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Old August 9th, 2010, 12:06 PM   #21 (permalink)
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CAPS!

Well, I'm gonna put in an order for some new caps. Seems I didnt have as much laying around the house as I thought.

I'm inclined to change the whole lot of caps. Looking to get some orange drops in place of the big blues.

Want to make sure that the ceramic disks can all be replaced with Silver Mica. Is this OKAY???? ...or should I stick with ceramic disk????

kM

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Old August 9th, 2010, 04:11 PM   #22 (permalink)
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Kosmo, as I noted, I am not a big proponent of changing these poly caps for grins.....just if they are bad or if I want a tonal change. YMMV. I like the effect that silver mica has for the treble cap. That .047mfd mid cap ....brown cap....has a bad reputation...noone seems to like them.
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Old August 9th, 2010, 04:19 PM   #23 (permalink)
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Wally,

I am a bit apprehensive changing all the caps for grins myself. I am of the school that "if its broke don't fix it".

However, I'm wondering if the original tone caps are bad / leaky - which could be giving me the terrible flub-out at higher volumes. This is why I'm thinking of installing new tone caps to see if the new ones at stock values resolve the issue. If not, then I'll move on to experimenting with different values.

I don't think my DMM can check capacitance - so I really have no way to check them unless I invest in a capacitance meter. Maybe I should get one? I'd rather go through and check the caps out to see exactly what I need to replace.

What do think?

kM
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Old August 9th, 2010, 04:51 PM   #24 (permalink)
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I'd say replace the tone caps with stock values, but solder in those quick connect thingies, so you can just swap them in and out in the future if you wanna test tones. Also, PM me the values you need for Orange Caps, I ordered several Orange Drops for tone caps in a guitar, but they were the 400v ones or whatever so they are huge. I might be able to save you some dough on shipping.
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Old August 9th, 2010, 04:54 PM   #25 (permalink)
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Quote:
Originally Posted by Commodore 64 View Post
I'd say replace the tone caps with stock values, but solder in those quick connect thingies, so you can just swap them in and out in the future if you wanna test tones.
"Quick connect thingies" ??? :)

Is there some kind of socket for this? I was not aware. I've heard of IC sockets...I think I may just stick with soldering.

kM
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Old August 9th, 2010, 05:54 PM   #26 (permalink)
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Kosmo, I think that putting in new 'stock' value caps woudl be aninteresting experiment. I am going to guess that things won't change much, but that is just a guess. ORder a few different values..... .1, .068, .047, .033 and .02 you can always find a use for them, and you might find that this amp needs a different value other than stock to do what you want it to do.
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Old August 9th, 2010, 07:19 PM   #27 (permalink)
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I ordered several different values or orange drop caps to give them a whirl.

Wally, are you thinking the stock caps are probably okay and I simply may need a tone mod to get past this trouble? I would have figured Fender wouldn't wnat to release an amp that's only limited to 3 or 4 on volume. Besides I know there are lots of folks here who say they can crank their PRs with great results.

Then again, this may only be the beginning of troubleshooting to figure this out. I would not be able to accomplish near as much without everyone's expert opinion and guidance. So again, thanks to Wally and everyone here for all of your help! :) It's means a lot that everyone takes the time to help a fellow fanatic!

kM
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Old August 12th, 2010, 03:16 PM   #28 (permalink)
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OK - I'm back and I'm armed with a multitude of caps and a free evening!

My plan is to try things out gradually to see what happens. I'm gonna start with tone caps (hopefully all I'll have to do is adjust these to taste) The more I read about about flabby bass, the more I realize its a problem that plagues many of the lower wattage BF and SF amps.

I'm going to replace the reble cap with a silver mica (stock value), bass and mid caps will start with stock values and then I'll experiement with the 3 or 4 values I have just received. Gradual, slow, and tedious, but worth it to me. My PR has been a very useful learning amp. Might as well not skimp here, either.
:)

I'll post my findings as soon as I can...

kM
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Old August 12th, 2010, 09:57 PM   #29 (permalink)
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Tone Caps: PART I

Tone Cap Adventure:

Changed the tone caps tonite with pretty good results.

