SuperBee
1558 posts
Dec 02, 2013
11:35 PM
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I recently picked up a couple 12ay7 tubes. Decided to try one in the SF PR.
It's been a long time since I inspected the tubes in this amp.
In V1 was a fender branded 7025/12AX7. Ditto V3.
V2 was a Chinese "audio classic" 12AT7. V4 was another 12ax7, a sovtek I think.
I put the new 12AY7 in V1. I also changed V2 to an old Phillips miniwatt 12AT7.
I was expecting to get more pre-feedback sweep on the volume. But no. I couldn't really notice any change. Maybe the old tube was worn out?
This amp has reverb I don't love, although I didn't mind it so much today. Also, the vibrato is non-existent.
I haven't scared up a schematic yet, but I think it's probably time to pay attention to it. It's had a little work done on it; new PT (mojotone), standby added, bias adjustment pot. This is a '78 model, pull-boost PR. I've read that replacing the volume pot can make them a little easier to get along with.
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5F6H
1704 posts
Dec 03, 2013
4:53 AM
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The Princeton Reverbs have a lot of gain, so subbing V1 may not have as pronounced effect on pot sweep as on other tolex Fenders...you should still notice a change in character though?
Try subbing V3 too, and/or tweaking the volume pot (tacking a resistor from centre to ground to tame sweep).
Check the .01/.01/.022 oscillator caps to see if one is broken, also check the cathode bypass cap at the oscillator tube's 3.3K cathode resistor...it is likely however, that the amp is biased hotter than designed & this has an adverse effect on the tremolo (bias trems are more pronounced at cooler idle currents, if you bring the current down, then watch dc voltages vs. tube & cap ratings).
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Rick Davis
2743 posts
Dec 03, 2013
7:19 AM
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I'm not sure this is helpful, but the vibrato is driven by 1/2 of one of the 12AX7 tubes. If the value of the tube has been changed (for example, to a 12AT7 or 12AY7) the vibrato will stop working. I discovered that the hard way....
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-Little Rick Davis
The Blues Harp Amps Blog
The Mile High Blues Society
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Greg Heumann
2491 posts
Dec 03, 2013
8:42 AM
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Dropping one tube form a 12AX to a 12AY will result in a nearly undetectable change. I just looked at http://fenderguru.com/amps/princeton-reverb
If that's your amp:
(Seen from behind, V1 is to the right side)
V1 12ax7 = Preamp
V2 12at7 = Reverb send
V3 12ax7 = 1/2 Reverb recovery and 1/2 Reverb mix
V4 12ax7 = 1/2 Vibrato and 1/2 Phase inverter
V5 6V6 = Power tube #1
V6 6V6 = Power tube #2
V7 5U4GB = Rectifier tube
That means the the ONLY tube you can swap and affect nothing else is V1. To make a meaning full difference you'll need the biggest drop you can get. Try a 12AU7 there. If that isn't enough and you're willing to sacrifice some or all of the vibrato performance, try a 12AU7 in V4 as well.
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***************************************************
/Greg
BlowsMeAway Productions
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Last Edited by Greg Heumann on Dec 03, 2013 8:47 AM
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5F6H
1705 posts
Dec 03, 2013
9:24 AM
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@ Greg "That means the the ONLY tube you can swap and affect nothing else is V1. To make a meaning full difference you'll need the biggest drop you can get. Try a 12AU7 there. If that isn't enough and you're willing to sacrifice some or all of the vibrato performance, try a 12AU7 in V4 as well."
Swapping V4 may well kill the vibrato? V3 will lessen gain & change tone on the reverb, but also reduce gain on the dry signal.
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Last Edited by 5F6H on Dec 03, 2013 9:24 AM
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1847
1357 posts
Dec 03, 2013
9:26 AM
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one thing to consider is the fact that quite often
nos tubes made in the fifties, or sixties, have much more gain than more modern tubes
simply replacing a 12ax7 from the good old usa,and replacing it
with its more modern equivalent, can sometimes reduce the gain significantly.
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i get a lot of request when i play my harmonica
"but i play it anyway"
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Greg Heumann
2492 posts
Dec 03, 2013
9:26 AM
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Thanks, Mark
I don't have that amp to play with. Are you saying a V3 swap WILL lower gain? I was just going based on the quoted description of tube functions above.....
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***************************************************
/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
BlueState - my band
Bluestate on iTunes
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TetonJohn
173 posts
Dec 03, 2013
9:46 AM
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FWIW, I gigged recently with my Blackface Princeton Reverb (very similar to SF): V1 AY, V2 AT, V3 AX, V4 AX (that's right to left of course looking at the back).
Everything worked fine.
(The reverb does get big really quickly, so I have thought of dropping V2 to an AY, but haven't tried that yet.)
Edit: Pretty sure I had AFB+ and LW Tone+ in the chain, definitely driven by Shure533S; IIRC, Vol. 5.5 (thank AY and AFB+ for that); Treb 3, Bass 7 (thank Tone+ for those); Reverb barely nudged up; Vibrato speed around 7 or 8; Vibrato Intensity very low,maybe 2 (was not looking for much vibrato effect).
