Champ components

[Psopho]

Hi,

First time valve amp builder here, though have built plenty of guitar pedals and smallish solid state amps.

I'm building a champ clone from this schematic: http://ampwares.com/schematics/princeton_5f2a.pdf and am wondering about the ratings of the components. Am I safe to use 0.5W resistors and pots and 16V caps unless it specifies otherwise?

Thanks in advance.

[Enzo]

The schematic and layouts say right on them, "all resistors 1/2 watt unless otherwise noted."

The filter caps are already spec'd at 450v. You will have an easier time finding 10uf and 20uf (or 22uf) than the odd values of 8uf and 16uf on the schematic. The caps they used then had notoriously sloppy tolerances, so those newer values would still fall within tolerance.

There are two small electrolytic caps on cathodes. They are spec'd at 25v. The one on the input tube could be 16v I guess, but I'd prefer the one on the 6V6 remain a 25v or higher type.

The film caps used as couplers and tone caps are called out as 400v, so use 400v or 600v types there.

[tubeswell]

Also might be prudent to put a wire-wound as the 470R cathode resistor for the 6V6. Depending on your B+ voltage that you ultimately get, you may want to put 500R or so in here (depending on how you feel about the bias once you have it running - but build it with the 470 first and see how she goes), and this resistor can idle at about anything up to around 0.8W dissipation (22V * .036A) and although technically a 1W rating should be fine, 2W - 3W (or even 5W, if you can't find a 2-3W one) has a bit more buffer.

And also if you have a ground connection in your mains supply, it is adviseable to wire the mains side of the PT as a 3 chord system with a chassis ground, and do not install the 0.05uF 600V cap that the schematic shows running from the fuse to ground. This is known as the 'death cap' and it makes the amp more hazardous if you have a 3 wire mains.

[Psopho]

Thanks Enzo. I can't believe I didn't notice the note on that schematic about the ratings I feel like a fool!

For the 450v filter caps, I'm only able to find 10uf and 47uf. Obviously the 10uf will work for the 8uf, but would I be better off with a 10uf or 47uf in place of the 16uf?

Tubeswell, I'm a fellow kiwi so have 3 wire mains. Could you elaborate on the '3 chord system'? Am I safe to just omit the .005uf cap and wire the mains earth connection to the chassis, or is there more to it than that?

I'm playing it on the safe side with the 1w resistors and using 5w instead.

[tubeswell]

Well I guess you know what I mean about a '3 wire' mains systems then. (Sorry got chords on the brain at the mo')

FWIW the way I do it is;

1 Earth (Yellow/Green) wire from the mains chord goes to solder tag on one of the PT bolts inside the chassis

2 The active/phase (brown) wire goes from the mains chord to the end of the fuse (the tag that is not at the 'cap' end of the fuse holder, but the other end - this is to prevent you accidentally zapping yourself with 240VAC if you take the fuse cap off and expose the live fuse contact inside the fuse holder), and then from the other side of the fuse to the power switch, and then from the other side of the power switch to one of the PT primaries.

3 The neutral (blue) wire just goes straight from the mains chord to the other PT primary.

[Psopho]

Thanks.

That is the way I thought it should be done. Was a little confused by the schematic I've linked to above as it has the fuse and cap on one wire (I assume the active) and the switch on the other (neutral?). That seemed a little strange to me.

Any thoughts on 10uf vs 47uf to replace the 16uf filter cap?

[tubeswell]

I built a 5F2A earlier this year (my first build) and ended up using 2 x Sprague Atom 20uF 500V in parallel (40uF) for the reservoir (input) filter cap with a Toshiba 5Y3GT. (I used 10uf for the other two filter caps) That gave it a bit of bite. No problems. There's a thread on that build under the cabinet forum

47uF might be going a wee bit higher than I would feel comfortable with tho (Might fry the rectumfrier). 5Y3Gts are supposed to have a reservoir cap of 32uF max, but there's a bit of slack for us gueetar amp guys ;-). I saw a guy on trade me selling 33uF 450V a little while back. He posts here sometimes (unparalleled) - you could PM him and see if he has any still.

[Enzo]

There is absolutely no reason 500v caps won't work completely as well as 450v caps, so if you cannot find 450v in a vlue you want, look in 500v. 20 or 22uf caps at 500 are one of the most common types. Many places that sell tubes for guitar amps also sell these caps. I would without hesitation stick a 22uf/500v cap in place of one of those 16s. And for that matter if you have a supply of 10uf/450v caps, you can also mount two in parallel wherever you want 20uf.

[Psopho]

I saw a guy on trade me selling 33uF 450V a little while back. He posts here sometimes (unparalleled) - you could PM him and see if he has any still.

I hadn't thought to check trademe for caps. Thanks for the idea.

Looks like I'll need to parallel a couple of the 10uf. hadn't considered the possibility of doing that either!

High voltage caps seem quite hard to come by down this way.

Thanks for your help Enzo and Tubeswell.

[tubeswell]

I PM'd you about an NZ source for valve amp parts