'69 Princeton Reverb.....Badly Burned, Brutally Botched, and (finally) Reborn

well it's time to open a box and this one could well contain a very interesting project it comes from a gentleman in Denison Texas apparently he rescued a Princeton or Princeton Reverb chassis from a fire and wants to put it back into a cabinet and turn it back into a nice fully functional Princeton Reverb that's my understanding at least so let's open the box and see what we've got now how about this for a first class packing job layer of plywood to protect the contents here from being crushed and then several layers of high-density foam over the amplifier which looks fantastic a silver face Princeton let's get it out of the box and see what we got alright we've got the amp up here on the workbench let's take a look at what we have to work with it appears that the fire damage chassis has received a brand-new silver face face plate and knobs and then it's been fit into a 68 silver faced with the aluminum trimmed grill cloth cabinet okay so far so good it looks really nice there is a tiny little blemish right here in the grill cloth but otherwise this thing could pretty well pass for new looks like probably a good used handle and then new metal trim strips here have been provided the tolex is good on the cabinet but it shows somewhere so obviously it's a used cabinet from 1968 but gently used let's turn it around and see what we can figure out from the rear view all right I removed that rear panel and we can see that the fire damage to the rear panel on the chassis is fairly significant only a portion of that sort of a cover plate has been preserved the chassis itself looks pretty shiny and nice underneath okay if any of you have or know where you can buy a replacement rear control panel for a silver faced fender Princeton Reverb please let me know because it would be a really nice touch on this amp looking downstairs we see a Jensen 10 inch a speaker strangely devoid of any perimeter markings but it looks like a suitable speaker for this amp and on the bottom we have a brand new for a b3 c1 b ack atronics reverb tank okay there are no cables that I can see but it does have the three more power cord and it has a replacement can cap already installed the output transformer seems a little loose the we have the reverb driver transformer and I see no tube shields for the 12ax7 s and 1287 we've got a whole slew of tubes with it hopefully we can come up with one good set from all of these tubes it's obvious that this chassis has been worked on as I mentioned earlier the caps been replaced and these are brand new toggle switches so some work has been done inside let's I've unplugged the speaker let's unscrew the chassis drop it down and take a look and see what's been done and what needs to be done all right let's pull those four chassis retention screws and drop this jewel down slide it out and see what we got we can see from the serial number that this amp chassis was created in early 1969 all right looking down here at the top of the chassis we see that all of the tube sockets have been replaced the reverb driver transform as fairly new what it would that be probably the 40th week of 2015 I'm not sure what the EIA code indicates 989 is a rather high number and I don't have that on my list so this is new as far as the power transformer it it and the output transformer were both made by Schumacher which is appropriate and it appears that the part number is correct so this could be an original power transformer I would say the same about the output transformer except I see a 905 here now maybe that 69 the fifth week but I tend to doubt it since the chassis was built in 1969 that would have been cutting it pretty close but it is the correct number so it's it's hard to tell it might be an original it's just been cleaned up and of course here is that replaced can capacitor now let's flip this over and take a look at what wiring changes have been made well it would appear that the wiring changes are substantial virtually what would you say 75 80 percent of all the components have been replaced even the bias supply looks like the new tube sockets have all been wired in okay let me make an observation now the reason I always get excited when these things are pristine and untouched is that I could trust the wiring because the amp worked at one time now once they've been extensively rewired like this all bets are off I pretty much I'm going to have to go in and check every single connection in the circuit and every component to make sure that it's correct okay and that's a whole lot more work but it looks like the parts used or a reasonably good quality I think one thing that might be good is to stick in some tubes plug it in and see what happens and go from there okay we're plugged into the new and improved current limiter speaker is connected and I'm going to turn this on I checked the original emails from the owner and apparently he had this worked on by some technician in his area and it was working for a while and then it quit okay so that gives us something to go on let's turn it on and see what happens now all right the tubes are illuminated I'm getting a power home from the speaker but even with the volume turned up it responds to the volume except no amplification when I ground the input you know it should hum loudly and it doesn't okay let's give it bass in trouble just to be sure there's nothing ok so just with that simple test