Blackface Princeton Reverb Service

[Steve S.]

Hi All
I have a BF Fender Princeton on the bench for service.  It has all the original Electrolytics in it still.  I would usually just change them all out but this amp has the original astron cap can [4x20uf @ 450].
Are these cap cans less prone to failure then individual caps?  Is there a reason that I should consider just leaving the can and changing the bypass caps and power and screen resistors only.  The amp sounds good, no particular problems other than pots which need to be cleaned and a grounded power cord.  I can get a cap can with the same values if I choose to do so. Also, I've read that a GZ34 is a better choice than a 5U4 for the rectifier because of the slower warm up.
Any input would be much appreciated.
Thanks,
Steve

[DummyLoad]

clean the pots and replace the grounded power cord. GZ34 will bump up the B+ so i'd leave it as stock as possible, 5U4 and all. 

--DL

[Fresh_Start]

I cleaned up an original Blackface Princeton Reverb a couple of years ago and wrestled with the same questions.  The amp shocked people, so a 3-prong cord was first on my list.  The filter cap for the bias circuit was toast and the power tubes were biased really hot.  The owner wanted adjustable bias anyway, so I pulled the entire bias board and installed a new one.  Easy enough to restore (other than solder joints).  Scratchy tone pots lead me to leaky caps in the tone stack - those got replaced once I narrowed it down to 2 caps.  One cathode bypass cap was obviously leaking goo and the paper cover rotated freely, so I replaced that too.  After those changes, the amp sounded fantastic so I left everything else alone.

What does the owner want?  A noodler/collector has completely different needs from a studio guitarist.

Unless the owner tells you to replace everything that might be a problem down the road, I'd only take out what was absolutely necessary.  That old rectifier tube is worth some money if it still does its job.  Leave it alone.  

Unless the power resistors have drifted so much that voltages are way off in the power supply, I'd leave those in there too.  

Screen resistors - the original didn't have any so if this amp already does, someone has been inside there before you.  The amp I worked on was untouched, but I did add screen grid resistors.

I mapped out voltages throughout the amp, all of the resistor values and the caps that I could measure in circuit.  Most of the resistors had drifted up in value, some more than others, but none were completely out of bounds to me.  (A 300K plate resistor for a 12AX7 clearly qualifies as "out of bounds" for me.)  Other than the leaky caps, the rest were reasonable too.  Of course, they'd generally drifted to lower values.  Sure, some voltages were "off" but none more than 20% after I was done.

Any chance you have time to map voltages, both on tubes and on the power rail?  I'd be really interested in plate resistor and cathode resistor values too.

Just in case you're interested, I've attached the bias board layout I used.  I did have to drill one hole in the chassis - with the owner's approval first!  Had to tweak a resistor value to get the right voltage range (18K net) and added a 100uf cap in parallel with the second filter cap (attached to bias pot) to clean up the bias voltage ripple.

Have fun in there!

Chip

[Steve S.]

Thanks for the responses,
So, you wouldn't worry about the original filter caps failing and damaging the PT?
 Some of the other Electolytics including the Bias cap, are showing signs of leakage so I think I will replace them.  I haven't played through one of these amps in a while, it's really sweet.
Steve

[cbass]

If I was working on it.I would definately replace ALL the old electrlytics.I would leave the 5u4 a gz34 will up the voltages a bit.
It really depends on what the customer wants.
A collectors peice to sit around and look at.Or a reliable amp to play through.

But what the hell do I know.

[Willabe]

I agree with cbass.

Those caps are too old if it's a BF. If a cap in the B+ is leaking current to ground, it's stressing the PT, by making it run harder. If a cap in the B+ shorts, it most likely will take out the PT. 

[HotBluePlates]

I don't believe a collector would have an issue with a replaced cap can. Find out if the owner is okay with changing it.

I'd probably change all the electrolytics, although the preamp cathode bypass electrolytics may be something you'd leave as-is, if the owner likes the amp's present tone. They tend to dry up and drop in effective value. That shaves the bass somewhat in a pleasing way. If you swap them for new ones, the amp will sound like-new, but it may also increase the apparent bass. If the owner really likes the current sound, you might not want to mess with them; or if you could measure their capacitance, you could swap with a cap with a similar value. In the Princeton Reverb I used to have, the bypass caps dried up to around 1-5uF each, and sounded really good that way.

