Princeton reverb II Re cap.

Bubbarock

Good afternoon ladies and gents!

In a trade I recently acquired a princeton reverb II, probably my perfect amp! A beautiful blackface clean channel with reverb to use live with some pedals for some extra dirt and a lead channel with master volume so it's still usable at home levels! However after a little bit of playing time I've noticed it just cannot put out any decent amount of bottom end on the lead channel and there are some horrible harmonics going in in there....

At first I suspected the speaker, but after trying it through a speaker I know is fine and having the same problem I figured i'd try a new set of valves, both power and pre (luckily I had all the valves I needed in my parts box) but again, no change.

So I can only conclude that it's electrolytic capacitors needs replacement. I was led to believe the amp had been recently serviced so assumed this would have been done, but after opening her up I can see that all the capacitors look to be original.

Being a hand wired amp I'm perfectly happy to do this job myself, I have abit of an electronics background, have built a couple of valve amps so understand the dangers and its always satisfying to do your own maintenance.

But I do have a few questions.

1. Does anyone have any other suggestions of what could be the issue?

2. Does anyone know of any reasonable priced supplier of cap cans in the UK? I've found a suitable replacement in the US at a fairly reasonable price, but would prefer to not have to import (specifically I need a 80/40/30/20uf 450v can)

3. Does anyone have any suggestions for capacitor manufacturers? I'm leaning toward sprague atoms, but I'm more than willing to have my mind changed!

Anyway, thanks for reading and any advice you may impart!

ICBM

Get the correct part even if you have to import it. Those can caps are hard to come by in the exact combination you need, and there's not a lot of room in a PRII to bodge it with separate caps - even if that was really the right thing to do.

Also make sure you replace the bias filter caps, and while you're at it, check that the 47-ohm resistor in the bias supply/footswitch supply has been uprated to at least 1W, preferably wirewound - the original is a 1/2W carbon which often burns out, and many techs replace it with another similar one, which just delays the return of the fault. If it fails, it usually blows the power valves since the bias is lost completely.

I've never particularly bothered about specific brands for electrolytic caps, so I can't advise on that, sorry!

Bubbarock

Great response, thanks ICBM. I'm perfectly happy to import if I can't find any in the UK, but I thought I'd find out first. Yes I've read about the 1/2 watt resistor being right on its tolerance, and it hasn't been changed in this amp so that will be on the shopping list too.

exocet

Tube Amp Doctor in Germany do them

http://www.tubeampdoctor.com/product_info.php?category_id=77&products_id=1917&mySID=Menyjuhogaqy-4OInHnLn3

jpfamps

I would agree with ICBM: it's much more straight forward to use an exact replacement part, especially in this case as using separate caps will be a pain to implement and you could run into grounding issues if you are unsure of what you are doing.

If I can I use German-made F&T caps for valve amp repairs. The specs of these are much better than Sprague Atoms (which are actually quite poor).

I always try to use caps with the highest temperature rating I can find (usually 105 deg C for 450 VDC and lower) as these will have much longer life than 85 degC rated caps (4 times longer in fact).

It does sound like your problem could well be the caps. Regardless even if it isn't, then you will have done some very useful preventative maintenance.

johnnyguitarman

Hot Rox in Nottingham have always been good for me.

Capacitors:

http://www.hotroxuk.com/amp-parts-service-equipment-294-0/capacitors-mallory-caps-sprague-orange-drops-6136-0.html

Resistors:

http://www.hotroxuk.com/amp-parts-service-equipment-294-0/resistors-7961-0.html

Bubbarock

I love Hotrox. I have in fact bought some from there now. Great service too.

Bubbarock

So she's had a partial recap (everything bar the cap can, that's next months job) and a new speaker as the original was suffering from a little coil rub. Blown away! Even the lead channel, that most sources on the net proclaim to be horrible, sounds great!

A little bit of routine maintenance and an amp that's been used and abused for 32 years can continue to make some great noise. 

jpfamps

Changing the cathode bypass caps in old amps always seems to improve the sound.

Bubbarock

I was hoping to take the amp out for a blast this weekend, but I was sent the wrong resistors to uprate the 1/2 bias resistor....lucky I checked them with a multimeter first.