Princeton Reverb DIY

[man-bot]

Cool - I see those Kendrick's are based off the ceramic jensens so I'm sure a broken in one will sound great.

I'll have to try the mid resistor change on my BF as with the bridge pickup on a tele it is tone all the way down or icepick city!  I was thinking of swapping in an alnico speaker (I have a tone tubby hemp cone kicking around) to try and tame it a bit.

You definitely have to let us know how it sounds!

 

[CJ]

ok we have some small items that take forever finished,

pwr switch, polarity switch and cap, 100 ohm resistors from heater to ground for simulated center tap-less hum, pilot lamp socket,

a few mistakes, wired the green heater wire to a grid which was kind of noisy,
left out a B+ wire to the 220k vibrato plate resistor,

we did add a  color dot mica across the 3.3 meg

sounds just great so far with no neg feedback, will dial that in tomorrow,

added a non lytic filter cap to the pwr supply, 10uf at 250 volts, preamp cap,

GZ34 makes this possible, solid state rect would pop the cap eventually upon start-up,

this was also an experiment in using 26 ga sheet metal, almost like tin foil,

all the components really stiffened up the chassis, pots, switches, 1/4 jacks, transformers, they all add up,

there is a 1/2 inch would panel underneath the main board for added stiffness,
capacitor boards also help,

less metal = less hum? maybe. lamination theory suggests a higher freq chassis,

 

[CJ]

bought a weird shirt for rock climbing, did not fit right so i hacked it up for grill cloth,

what color should i paint the faceplate?

amp sounds killer, pleny loud, sounds best on about 5,

so maybe switch from fixed bias to cathode bias on the pwr tubes,

that would be the only mod this thing needs,

vibrato is great but i think that the VibroChamp is the king of the vibrato amps,

winding the reverb transformer on a bigger core, 375 EI so i can use less turns and bigger wire, original uses 5000 turns of fine wire, don't want to snap a wire half way thru,

had a coupling cap give  up, one of the tubes started freaking out with a light show and weird sounds, so i upped the voltage from 400 to 1000 volts dc.

 

[buildafriend]

cool grill cloth.

 

[ed rees]

8) Clever idea to use that shirt for the grill cloth.  Blackface it.

 

[CJ]

blackface it is. or will be.

when the shirt gets all tore up, you just put a different one on.

i mean tie dye grill cloth is so 70's, 

 

[buildafriend]

I wonder what your gear list looks like.. I second black face.

 

[ed rees]

Find a British flag shirt for some Townsend vibe. Just don't get the urge to shove your headstocks into the baffle...   Just noticed the mis-matched knobs, love it. My Deluxe was the same way until I remembered I had a '46 Sylvania tube tester with Fender spec chickenheads.

I found eyelets in the leatherworking section at Hobby Lobby. $1.75 for a bag of 50 or so.  They're a little bit bigger than Fender's originals but get the job done. One of these reverb/trem units is next on my list, to fully compliment the new 5e3. I crave that boinky surf 'verb.

http://taweber.powweb.com/store/5h15t_layout.jpg


Can you share your specs for the reverb transformer CJ? I might have a go at winding my own when the time comes.

 

[CJ]

reverb transformer is super easy, 40 turns of #27, 2000 turns of #34= 50:1 ratio,

square stack of 375 EI with .003 gap pumps out 27 henries just like the original,

darn things are only about 17 bucks, i do not know where they get their copper, 

terrible frequency response, starts rolling off at 7000 hertz,

which is probably fine since reverb pans have a limited bandwidth,

there is a 500 pf feeding the 12AT7a in the reverb circuit, this limits the bottom end,

so top and bottom are chopped big time before entering the pan,

note that phase does matter with the reverb driver xfmr,

you will still get reverb, but volume will drop quite a bit due to the out of phase reverb signal getting mixed with the in phase dry signal. noise will drop substantially, if you want to do a weird studio trick for low noise reverb,

eyelets sound cool, how do you crimp them?

