Forming Caps correctly In Fenders

[ckpop]

Ok eveyone I have a question about Cap forming in fender amps
Using the 100K 2W resistor method in series with the first filter cap for forming caps do I lift the the 220K resistors that are balancing the caps in series. Are they just balancing the caps in series or do they also act as bleeder resistors that must be lifted to get the correct voltage drop across the 100K 2W ?

I know the method for forming caps in marshalls but I want to be sure in fenders if the rules change if you first two caps are in series vs Parallel.

Thanks

[meddler]

Yes, lift the 220K balancing resistors or they will add to the leakage reading.

[Phil_S]

Please forgive my apparent ignorance, but why is cap forming necessary? Maybe in the olden days, but, as I understand it, new caps are ready to go. I've never bothered with cap forming. I can understand maybe wanting to reform old caps in a pinch, but new caps?

[tubeswell]

Phil's right - new (modern) filter caps don't need forming. It only works (sometimes) for tired old caps

[selloutrr]

personally i put the amp on a variac and bring it up slowly to the 117vac-120vac (USA) partly to not shock the caps with full voltage but more to test the circuits and meter the amps being drawn as the voltage is aplied, so if there is a problem i can spot it before it's to late and does serious damage. If you want to reform a bad or old cap i've had the best luck with the sencore 103 just press the test function until it reforms and goes from a bad to a good reading. reforming a cap is like using the green gunk in a can for a flat tire it might get you back on the road but it's not a perminent fix.

[ckpop]

Well The main quesion was more or less if I had to lift the 220K resistors on a series cap setup.

I look at forming caps for amps that just has not been used in a while, older caps, and when I do a builds. The question is and I will agree, is it necessary to form new caps ? No. Do I do it for other reasons ? yes

Anyway keep the ideas, opinions and comments posting because I will be checking in later

Thanks

[meddler]

Forming caps using the technique the original poster is using is definitely useful for amps which have not been used in 6 months or more. Will it restore filter capacitors in amps not used for 10 years to their original glory? Of course not. Is it necessary for new caps? Probably not but it won't hurt either. Using this method over the variac, it automatically self-limits the voltage being applied to the caps (remember: V = I*R) and you can measure the current at the same time by measuring the voltage drop across the 100K resistor. If the voltage drop does not fall below 40 V after 48 hours, for example, you know there's a current leakage that needs to be fixed by potentially replacing the caps.

[ckpop]

Thanks meddler

For Amps I build I bring them up slowly on a variac and let the amp burn in. As we all know components change slightly, voltages until they are settled in. Then adjustments I make from there. I make measurements to see and know exactly what is happening with every amp I build just for a consistency factor. This is not for everyone and is time consuming but it helps when you build two identical amps, same circuit, same parts and one sounds different then the other. At least know I can narrow down and eliminate based on all measurements. Without that you are guessing or back tracking.

Thanks

[hitchcaster]

couldn't you just leave in the 220K totem/bleeder resistors anyway and just get different readings but still form the caps correctly? i was thinking about doing this on a dumble build but its got alot more of those which makes it more of a pain to do. good question though, thanks!

[ckpop]

Lifting the 220ks is just to get a proper reading when you add a 100k to monitor your votage drop across it when reforming older caps. This is not necessary when forming caps, just to monitor it and measure.

[hitchcaster]

thanks that what i thought. it looks like if you just attach the resistor to the standby switch you'll form the 20uf caps but not the first 2 larger series caps. unless im missing something

[ckpop]

Yes you are correct, I need some guys to chime in. The 100K resistor must before the two caps in series to reform all caps.

Thanks I forgot about that.

[meddler]

Preferably, you want the 100K resistor right after the rectifier or before the first filter cap. I generally just use alligator clips to stick the resistor in the circuit after desoldering the relevant wires. (danger: high voltages present. always discharge the caps before removing the resistor!). Some circuits have the first filter cap right after the standby switch (Marshall JTM45 & some Alessandro amps come to mind) so you don't even need to disconnect any wires. Stick the resistor across the standby switch in that case and leave the switch in standby. Also remove all tubes other than the rectifier tube. If you are concerned about leaving the rectifier tube in, temporarily connect some diodes across the relevant pins.