1965 Fender Vibro Champ Amp, low volume, What are some people thinking when they mod an amp????

 

This was a repair amongst many I did for Southside Guitars last month over a marathon 2 day period. Simple amp, somehow it took up more bench time than the rest!

You're looking at a run of the mill used 1965 "Fender Electric Instrument Company" pre-CBS Vibro Champ. Lovely little buggers. The first time I ever played one was at this guys apartment in the 80's in Boston. He used a TS9 Tube Screamer into it to record my guitar tracks when I was a Berklee student. I was pretty floored at how good it sounded. I think he had a reel to reel or one of those cassette 4 track machines we all had in those days, the Fostex X15 or Tascam something or another. 

Anyway. This one had very little volume. I popped the chassis and, well, you tell me what you see:

Yeah. Seriously. What the eff? That nice bit of lamp cord poorly soldered between the input of the volume knob directly to the 15k midrange resistor which goes to ground. So where you want to see up to 250k between you and ground, you get 15k maximum. I just don't understand what they were trying to achieve here! Did they put it together and say "Yeah, now that's the ticket to Hollywood baby?" 

Why didn't they just undo it? Ugh......

That was the first thing to take care of and while it helped tremendously, there were plenty of other bugs.

Always start from the back and go forward. The speaker was an old alnico full range 8 ohm job with a whizzer cone. That wasn't helping any so I just threw in what we had that day, a reissue 4 ohm used Jensen Alnico speaker. That helped but still, meh.... blah.... lackluster. No bass at all.

They had replaced every single signal capacitor with used / leaky weak caps that were also the wrong values. I used Mallory 150s and if you notice, I grounded the amp and used the .047 "Death Cap" at the midrange capacitor cuz, why not? Reduce reuse recycle! And those never go bad and have a special sound. 

I also replaced the tremolo caps with fresh ones as that was super weak and they had some weird stuff going on in there too. 

But still, no bass and pretty sad tremolo.

Cathode caps..... no bass, sad tremolo. Tremolo is just an LFO right? Better have fresh cathode caps to get it strong. In fact, just replace them already! This amp is 56 years old now.

After that I still wasn't happy though. Tremolo was working albeit, just not awesome, and the bass would vanish when you turned the treble up. Weird......

So upon closer inspection I noticed the tremolo intensity pot was dated 1966, but no indication of the ohms. They always say 25K RA. So I pulled it and measured the thing. 500k. WRONG! 

I dropped in a 50k pot since that was all I had on the shelf. Those 25K reverse audio pots are hard to find. The 50k pot isn't correct but it did make the tremolo work better. With the 500k pot there was almost no tremolo until you got up to about 8. So, while not satisfied this would work. I could also bypass it with a 47k resistor to make a fake 25k pot. I don't like doing this but in a pinch it does work and only the rabid amp fanatic can usually tell.

But that treble control bit was really driving me bananas. The amp still seemed weak too. So I then figured if the Bozo that modded this amp changed that tremolo pot to something so wack, maybe they changed a tone pot.

They didn't. The tone pots are the original 250kA pots dated 1965. 

So I checked the volume pot. Lo and behold, it was the 25K RA Intensity pot I needed! Captain "Wha? TechNo No No" used the 25k pot and scrapped the 1Meg pot that was in there. I took that 25kRA pot and put it where it belonged and threw in a new CTS 1 Meg volume pot. 

Now we're cooking with grease...... GREAT tremolo and GREAT tone!

The power transformer was replaced sometime in the 70's with the correct part. I did clean up the wiring there too. Come on folks, twist those AC lines already!!

In conclusion. I do not know what this person was trying to achieve with the mods they did. If they were trying to make the amp play with little volume, why not just turn the volume knob down? I imagine they may have been trying to get more tremolo so they used that 500k pot. Nope! 

If you do not know what you are doing then please consult someone! Otherwise a fella like myself can spend hours undoing your ill conceived ideas. Seriously, ask. Most of us are happy to share information....

J

Plush amp circa 1969 baby!

 This is from the pile o' bigass amps at Southside Guitars. 

Not going to get all technical, nothing new to write. I just wanted to share some photos of this behemoth!

This was a "just get it going" job. It didn't take much. I just replaced the electrolytic caps. Fortunately that quartet of beautiful GE 6L6 tubes was in great shape. Most of the preamp tubes were good too. I replaced the first stage filter caps and one of the new ones was bad! 120 cycle hum and low b+ voltage around 380 or so. Good indicator it's the first stage. Replaced it and voila. 