did you know that Fender boards can become conductive over time?

they sell a non conductive black board now, some old fender will act up when the boards age,

cool on the Weber circuit,  best Vibroverb is the first one, has 2 10's, better vibrato, and slightly lower plate voltage on preamp tubes which drops highs a bit and gives just a tad more crunch,




the Grovin Ghoulies are always changing cloth, plus they add stuff>

 

[CJ]

ok after a few days of ripping my hair out, i got this amp working right.

cj gets bogged down by a tube circuit, what the...  :-\

i changed the 10uf/400 volt ERO "designer cap" into a 150uf/350 volt cap.

there was 100 mv of AC hum on the last cap,  too high for this position in the pi network,

the new cap drops it down to 2 or 3 mv, which is normal,

there are 5 tube sections connected to "D", which is taken off the last cap.

one of them is the phase inverter, so there is a lot of stuff going on at 240 volts DC.

the amp is a lot more quiet, and still sounds great!

now with this stock AA-1164 Fender circuit, there are some problems.

when you turn up the reverb, you lose a lot of volume, and the reverb comes on quick.

now if you like Dick Dale reverb set on 11, don't touch the amp.

if you look at the circuit, there is a 100 K reverb pot and a 470K resistor off the wiper.

this is a resistive mixer, the reverb goes to the grid of the next tube via the 470K.

it is mixed with the dry signal coming thru the 3.3 meg.

this is where things get a little complicated. 

when the reverb control is set full on, all the reverb coming off the .003 goes to the grid of the next tube. this is a high Z node and there is no voltage divider.

so  the xtra strong reverb swamps out the dry signal which causes a volume drop.

there is a voltage divider for the dry signal, but not for the reverb signal.

i guess you need the dry signal to give the reverb signal some volume.

remember when i said i thought i had the reverb driver xfmr wired out of phase?

well it was really the circuit that was causing the problem, not the phase issue.

i should have known better, once you run a signal thru a spring delay there is no phase.

remember when i thought the amp was already loud on 5?

and also said that the Vibro Champ had better vibrato?

well all these problems,

1) loss of vibrato when reverb is turned up,
2) amp already clipping on 5
3) reverb too strong, pot comes on too fast

can all be fixed with one 220K resistor.

after checking other Fender schematics, i notice that all the other amps with reverb had a 220 K resistor coming off the 470K to ground.

it it fixes everything by forming a voltage divider to the reverb output signal,

and it also changes the voltage divider on the dry signal.

when the reverb control is set on full, the 3.3 Meg is forms a divider with the 470K and the 100K pot, = 570 K for the dry signal

when the pot is all the way off, you will have a 3.3 meg / 470 divider for the dry signal.

with the 220K installed, you have a more constant divider of 3.3M and about 150K.

this cuts the dry signal like we needed and cuts the reverb like we needed,

the reverb divider is gonna be 470k on top and 220K the bottom,

this means that about 1/3 the total reverb signal will appear at the next grid,

for some reason, vibrato does not modulate a reverb signal well,

so with the drop in reverb compared to the dry signal, the vibrato starts working again.

so everything is cool now, ready to paint the face plate and bolt this thing up.

does sound wicked good with a maple neck tele.  ;D

pic of the mod shown below.

you do not have to hang the resistor way off the board like the drawing,

this is just an aid to locating the tie in points.

and don't worry about the phase of the reverb driver transformer.

you could have a switch on this resistor for xtra strong reverb and a lot more crunch.

most people like the long tail inverter, i like the crunchy Princeton inverter running off 240 volts.

if you want to see a trainwreck vibrato circuit, look at some older Showman circuits.

there is a mod to get more vibrato for this amp, here is a really cool mod link.

and they use a 12AU7a instead of the 12AT7a to fix the reverb problem.

notice that there are many other Fender models on the home page>

http://fenderguru.com/amps/princeton-reverb

when troubleshooting, realize that you will have dry signal coming up thru the .003 cap backwards, i clamped the reverb driver tube grid to ground in order to isolate the problem and i still had signal present, wtf, over? 

