Blew my filter caps after upgrade

[Naur]

Hi guys,
First post here, love the forum and all the great reading it has given me so far, thanks!

So, my amp, a 1982 Fender Princeton Reverb II, kept blowing the fuses. So I decided to open her up, and from what I could tell three of the caps had gone bad. I decided to switch them with some new ones.
The old ones read 25uF 25V and the new ones I got were 22uF 25V, but the guy at the store said I was good to go anyway.
I desoldered the old ones, and put the new ones in their place, paying attention to direction, so minus and plus where it was before.
However, 30 seconds after I powered up the amp there was a distinct pop, and smoke coming from the caps. So I cut the power, and here I am.

I know I should take to a tech, but I am trying to learn more about amps work, and I would like to try and see if I can fix it by myself. I don't think the amp is damaged apart from the caps. So if you have any ideas, or inputs, I'd love to hear them. And if there is nothing I can try out before handing it over to a tech, I'll just head there straight away. Let me know what you think.

Greetings from Denmark!
- Peter Naur

 

[jhundt]

I think that maybe you replaced the caps that did NOT need to be replaced, and you did NOT replace the caps that NEEDED to be replaced.

I don't know how you determined that these caps needed to be replaced. It is probably a good idea to replace these caps after 34 years, but they were probably NOT the cause of your problem. These are cathode-bypass caps, and they rarely cause problems like you described. The guy in the shop was correct, the replacement caps he sold you should work quite well in those positions.

There are many possible reasons why your amp is blowing fuses. I wonder if the filter caps and the bias-supply cap have ever been changed? I wonder what tubes you are using... I wonder about a lot of things here.

I also own a 1982 PRII (mine is extremely modified) and I am quite interested in your problem.

 

[telemnemonics]

Are you sure they were 22uF 25v and not 22uF 500v or maybe 450v or 550v?
Could possibly be 525v with the first "5" obscured.
These are the filter caps that are highly stressed and need regular replacement, well, vintage Fenders are typically 20uf 500v and 70uf 500v, sometimes 450v, and 20uf might be 22uf, while 70uf might be 100uf replacements.
Maybe the old writing was obscured by leaking electrolyte?

Or what jhundt said.

Also, the 500v caps need to be formed (in the amp) on a variac, raising the voltage slowly.

 

[Papa Joe]

You definitely should take it to a pro..
The problem could be in any number of places..But you have a short somewhere in the circuit..
Poking around inside a tube amp can be very dangerous...

 

[Naur]

Thanks you guys for replying.
Ok, so jhundt was right, in a way. I'm still learning when it comes to electronics, so bear with me. It turns out I got the values mixed up, so I should have bought a 4uf 450 V cap and two 70uF caps 100v. I'm a fool, I know. However, the good news is, I soldered the old components back in place and she plays again, so no harm done.
Now the problem is the same as to begin with: I keep blowing fuses. It happens when the amp is off (?!) that is, i play the amp with a new fuse, and everything is great. Then I turn it of, and the next time I flick the switch, it's dead. Fuse blown. Any ideas?
Anyways, thank you so much fellas.
And jhundt, great amp, isn't it? I know I love mine
Danish greetings

 

[BobbyZ]

Looks like there's a cap can in there or at least it says "filter assembly" in the schematic. Looks like a three section cap with an 80,40 and 20uf @ 450 volt. Then there's another 20uf and 4uf both @ 450 volt. The two 70uf @ 100volts are in the bias supply.
Here's the schematic I'm looking at. http://www.stratopastor.org.uk/strato/amps/prii/schematic/schematic.html
Looks like a fairly complicated amp with a lot going on inside.

 

[printer2]

Play your amp then when you turn it off watch your output tubes and see if they redplate.

 

[jhundt]

there is a site dedicated to these amps:
http://www.stratopastor.org.uk/strato/amps/prii/PRII_hub.html

Bobby Z said:

Looks like a fairly complicated amp with a lot going on inside.

and that is the truth. In fact I took most of that stuff out of mine... you can read about it on the PRII page (http://www.stratopastor.org.uk/strato/amps/prii/modifications/stripped/stripped.html)

The filter caps are in a can, original values 80-40-20-20 at 450V. I replaced the can with this:
http://www.tubeampdoctor.com/product_info.php?category_id=77&products_id=1917&language=en
which gives 80-40-30-20 at 525V.

But this is a pretty serious job, and if you are not experienced with this kind of work you should find a tech shop that will do it for you.

 

[Naur]

Ok, thanks for the tips. I just discovered two things: one, one of the power tubes has been inserted wrongly by the previous owner, so that the key was not in the slot, but opposite to it. Two, the power tubes are 6l6 and the amp should be fitted with 6v6. Could this be the reason why it's been blowing fuses?
And jhundt what's to gain from stripping the amp?
Thank you all again for helping out!

 

[jhundt]

that power tube that was plugged in wrong could certainly cause some problems.

the amp was designed for 6V6 tubes. It will also accept 6L6 tubes. They will draw more current, and some people think this can be bad for the power transformer. I used 6L6 tubes for several years with no damage, but since I discovered JJ 6V6s I am using them. They are a 6V6 that sounds very much like a 6L6.

I stripped the amp because, in my opinion, the original design was full of things that I did not like, did not use, or that detracted from the pure Fender sound.

I did not like the "boost" feature at all.

There are many little caps and components that the designer added to 'improve' on the Fender design, but I felt that the amp was rather 'sterile' and lifeless compared to the blackface Super Reverb that I was used to.

At the time, the PRII was not valuable, and I decided to simplify mine. It is now very similar to an old blackface amp, and I think it sounds really nice this way.

I don't think I would do this today; I would probably leave the amp the way it is. But another guy contacted me recently and asked about my changes. He then did the same things, and is also quite happy with the result.