Fender 1966 Champ Cap too large?

[Gene]

Hi,

I've just got a 66 Fender Champ. As I was giving it an once over, I saw that someone added an additional 40uf PF cap in parallel to the first 20uf PF cap coming from B+. Then the power resistor from this 20uf cap has been changed to 10k. The original schematic calls for a 1K.

The amp sounds fine but I am wondering if there is any sonic benefit from this change? Less hum? It is dead quiet.

Also, the other electrolytic caps have been changed to similar uf rated caps but with like 1500v ratings. A little over kill? Does this crush the "mojo" :D ? I know upping voltages are fine but this seems a bit much. The originals were like 25v. Any sonic differences in a 1500v and 25v caps?

I am leaning on reversing these and making it as original as possible.

Here is the schematic but I'm sure you all know it by heart: http://www.kbapps.com/audio/schematics/tubeamps/fender/champaa764.html

Regards,
Gene

[phsyconoodler]

that's way overkill for caps,that's for sure!The original champ values are best for the loose sounding grit the Champ is famous for.The voltag rating has no effect on the way the electrolytic bypass caps work,they are probably huge though!
Get a new set of the proper caps and get this baby back the way it was meant to be.
you might want to check the plate voltages of the amp and see if it's in the ballpark for a champ.If someone changed the power transformer it may be the reason why the caps were upped in value.However,the 40uf cap will likely eat the 5Y3 rectifier soon.It's just plain too big.

[Gene]

Thanks for the replies. The plate voltage is 380v. The PT looks original with a date code of 6601. Same as the OT.

I misread the caps. Not 1500 but 150v. Time for glasses maybe. The info was sort of underneath the cap and I used a mirror to see it. There is a .022uf cap in the middle of the tag board with 1000vdc. THe schematic calls for 400v minimum. The others are 25v on the schematic but 50v won't hurt, I'm sure.

I will revert it to original. I wonder what the previous tech/owner was thinking with the additional 40uf cap in parralell. I measured it and it is 70uf there.

[epluribus]

...total capacitance of 60uF- that's what the 5Y3 is having to "work with", and this is past max limit for a 5Y3 for sure.

Hey John.

How do you determine the max limit anyway? Haven't fooled with rectifiers or power rail structure a whole lot yet.

--Ray

[phsyconoodler]

You need a tube manual or go to "Franks Electron tube data sheets" .google it.Look up 5Y3 and see what it says.

[epluribus]

Hey guys, 'preciate your patience with my question. Actually, I looked on several pages (TDSL is one of my favorites too.) for 5Y3 variants, and I only saw a couple that mentioned filtering. (That I understood, perhaps. :)) This one, for example:

http://www.nj7p.org/Tube4.php?tube=5Y3GT

I note that they give several sets of specs depending on how you run the tube, but that the highest filtering figure they publish is 20Mfd. So I gather I'm just missing something, since that's obviously a "limit" that they run above regularly. The reason I'm interested is that I have an obscure rectifier tube I'm working with, a 6X4, and as yet have found no filtering data for it.

Nothin' like a bit of mystery to keep life lively. :) Thanks for the help, gents.

:BEER

--Ray

BTW, this is a new subject I'm only just now broaching. The spec is for the first filter cap only, true? I gather you can filter to death after that if you don't mind what it does to the tone.

[Dana-L]

I have a silverface Champ that I got brand new in about 1981 and although every schematic I've seen for this model indicates that it should have a 20/20/20uF cap can, it came from the factory with a 40/20/20 can.

Cheers,

-Dana

[Gene]

That's interesting. The cap can in mine is original and has 3x20uf caps. I guess the previous guy wanted to kill the hum. I took off the 40uf cap in parallel to the 1st 20uf and now it has that slight vintage hum.

[Axekisser]

The guy probably wanted to slow the charge time of the first cap and that's why the PS current limit resistor is 10k instead of 1k. That should help keep the rectifier from being killed. I usually use 40uf as the first cap on Champs and Princetons and never have a problem. Most/some rectifier tubes slowly turn on and that helps too.

[Gene]

Is there any sonic difference? I took out his "mod" and the amp sounds pretty much the same to my ears but with a little more hum.

Is it okay to put a 40uf first cap without increasing the 1st resistor (the 1K resistor)?

[John Phillips]

You may find that whoever put in the 40uF cap was effectively 'replacing' the first cap in the can. This is common practice if just one cap section fails and you don't want to replace the whole can. (Not good practice really, but common.) I would expect it to hum a fair bit without the extra cap if that's the case. In a single-ended amp, the size of the caps doesn't much affect the tone BTW, since the current draw is more or less constant.

It's possible that the resistor value was increased to reduce the voltage on the second filter node, which supplies the tube screen. This will make it less hard on tubes (especially new-production ones), and should also make the amp sound a bit softer and more compressed when overdriven.

It's also possible that the 10K resistor was a mistake - how 'red' is the orange stripe on it? Don't count on people to know what they're doing! Especially with tube amps, since they're fairly easy to tinker with and there are a lot of self-taught techs around - and maybe even more so in the past.

I'm largely self-taught too BTW, that's not a put-down in general, but I've seen some very odd, questionable and even downright dangerous things done by other people who probably didn't really know what they were doing or what constitutes 'best practice'.

[Gene]

Thanks for the reply. The thing is I measured close to 70uf with the 40uf in parallel to the 20uf. The hum to me seems negligible without but with the 40uf cap in there, it was dead quiet. The 40uf measured 45 or so and the 20 measured about 24 or so. Within tolerance, I suppose.

So it is okay to not increase the resistor when increasing the 1st filter cap? I might add another 20uf to make it 40.