys, the title was talking about a particular company... Of course, it took days for me to catch on to that. :rolleyes:
There once was a company called www.Handwoundtransformers.com, which I think claimed to be about the greatest thing since Mercury Magnetics. The site is now defunct, so I guess folks got tired of waiting for their ordered transformer to be wound.
Yeah, what everyone is telling you is right. You have to approach it like the unknown transformer that it is, but at least you have the info that it is meant to be a power transformer for a 100w amp.
Just remember, unloaded voltages from any transformer will be higher than what you will see when current is being pulled from the transformer.
The problem is the Blue & Yellow pair B+ is too high. Your filter caps are rated at 450V and I get 450V with only 90VAC input.
So this transformer may sag down to 450v or less when current is being pulled by a circuit. But it also points out a key part of amp design: you don't spec the power supply parts for the loaded B+ voltage, you spec them for the worst-case voltage when the amp is turned on and the tubes haven't started pulling current yet.
But Marshall didn't use a single filter cap in the 100w amps, they stacked filter caps to get a higher effective working voltage. Just like Fender did with a pair of 350v caps in series to get an effective rating of 700v with half the capacitance of the individual filter caps.
-
EUREKA!!!!!
(http://i193.photobucket.com/albums/z123/crashtm1/th_Hamdwoundtrannie.jpg) (http://s193.photobucket.com/albums/z123/crashtm1/?action=view¤t=Hamdwoundtrannie.jpg)
-
The guy just had to be different... and not color-code his leads like anyone else in the world!!
-
Here's the latest... BTW I'm using the Weber caps...
*****
Todd...this pt won't work because it puts out too high of a voltage. It is 400-0-400 RMS which becomes 400 x 1.4 = 560V peak. This is the voltage the bridge rectifier rides at (a little less). So the CT makes it approx. 560V across the caps. I already blew one of the caps because they are rated at 450V. Anyway, I think what you need is a 325 - 0 - 325 which would work out to be approx 450V. I will replace the cap for you. Does this make sense to you??