1978 Fender 135W Twin Reverb Kicks Ass.....in more ways than one

well greetings out there in YouTube land and welcome to today's video recently I was approached by a viewer too and asked if I would repair their Fender amp and to save money I had him sent just the chassis and the river tank and now you're wondering what type of Thunder chassis travels in a box at this size and weighs close to 50 pounds okay if you're as curious as I am to see it let me set down the camera open the box and we'll check out what's inside well first off I gotta say that this gentleman could give lessons in how to pack amplifiers look at this everything is surrounded by foam with inlets for the river tank a tube set a letter here of instructions this is probably give a giant schematic and want to say giant I mean it you can use this for wallpaper okay let's get all this laid out and see what we got and now lo and behold after unpacking all the miscellaneous accessories we see a 135 watt Fender Twin Reverb from the top here it looks pretty darn clean and nice okay we'll go through the letter and discuss what's wrong with it and we'll check out the parts that were sent and then we'll flip it over and get started and to get this out of the box it's probably gonna take an engine hoist I don't think I've ever seen a power transformer any bigger than this one or an output transformer absolutely incredible this is probably where 40 of the 50 or so pounds of weight is located right here at this end all right I've laid everything out here on the workbench of course the chassis the old tube set and look at that talk about a mismatched set of 6l6 GCS there's no two that are remotely alike okay a set of original the smaller tubes of 12ax7 s no doubt maybe an 87 and here is a new tube set the original reverb tank we'll have to test it to see if it's still working the reverb cables and a fuse holder and a monster a schematic suitable I guess in dimension and scale to the amp itself this should be easy on the old eyes to read and we also get a letter that describes the problems with the amplifier let's take a look at it okay let's go through this quickly the amp comes from 1978 obviously a silver face it must have been used in some disco punk band lots of burn marks of wax covering the insides maybe from melted capacitors or transformer insulation of the potting also there was a marijuana cigarette inside I'm glad the drug sniffing dogs at the airport didn't pick up on it now apparently he fired it up literally set the volume to five and strum Danny chord and pretty much demolished his house and his relationship with his neighbors I'm assuming this thing may even be louder than a Marshall amp there is a horrific hum okay he tried to bias it and the hum persisted naturally we're going to be thinking of electrolytic capacitors only here at home depending on the frequency 120 Hertz and we'll be thinking electrolytic s-- 60 cycle and it's every man for himself there's about 900 different reasons okay so that's about it let's go ahead then and get started we'll put all of our miscellaneous parts to one song let's take a look in the doghouse and see and if we're looking at a whole set of brand-new Mallory filter caps then that's gonna kind of shatter my theory about electrolytic being at fault but let's clean things up and then take a look inside the doghouse oh and one more thing before I get started on the dismantling he wants the master volume control removed from the circuit okay that's kind of the downfall in any of these later 70s large silver face amps is when they started putting master volume controls in them the value of the amp just dropped precipitously because people just don't like it okay it's it's like if harley-davidson started to sell a steam-powered motorcycle or something people just aren't going to accept a whole lot of fooling around with a classic circuit okay so he wants that removed naturally once it's removed this won't suddenly double in value but I'm a feels it interferes with the use of the amp and so we'll remove it any of you who would like to join in on that then we'll see how I do it on this one and it might help you if you choose to remove the master volume control on yours if Jack is in the mood he's going to demonstrate one of his greatest tricks for us apparently he's not in the mood good boy Jack where is he now he's vanished where's jack well I think the chances of any speedy easy repair just went out the window when we look in here and the electrolytic have all been replaced with F and T's and a couple Asian caps here which are just fine in my book so unless one of these is installed wrong which I'm going to check right now I think we can pretty well eliminate electrolytic as the source of the problem also it might be a good idea to test each one of these to make sure that it is fully functional sometimes new ones aren't it's very rare I almost never see it anymore but you never know I'm pleased to report that that serious home problem was quickly and easily resolved simply by using the output tube hum balance control right here to balance the two pairs of 6l6 GC output tubes as many of you know one of the big advantages of a push-pull circuit is that the 2 out of phase output tubes tend to neutralize any hum that might be presented to them by the power supply this is why you're able to get away with sending relatively unfiltered B plus up here to the plates of the output tubes I don't know if you ever wondered about that because this really is not well filtered B plus at all that's being sent to them but it's not an issue normally because the out of phase tubes will neutralize that hum but if the pairs in this case or single outlet tubes in most cases are not balanced then there is a differential between them of home that is actually amplified so it's essential that you especially with for altitudes like this that you balance the pairs all right the amp is functioning you can hear the signal generator tone in the