[TUBE] Champ power supply

[jimmy]

hi everyone

im looking at building a fender champ for one of my next projects. i was looking at the power supply and i was wondering, with the tube recitifer, how would you connect everything if you were to use the back-to-back transformer method? or is it worth paying the extra and getting a purpose built power transformer?

thanks
Jim

[Mike Burgundy]

as long as you get within a reasonable range of the original supply, and don't exceed any ratings, you should be fine. Just use back-to-back transformers (MAKE SURE YOU GET THE RIGHT RATING). Watch out though - not all tubes have two filaments to put in series - you might have to regulate it down for powertubes. You can use Duncan's powersupply sim at duncanamps to fiddle with your PS design.
Keep in mind a SS rectifier will not have the natural soft-start a tube has, so you might want to inlude a stanby-switch.
Have a look at AX84 as well.
hih

[jimmy]

excellent! thanks for the info. the transformers i am looking at using are 2  x 12.6VAC 500mA. is this enough current for everything?

and where the rectifier tube goes back into the transformer, what is the purpose of that? im a little unclear.

thanks again
Jim

[The Tone God]

Look at the ever knowing datasheets. First thing you should do is figure out your filimant situation.

6V6: 6.3V @ 450mA
12AX7: 6.3V @ 300mA or 12.6V @ 150mA
5Y3: 5V @ 2A (AC!!!)

You going to have to generate 6.3V @ 750mA plus 5V @ 2A AC just for the filimants and thats before the B+ you need. You need more transformer(s). If really want to do all of this it might be worth just getting a purpose built transformer.

Andrew

[jimmy]

:shock:

can anyone direct me to a place where i could get such a transformer for a reasonable price?

thanks
Jim

[cd]

If you're in Australia your best bet is a Hammond "Classic" power transformer (www.hammondmfg.com).  Normally I'd say use the Fender replacement transformer (they're normally $50-60) but they don't have export mains.

[jimmy]

aaahh right. thanks!!

whats export mains? is that just mains voltages that arent 110VAC?

cheers
Jim

[sir_modulus]

(sorry i tried to post this earlier, but my comp failed)
Hey Jimmy,

I just built a champ too.
First off, the B2B PS setup you have will not work well.

1) Take 120 - 12 and you get like 16 v (as you are not comsuming the rated MA, the voltage produced is higher) then you put a 1:10 trannie on it (12 to 120), so you get 160V. With this setup you need to use a solidstate rectifier, as a tube will drop the voltage down by 50 Volts. When you rectify 160 V with a solidestate rectifier (BRIDGE) it is multiplied by 1.4 making the final voltage 224, then you drop about 20 volts due to filter caps and resistors, making your final voltage 200 V DC which is 50 lower than the B+ in the circuit (needed for the 6V6). This will make you amp distort easier. You also need 6.3 volts for the filaments which is another trannie.

2)Get a real PT. If you can't order a Champ Power Trannie from Angela music or Hoffman amplifiers, just get a Hammond one. A hammond 272X has 310-0-310 @ 70ma and 5V @ 2A and 6.3V @ 2.4A. That should be good.

3) For your understanding, There are two sets of wires coming from the trannie to the 5Y3GT. First is the B+ to be rectified, next is the filament voltage (the ones to heat the 5Y3GT (5 Volts)). Those are the ones that go to the spike.

P.S. are you using the GT version of the 6V6 and the 5Y3?

EDIT: As a plus, the PT I recommend can handle a couple more tubes incase (like me) you want to eventually upgrade your champ to a vibro Champ (more 12AX7A's).

[jimmy]

now i understand. yes, i will be using the GT versions of the two tubes. i am now weighing up building a marshall 18watt instead, now noting it wont cost that much extra. hopefully. what was the B+ for the 18watt originally? if i was to build that, would a hammond 372BX be alright?

thanks
Jim

[jimmy]

by the way, what did you think of the champ?

[puretube]

New Sensor (=Electro-Harmonix) got PT & OT for Champ:
http://www.turnstyle.com/nsc/catalog.asp?raster=96F5.0x28D049FC776F3313.AC2B.0.E48E&category=fenderxfmrs

[jimmy]

are they alright for a primary of 240 volts? thats what i need, because im in australia.

cheers
Jim

[puretube]

I`m afraid 120V only... :cry:

[Dan N]

If you can find an old '60's tape recorder, they often have nice iron for small DIY amps.

[jimmy]

another question: what does it mean when a transfromer is rated at

5,000 ohm ct 30 Watts

? i get as far as 5,000 ohm centre tapped 30 Watts. i dont know what it means though.

how do you know if your amp has 5,000 ohm or whatever?

help?

thanks
Jim

[jimmy]

anyone?

[puretube]

another question: what does it mean when a transfromer is rated at

5,000 ohm ct 30 Watts

? i get as far as 5,000 ohm centre tapped 30 Watts. i dont know what it means though.

how do you know if your amp has 5,000 ohm or whatever?

help?

thanks
Jim

those 5kOhm will be your plate-to-plate resistance for push-pull configuration.
your tube-datasheet will tell you which B+ you can apply, and which bias to use with this anode-impedance for a suitable tube.
your schematic will tell you which OT to use.
your transformer manufacturer will tell you, which tubes the thing is made for.

[jimmy]

ok i sorta get it. the number at the beginning has something to do with the resistance of the tubes resistances to each other. ok ill puzzle that one out.

thanks
Jim