[Steve S.]

Thanks to all of you for your input.
The owner is open to whatever I recommend as far as replacing components. He wants a reliable amp to play.  I feel like its better to have non original filter caps than a non original PT due to cap failure..
I guess my main question is; do the cap cans dry up and fail at a certain age same as individual caps or do they tend to last longer, even indefinitely, because of the sealed nature of them.
Steve

[PRR]

The cap-can is not "sealed". It is the same stuff as single caps. But in one lump. To save money. To be cheaper than single caps, it can't be higher grade.

Yes e-caps fail on a timer. But nobody knows what the timer is set for. 40+ years if everything was just-right. 3- years if you got a bad batch (like the Dell and other PCs of a few years ago).

I tend to let healthy caps stay, but for a non-museum working-gigs amp going past 40 years, I think replacement may be the best path.

[Fresh_Start]

I'll gladly defer to the others - especially PRR and HotBluePlates.  Replace all of the electrolytics on the power rail i.e. the cap can unless the customer objects (mine did BTW).

Also like HBP said, the cathode bypass caps in the one I worked on were all in the 2-10uf range and I left 'em alone except for one which was visibly leaking goo.  The owner liked the way it sounded.  He just didn't like getting shocked if he plugged it in wrong!

If your customer will go for it, adjustable bias is a big plus in this amp.  Especially with the bias vary tremolo.  Bias too hot and you can't hear the tremolo; too cold and it just sounds bad.  Frankly, I think that's part of why Fender changed over to the opto oscillator diddling a preamp tube instead.

Cheers,

Chip

[HotBluePlates]

Okay, since you're gonna swap that can cap, a couple tips:

- You'll need a solder gun, or a BIG iron that's 100w+. The tabs which form the - end of the cap are also part of the outer casing, and tend to be folded over and soldered to the chassis. That's both to hold the can in place and form the connection from - to ground (in this case, the chassis). A 100w+ iron is needed to heat the chassis & cap enought to avoid cold solder joints in this area.

- Get a 4-section can from Antique Electronic Supply. Hoffman has a less-expensive multi-section cap, but the original mounts in a 1-3/8" hole, and the JJ cap from Hoffman needs a 1-1/2" hole. AES' CE Manufacturing 4x20uF, 475v cap is an exact match for the original part. Select "Capacitors" -> "Multisection" -> "FP Type".

[Shrapnel]

Okay, since you're gonna swap that can cap, a couple tips:

- You'll need a solder gun, or a BIG iron that's 100w+. The tabs which form the - end of the cap are also part of the outer casing, and tend to be folded over and soldered to the chassis. That's both to hold the can in place and form the connection from - to ground (in this case, the chassis). A 100w+ iron is needed to heat the chassis & cap enought to avoid cold solder joints in this area.

- Get a 4-section can from Antique Electronic Supply. Hoffman has a less-expensive multi-section cap, but the original mounts in a 1-3/8" hole, and the JJ cap from Hoffman needs a 1-1/2" hole. AES' CE Manufacturing 4x20uF, 475v cap is an exact match for the original part. Select "Capacitors" -> "Multisection" -> "FP Type".

OR if aesthetics are important, leave the cap can (if you don't have a replacement one that fits or want to mess with trying to remove and replace) and leave it unconnected (or shorted to ground - no other connections) and replace with individual caps inside the chassis. Either way.

[Steve S.]

Thanks for the tip HotBluePlates!  I have my Dad's great big Weller iron, he repaired TVs and radios for 35+ years.  I think I will replace the can since I can get a drop in replacement so easily.
 On some Silvertone 1482s and 1484s I have replaced the can with individual caps located inside the amp and left the disconnected can in place.  I use Zapadappa glue to glue them to the chassis, it sticks really well and stays somewhat soft so you can remove it if you need to.
Thanks so much for all your input and info!
Steve