 

[Ego Tripper]

eyelets sound cool, how do you crimp them?

With one of these:
http://www.tubesandmore.com/products/S-T1715

 

[CJ]

cool!

almost looks like a transfer punch

i think we could DIY that, 

here is a bias link>

http://www.tone-lizard.com/Princeton_bias.htm

i did use a ERO "designer" cap non polarized 4.7 uf at 63 for the first signal amp,



this was to limit bass to the beefier OPT so that i could use a bass guitar without popping the speaker, and the reduced speaker flutter means a tighter low end for guitar, and non polarized makes most effect in the first small signal amp.

oh, and a 6072/12AX7a first preamp tube for the funk.

and instead of installing an extra switch for the 220K boost, i used a 330 k to retain a little boost at the sacrifice of a little vibrato at max reverb,

here is a discussion on losing the vibrato when switching to cathode bias.

http://archive.ampage.org/threads/0/ampgen2/006550/Re_Fender_Princeton_Reverb-3.html

"trainwreck" vibrato circuit may be "real" vibrato, ie, vary tone by phase shift,

see Magnetone amps for cool vibrato circuits.

Fender liked the "bug" vibrato because it slammed the grid shut, other circuits did not have as much control over the signal,

now i can relax and get back to something really important, El,

crank up the Reverb for Rock Hula Baby>

 

[CJ]

still tweaking this amp, think i got it now, 

changed to cathode bias, sounds great! no more harshness, lots of tone,

i stuck in a 2200uf cathode bypas cap across the 270 resistor,

why? because this is also a killer bass amp! smooth, deep tone,

but you have to use an ext spk, like a 15 inch,

used it at open mic last saturday, asked the crowd how the mix was,

some guy in the balconey said turn down the bass! 

i figure it was the Princeton, not my playing, so enuuf bass to fill a small room, 

so i was thinkin that if if it is too loud, i can drop some pwr and switch to cathode bias and still be able to use it at the coffee house for bass.

now with cathode bias, i had to use the trick of grounding the  intensity pot as linked above to keep the vibrato workin,

and it worked! then i noticed that with this vibrato circuit, i could really lower the 1 meg resistor that keeps the fixed bias from getting modulated when you have the pot turned off.

so i stuck a 22k in there, and now i have a vibrato almost as good as the vibro champ! 

this means it will modulate a full on reverb signal without dropping out completely like before,

you know you have the vibrato runnin good when you can see the pilot light modulating with the vibrato, and the pwr tubes have that blue glow blinking pretty good also.

so very happy, great sound, reverb and vibrato workin better than ever,

now i do have a bigger output transformer, so your mileage might vary as far as using this amp for bass, but if you use a good 12 or 15 inch, it should work great.

ripped out the fixed bias components and use the board for the parallel cathode resistors, had a 400 and a 800, which makes about 270 in parallel and wattage to spare,

i might try the original OPT in there, maybe mount both and have a switch if it gets me some geetar mojo with the smaller Fender OPT,

so switch in the bigger OPT for bass and clean guitar, switch in the smaller OPT for distortion,

also thinking about running fixed bias and cathode bias at the same time for more wattage while retaining  some  compression, (see Drawing Room)

does anybody ever use the Intensity control at half volume? i always use it full up,

if so, why not combine a 3 Meg Speed pot with a switch, this would save space for maybe another control or a switch for some mod,

saw this done on an old Realistic transistor amp that i fixed the other day,

another note: use a single stereo phone jack for Reverb and Vibrato pedal,

saves room, hardware, and cabling will be neater,

i also added some wood rails for the amp to slide on when installing into cabinet,

do not like the Fender amps where you have to balance the amp till it gets to the front,

what do you mean just lay the cabinet flat when installing chassis, to easy!