background but we've got a serious problem with micro phonics listen to this for that coming through the speaker this one to a lesser degree bad listen to this one virtually nothing nothing see normally tubes are just not going to make noise through the speaker when you tap on them but this one has some serious problems now what can you do about it actually replacing the tube or putting the tube in a different location may help okay but it's an internal problem with a tube that you cannot solve externally and in this case it's really serious look what happens when I tap on the table here it [Music] kind of an equi sound on the speaker this tubes got a problem alright let's flip the chassis over and take a look at the circuit itself you can see down here and the power supply where you have your full wave bridge rectifier and looking at the eyelet board it appears that every single non polar cap and electrolytic has been replaced okay now that's a mixed blessing if it was done properly great okay and it looks like good quality orange drop caps but if it was done improperly then the nightmare begins they're trying to hunt down where the mistake was made now this is why unmolested amps are always much easier to work on you can't assume that when it left the factory it worked therefore even if you have a faulty component at least it's the right value and in the right place with an amp like this it's been worked on a great deal there are no guarantees in that regard I think step one will be to clean all of the potentiometers I've noticed all kinds of problems with the volume control where you'll get volume at one and then no volume at two or three things like that and I think the pots are probably filthy and the tube sockets probably are too so let's clean all the tube sockets all the parts and elite eliminate that as a source of error once I started cleaning the pots it became evident that every single one of them has been replaced these all appear to be brand new potentiometers and knobs all except for the master volume control which is the old dingy pot with a switch on the back you can see the difference these are all brand new that just exacerbates the problem because how now I have to check that every one of them is wired correctly that every one of them is of the right value okay this looks to me like just about the worst possible nightmare for a repair person in that you really have no basis upon which to start because you can't trust what's here okay this amp has all sorts of issues now when you get one like this which has had extensive work and yet they still send it to you that means all of this didn't solve the problem whatever the problem is it's up to you to find and God help you okay so this is why I always marvel when I flip one over and I say oh good its unmolested because it won't present the almost unlimited number of problems that a an amp like this will well enough moaning and bitching let's get started and see if we can't make this thing work all right now I'm working on the tube sockets first drain all the electrolytic capacitors because otherwise you're gonna reach into the plate socket hole right here while you're touching the body of the amp and get blasted across the room okay so drain the filter caps then you go in with a small screwdriver and reduce the interior diameter of all of the clips in here that will be holding the tube pins look to see if any of them are burnt or disfigured and I guess then you'd have to replace either that pin or the answer our socket okay with the 12ax7 s and 1287 s you can look in and see if these sockets appear to be intact they do in this case but they're very dirty so next we'll clean them then we'll insert the tube set and see if we've improved the performance any okay the problems just keep piling up I'm in the vibrato Channel listen to the erratic response to the clean actually brand new volume pot it just shifted to much higher volume also look what happens to my current limiter there's a cranked up the volume growing a whole lot more current until at ten it shuts off I've never seen a product like not a brand new clean one very strange now let's watch here in the normal channel I cranked it up goes way up to starts to increase it three and as a volume increases so does the current draw by tremendous amount in the normal channel again I've never seen a pot behave like that especially not a new one then check this let's turn on the bright switch well there goes our all our sound I shudder to think but I believe the bright switch is new too God knows how it's been wired ok to all this I have the master volume at 10 otherwise you get nothing this is kind of interesting too I'm going to increase and decrease the amplitude of the input signal and watch how it affects the current drawing naturally on a fixed bias am the harder you drive it the more play Kirchhoff whoa but this seems extreme to me well I removed the volume control pot that had been installed in the app it's looks like a code for 50 I'm not familiar with the manufacturer but it's extremely cheaply made and has a real gravelly rough rotation with no response until you've turned the knob like a quarter of an answer so whatever this is I absolutely don't recommend it i installed an alpha 1 Meg pod here which is no high-dollar pot but it feels like oil on glass compared to that other one and now we have a nice uniform volume control now in the vibrato channel the same problem exists no law in control like a little speed bump right there so I'll replace this one now and one unforeseen benefit is that the bright switch now works with the new pot so two things fixed for the price of one okay the second of these horrible potentiometers have been replaced here and the vibrato volume control got the amp on standby you can hear the input from the signal generator [Music] well upon close inspection and to no surprise it appears that the wiring on the replaced pots is all screwed up let me give you an example