it looks like the output tubes are probably functioning fine I'll need to check the voltages but the problem is going to be down there probably in the preamp section and to narrow it down even more I changed from channel 1 to channel 2 channel 2 seems to be working so we need to look here with a channel 1 input there is a problem and I am also smelling some heat here so we'd best check into the biasing of the output tubes 2 while we're at it well I made the huge mistake of touching the output tubes to see if they were hot and after a quick trip to the emergency room for skin grafts I can tell you yes they're hot and look at this this is kind of spooky look how the plate connection the tip of the wire protrudes out here and touches over to the filament connection that can't be good and they are in contact okay that's that's just terrible let me clip that little toenail off there here's another discovery look at this it looks like a short between the two input jacks here I tell you this not to just strictly be critical but when you are wiring an amp it's very easy to bend the contacts away from each other so you've got a little air gap there and you can eliminate some short circuits well the whole problem with channel one not working has been explained by that inadvertent short I just showed you and it wasn't inadvertent the shield here from channel 2 was actually hardwired to the signal output from channel 1 so all the signal from 1 it says do I want to go to the 7025 or do I want to go to ground well guess where it went so now that that gap has been opened there and that wire has been removed I think our channel 1 now will be able to produce some sound unfortunately though the bias is at about 900 watts from what my finger will attest from touching the 6 v 6s but let's let's look at the plate current here just for the heck of it in these output tubes here's the resistance of the half winding of the output transformer to the left 6 v 6 and to the right 6 v 6 you see they're very close now let's see what kind of voltage drop we get across these resistances okay we've got a voltage drop around sixteen point eight volts let's calculate the plate current well I think we have a winner in the highest plate current I've ever seen on a 66 how about a hundred and nine point five milliamps whereas normal is what twenty five to thirty okay you can see why the tubes were red plating and roasting my fingertip okay let's check the bias supply and see what's going on with it okay now look you just go sit down somewhere so you don't faint let me show you something here this is pen five of the left hand sixty-six that's the grid do you see any bias supply or signal input to the grid I don't do you know what happens to a 6v6 when you don't have any negative bias on the grid well let's see I'm thinkin one hundred nine point five milliamps okay now for the first time in recorded history there actually is a wire running from the output here to the grid of the left-hand 6v6 let's hook on a clip and check to see if there's a about a minus 34 volts DC grid bias being applied to this tube pin well it's still rather low it's minus 28 and minus 34 is about right let's check the bias again now with some negative DC being applied to the grid well a little better than negative sixteen point eight volts minus five point two let's see what kind of plate current we're looking at what a difference of wire makes now the plate current on the Left 66 is thirty three point nine milliamps on the right is twenty seven point one times the plate voltage of three forty six point three and we've got plate dissipation on the Left oval eleven point seven on the right of 9.4 not a great match but it's a whole lot better than it was before okay so that missing wire really made a difference our plate dissipation was simply too high on our 66 s as I recall is around an average of ten watts so I increased the resistance from 22 K up to thirty K here in the bias circuit you notice I didn't have at the decay resistor so I did two 15s in series and now let's see what our final two bias is okay here's the final two a bias with the 30k resistor in the Maya circuit as you can see the plate dissipation is eight point nine and seven point nine which averages out to a perfect eight point four watts now some of you will say well that's not a very good match but I'm going to say something now that's sort of a blasphemy probably too accepted thinking and that is with guitar amps I don't think that perfectly matched output tubes are necessarily a good thing okay in hi-fi circuits yes you want to minimize your distortion and home completely but in a guitar circuit first off a mild mismatch like this or even if this were six point nine you won't get really noticeable home and you're gonna get a whole lot more complex harmonic spectrum output with mix mismatched output tubes and you will if they're perfectly matched okay now I know I'll probably get some disagreement in the comments but this is something that I kind of picked up just by listening and experienced I know it's counterintuitive from what we're all told but trust me on this try it yourself set up an amp with mismatched output tubes and see what you think to me it actually in many cases sounds better we'll take a gander at the box that the Postal Service dropped off here sent by second day air all kinds of fragile markings and electronic equipment which really piqued my interest let's get out the old