Princeton Reverb Cap Job

[RightLurker]

I'm replacing the electrolytic capacitors in a BF Princeton Reverb. Will it hurt anything to use 25 mfd/50 volt bypass caps in place of the 25 mfd/25 volt caps called for in the schematic? I assume it won't make any difference, but thought I'd better ask some people who know what they're doing before I stick them in.

Thanks.

[Randy Magee]

Won't hurt a thing...

[selloutrr]

It's safe to increase the voltage just not lower it.
As long as you keep the mfd the same the tone will remain unchanged.

[selloutrr]

Just a heads up incase you didn't know the "Y" shaped 25-25mfd at 25volt caps. F&T makes a direct "Y" cap replacement for around $8 if looks matter but if performance is all just use two 25mfd / 25 v and share the ground.

[RightLurker]

Thanks for all the replies, and nice to know about the dual caps - I didn't know anyone made those anymore. I have a bunch of 25 mfd/50 volt Spragues, so that's what I'll use.

[RightLurker]

What effect on tone, if any, would raising the value of the filter caps in a Princeton Reverb from 20 mf to 80 mf have?

The Princeton Reverb I'm working on was serviced by a nationally-known "guru" about ten years ago. He replaced the original can capacitor (4 x 20mf @ 450V) with four, made-in-Japan 80mf @ 475V units, which are held in place with heat-resistant silicone. (For reasons I don't understand, none of the other electrolytics were replaced - they're original, from-the-factory parts.)

Aside from the fact that the filter caps look sloppy with all the silicone goop, they're wired in just fine, and the amp functions. So my question, again, is what effect, if any, would raising the value of these caps from 20 mf to 80 mf have? I'm tempted to leave the four 80 mf units in place, although I do have a new 4 x 20mf @ 475V can on hand.

Thanks.

RL

[krash]

Increasing the capacitance of the filter caps will change the tone noticeably. It has less of an effect downstream of the first one. I typically don't go much larger than 47 (aka "40" or "50) uF for the first filter cap in any amp, but 80uF is not off the chart. However, 80uF for all four is probably overkill.

If the amp works I wouldn't mess with it.

I would check the bias supply cap and make sure it is fresh or at least a decent cap. While you are at it, there is a diode on the bias supply board that bears replacement. Heck, replace the Rs too.

The other electrolytics in the amp are cathode bypass caps that are large enough in value that even if they drifted to 50% it would not make a noticeable difference in tone. So there is little value in replacing them unless you have a reason to know they are bad, such as they are leaking DC or they are shorted. If you are going to go to the trouble of replacing them, there is no reason you HAVE to replace them with a) electrolytics and b) caps of the same value. And most of those are in circuits where they will see a working voltage of no more than about 3 volts, so 25V is also overkill but a 25/25 cap is small and readily available. I think most people would find a PR to be improved if the V1A cathode bypass cap were replaced with a 0.47uF-1uF film cap, and perhaps even if the others were replaced with 2.2uF or 4.7uF film caps.

Of course, don't bother listening to me. I have done a TON of mods to my PR. I didn't like it stock, not even close.

[krash]

BTW the effect on tone of replacing the filter caps with 80uF will be that it will sag less and generally feel/sound "stiffer".

[David Root]

Also probably reducing your rectifier tube life, 80 uF is a tad large for a 5AR4.

[RightLurker]

Thank you kindly, Krash and David. Interestingly, the amp guru who worked on this thing previously swapped the GZ34 rectifier tube for a 5Y3. All of the bias circuit components - with the possible exception of the diode - were replaced with metal film resistors and Sprague caps (he put in an extra cap). I haven't checked the bias voltage yet. I think I'm going to recap the amp (I didn't know you could use non-electrolytics for bypass caps - I'll be sticking with electrolytics because that's what I have), change the rectifier back to a GZ34, put in some fresh tubes, and take it from there.

Again, thanks for the help.

RL

[10thTx]

80uf filter cap exceeds the max rating of capacitance for 5Y3GT.

with respect, 10thtx

[RightLurker]

Thanks, 10thTx. That tears it. The whole amp is going back to stock, or as close to stock as current parts will allow, with the addition of 470 ohm screen resistors.

RL

[Structo]

Does the name of the nationally known "guru" start with Gerald Weber?

[David Root]

Changing a 5AR4 out to a 5Y3, then putting an 80 uF cap on the 5Y3 sounds like Gerald! I know that he has done that kind of thing before.

[Firestorm]

With rectifier tubes, you can increase the "maximum" capacitance of the first filter if you increase the total effective supply resistance. Check to be sure your "nationally known guru" didn't add any extra resistance to the primary or secondary of the PT (if it was Gerald, I'm sure he didn't). One of the problems with using a can cap is that all of the grounds have to be in the same place, which in a Princeton puts the preamp filter grounds too close to the output tube ground, so output tube cathode currents can find their way into the preamp. If you are recapping with individual caps, you might want to move the preamp filter grounds closer to the input ground. As to values, stock should be fine. As krash says, more filtering will stiffen the power supply, which is noticeable in the output. Larger than stock caps also have a less noticeable effect on preamp response: since power supply nodes are all AC grounds, the caps and plate resistors form low-pass filters in series with the signal so they can affect the frequency response of the stage.