here let's look at the base pot now it's simply a variable resistor in other words the wiper has varying resistances to this love this lug is not connected now let's look at the base over here on the vibrato channel and look what they've done they've wired to the wrong look okay now I'm going to guess that a whole bunch of these are wired wrong and I'm not going to bore you with going through it or for about the next hour I'm just going to correct the wiring done on these new install pots ok I've corrected for different wiring errors committed during the installation of the new potentiometers now they are wired properly as per both the schematic and the layout diagram as a result the volume control now works in the vibrato Channel and the bright switch seems to function as a bright switch should know it's director attention at the removal of the master volume switch and pot ok this is discussed on the internet a lot but I could never find any clear concise explanation of how to remove it so let's go through the thought process involved to figure out how to remove this on the circuit and still of the circuit work at least as well if not better first let's figure out how the pull push boost works now a lot of you are familiar with boost pedals which are like a little small amplifier that boosts the signal so that when it finally gets to the output tubes it's much higher than it would be if it were left up to the amplification of the main circuit of the amplifier well that's exactly how the boost works in one of these large twin reverbs and other similar amps that have the push-pull boost switch and where it gets that amplified signal is actually very clever I think a lot of you know that the reverb section in an amplifier circuit really is a separate amplifier you run the signal through the 1287 both triodes are hooked together at the both the cathodes and the grids and you feed what amounts to an output transformer and that output transformer right here drives not a speaker like normal output transformers but it will drive the reverb tank now what if we were to take this boosted signal that is sent over here to what amounts to an output transformer but instead of sending it down here to the river tank what if we were to send it up to the grid of one of the amplification tubes don't you see that a very boosted signal then which is the same as the music signal would now arrive here and be much stronger than if it had just come through the regular amplification channels to get to this point okay so and that's exactly how they do it when you pull for boost you're simply sending the output from your reverb driver up into the amplification stages of your amp now this is going to be a cinch to get rid of let's simply from this point right here remove all of the wiring that connects it to the amplifier circuit if we take this out it will be like the blue switches and it permanently off position now what's wrong you may ask with having deep blue switch connected well the main thing is that the fidelity of the signal that is sent up here into the amplification circuit is not a particularly good quality it in effect degrades the already amplified signal somewhat and you end up with a more noisy kind of trashy output it just happens to be real loud okay which is not a good combination so by eliminating this we're going to either be forced to just use the ungodly output of this amp as is or by a separate boost pedal that can create a high fidelity signal that will boost our output okay so let's go ahead and simply remove this wire now there'll be three steps and the removal of the master volume control and push-pull boost we'll have to remove this section here right up here we're going to have to remove this section and we'll have to remove the potentiometer up here at the top so let's start off with step one and I'll show you exactly where it's located in the chassis let's look at our landmarks we've got a green wire okay going to our reverb tank and we've got a 1k and 470 K resistor so from that point to that point we want to remove it we look up here and we see that here's the green wire to a river tank we're going to cut this wire right here this is the output to the reverb tank and also we're going to come down right over here where we see the 470 K and the 1k resistor and we're going to cut this black wire okay this and cut this and we will eliminate stage one of the master volume control let me back away so you can see where that's located pretty close to the center of the board okay and these two resistors are your landmarks so let me get out the snippers okay so now that black wire is removed from that junction and the white wire is removed from the output jack to the reverb tank that has the green wire attached step two will be to remove this section right here and our landmarks will be the one Meg resistor and the point of 1 micro farad cap now we'll follow the remaining shielded cable down here to this point and we see here is the point O 1 micro farad cap and the 1 Meg resistor so we're going to cut the signal wire right there and we're going to cut where the shield is grounded right under here okay snip snip and the signal down that would normally have come down here to be injected into the amplifier circuit will no longer be able to do so where's Jack I see a tail but I don't see jack okay the shield's been cut and the signal wire has been cut and how it comes now we have completely removed the booster switch from the circuit and also to finish the cleanup after the removal of the shielded cable here in step number two we're going to remove the 12 K resistor the 0.01 micro farad cap and the 1 Meg resistor to ground this is no longer needed now the reason that we have to eliminate these three components the 12 K resistor 0.01 micro farad count 1 Meg resistor and of course the boost switch is that this is a grounded line and we do not want it to be able to interfere with the passage of our signal from the plate