here is a pic of the final chassis guts,

 

[CJ]

getting the last 0.5% of improvement out of this amp, which takes the most time,

2200uf was overkill for a pwr tube cathode bypass cap,

let us calculate the correct value,

what is minimum freq that we use thie amp at?

well, since we want to use bass thru this thing, we need the f of the lowest note,

who memorizes stuff like that? easier to use the web:

http://www.gollihurmusic.com/faq/25-FREQUENCIES_WHAT_ARE_THE_FREQUENCIES_OF_BASS_NOTES.html

ok, the E string = 41.2 hz if you are in tune.  what about Geezer Butler?

what if we want to play

"Rocket ships burning fuel so fast, up into the night sky they blast"

that sucker is in Db! (Master of Reality-Into the Void)

so lets use 30 hz to cover all the bases,

divide the ohms of the pwr tubes cathode resistor by 10.

270 ohms/10 = 27 ohms. 

now the degeneration on the cathode is reduced since the bass current goes thru 27 ohms, not 270 ohms

so if Xc=1/2pifL, we want Xc at 27, so 27=1/6.28*30hz*C

27=1/188*C, multiply both sides by C, C*27=1*C/188*C, C cancels on rt side,so

27*C=1/188, 27*C=0.0053191, divide by 27 to isolate C, C=0.0053191/27,

C=0.000197 in Farads, move the decimal back 6 places,

C=200uf.  anything bigger is Overkill, which is a Motorhead song. 

we have the bypass cap installed, but our pwr supply hum goes way up since hum is at 60 and 120 hz, which will get amplified more with the bypass cap,

this will get even worse if the tubes are mis-matched a bit, why?

well in a Push Pull circuit, hum is cancelled out in the output xfmr, but not if the tubes are off a bit,

so now we need a balance control for tube bias so we can match the tubes and really cancel hum,

how do we do that with a single cathode resistor?

we have a 25 ohm wire wound pot in the junk drawer, try that.

not only will most of the hum go away, thae bass sound will get better too. why?

less unbalanced DC in the OPT.  P-P transformers hate DC because they are ungapped.

just a little DC and the bass goes down,

so we installed the pot, now we have a quiet amp with a killer bass sound,

now we can run a JJ 6V6 on one side and a GE black plate on the other,  ???

 

[PRR]

> you will have dry signal coming up thru the .003 cap backwards, i clamped the reverb driver tube grid to ground in order to isolate the problem and i still had signal present, wtf, over?

That's normal. The plate node is maybe 40K (100K||60K). So the large signal goes through the 3.3Meg, the 470K, the 0.003u, and hits 40K at about 1/100th the level into the 3.3Meg. (I've skipped the 100K pot, leak-back is a little less.) (BASS leakback will be large downstream of the 0.003u cap, which is half-meg at 100Hz, swamps the 40K tube everywhere below 1KC.)

Grounding the reverb recovery grid does very little to change that. (If it does, signal is leaking some other path.)

The 220K *is* needed in the TWO-channel Big Reverb amps, which have another gain-stage (especially when they have opto-trem).

I don't see a problem. It's all the same player and axe. Not like we want to tap-out pure reverb with less than -40dB corruption from the dry signal. It isn't even an intermodulation issue. While dry/wet IM happens in V3B, it's gonna happen more in the power out section.

 

[PRR]

> 2200uf was overkill for a pwr tube cathode bypass cap

It is un-commercial.

But it may have musical effect.

> ....the ohms of the pwr tubes cathode resistor...

No, what the cap is facing is the *tube* internal cathode impedance, 1/Gm, twice, and the cathode resistor.

6V6 at some nominal current is 5,000uMho or so. Which is 200 ohms. The Fenders tend to run higher voltage and lower current, so lower Gm, but no large difference and the error is in our favor.

200||200||270= 73 ohms.

