when do you need to replace Fender filter caps

texwest · Post by **texwest** » Mon Nov 28, 2005 1:10 am

Here's a question for you guys. My blackface princeton reverb is completely stock. I have used it almost everyday for fifteenyears for practice and teaching. I'm wondering if I should replace that 20+20+20+20 can. I've heard that if a filter cap goes out it can take out other parts or tranformers. Is this true? What problems can I have if I don't replace it? Are there any sure signs that a filter cap is problematic and shoudl be replaced? It still sounds so good I hate to change anything on it.

What about those paper capacitors on the board with two leads coming out of one end and one lead coming out of the other? When I sent my twin to kendrick for an overhaul about thirteen years ago he replaced all of those. I think with sprague atoms. Thankfully he left the cool blue capacitors. Is it important to replace those paper ones?

Wes

rjgtr · Post by **rjgtr** » Tue Nov 29, 2005 5:41 pm

A blackface Princeton will be over 30 years old by now. So the electrolytics probably should be replaced.

However, if you like the sound, then go right ahead until they die or the amp starts sounding bad. When they die they can take out other parts, because the other parts have to work harder. They can also leak or explode causing a big mess (which is why on bigger Fenders the PS electrolytics are in an isolated pan under the chassis.

I'll bet if you replace all the electrolytic caps the amp will sound a lot punchier and clearer.

novosibir · Post by **novosibir** » Tue Nov 29, 2005 7:01 pm

Don't worry too much about the four section can. If you own a LCR meter, go and measure the capacity of each section of the cap, but make sure, that the cap is discharged.

If only one ore more section(s) measures less than 16µF, remove the can.

If not, make the leakage current test:

- Pull all the tubes out of the amp (chassis removes from cab)
- Jumper the standby switch with a 47K/5W resistor with alligator cables
- Fire the amp up, but ONLY the power switch (standby = OFF !!!)
- Measure the voltage drop across the 47K
- Again after 5 minutes, again after 15 minutes, again after 30 minutes

If the voltage drop across the 47K with the total capacity of 80µF is less than 16V and doesn't rise more than 20% after 30 minutes, leave the can in place. If the voltage drop after 30 minutes is less than at the beginning, even better!

No need, to plug the speaker to the amp while this tests!

Larry G

texwest · Post by **texwest** » Wed Nov 30, 2005 1:05 pm

Here's a technical question. Please excuse my lack of knowledge on this point. Are the electrolytics the paper ones on the circuit board? Are they the filter caps as well? What are the differences between differnet types of capacitors? So what exactly makes a cap electrolytic as opposed to some other type of cap? Should I replace those paper ones on the board?

Novosibir, I really appreciate your explanation, but a princeotn reverb doesn't have a standby switch. Is there another way to do it? Although I'm thinking about just replacing that can anyway.

thanks

Wes

novosibir · Post by **novosibir** » Wed Nov 30, 2005 1:35 pm

texwest wrote:Novosibir, I really appreciate your explanation, but a princeotn reverb doesn't have a standby switch. Is there another way to do it?

Ooops - I forgot, although I've already serviced a lot in the past! Sorry!

Another way is, to put in temporarilly a "standby switch" - but NOT as a switch. Disconnect the cable from the rectifier tube, which is running to the four section can - and in this disconnection then put (temporarily) in the 47K/5W resistor, across which you couldt measure the voltage drop.

texwest wrote:Are the electrolytics the paper ones on the circuit board? Are they the filter caps as well?

These aren't filter caps - these are the (two section) cathode bypass caps. You have to unsolder them at their ground side, that you couldt measure them correctly!

Larry G

texwest · Post by **texwest** » Wed Nov 30, 2005 8:40 pm

Actually I know that the paper caps on th board are not filter caps. I'm just asking a general question about caps. Are the filter caps in amps electrolytic type caps? What are the different types of caps besides electrolytics? What makes each type unique?