razor blade and see what's inside how about this for a beautiful packing job all sorts of foam protecting the contents this gem was sent by Jack Allen in Oklahoma and I can tell you an advanced Jack I think I'm gonna really appreciate it let's dig down through all the foam and see what's inside well this looks like quite the surprise package a bunch of miscellaneous small items nutone lighted push button looks like a doorbell from the old days look at the price of taller 69 looks like one less old l-pad type of intercom or music volume controls a couple stereo headphone junction boxes and now this is nice an old fashioned reverb tank which I believe is going to go with the larger object that's buried in here so let's get this out so we can excavate down and see what the main contents of this box are well I guess Jack realized that I have a weak spot in my heart for Alamo amps because they're made in Texas and look here this is a Futura reverb okay some of the components have been borrowed it looks like but they're very basic ones input jacks and toggles look at that chassis it appears to me that this has a transistorized preamp so it's a hybrid circuit with a tube output section here which was typical of Alamo in the latter half of their period of existence you can see the output tubes here probably a phase inverter and good stout transformers this looks to me like it's going to be the basis one heck of a neat project check and for that reason I want to thank you and I know the viewers want to thank you for your generosity and sending it to me I envisioned maybe even designing like a tube preamp over here to feed the tube output section down here but time will tell we'll see meanwhile oh thanks again I feel compelled really to start checking every component and one of the first ones I checked is this resistor is supposed to be 18k but as you can see it's not so I'm going to replace it so now we have a proper 18k resistor here two watt metal film I measured this one and it is 1 K so that's correct now I'm going to go down the line and check every component on the eyelet board to make sure that it is a proper value and connected in the proper place ok I'm using the signal generator to put out a 200 cycle per second tone into the formerly dead input 1 and we see that we can hear just bond tone controls have some effect let's check the river I've got cables connecting it to the tank and of course it's stone-dead okay now let's try the tremolo oh it was already on and I sure don't hear it so Tremeloes dead reverbs dead but now channel 1 & 2 or generating decent amplified tones and the app itself is nice and quiet now that I found a way to ground the channel 2 input jack when it's not in use this self grounding feature had never been connected on it ok I pulled out the reverb tag to see if it had any the packing still in it and it doesn't it's loose and ready to work and I'm putting grommets in each of the screw holes gonna add two back here so that you have some rubber cushion between the tank and the cabinet the grommets are in place and it's time to screw the tank back to the floor of the cabinet loosely I don't usually tighten the screws down real tight but keep it where the tech can kind of wiggle around a little bit also a point to be made is that you see this jack here on the tank is labeled output now common sense would say that the output from the tank would go to the input of the amp and the output of the amp would go to the input of the tank but just forget all that because no common sense rules here the way it's done is output to output input to input ok counterintuitive but that's the way it is well a tremolo is just giving me fits ok I believe I have the reverb working but I even changed all three of the capacitors at 0.02 and 2.01 some of the wiring resolder the connections but I found something that makes me a little nervous and that is this look here this is how a socket pin axe when it's really connected to the tube pin now look at these two here watch this look at that it just flops around that one and the one next to it like they're not connected to anything so I'm gonna have to turn this over and see if there's something wrong with these two socket pens now I know this is just way too easy and it will have no effect whatsoever but I tightened using a little tiny screwdriver each of those clips that grab a hold of the two pins so now I'm going to reinstall the tube and I'm gonna try I tried two different 12ax7 so I'll try a third one just for the heck of it okay and we'll see what happens all right two pins are grew in the grip of death now there's really good contact turn upper tone let's crank up our intensity and speed nothing debtor and adorn it I think I may have found the problem here with the tremolo if you look right here at the junction of the one mag resistor the point one cap and the point O to cap the B+ is connected there now I'm going to show you the schematic in just a second and you'll see that the B+ was supposed to be connected right there okay let's take a look and see what you think okay here's the junction of the point one the point O 2 and the 1 Meg resistor I don't see any B+ connecting there but look here at the end of the 220 K resistor there it is I believe then that there is a serious wiring error here and that this red wire should actually connect over here I'm going to do that and we'll see what happens it's either going to be great from a low or a lot of