How are these amps? They're like a janky ass Fender that looks soooooo glam. Made well enough and not one bit better! I've worked on many including the "Earth" amp series built later. They were made in NYC. This one boast a 2x15" cabinet. I cannot imagine anyone in  2021 being interested in hauling one to play in sad backroom gigs where they have no chairs but 200000 watt PA systems that sound stellar, but please, someone start a band with these as the backline so we can see a real film, no filter photo!

Tremolo and reverb are good and lush. Get yerself a fuzz box and a wah wah and write yourself the ticket to the sun baby!

The hardest part about working on these amps is getting the chassis out of the head box. There's literally no room at all! I always wind up pulling the power tubes out then taking a dead blow hammer to knock out a piece of wood that holds the back grill in place cause it is dead in the way of your power transformer! 

So if you come across one, re-cap it, bias it and test test test it before you put it back in the box. It's like a forest grows around the damned thing minutes after you box it up and if something is wrong, a gotta open it back up again!!

Go big kids! You have strong backs still!!! And big amps are kinda cheap these days!!!  

J

1967 Ampeg B15N 2x15".... rare bird!

 Just had a day servicing two huge amplifiers made in the day of, well, huge amplifiers. 

This may easily be the coolest amp I've ever seen from Ampeg:

I believe this may have been a custom order. It's a later model, with 7027 tubes rather than 6L6, and a 7199 driver tube. Grid bias, 5AR4 rectifier and those lovely 6SL7 preamp tubes. The tweeters are a cosmetic addition having never been hooked up. 

Not going to get into anything technical, I just wanted to share this behemoth with y'all.

The cabinet has that ultra cool Austin Powers era cosmetics. I have a friend who has a single 15" version as well. Big amps really are no longer in vogue in NYC. This one was on the Reverb cheap as chips and still sitting there. Today the world is about small amps and efficient PA systems. I mean, who wants to move a refrigerator every time you have a gig?

That being said, if I owned a little coffee / wine shack where I featured jazz and folk music, it would be kinda against the law not to own this and stick it in the corner next to the house Slingerland drum kit.

One interior shot:

I did grow up in the last age of the big amp. I used to play a Marshall half myself. Can't say I miss moving these things but, damn. Nothing sounds or looks as cool as that oversized backline!

J

 

My favorite amplifier was stolen, please help! Magnatone A-646

 Hi all. About 2 weeks ago my favorite little amp was stolen from me. The photos here are from about 5 years ago and it's more beat up now. I also replaced the knobs and the pilot light which it sky blue, smooth glass. 

Any info would be much appreciated. I've filed a report with the NYPD......

Thanks! 

Bronson Amplifier circa 1949, in the original box with the matching lap steel

 Today I got to see a stunning piece of history. This Bronson amp dated 1949, still with the original shipping box. It spent some of it's life in New Mexico at the Aloha Conservatory of Music. Apparently this school was affiliated with the Bronson brand. I would love to know more! American musical instrument manufacturing is a fascinating subject for me. Especially when it is tied to schools. 

Nothing to really write here, the amp isn't NOS, but it's as clean as one can be for a 72 year old amplifier. The customer wants to use it so sadly I needed to change every capacitor in the house. All of the wax caps were leaking badly (yes even with little use, they likely leaked right out of the Valco factory!). And I also replaced the electrolytic caps and the power cable at the customers request. Easy job. These are so well made. 

So the insides are all "before" pictures for your amusement. 

How does it sound? We happen to have a lovely old Kalamazoo model A with a real Charlie Christian pickup. Lovely combo. I'm a fan of a good field coil speaker which this has. Nice low end.

The filter cans are as follows: 40uf (big for the time!) and a dual 10uf can for the preamp and screens. 

Tubes are 5Y3, 6V6 pair, 6SC7 phase inverter and 6J7 pentode for the preamp. I'm a big fan of this arrangement having built a few amps with a 6SJ7 preamp into a 12AX7 or 6SL7 phase inverter. Big, warm sound with nice low end. 

Valco knew how to build a good amp. This thing runs pretty quiet. And it's a nice thing to look at. Easy on the eyes. An amp should be able to double as furniture so it doesn't get stuck in the closet when not being used. Potted plants should go near it alone with your sofa. These manufacturers had style back then. Not an ugly rock box or just some industrial looking thing that belongs in the garage. This is something you want to relax with and play some sweet music through. 

J 

Kalamazoo KEA Amp revisited. "Why didn't I think of that???" Bad fuzz in the sound, let's get that out with a mod.....

 The Kalamazoo KEA amp now lives with my good friend Eric. It's become one of his favorite little amps to work with. We both think it's special sounding, very rich and clear. But alas, it still had some annoying issues.