Treble Cap: 250pF (same value, but changed to new silver mica)
Mid Cap: .022uF
Bass Cap: .047uF

I was able to get the amp up to 6 before my upstairs neighbor came knockin'. Needless to say my tone tests lasted only minutes. But I was pretty happy while it lasted. I'll have to go to my rehearsal space to really see how the amps acts when its cranked.

Sounded good so far! No more flub...at least up to Vol 6. I used to get it pretty bad anywhere around 4 and up on the volume.

I did have to keep the Bass knob down pretty far - was able to experiment between 1 and 3. Its touchy, very touchy though. As I eased the bass knob closer to 4, the bass response really starting coming through. So I wonder if I can get a more gradual bass control if I went to a lower value - like .022uF???

The mids seemed alright, but again I really didn't have the opportunity or time to test without p*ssing off my neighbors. That's Chicago for you!

If I can make it to the space this weekend I'll post full results! Can't wait.

Thanks again to everyone here! I think I'm on the right path to Princeton redemption!

kM

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Old August 13th, 2010, 01:49 PM   #30 (permalink)
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Your on the track.....
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Old August 13th, 2010, 02:02 PM   #31 (permalink)
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I've been reading.....a lot. But nothing better than experimentation!

In my studies, I've read that some people have been changing values to the coupling caps in phase inverter circuit.

Should I give this a whirl as a well or try to stay focused and accomplish this through changes to the EQ circuit? So far I'm happy just being able to change the bass cap to do this, since I'm not disturbing much else or changing out good, pre-existing caps and such.

I definitely think I'm getting there, but it would be nice to have a more gradual bass knob. Would this entail changing the resistor value on the pot?
I can always try to reduce the bass cap down to .022uF, which is likely what I'll try first. But I still need to run the amp through the paces before I do anymore though. Need to see how much higher I can get it before it flubs out. If I can reach a happy medium by changing tone caps and adjusting my Bass and Treble knobs I'll be in business.

Just taking a stab....
????

kM
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Old August 13th, 2010, 03:24 PM   #32 (permalink)
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Kosmo, I noted those possibilities way back in this thread, too. There are a lot of ways to affect the problem you have. They all have their benefits. IN that amp, the 5U4 would allow you to bump up the capacitance in the first stage of filtering to 40 or 50 mfds. This tightens low end considerably. You coudl simply parallel another 20mfd/500 cap onto that first stage section of the multisection can.
Changing those .1's in the PI/power section to ..02's also helps the low end stay firm. Lots of options here. Get busy....(;^)
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Old August 13th, 2010, 03:33 PM   #33 (permalink)
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As always, thanks again Wally. I got some experimentin' to do....

btw - I did discover that the 5U4GB IS the correct rectifier for my '72 PR.

Any of these possible changes recommended over the other?



kM

Quote:
Originally Posted by Wally View Post
Kosmo, I noted those possibilities way back in this thread, too. There are a lot of ways to affect the problem you have. They all have their benefits. IN that amp, the 5U4 would allow you to bump up the capacitance in the first stage of filtering to 40 or 50 mfds. This tightens low end considerably. You coudl simply parallel another 20mfd/500 cap onto that first stage section of the multisection can.
Changing those .1's in the PI/power section to ..02's also helps the low end stay firm. Lots of options here. Get busy....(;^)
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Old August 13th, 2010, 04:02 PM   #34 (permalink)
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The only options if you are looking for strong low end would be a GZ34 or a solid state. IN both cases the voltages woud go up, so you have a consideration regarding the power tubes ability to handle the voltage to take into account. JJ's would handle it, but everything else would be a question mark???
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Old August 13th, 2010, 05:10 PM   #35 (permalink)
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Not so much looking for a ton of strong low end. I really just want this amp to be able to handle being cranked and not flubbbb out.

I guess I'll have to see how it performs for a while and go from there.