Wes

novosibir · Post by **novosibir** » Wed Nov 30, 2005 9:04 pm

texwest wrote:Are the filter caps in amps electrolytic type caps? What are the different types of caps besides electrolytics? What makes each type unique?

To your first question I can give you a straight YES!

To answer to your 2-nd and specially to your 3-th question, I couldt write you at least 10 pages and wouldt need maybe a couple hours for it.

IMHO it's not the intention of this board, to be a online skool, but that give users and fans of the real ampsound helping hints each other. So I'd suggest you, to take a look in some of the right books, there're a lot of them available in these days.

I hope, you couldt understand my German's English

So long,

Larry G

Billy Batz · Post by **Billy Batz** » Wed Nov 30, 2005 10:07 pm

For a very general description- it depends on their composition and type Wes. Theres thousands of kinds of caps. Suffice to say that in amps your mostly dealing with film caps for values from .001(1000p/1n) to 1u, Ceramic or Mica caps for 1000p or lower, and electrolytics from 1u up. The values that overlap can be either (most 1u bypass caps Ive seen are film.) The cathode bypass caps are electrolytics and need replacing eventually but there not filters. The cathodes are typically low voltage (thats why a 330u bypass cap is tiny compared to a 16u filter can), the filters high voltage obviously, and the signal caps taht are film or mica are 400-630v. You choose caps by their value then typically use the smallest voltage you can (to keep them as small as you can and cheap) while not going below the voltage the cap may actually see while playing or repairing. Most caps like ceramics, micas and films are well beyond the voltage they need to be.

sub · Post by **sub** » Wed Nov 30, 2005 10:27 pm

novosibir wrote:
texwest wrote:Are the filter caps in amps electrolytic type caps? What are the different types of caps besides electrolytics? What makes each type unique?
To your first question I can give you a straight YES!

To answer to your 2-nd and specially to your 3-th question, I couldt write you at least 10 pages and wouldt need maybe a couple hours for it.

IMHO it's not the intention of this board, to be a online skool, but that give users and fans of the real ampsound helping hints each other. So I'd suggest you, to take a look in some of the right books, there're a lot of them available in these days.

I hope, you couldt understand my German's English

So long,

Larry G

Larry G,

I think your english is good ..... look at my f#&in bad english

sometimes only one person understand my post -> me

Anyway.... are you electrical engineer? this is how i see it you are good at electical stuffs....

Schöne Güße

Sub

novosibir · Post by **novosibir** » Wed Nov 30, 2005 10:52 pm

sub wrote:Anyway.... are you electrical engineer? this is how i see it you are good at electical stuffs....

From the base I've been bodyworker on cars, absolved master, engineer and CAD designer later and have designed cars at AUDI in the mid of the 80's for a couple years.

But from my youth on I've also been a guitar player and hobby technican, started servicing and modding Marshalls and Fenders also in the 80's together with Bernd Stephan (Steavens), Reinhold Bogner and Peter Diezel (my friend) here in Germany. Since '89 I'm building my own tube amps, although meanwhile my philosophy in building tube amps is huge different from the guys mentioned above.

But I've never learned electronics anywhere at school and I believe, that referring to building great tube amps, the best way is the way of the autodidact.

Larry G

sub · Post by **sub** » Wed Nov 30, 2005 11:09 pm

Audi, Bogner, Diezel .....

Ron (Proguitar) Andreas (TAD) Dirk (TubeTown) <- i think they are familiar for you.........

novosibir · Post by **novosibir** » Wed Nov 30, 2005 11:21 pm

sub wrote: Audi, Bogner, Diezel .....
Ron (Proguitar) Andreas (TAD) Dirk (TubeTown) <- i think they are familiar for you.........

...... they are

Larry G

sub · Post by **sub** » Thu Dec 01, 2005 8:33 pm

Billy Batz · Post by **Billy Batz** » Thu Dec 01, 2005 8:56 pm

Mayeb you can explain what the hell is going on in a Diezel

sub · Post by **sub** » Thu Dec 01, 2005 9:09 pm

http://www.diezelamplification.com/