This amp initially was a basket case. Doomed to the "part it out" pile. Missing speaker and all. I wanted to save it so luckily I found deep under the bench at Southside Guitars an old ceramic square magnet 10" that was shallow enough to fit in there. I replaced the field coil, with a 1k 10 watt resistor and yadda yadda, re-cap blah blah the usual.

In the end it became a lovely sounding amp. Impossible to make a buck off it so thankfully Eric wanted the thing. 

The volume pot was scratchy so I replaced that. The schematic called for a 500k, but what was in there was actually a 1 meg. I used one of those to keep the integrity of the amp. You need to remove the power transformer to access the pot! Easy enough but it's one of those things where you realize what a genius Leo Fender was! 

The pot was worn out but so was the .05 cap feeding it. That was leaking causing some scratchiness so I replaced that. He was stoked cause the pot was getting worse and worse, it only worked in certain positions.....

But it still had some rattle issues. Most were easy: microphonic old RCA 6V6 and very microphonic 6SJ7. Piece of cake!

But then I get that call..... something in the background sounds like a Fuzzface when it's out of batteries. The decay is like an amp that is biased way too cold. 

I was praying it wasn't the speaker. We paid good money to have that dead old speaker rebuilt....

So first thing was I re-doweled all of the screw holes. Some were loose after 70 years. That helped. The chassis was vibrating. 

But not 100% which is what we're shooting for.

So I figured let's lose that metal speaker grill. These amps likely weren't turned up to 10 back in the day. Those metal grills can cause issues. And we'll replace it with a boss looking vintage grill cloth. This amp sells for less than $500 on a good day so before you scream at me for changing that grill, I'll just say I DON"T CARE! And neither does Eric. This is an amp being used by a pro......

Anyway, that helped! Buy that decay was still bothersome. It didn't happen all of the time but it was annoying when it did.

So, next was more cabinet work. I mean, why didn't I think to glue that big ass crack in the baffle???

Done! Helped! Not perfect!!!!

So at this point I'm ready to throw in the towel. I sat down to lunch and had my Kabab Shack platter (Damn they are good there! 10 year customer!!!) when I had a thought:

On 70's Champs I always remove this one little capacitor that they added during the CBS era. It's a 330pf cap going from the control grid (pin 5) to the cathode (pin 8). This shunts some of the very high frequencies to ground, taming that sine wave if you will. When I worked for Budda we had a similar thing on the plates of the EL84 tubes to ground. 

Some Champs I wound up putting it back in. Others not. Why replace it? On some Champs I got this ugly distortion, not unlike a Fuzzface with a dying battery.......

CBS sucked but they did a few things right.

So after lunch I did two things: 

I added a 1 Meg resistor from the center tap of the volume pot to ground making it effectively 500K. I figured the schematic had a 500k for a reason. It's also not unusual to see values vary on amps this old. The pot was clearly original to the amp.

Then I also put a 100pf cap across that resistor. 

And..... that did the trick! No more nasty distortion getting in the way of Eric's playing! And on top of that the amp just has a warmer tone. It didn't lose any volume or any bite for that matter, it just simply works better.

So now this "one foot in the grave, another on a banana peel" amp is ready to cut tracks. I didn't think I could like this amp more than I already did. 

J

 

My ReMus preamp. Pandemic project number 3,84whatever

 Today was pandemic project number three thousand two hundred and whatever. It entailed me spending a whopping 2 hours re-vamping the ReMus preamp I built way back in 1998 when I lived on the corner of Missouri and Failing (yes it actually was a dead end!) in Portland Oregon. 

It was an early build of mine with the classic "bowl o' spaghetti" construction. But it worked shockingly well so I never bothered to fix a few annoying issues. 

First was eliminating the tape out buffer. This preamp has one too many gain stages. The beauty of it is it's simplicity (I'll provide an updated schematic here).  So I put this part of the circuit on a switch as I like to use it for putting vinyl to digital. But audio should not pass through it going to the output. It's redundant and likely causes phase issues. There was something about the original design that I simply didn't like but couldn't put my finger on. I figured less is more and this did the trick. The whole thing sounds dramatically better.

Second was back in the day I used 1Meg pots. The original calls for 250K. I couldn't even turn it up past one before it gets too loud for my room using the phono stage! I fell into a rabbit hole trying to decide what to throw in there. This is my hi fi.... guitar amp pots aren't gonna do. One can obsess over this stuff. I wound up using pots by Tokyo Cosmos cause, well, Cosmos. I like the name!! I got them through Antique Audio Supply:  https://www.tubesandmore.com/products/potentiometer-tocos-rv24-audio-10-6mm-shaft

Again, major improvement. Everything sounds better and is much more in control!