Last Edited by TetonJohn on Dec 04, 2013 9:29 AM
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jukelemon
36 posts
Dec 03, 2013
10:04 AM
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Bee:
My .02.
If you are trying for the most volume without help from a pedal, change out your v1 to a 12ay and sub in a 5y3 rectifier for your current 5u4.
You may need (likely) to rebias your tubes after that as plate voltage will drop across the whole circuit.
The PR is a very loud/powerful/clean amp for its size - it has way too much plate voltage to get dirty on its own.
Report back :)
Last Edited by jukelemon on Dec 03, 2013 10:05 AM
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5F6H
1706 posts
Dec 03, 2013
11:47 AM
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@ Greg "Thanks, Mark. I don't have that amp to play with. Are you saying a V3 swap WILL lower gain? I was just going based on the quoted description of tube functions above....."
Hi Greg, yes it should drop gain a bit...half V3 is in the wet signal path, but the other half is also in the dry path, so reverb apart, it is still one stage you can mess with...may not be enough & may not be to preference for the wet tone, but worth a try.
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SuperBee
1559 posts
Dec 03, 2013
12:27 PM
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Thanks everyone who commented!
I haven't checked the bias, but I think it is a likely place to start looking regarding the lack of vibrato. I can barely discern any effect at all from the speed and intensity controls. Perhaps there is a change in tone, almost a 'prescence' effect, but no oscillation I can hear as such.
It has a bias adjustment pot located where the original extension speaker jack was. I have never touched it. Previous owner told me the full range of adjustment was acceptable, but I'd rather confirm that for myself.
This amp does not present any problems in terms of being too clean. I'd probably rather it were a little cleaner if anything. I can turn it up to 3 with a fairly hot CR/CM mic. Yes i expect there was a tone change when I subbed in the 12ay7.
I'm home alone the next couple days. I'll try a couple of things.
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Greg Heumann
2494 posts
Dec 04, 2013
7:52 AM
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Vibrato and tremelo circuits depend on an oscillation-generating circuit which usually involves a tube and an array of capacitors and resistors. At least on the Kalamazoo tremelo, it is heavily dependent on the tube's gain - change out the Zoo's 12AX7's for anything else and the tremelo won't work. That's what Mark and I were trying to suggest above - you have to be pretty selective about which tubes you swap in that amp without affecting one, the other or both of those auxiliary functions. When they're weak it CAN be a tube issue but can also be a bad capacitor or (less likely) resistor in that oscillation circuit ....
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***************************************************
/Greg
BlowsMeAway Productions
See my Customer Mics album on Facebook
BlueState - my band
Bluestate on iTunes
Last Edited by Greg Heumann on Dec 04, 2013 7:52 AM
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HawkeyeKane
2191 posts
Dec 04, 2013
8:30 AM
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Does the PR use a photoresistor on the oscillator?
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Hawkeye Kane
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5F6H
1709 posts
Dec 04, 2013
10:50 AM
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No photoresistor, it's a bias vary trem. The hotter you idle the power tubes the less the trem can swing.
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SuperBee
1566 posts
Dec 04, 2013
5:39 PM
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yeah i reckon thats the most likely reason the trem doesn't work on this one. what you've never known, you dont miss...
but i had better investigate the bias question. i'm probably not getting the best from it
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jukelemon
39 posts
Dec 04, 2013
6:32 PM
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5F6 is spot on with his concern about the Trem in general.
However, in my experience I have never known a bias type Trem to be biased so hot that there was not at least something. You could at least hear a faint trem with the Intensity all the way up.
I bet you have something else going on in addition to the bias issue or as the root cause.
If one of the caps in the trem circuit is leaking, you will get nothing as if it doesn't exist at all.
Should be a easy fix either way.
Just another perspective/road to look down.
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SuperBee
1567 posts
Dec 04, 2013
6:38 PM
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yeah good call. its due for service anyway
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Barley Nectar
204 posts
Dec 06, 2013
12:28 PM
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SupperBee, Here's a link for ya. Scrool down to "Weak Tremolo". I put a 270K R at the point indicated when I built the Super Princeton. Dude, the trem on this thing will make women glassy eyed. Give it a try...BN
http://fenderguru.com/amps/princeton-reverb
Last Edited by Barley Nectar on Dec 06, 2013 12:29 PM
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SuperBee
1684 posts
Feb 19, 2014
12:31 AM
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i tried some different tubes with the amp, included a JAN Phillips 5751 in, V1 which seemed almost exactly where the original 7025 was in terms of gain...
nothing i did made much difference...until today when i put a 12au7 in v1.
it was weird though. when i first fired it up, i had volume up to around 8, but later i noticed i had reduced the volume to 5 and the thing was still plenty loud...
it sounds good for harp but i think it probably is defective. folks keep telling me how its gonna be clean and hard to push into break up...but its not clean-sounding. even with guitar it is hard to get clean tones. and the trem is just not there.