smoke okay as you can see I have removed the B+ from this junction and moved it over here to the tail of the 220 K resistor as it shows in the schematic okay you're seeing it at the same time I'm seeing it signal generator is on crank up our tone ready one two three and we have tremolo with a nice intensity at 10 speed it's actually too fast I might slow it down and you know how that's done is we're going to increase one of the point Oh ones to 0.02 and that will slow it down but listen that sweet tremolo what a relief you know situations like this make me think that when I'm giving estimates to people on amp overhauls I think one of my questions should be has it been worked on before by a and I could it in quotes technician and if so I'm gonna double the okay as I've explained in previous videos by reducing the value of the this resistor I'll show you on the schematic which one from 1 megaohm to 500 K we have greatly increased the intensity of the tremolo now it's time to slow it down by changing this point a 1 microfarad cap to 0.02 I'll now demonstrate the difference with it of being point O one versus point O 2 okay here's the speed at point O 1 there is a speed at point O 2 now when we go all the way up to 10 it's not fluttering there's actual individual pulses and at five it's a good usable level and you can go real slow down at one the 1 megaohm resistor that I'm reducing to 500k is this one that is exiting from the plate of the 12 X of an oscillator tube in the circuit it's right here and it points right at that nut on the tremolo tube which is the fourth one from the left so now the tremolo has been optimized we're at a setting of 5 now it's as stout as it was at 10 before and the speed is probably about half what it was I can see that the three wire power cord wasn't wired correctly the switch is in the hot wire the fuses in they return which of course isn't a good idea so I'm gonna have to rewire all of this primary circuit all right now the black hot wire passes through both the power switch and the fuse before it connects to the primary winding of the power transformer the white return wire connects directly to the other primary transformer lead I also see that there is no grounded center tap for the 6 volt filament winding there might have been a center tap but it looks like there's a wire that's been cut off short so I'm gonna create a virtual Center tap for the filament circuit and try to reduce the home I'll also explain to you how it works okay here's a before without the center tap for the filament winding listen to how the home response to the volume control and gets really nasty up here around 8:00 okay now let's listen to it after the virtual Center tap has been added okay I've installed the virtual Center tap 2 to 100 ohm resistors across the terminals of the pilot light which is really the most convenient way to do it and the other end goes directly to ground also I added a shielded wire from the input jacks all the way back here to the grid of the 7025 now listen barely audible power hum and that really awful buzz is gone ok now let's discuss how this virtual center tap works as you may know when you have a center tapped transformer winding you create two separate half windings and each one of them will produce a signal that is 180 degrees out of phase with the other winding this is the way output transformers work this is the way that the high voltage winding works so that we can use full wave rectifiers on these two signals and in this case since we had no center tap for the filament winding the output was in phase both of them okay so by creating this virtual Center tap with a 100 ohm resistors we've created opposite phases in the filament wires now when you wind them like this the opposite phases neutralize the electromagnetic field that they can produce and therefore eliminate the 60 cycle hum that you were hearing just a few minutes ago okay I've been through every component wire connection the river tack the driving transformer I've tested the tubes I can find nothing wrong now with the reverb circuit and yet it's extremely weak at 10 it's like about a 1 setting so then I started coming up here tracing this wire up to see how it fit into the circuit and of course there is nothing here between it and the grid but to the left I saw a 3.3 Meg resistor but look in here at the circuit this is it right here panas 3.3 k not Meg and I'm wondering you know I'm grabbing at straws but I'm gonna put in the proper resistor here and see if it makes any difference okay I replace the three point 3k resistor with a 3.3 Meg resistor as is specified in the schematic it's in place so I'm gonna put this camera on a tripod I'm gonna turn this thing on and hook up with guitar and strong strong a few chords and I swear to you if this doesn't fix it I'm gonna take an axe to it okay and I'll just say it was damaged during shipment or something like that okay next scene it's gonna be a do or die for this amp okay I have the amp plugged in the guitar is plugged in I'm going to turn it up to a volume of like 3 it's absolutely dead quiet now and it was humming before I know you probably noticed that dead quiet let's just do a couple course to get a volume check [Music] okay now with the axe and one hand and the guitar and the other which sounds redundant I'm going to turn the reverb up to five here goes a little bit of oscillation let's try 10 sounds like we've got reverb the amp is spared let's take