My preamp is kinda "dual mono" rather than "stereo". In other words I have two mono volume pots for left and right rather than a dual pot. This eliminates the possibility of having one side behave differently than the other. 

I also replaced the output caps with Sprague Paper in Oil. Not Vitamin Q but I'm sure very similar. They're 1uf@ 400V.  Before I had these beautiful Angela polypropylene caps that were 1.5uf. I love those but wanted to try something else and love the Spragues more. The rest of the caps are Angela / Jensen PIO caps. Sadly none of these are made any longer. I think the business has gotten so spread out with competition. I don't know that anyone is making better stuff than what he was offering in the 90's.

Last but not least..... I used some cheapass RCA jacks. Total Jive dig?!? 

What was I thinking? They were always getting loose! The phono, line and output stage I did years ago with nice Switchcraft 3501. So today I finally did the rest, 24 years later! Typical tech. The last thing you want to do after a long day is your own stuff so it's easy to tolerate. One of my favorite luthiers plays bass. I asked him how his basses play and knew the likely answer: "dude they all play like crap!" he replied laughing.

Much better! I also added a center channel output should I want to experiment with a sub woofer. I've been fantasizing about that. Building a 50 watt mono block only to be used for symphonic music, reggae, soul, Afrobeat, electronic stuff. I'm kind of afraid I'll like it too much and get addicted to it but hey....

I also finally color coded my wires, left and right channel..... It was difficult to get Mogami hookup wire back then. Finally have a source and they have plenty of pretty colors! Cheap as chips too. That stuff is great. Warm, rich, full bodied wire and easy to work with. I could only buy it in blue back then!! 

Buy it here: 

https://www.performanceaudio.com/mogami-w2514-hi-fi-red-hook-up-wire-by-the-foot.html

Apparently you can get this on Amazon now but, well, ew......

So now I have one final task. The power supply is separate. It uses a 5U4 tube with all oil filtering, two chokes (another choke inside the preamp housing for a total of three!) and 6.3V DC filament voltage. I built it this way so I can have it far away from the audio. No AC voltage enters the preamp except for the signal itself. But...... I used an old Kenton transformer I scored at a Ham Fest.  And it buzzes.... I once had a 1967 Plexi 100 Watt Marshall with the same annoying problem. I couldn't do anything about that one cause no one made a replacement part for that old beast and, well, it was vintage. I think I sold that one to Mick Mars. He bought a few from me back then. Nice fella!

Anyway, that transformer has gotta go. It's ironic to have gone to such great lengths to create something quiet only to live with a stupid mechanical buzz! That will be my mid-April project.

For those who are interested in the schematic, here is the modern "no tape buffer' circuit. Brilliantly simple.

https://www.lencoheaven.net/forum/index.php?topic=28010.0

My own version used octal preamp tubes, substitute 6SL7 for 12AX7 and 6SN7 for 12AU7. I prefer these for hi fi. The modern little 12 volters sound a bit hyper for my taste. I like a bit more euphonic presentation. Plus I can still find high quality octal preamp tubes for much cheaper than tubes all the guitar freaks need! And they tend to last forever.

So there you have it. In it's 24 year life it burned out two resistors (the 22k cathode follower resistors on the output tubes.) I upped those to 5 watt wire wound resistors, nice Allen Bradley ones. Yes, I like to overbuild things. I don't want to open this up until 2031 now. I've changed tubes a few times just for fun. I think this set has been going since I moved to Brooklyn in 2011. Still making music!

I also didn't use their power supply schematic. I just went with a simple 3 stage dual choke loaded power supply like they've been using since the 1920's. Why so many chokes? Why big oil caps? I used to live by a surplus shop, actually two: R5D3 on 82nd Ave and Cascade Electronics way up in the northernmost tip of Portland and they had so much of this junk! These oil caps will last forever and they look super hip. Plus, again, relaxed sound...  I avoid electrolytics when I can, and sometimes you really can't, I find they just don't work as well. Besides, if you are only going to build one thing, why not go all out into the land of the ridiculous??? Just please, don't do what I did and skimp on cheap RCA jacks that get loose when you look at them wrong, or right, or whatever....

J

1964 Fender Deluxe Reverb. Re-capped but still too much hum. What can I do?

 Here's the desert island amp for many of us and I think after all these years I'm one to be on that island myself. A really clean 1964 Fender Deluxe Reverb. 