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SuperBee
1940 posts
May 03, 2014
5:17 AM
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The ox is slow but the earth is patient:
Found the PR has B+ 320vdc.
PT producing 335 vac from the secondary. Mains voltage running a little low today.
Schematic indicates b+ 420vdc.
Amp was running 5u4. Is GZ34 'correct' in this model? (78 pull-boost)
Possible explanation of why amp is not bright and clean.
Bias around 12 milliamps. The bias pot was able to create a difference of 1.5 ma.
Pot appears wired incorrectly, centre bridged to earth via left tab and 22k resistor.
Last Edited by SuperBee on May 03, 2014 6:50 AM
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Barley Nectar
379 posts
May 03, 2014
7:39 AM
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Supe, The bias is way cold. Your tubes are dissipating 3.84 W. those 6V6's are rated at 12W max unless there are JJ's which will handle 14 W+. You want 50%-70% of 12 W. Also, does that amp have the original power supply caps in it? You are loosing a ton of voltage somewhere in your power supply. All the electrolytic are too old if stock and need to be replaced. If original, that is probably part of the problem. That bias pot is wired wrong also, There should be way more adjustment then 1.5mA. That 5U4 is correct. I think you need a good tech there buddy...BN
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5F6H
1776 posts
May 03, 2014
1:04 PM
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+1 on what BN says, something is eating up over 100vdc of your B+, don't worry about bias until you have sorted the bigger issue.
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SuperBee
1941 posts
May 03, 2014
3:47 PM
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Too right.
Yes, I believe the technician had the wrong schem. Regardless, the rectifier tube would not affect B+ to the tune of 100vdc.
Bias adj pot mod has been incorrectly executed.
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rockmonkeyguitars
73 posts
May 04, 2014
3:20 AM
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Although a 5U4 rec is technically correct for later models, early Princeton reverbs used a GZ34 and I highly recommend them over the GU4. There is a dramatic boost in the tremolo effect with a GZ34.
There are also a couple very simple and easily reversible mods you could do to the amp to help your vibrato. There if you were to open your amp and follow the wire coming from pin 1 of valve 4 you would see that the wire connects to a 1m resistor and a .02uf cap that connects to a .01uf cap that connects to another .01 cap. If you bridge the 1m resistor with a 470k resistor that will deepen your vibrato intensity. If you increase the value of the .01uf caps it'll slow down the speed. On my princeton I actually use a 330k resistor and .02uf cap and it gives me a vibrato with the depth of reverb amps that use an opticoupler and the speed you would expect from an 18watt marshall.
Any reasonable tech would be able to do these mods and it would take longer to unbolt that chassis than it would take to actually do the mod. Because it sounds like your amp needs a bit of TLC anyway. I suspect the filter caps are old and need replacing, this is common for any amp that hasn't been used in a few years or if the amp is regularly used they still get old ater 10 to 15 years and should be replaced.
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5F6H
1777 posts
May 04, 2014
3:38 AM
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@Superbee "Bias adj pot mod has been incorrectly executed." That's not possible to determine at this time. It actually sounds correct - 2 terminals in use (3rd terminal bridged to wiper) in series with a 22K resistor to ground?
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SuperBee
1944 posts
May 04, 2014
4:09 AM
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Amp is in shop right now. Unable to confirm any details. In any case B+ is primary issue. What I can say for certain about bias mod is that it made very little difference to bias. Whether that is due to wiring error or complication from another problem I don't know. According to quick look at mod diagram wiring appeared incorrect. But I concede there are often more ways to kill a cat than by choking it with butter.
Edit: iirc though, there was a lead from one end of pot to the intensity pot. The other 2 terminals were bridged to ground via 22k resistor. This seems to be lacking a connection to the bias current.
Last Edited by SuperBee on May 04, 2014 5:55 AM
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SuperBee
2005 posts
May 20, 2014
4:30 AM
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the cat came back.
B+ was a red herring. Measured at the wrong point. B+ is actually 418 volts.
Tremolo problem was a cap which had come unhooked.
Bias adjustment pot was incorrectly wired. Now fixed.
Amp is now developing 12 watts..
Running a 5751 in V1, 12AT7 V2, 12AX7 V3, 12AT7 PI.
'Original' (who knows) 6V6 tubes are well matched and test like new.
All caps and resistors test good,
Weber 12A125-O sounds good
Tested tonight, all sounds good, but couldn't really test the feedback threshold. Still, I could tell I'm happy with it. Nice to have the effects back in action.
All my amps are just workaday-type units. This old thing is the closest to collectible I own.
Now, back to wiring the heaters in my 5F1 project...
Last Edited by SuperBee on May 20, 2014 5:56 AM
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Barley Nectar
397 posts
May 20, 2014
8:19 AM
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Nothin like a healthy Princeton. Cool. Harp on Bro...BN
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