Champ Power Transformer

[unclej]

well, i promised myself that i wouldn't ask you guys too many questions while i assembled my little champ project and i haven't. it's complete and working but not quite right. i have good clear sound but not much volume. i've double checked and triple checked all routing and i've re-done all connections to make sure i don't have any cold solder joints.

while checking the actual voltages against what they're supposed to be i may have found the problem and would like confirmation please.

http://www.ampwares.com/ffg/

starting at the bottom left hand side of the tag board and reading to the right i found these discrepancies in what the voltage readings are supposed to be (first number) and what they actually are (second number). 340v/394v, 295v/351v, 250v/336v, 15v/20v, 150v/296v, 150v/307v, 15v/3v.

the ce distributors webb site listed the transformer that i purchased as a champ replacement so i'm afraid i didn't double check against what the plans called for. the specs on the one i purchased are as follows: 120v, 60Hz. 650V ct @ 70 mA, 6.3v @ 2A out and 5v@ 2A out. the plans i'm following call for a 300V ct @50mA with 6.3V and 5V @ 2A out.

so i'm assuming that it needs to be replaced before i burn everything in the amp up but will dropping to the 300v center tap instead of the current 650v correct the volume because everything's working the way it was designed?

[lovekraft]

Are you using a tube rectifier? Between the 5Y3GT and the choke, you should be dropping into the ballpark. keep in mind that if you're measuring the voltage with no tubes in place, there's no load on the transformer, and everything will measure quite a bit higher.

[unclej]

yes, i am using the tube rectifier but there is no choke as such on this model, the champ 5f1. and the measurements were taken with the tubes in place. and excuse the link please. i thought it would take you to the layout but it's just ampware's menu.

so i think i just figured out that the transformer that i bought is supposed to put out 300 or so of b+ voltage not be a 300v ct transformer. so even if i have the right transformer it doesn't account for some of the readings being twice as high as they should be. any ideas where i should start looking? i'm more than willing to do the homework but i'm at a lost as to what the next step should be.

[lovekraft]

I'm sorry, unclej, I thought it was a 5E1, which does have a choke - my mistake! A 650vct transformer is exactly the same as a 325v-0v-325v transformer, which is what the Blackface Champ/VibroChamp uses. I think adding a choke and another filter cap shunted to ground after the first filter cap will take care of the problem (the 6V6 is running Class A, so it shouldn't affect the sound - in fact, except for eliminating the need for a standby switch due to their "soft-start" behavior, the tube rectifier is wasted on a single-ended amp). Let me see if I can find out how other people are handling the voltage difference - I'll give you a shout with whatever I find out.

[ansil]

i got a 68 vibro champ and i don't get the 650v specs you posted above. but according to what i could find out about the thing it was around 190-0-190 give or take. still the same replacement one from antique got me 440v on a silicon rect. around 464 when it first kicks on.

[unclej]

thank you both..i'll be interested to hear what you find out lk..i need a little break from it today anyway. i've had my head buried in it for way too many hours the last two days. did i mention that i become obsessed with new projects?

[lovekraft]

Well, I found this

"A poster to the Ampage posted this information on a '56 Champ PT:

110 -> 240-0-240

117 -> 255-0-255

120 -> 262-0-262

123 -> 268-0-268"

I have also read somewhere that it was a 300-0-300 transformer and the layout at ffg shows 340 volts at the plate.

and this

...I haven't been able to get my 5F1 to sound right (pure clean headroom, like a silverface champ) due to the extremely high voltages from my champ/princeton/tweed deluxe replacement transformer. With a "by the schematic" build and a NOS 5Y3, my EH6V6 tube was dissapating something like 18 watts...

So it appears that the BF/SF Champ transformer is a little hot for the Tweed models. Adding a Deluxe choke and another 16uF cap before the output transformer/plate power supply point, a la the 5E1, should take care of the problem, or you could replace the transformer with a 300-0-300 volt (600vct) or lower voltage model. It appears that most everybody who's homebrewing them is using a lower voltage transformer (250-0-250 or 225-0-225) and silicon diodes - sorry, I'm not much help. FWIW, the SF Champ appears to run the 6V6 plate at 420 volts, so it may not be so bad, but it'll definitely increase the clean headroom, so if your customer is looking for tweed crunch, you'll probably have to lower that voltage.

[unclej]

thanks again..i think i will replace it and save this transformer for something else. my "customer" is my partner here at the music shop so he gets what i make him but i'd like for it to sound good.

i may experiment with the choke and cap while i'm waiting for delivery though..good learning experience.