About the amp: Transformers are original, speaker was replaced with a Celestion G12M greenback 30 watt. I'm a fan.... Amp was re-capped well enough. All of the electrolytic caps were top shelf expensive Sprague Atoms. Some of the work wasn't exactly tidy but done well enough.

But, I just didn't like the amount of hum I was still getting. I right away saw some things to improve. First is the bias supply cap:

Whoever re-capped this amp went with the original value. I mean, why wouldn't you. That's what Fender used in 1964 so.... 

Nah. When I worked for Jeff Bober in 1994 he just told me to upgrade it to 100@100v rather than 25@50v. I've been doing it ever since. Better regulation and less hum. I do not detect any change to the sound of the amp and I have well trained ears. And for what it's worth Fender upped it later to 70uf then 100uf cause they could. Caps got a little better. So just do it. If you think that's blasphemy try it anyway and put the old value back in if it freaks you out.

Yup, being dramatic here but I'll share a story. I once had a client who paid me to convert an AB165 Bassman to an AA864. That's no small task. Big job. I made one mistake and one executive decision. He took the amp home and inspected the work under a microscope. The next day I had an irate customer who brought his amp back and he (understandably) expressed his trust had been burnt. So I gladly changed the one pot value that I missed to the correct one. But he was really upset about my little executive choice: using a 100@100v for the bias supply. I explained to him why I did this: with a 25@50v cap it will hum. He didn't believe me one bit and I could tell I was losing his business.

So we gave it a listen, I then replaced it with a 25uf cap.... HUM! And not a teeny tiny difference. It was significant enough that he heard it and looked a bit embarrassed. He asked me if it would make a tonal difference and I said no but if he thinks it does bring it back in a week. I put my little 100uf cap back in, fired it up and he was happy with the quiet. I never saw him again. All is well that ends well!

So here ya go. Use a pretty blue Philips Vishay cap if you will! I like them cause they look nice and European:

But wait! We're not done yet. Amp was much improved but I still wasn't 100% satisfied. This next part involves the signal caps.

If you have been inside as many old Fender and other high quality amps you'll notice even the signal caps have a direction. No they're technically not polarized like an electrolytic cap is, but there is a direction you want them to go in. The have an inside foil and an outside foil. Some old caps like my favorite Ajax blue capacitors you find in Fender amps even tell you which side is the outside foil.

Rule of thumb: always have the outside foil closer to ground. In other words, if one side of the cap is on the 100k plate load resistor and the other is connected to the 1Meg volume pot, outside foil goes on the 100k resistor. Fender almost always got that right! I've only seen a few exceptions. 

Another example is the tone stack. The .1 bass cap goes to a 250k pot and the .047 mid cap goes to a 6.8k resistor. On the other side is the 100k slope resistor then the 100k plate load resistor for a total of 200k. So the bass cap should have the outside foil attached to that slope resistor and the mid cap should be going the opposite direction with the outside foil connected to the 6.8k resistor on the bass cap. Like this:

The phase inverter has three .1 caps. Two are connected to the plates of the 12AT7 while the third is connected to the grid. On the plate load side you have a 100k and an 82k resistor going to each plate. On the other side you have two 220k resistors going to the bias supply. So the outside foil goes to the plates, inside to those 220k resistors. The third cap however is connected to a 1meg resistor on one side and a 47 ohm resistor going right to ground on the other side. So that cap goes in the opposite direction. In this case the previous owner used my least favorite cap, the Sprague 715P. No indication for inside or outside foil so you just read it from left to right. It will look like this:

Notice how the print is going in the opposite direction.........

Here's the dry channel tone stack. I had a spare old polyester orange drop cap that has a stripe on it for outside foil. The mid cap you'll notice reads left to right with the left going to the 6.8k resistor. If you are experienced you'll notice the .047 600v blue Ajax cap for the final stage. Sadly entire channel was orange dropped. But I have a bag of .047 Fender "Death Caps" from the AC balance. I never throw these out when I ground an amp. I save them for classy amps like this one!

One cap backwards will not make all that much of a difference but in this case I counted 5. It adds up. In the hundreds of old Gibson and Fender guitars I've seen over the years, the tone caps are almost always done in this arrangement: outside foil to ground. If the good people at those companies were sticklers for such a detail on the treble bleed cap, there's something to it.

So there you have it. The amp behaves like my own '69 model now. Quiet! Of course it's not possible to eliminate all hum and noise but it is possible to reduce it using these little steps. And the less noise the more music dig?!

Thanks for reading, hope this was helpful and feel free to leave a comment of ask a question below! J