a few moments here to analyze why this resistor which is actually before the reverb circuit enters into the regular amplifier circuit why that value would make such a huge difference in the reverb now here is why this resistor is critical to the reverb circuit it creates back pressure in the preamp circuit okay it's without the resistor here or with a three point 3k resistor the current will flow straight through to the output tubes but when you put the beaver dam right here you force a bunch of the signal to come down here and go through the reverb circuit then the amount of it here is controlled that is sent back up to mix with the dry signal so what you want here is a resistor of sufficient value to force signal to come down here through the revert circuit but not so high resistance that it doesn't allow enough dry signal to come through to mix right at this point right here in many ways you could think of this resistor as a overall plate resistor for the entire preamps circuit because it functions like a plate resistor in that it forces signal to take two separate paths and it controls the amount of signal that takes each path thus with the properly chosen value for this resistor you have a certain amount of dry signal arriving at this point and you force a certain amount of wet signal which is moderated down here by the reverb intensity pot to arrive at the same point for mixing I noticed in later versions of this circuit that a 2000 Pico farad capacitor is placed between the tank input to the 12 ax7 and ground I believe that this is to cut down on the oscillations that tend to occur in river tanks we heard a high frequency oscillation in that first test of the reverb so I'm going to place a 2,000 Pico farad cap in this position on this circuit and see if we can't eliminate that oscillation how it works is that as you know high frequencies can pass through low value capacitors so if an oscillation is trying to set up here instead of being able to come through here to the grid the high frequency oscillation will take this shortcut to ground to that capacitor and here is the point O 2 micro farad cap between the grid of the recovery triode of the 12ax7 and ground now let's strum a few cores and see how our reverb sounds ok let's have a preliminary test here of all looking trolls and effects this is with the volume at four and I'm using humbuckers treble and bass or at five [Music] now let's try the tremolo it five and five [Music] now the tremolo at 10 intensity five speed [Music] now the reverb at five [Music] now the reverb at eight [Music] Reverb attend [Music] now the combo platter reverb at five and tremolo speed and intensity at five [Music] okay let's try some single coil pickups now at a higher volume I'll crank it up to seven with the treble and bass still at five reverb and tremolo off [Music] [Music] volume at 6:00 reverb at 5:00 [Music] [Applause] [Music] volume at 6:00 reverb at 5:00 [Music] well before we turn this over to all and Jack for a couple of their Tunes let's discuss what's happened here first off the nightmare that will occur when you turn over an amp for extensive repairs by an incompetent person it makes it much much worse and about 10 times harder for the next guy to fix on the plus side however an amp like this with incorrect value components and and bad wiring gives you all sorts of insights into proper repair practices and unusual things that might pop up from time to time that you'll have to fix yourself I guess the bottom line here is you don't take your kids to some back-alley pediatrician and you shouldn't take your amps to some shady tech okay because you really don't get what you pay for you're gonna pay probably a fairly high price to get some really shabby work and it's gonna be a setback for you you're paying in other words to be harmed and now that just doesn't make sense so without further ado let's turn this reborn circuitry here over to the tender mercies of Holly and Jack for a few familiar tunes and then we'll sign off also I was pleased that that capacitor that I added right here into the revert circuit did seem to eliminate that high frequency oscillation that we heard in one of the early tests [Music] [Music] [Music] [Music] [Music] [Music] [Applause] [Applause] [Music] [Applause] [Music] [Music] [Applause] [Applause] [Music] [Applause] [Music] [Applause] [Music] well I guess that's about it for this lure its saga bad wiring incorrect components and seemingly insurmountable repair obstacles I hope you were not overwhelmed by the drama and survived to share with me the joy of victory I also wanted to express my appreciation to my paypal contributors and my patreon patrons whose generosity has kept us on the air for another month advertising free for those of you who would like to join them check the links in the video description also if you have any old amplifiers that you think might make interesting video subject matter please contact me so we can discuss it meanwhile thanks so much for your time and interest appreciate it and hope you all stay healthy and stay tuned bye for

Channel: Uncle Doug

Views: 108,522

Rating: 4.9545012 out of 5

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Id: 3kduQMM4qOI

Length: 44min 47sec (2687 seconds)

Published: Tue Mar 05 2019

Reddit Comments

[u/shrugs27]

This dude gives me serious ASMR and teaches me cool stuff, I love it.

[u/digbychickencaesarVC]

always a happy moment when he uploads a new video