In a push-pull cathode-bias amp, with reasonably balanced drive, this capacitor has almost no effect on signal gain. (If you have freq-resp test gear, try it.) (Yes, it may have effect on PSRR.)

If the amp is perfectly biased and loaded, the cathode voltage hardly changes with signal from silence to clipping.

Guitar amps are biased and loaded MORE!!, and then taken far into clipping. The cathode voltage does change.

As the cathode voltage changes, the input clipping level and clipping character changes.

Guitar is a fast-attack slow-decay sound.

Naked guitar lacks the tonal range which woodwinds and singers get by overdriving their pipes; such musical expression is gotten by overdriving the amp.

So the way the bias and clipping changes over the envelope of a guitar note is important, and depends on the value of that cap.

For "even smooth tone" we might want a cap SO large the cathode voltage hardly changes a bit. Time-constant on the order of a second, more or less. Since R is about 73 ohms, this calls for 14,000uFd!! Your 2,200uFd gives 157 milliSeconds, longer than a plucked-string attack but shorter than string-ring decay.

Interestingly, conventional practice is 20uFd-50uFd, a couple/few mS, which is right there in the pluck-attack zone. This is of course also "large" in terms of early-1950s cap-prices, when 2000-14000uFd would have been absurd.

Note though that the better-loved old push-pull cathode-bias amps were also not so MORE!!-biased as later versions. Again if the amp is not so over-volted and over-loaded, the cathode voltage changes very little, and the cap does very little.

 

[CJ]

OK, PRR bails me out again! 

this darn amp, it has a problem so i mod the circuit to make it work anyway,

troubleshooting, you go from self doubt and introspection to joy and gleefulness, it is a vicious cycle that can make you pray to Jesus,
and this was no different, probably the weirdest problem i have ever solved in a tube circuit, and i built my first heathkit back in 66,(regen radio, worked like crap but picked up KFRC at night) so i have been at it a while,

here is what happened:
on just about every Fender schemo i have checked, the cathode resistor and cap layout for each tube section progresses in a normal matter, that is, on the circuit board, cathode hookup point goes V1a, V1b, V2a, V2b,

well, on the Princeton Reverb, the cathode hookup point for V3a and V3b are reversed!

this is because there is a 47 ohm resistor that goes to ground on V3b, so the layout is designed to make this happen in a neat way.

so i swapped the cathode hookups on V3. 

this means that the negative feedback from the output transformer now goes to the reverb driver tube, which now has the 47 ohm resistor hanging off the bottom.

now when i grounded the grid on the reverb driver, i still had a dry signal on the cathode of the reverb driver, which modulates the tube with dry signal.

remember what PRR said up there>

"Grounding the reverb recovery grid does very little to change that. (If it does, signal is leaking some other path.)"

well mr psychic, "some other path" was the mis-wired negative feedback,


so when you turn up the reverb pot, this dry signal gets shipped up to the end of the 3.3 meg. well when you modulate a cathode, there is no phase reversal like a normal triode stage, the signal off the plate will be in phase with the signal on the cathode, which in this case, happened to be out of phase with the dry signal coming in from the preamp tubes, so when you turn the reverb pot, you are adding amplified neg feedback coming from the reverb driver tube to the dry signal,

why didn't the vibrato still work with the reverb turned up?  this is interesting, vibrato does not work on a reverb signal that well! the echo covers it up, so when you use vibrato with reverb, you hear the dry signal with vibrato, not the reverb signal, but since they are mixed, it works fine, since i had all reverb signal and no dry signal with the reverb pot turned up, the vibrato could not be heard that well.

so what were my symptoms that were posted?  too much volume, no vibrato when the reverb is turned up, and also a loss of overall volume as the reverb is turned up, as well as too much reverb!  4 problems for one swapped lead.  :

so i tried to compensate by putting in the 220k to ground, which cut down on the out of phase signal which helped the volume drop  with the reverb pot turned up,and  which also helped cut down the excess reverb, but did nothing to help the vibrato problem with reverb turned up.

