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FDP Forum / Amp Mods, Repairs, and Projects / can anyone explain or point me to a site that explains Filter caps or RC


LA , Calif

I try my best
Nov 8th, 2016 03:14 PM   Edit   Profile  

I ask because I get so confused. My 73 SF champ came stock with a 40/20/20uf original cap can so I replaced it with the same value only it had higher voltage ratings . It had an RCA 5Y3 that also seemed original. I've read 20uf is the largest reservoir a 5Y3 can handle yet the fender Bronco schem shows 40/20/20 with a 5Y3 and a 6G2 Princeton has 30/30/30 UF .

I built a clone of the 73 I had because I liked the sound and used F&T 47 UF reservoir and 22 uf and 22 uf .

Since the 73 Champ didn't kill the RCA 5Y3 was this mainly because the old 5Y3's were better tubes that what you can get now days?

Another thing is why an F&T 47UF 500 volt cap is so much smaller than the F&T 30 UF 500 volt that just does make sense to me the 22 UF are small and the 47 UF is not much larger in diameter but the 30 UF is enormous compared.

I didn't have trouble with the 47 uf F&T in my champ clone and when I tried a 30 UF just to bring the valve closer it was so large I could only tack it and try it but lost bottom end.

I have a spare 22 UF so it would be more friendly to a 5Y3 and I do have a NOS 5Y3 in this build yet I fear it will lose what bottom it has even with a 10 " speaker. I know the Jensen C10R is a bright speaker and has most to do with the bottom end and wondered if the 22 uF and a Jensen jets Falcon 10" 40 watt would be better or would you just change the speaker first then see since the C10R is listed as bright and their sound clips prove that. So far with the 47UF I have had no issues with the NOS 5Y3.

Just to add this chassis I used is a narrow panel tweed Allen amps used to sell and it had everything I needed for this build and it has a standby switch hole so I put a standby switch in there but never use it since I've read they are not really a good idea and didn't come on Champs ever. Same thing with my AA 964 Princeton build crammed into a MM bass amp chassis , I was stupid and added a standby because this seemed to be the thing to do and only used it a few times as with this Champ build. Now I have a hole in the MM bass amp chassis if I remove the switch or just simply don't use it. Just to add years ago I posted on another forum and asked about the 5y3 with a 47UF reservoir and Bruce from Mission amps told me just to put a 47ohm resistor between the B+ and the reservoir to slow in rush so I did if I was concerned.
Sorry one more detail Since I have a NFB switch on my AA 964 and I use a 12AY7 preamp for V1 only because I have only a vol and tone on the MM bass amp so it's wired like a 6G2 tone stack and it sounds better with no NFB so I added a switch just to have the option. This switch is on the rear near the 2 output jacks . Since the standby is near the fuse and power cord and say I want the option of the allen amps TO 25 4 and 8 ohm would using the switch near the fuse and power cord to switch between 4 and 8 ohm be an issue?

What I did was install two output jacks and the 8 ohm has a switching jack and the 4 ohm does not so I need to use a dummy plug in the 8 ohm tap to break ground to use the 4 ohm tap. It works yet I prefer one jack. I know it sounds stupid since I used to use 6L6CG's and they need a 4 ohm load but this was long ago and I don't have any speaker cabs and only have 8 ohm speakers.

Did the same with my champ build since that chassis had 2 out put jack holes and I needed the 8 ohm tap that the Weber OT had both 4 and 8 ohm taps I only used the 4 ohm for the NFB so I could use the same value NFB resistor. Not many choices on 4 ohm speakers from Weber unless I got to an 8 " Sig series or spend a fortune on Weber's higher end speakers , not about to do that.

I'm almost tempted to remove the 4 ohm tap on the AA964 and tape it off and just have the 8 ohm and and ext speaker jack per most fender amps since the bloody hole is already drilled or get one of those chrome hole plugs and remove the other jack completely , don't like open holes in a chassis and I drilled the thing. I still have a set of JJ 6L6GC's and now Allen amps has a OT that supports 8 ohm for both 6V6 and 6L6 yet JJ 6V6's are plenty loud here in fact real loud as it is it puts out 18 watts.

Sorry for the long post hope it's not to much yet any info would help. I know it's my choice just needed some input.

I don't gig any more and don't use the amps all that much for more than an hour.

Contributing Member


Christian Slater
Nov 13th, 2016 03:41 PM   Edit   Profile  

Your question is long and arduous like my marriage was.


Best references for your questions is the ax84 site. It's *the* online community for amplifier design, construction, and troubleshooting/repair.

They have a search function that will lead you to a better understanding of all this stuff.



Nov 19th, 2016 08:01 AM   Edit   Profile  

"I've read 20uf is the largest reservoir a 5Y3 can handle yet the fender Bronco schem shows 40/20/20 with a 5Y3 and a 6G2 Princeton has 30/30/30 UF .""

That's just plain wrong information. Don't wring your hands over it.

Contributing Member

USA / Virginia

Nov 22nd, 2016 05:12 AM   Edit   Profile  

The tube data sheet shows 20uF, but that is in the section called "typical operation". Chart 3, capacitor input filter is used but using it requires knowing the turns ratio of the transformer, and the DC resistance of the primary and both secondary windings. The maximum is stated in terms of peak transient plate current = 2.5 amperes.

Kinda complicated.


LA , Calif

I try my best
Nov 22nd, 2016 01:00 PM   Edit   Profile  

Yes it is very complicated. Plus any electrolytic cap rated at a specific uf varies a lot. I never had any issues with the 73 SF fender champ which had a 40/20/20 uf cap can and replaced that one with what came stock.

When I enter these vast amp forums I end up more confused than I was before. Everyone has their own way and thought process on what's best when what's best is what works for you. The only time I built an amp from a kit was the first tube amp I built a form of a 5F2 Princeton then I decided to go with a parallel 6V6 version and needed to find out what Hammond PT I needed because the one that came with the kit was not going to cut it also needed a larger OT so I went with the large one Allen Amps offer's. I just built a head. Later I got into just copying fender amps and merging part of one tone stack with a different preamp and power section . The 73 SF Champ I had sounded great yet I never measured the screen or voltage on the 12ax7 so I have no idea if the one I built is close or not in that respect. I've had three SF Champs and each one sounded different and all were stock when I got them.

FDP Forum / Amp Mods, Repairs, and Projects / can anyone explain or point me to a site that explains Filter caps or RC

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