I was really pleased with the results from merely "halving" the bass and mid caps. Still had to keep the Bass knob low as I turned up louder, but I can live with that for now. So far so good....

kM
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Old August 13th, 2010, 05:18 PM   #36 (permalink)
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YOu can make as many changes as you want ...one at a time.....and you will learn a great deal about your amp. IMe, turnign the volume up on BF FEnders necessitates keeping the tones lower. AT lower volumes, turn the tones up to maintain presence. I guess this is why I don't have much trouble with 'flubbing'.
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Old August 13th, 2010, 05:21 PM   #37 (permalink)
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You know...at first I didn't buy it. I really didn't think I would have turn down the tones below 5 at higher volumes. Then I started hearing how it helped. But, seriously, without lowering the bass and mid caps I was in FLUBBBB city.

Now that I armed with some knowledge on these caps, I have a whole new set of tweaks to try....one at a time, of course ;)
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Old August 19th, 2010, 12:24 PM   #38 (permalink)
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Update!

So i finally had a chance to test the PR in a full volume situation at rehearsal last nite.

I still have some problems with crackling / speaker flub at higher volumes.

Started at Vol 4 and all was good. Even started breaking up nicely with my tele bridge PU when I hit a hard Emaj chord. Moved to Vol 6. and above. Its okay when I play lightly, but when I dig in I still get the same problem. The crackle/fartiness is not as present right away, but instead appears as the notes ring out. Its like its being choked or something. Definitely sounds bad.

I had my bass knob on 2 then rolled off all the way. Treble was on 4. No verb or trem for the tests.


Does this still sound like something tone caps could fix or could it be something else as well? I guess my next logical step to the best of my curreent knowledge would be to try an even lower bass cap value. I'm at .047uF right now. I have .033 and .022 caps to try out....but the bass seems well enough reduced with the .047uF in there. The tone is acceptable, but this crackling is still driving me crazy.

Could my tubes be biased too hot?
They're currently at 392/393VDC 26.3/23.3mA respectively.


kM
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Old August 19th, 2010, 07:32 PM   #39 (permalink)
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Truthfully? You are seeing why I sold my Princeton Reverb. I could never get rid of the flub no matter what "fixes" I tried.

I wish you all the luck in finding the answer to this. They really are great sounding amps up to a point on the dial.
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Old August 19th, 2010, 08:04 PM   #40 (permalink)
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I personally think you're biased too hot, but it probably isn't causing all of your "problem". If you do the math using your hotter tube you are biased at 72% of max if you use 14 watts as the basis for the tube (which isn't super hot) or you're at about 85% of max using 12 watts. If you're running JJs your probably okay with this setting, but unless you really liked the tone better than running the amp cooler I don't think you're gaining anything by biasing the amp that high.

I usually run my '67 PR between 14mA and 19mA and only run it at the higher end if I notice a tonal difference that I prefer. I think running it lower also smooths out the trem and give you more modulation in most cases. Just for fun it may be worth dialing yours down to where the middle ground is about 19mA to see if anything changes.

I also mentioned this in one of your very first threads and I'll repeat it again in a slightly different way. I think you need to very carefully listen to your volume level starting at the point on the dial right before you start hearing your farting and flubbing. My guess is that the amp isn't getting very much louder when it first edges up to the flub and that it isn't getting any louder at all beyond that point. Basically, I'm betting that you're amp is very close to being dimed at this point by virtue of the pot taper. It would be interesting to put a multimeter on your volume pot to see how much the resistance changes between 6-10. The pull-boost PRs that I have had sitting next to my PR all have the same symptoms as yours whereas it doesn't start until 8.5 or more on my amp. The relative volumes of the amps are the same when the flubbing starts, however, if you ignore the numbers on the dial. The '70s amps may just have a more linear taper volume pot where I get all of the fine adjustment before 8 when the amp is running clean and all of your adjustment happens to come when it's close to putting out it's maximum.

I could be wrong, but I think that at this point you're pretty much fighting the taper of the volume pot and the nature of the beast even though you might find a slight improvement with cap changes. It's hard to fight your eyes when you see 5 or so on the dial and the amp is breaking up terribly. I really thought that there was a problem with the last pull-boost PR that was in my living room until I listened with my ears instead of my eyes.

This is all personal opinion, and I'm not a tech, but I think you may find that there is nothing to gain after that point on the dial if the amp is electronically almost maxed out.
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