so i tweaked the vibrato circuit to compensate for that also.

but the circuit still had traces of the problems encountered.

so i tried changing the .003 to see if leakage was reducing gain on V3b by leaking DC onto the grid, but no joy.

finally i spotted the error, rewired the cathodes correctly, and viola! everything is working, gain of the amp is back to stock with the neg feedback on the right cathode, no volume reduction when the reverb is turned up, reveb pot works great, too much vibrato since i modified it,  ;D,

i was wondering why changing the feedback resistor value did not do anything to the dry signal, well with the reverb pot turned off, the neg feedback does not hit the 3.3 meg,

here is the layout so you can see the trap i fell into, look at the cathodes for the third tube from the right:

http://www.thevintagesound.com/ffg/schem/princeton_reverb_aa1164_layout.gif

now i have a vibrato that thumps real nice with the intensity turned all the way up, think i'll leave it for special effects, 

some Fenders have an even amount of gain stages which means that the amp plays "forward" or in phase.
some Fenders have an odd number of gain stages, blackface amps can have normal and vibrato phases different, so you can not daisy chain a Y cord into both channels, the signals will cancel and sound horrible,
OPT's are always wound so that pri and sec are in phase, you cn verify the phase by checking feedback polarity,
watch out for speakers, some old jensen types have reversed polarity,
stick a 9 volt batt, + on spk +. and see which way the cone moves, if it moves out, the speaker is plus polarity,
Marshall amps play out of phase, so watch it when combinig Fender and Marshall.

i had the OPT leads swapped, which resulted in neg feedback as i was injecting it onto the grid and not the cathode due to the reversed cathode leads.

gonna  yank the 220K to see which way is best,

here is a pic of how i had this amp wired, the U's are phase trackers>

 

[CJ]

that's a wrap!

this thing is bolted up and done.

last mods were:

neg feedback switch on the back of the amp, switches the 2.7 k in and out.

feedback sucks on the guitar, but improves sound when a bass guitar is used,

so now we can use both bass and git with no compromises,

12AY7/6092 plugged into V2, where the 12AT7 used to be. reverb pan driver.

tried a 12AU7 but not enough gain. 6092 sounds great.

auditioned rectifiers, using Sovtek 5Y3. GZ34 was too harsh, not enuff droop,

383 volts with JJ GZ34, 370 with Sovtek 5Y3, 343 with Sylvania brown base JAN 5Y3,

336 with what must be a tired RCA 5Y3.

higher voltage adds more highs, so you can tune the amp tone with the rectifier,

lower voltage means more compression, so we are right in the middle,

removed tube shield on V1. has influence on sound, capacitance and neg plate to e-,



loving the cathode bias, as note decays, less degeneration,

so gain goes up as the note volume goes down, this means more sustain, nice ring,

put a tele pickup on a L A G 12 string acoustic, plugged it in,

sounds wicked with reverb and vibrato, listen to Fred Neil, The Dolphins,

http://www.youtube.com/watch?v=FO6CNj5YVF8

30 minute DIY pickup job mount needs cosmetic improvement, 

saved big money on a new pickup and install job, plus no holes in the guitar>

 

[CJ]

this amp again,

ever try the XLR to phone jack adaptor cord for running a mic into an amp?

it kind of works,  Green Bullet harp mics sound ok, but vocal mics can leave a lot to be desired, so we yanked out the #2 jack and put in an XLR, nobody uses 2 instruments at once anymore, except the garage band back in the 60's when parents only had enuff cash to try their hand at getting their kids as rich as the British bands so they could steal the money and retire early,

should be interesting, the Neve input xfmr package is small so that you can cobble it into the inside of the chassis without using a jack hammer so we will wind on the DU core,

maybe scaling resistors to get the mix right or even a secret vol control on the back, maybe some tone shaping caps to get it to work with the guitar tone stack, ok then...