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