of v4 be up here to the phase inverter okay so we will eliminate that shortcut to ground to prevent it from interfering with the operation of our circuit and as you can see it no longer does exist everything's been cleanly removed from the circuit next we will address the potentiometer here that controls the master volume now this part is going to be just a little bit trickier look right here and you'll see that we have our negative feedback loop coming in and going down to our long tail pair to feed into our 1287 of phase inversion trials we don't want to affect it okay we want it to be able to come in here and down here so that much has to stay intact then let's look at how the master volume control works look we have the plate here of this puppy of 70 25 goes through a coupling cap 220 k resistor it comes up here and a certain amount of it depending on the position of the master volume control will be fed into the grid of the phase inverter trials okay so what we'll do is when this wiper or that little arrow is all the way over to the left side then all of the signal goes to the phase inverter that's as if the master volume are set to 10 if that little arrow was all the way over here to the right then the it's as if the master volume was set at zero so all we're going to do is eliminate the pot and this little kind of bypass cap right here and we're simply going to wire from here to here okay eliminating the potentiometer eliminating this capacitor and in effect leaving the master blind control at a setting of 10 all the time so let's see exactly how we're going to do this first off 120 picofarad cap is easy to get rid of because it's connected right to the pot so when you remove the pot you're removing the cap no problem they are next we're going to look for the intersection of the two 220 K resistors and we're going to hold wire from that intersection over to the point O 1 microfarad cap so it's going to be hardwired from here to here and then this is gone or NFB loop remains intact and this is gone let's take a look exactly how we're going to do this come over here and you see that there is the intersection of the 220 K resistors and right up here is the point of 1 micro farad cap so all we're going to do is make a jumper between here and here that's it so now it's removed you see that end was released here from this eyelet this end down here came from that 220 K Junction and there was a shield for the signal wire that was here to ground so now the master volume control is completely removed from the circuit but we're not quite done because we have to put in a jumper from here to here let's do that okay and there's our jumper from the 0.01 micro farad cap down here to the junction of the two 220 K resistors now you're probably saying great that wasn't too bad but now I've got a gaping hole in my control panel so here's what I propose what if we put a pot in here with one of these knobs and use it as a variable negative feedback control what do you think of those potatoes here comes the negative feedback signal from the secondary of our output transformer down here through an 820 ohm one watt resistor and then right into our long-tailed pair phase inverter just like it does on the black face and pre master volume control silver face amps now to make it variable we're going to come up with a combination of potentiometer and fixed resistor that will allow us to go way below this amount and way above this amount the higher we go we'll shut down the negative feedback the lower we go the more negative feedback we'll have okay so let's all look in the parts drawer and see what kind of potentiometers we have and how we can do this okay my parts choices sometimes are predicated on what I have in stock and I have a 10k linear part which is really going to shut down that NFB loop and I've got a 470 ohm resistor to establish the the maximum amount of negative feedback remember that originally we had an 820 ohm resistor so this will allow much more of the negative feedback to come through than the 820 ohm resistor so we'll go from higher negative feedback to much lower negative feedback depending on the setting of the pot so instead of the NFB loop coming down here to a fixed 820 ohm resistor we're going to have it come here through a 10k variable resistor out to the wiper to the 470 ohm fixed resistor and then on to the longtail pair and since thunder thought it was important to put a point on one micro farad cap here in parallel with the resistance will do the same okay and then after all this setup will jumper it and see if it makes any difference okay I'd like to experiment okay now so this is the way it's going to be set up let's install our parts and see how it looks now looking at our eyelet board we see here is the negative feedback resistor gray red Brown 820 ohms with the parallel point o 1 microfarad cap that we saw in the schematic also just to further verify it's the right when you see the wire here comes from the speaker outlet as you would expect okay the secondary of the output transformer now let's remove this resistor and in that gap let's wire in our negative feedback pot here with the fixed resistor in series now when our pot is at one you'll have almost no negative feedback why because you've got the 10,000 dogs here plus your 470 ohms eliminating it then as you turn this up to 10 you're down here to 0 ohms and so now the total net resistance in the NFB line is only the 4 and 70 ohm resistor that we just installed so this is lower resistance and this is much higher resistance than we had before therefore the NFB loop is now variable when testing the amp it seemed to me that the reverb was a little weak when compared to other Fender reverbs and I think I discovered the reason if you look here at the 1287 that drives the reverb driver transformer and the output tank you'll see that it has a 680 ohm bias resistor and no bypass cap whereas on the one in the blackface circuit the 1287 has a 2200 ohm resistor and a 25 at 25 bypass cap so I think to pep up the output here of this 1287 to make it drive the reverb tank harder we're going to put in a 25 at 25 cathode bypass cap on the cathode of the 1287 okay I've got the amp on here or the volume is about 1 in 3/4 which is about all I can stand this thing is extremely powerful very very loud okay so keep the volume moderate we'll turn the reverb just to 5 okay now I do not have the bypass cap wired in let's here before with no bypass cap hardly any reverb really now let's go ahead and connect the bypass cap difference a night-and-day I think it's a really good improvement and it strengthens the reverb so let's hardwire that in okay for those of you then who want to spruce up the reverb on your master volume control fender silver phases here is the modification on the schematic you see that we run a 25 micro farad a 25 volt bypass cap in parallel with the 680 bias resistor to ground let's look in the chassis here and see where it is if you look in we're right let's see that's the three just down below and you'll see here we have the blue grey brown 680 cathode bias resistor and then we have our bypass cap here positive end toward the tube negative and down here to ground now the reverb is it but say at least doubled in intensity alright now we've gone from an amp and which absolutely nothing worked into an amp in which I believe everything works and just to prove it let's start and go down the line here and try every single adjustment and setting and see if it doesn't have the appropriate effect alright the amp is on we're in the normal channel and the volume is set at 2 let's try the treble off [Music] trouble at 10:00 well that was fun about middle I kind of like it with the middle turned up okay and now the base [Music] now the bright switch [Music] okay so much for the normal channel I think everything works great now let's try that by bra totes okay we've got the volume set at two just like we did in the normal channel troubles at five let's try trouble at ten whoa hurt my fillings back down to one trouble only five or six it sounds about right how about the middle now the bass [Music] [Music] now the enhanced reverb will go to six [Music] and ten really too much down to say five sounds about right to me now let's try the tremolo speed and intensity alright now it's time for the tremolo and I have created a little RCA plug that's shorted here you have to have that's a type of jumper in place or a foot switch in place for the tremolo to work let's crank intensity say two five and the speed to five [Music] [Music] [Music] boy is that swampy a so I would say both the intensity and speed seem to work just fine on the tremolo and last but not least we have the newly added adjustable negative feedback control it's at a setting of 1 which means no negative feedback [Music] that was men on this is maximum negative feedback [Music] [Music] see how that negative feedback tames the higher frequencies especially but really compresses everything down and kind of keeps it under control keeps it cleaner and pretty much eliminates any sort of distortion well I almost forgot the all-important brightness which on the vibrato Channel here's without and it definitely worked well I think we verified that all of the controls now work and that the amp is functioning very well I think it's time now to turn it over to Jack and Holly so they can really ring it out and we'll see how it performs playing some hopefully familiar tunes looks like Casey's gonna go over to her personal gym for a quick workout scratching post quick chase bye jack and back to the couch okay we're out in the workshop here with the am parole a 50 watt Jensen ceramic speaker cabinet that I made and featured in earlier videos and because it's only a 50 watt up against this monster of an amp here we're gonna have to keep the volume to sort of reasonable levels okay not quite seismic level so Jack and Ollie are all are ready to go so let's play a few tunes [Music] [Applause] [Applause] [Music] [Music] [Laughter] okay now we'll switch to the vibrato channel and a scallion jack to do a few tunes [Music] [Applause] [Music] [Applause] [Music] okay let's try the reverb now it said a fairly conservative setting of four [Applause] [Music] [Music] [Applause] [Music] [Applause] [Music] [Applause] now let's char the tremolo we'll do it at 5 and 5 on the intensity and the speed [Music] [Music] [Applause] [Music] [Applause] [Music] [Applause] [Laughter] well I guess that about does it for this very lengthy video on the challenging repair of the 135 want Fender Twin Reverb I'm sorry that it led us down there at the last minute but that's the way things go and the amp is and will just give us something to work on another day now I'd like to thank all my patreon patrons and PayPal contributors for keeping us on the air and advertising free for another month any of you who would like to join them will please look in the video description for links that will enable you to do so and I think it's time for Jack and me to jump in the old hot rod and take a spin in the country to rest up for our next video so until we meet again on the sunny shores of YouTube land [Applause]

Channel: Uncle Doug

Views: 203,748

Rating: 4.9347606 out of 5

Keywords:

Id: Q6YJfWCi_uE

Length: 49min 27sec (2967 seconds)

Published: Sun Jul 14 2019

Reddit Comments

[u/[deleted]]

The ultralinear Fenders have received a good bit of hate over the years, but if you're looking for the cleanest of clean cleans that will shatter windows long before it ever breaks up, (and your back can handle the weight) these are great amps.

[u/Takeiitaway]

But does it really kick the llama’s ass?

[u/[deleted]]

The variable negative feedback mod is interesting. As the owner of one of these amps, I'd be interested to know if there's a discernible difference between just leaving the master volume at 10 and removing it from the circuit altogether (without the variable NFB mod.)