Early 50's Magnatone A-646. Perfect tiny amp!

Just finished this one, a very rare Magnatone A-646 amp from about 1951. This one us a real gem and is visually striking as well.

I bought this amp by mistake on Reverb this winter. I put an offer on a Varsity that I wanted as a backup to my main one and the owner declined. It wasn't a bad price and I love those early Varsity amps and it was pretty enough that I was willing to pay his price anyway. This one came up in my feed and I was curious so I put an offer on it. The owner said he would think about it. First amp wound up coming through at my price so I bought it then.......so did this one!

This one arrived first. It was noisy, had some really poor tech work done to it so I got it going. I wasn't all that impressed with it though and even after re-capping the power supply it had a considerable amount of hum. I even added another filter stage. I knew there were other solutions, maybe filament, maybe ground loop but I shelved it. If it's mine I tend to just put it away and forget about it.

So today I did a better re-cap, with a nice expensive C.E. can, an additional filter stage and while it got better and was certainly tidier, still hummed at an unacceptable level. So I disconnected the center tap of the 6.3v filament and added a hum balance: 2x 100 ohm resistors, one on each side of the filament, the other sides both going to the cathode bias resistor on the 6V6 tube. Looks like this:

When I brought this up on my Variac I thought "dammit! I killed my amp! Time for more troubleshooting!" I just wanted to be done with it. But reality is, the amp is just so quiet now! Hum free with only a little bit of noise when it's turned all the way up.

This amp I believe is the precursor to the Varsity Deluxe I reviewed here. Similar circuit, 40's style box, single 6V6 and 5Y3 but driven by the darker more euphonic 6SL7 tube rather than the snappier 12Ax7. It has a single 8" Alnico 5 Jensen speaker.

More pics:

How does it sound? Like the early version of the Tweed Fender Princeton. It's a similar circuit, but this plays cleaner and more articulate. It has a big sound for such a small box, warm but not fuzzy. Ideal Wes Montgomery or Charlie Christian sound. I'm a big fan of the Magnatone amplifier, especially the early ones. They are a bargain. I may sell this one but not for cheap! I like it a little too much.

The only drawback is the 6SL7 tube can be microphonic. If you turn both the volume and tone all the way up it may sing a song. The 12AX7 is a better tube for guitar amps. You could simply use a 6SL7 to 12AX7 adapter available on EBay, or you can just roll a little bit of the tone control back and enjoy that big warm sound.

If you like something different, give one of these a try! They are lovely little amps!

JB


8/29. After tracking with a Les Paul all weekend I've decided this is my best sounding amp. Bigger tone than a tweed Champ, smoother top, goes to tape beautifully! It's a keeper! Do look out for one of these. I did some reading about the hum and one tech mentions he struggled with his as well. he resorted to installing a big ass choke and extra filter stage. That should do the trick but try the hum balance first along with adding one filter stage. Quite happy with the results here. JB

Bad Cat 5R, adding a tube rectifier and making it sing like a bird

So a client of mine has this amp for his office use. It's a Bad Cat 5R. Don't know much about them but have heard they are related somehow to Matchless. This is my first venture into a Bad Cat and having worked on many Matchless amps I can say they certainly are similar.

The job: Modify the crap out of this amp!

My client has recently become hooked on old amps, Marshall, Tweed Fender, Supro, 50's Gibson, Ampeg. This amp just isn't doing it for him any longer. It's stiff and rather dull.

Now, it's a very simple circuit with only 2 tubes. A 6V6 driven by (interesting choice that I like!) a 6SL7. There is only so much modifying one can do. When I fired it up the first thing I noticed was what a lack of distortion there was. I like my little amps, distortion (Gibson GA5 from the 50's) or no distortion (Magnatone Varsity) to have a warm singing quality to them. My client is into warmth more than over the top blast so the goal is to get it to do both. I found the amp to lack warmth.

I have a back to front approach with amps whether I'm trouble shooting or modifying. Example: first thing to look at with a dead or dull sounding amp is the speaker and/or the power supply. So I started there. First thing: Why use a Celestion Vintage 30 on a 5 watt amp? If you want to go for cleaner tones that's great and I'm not objecting their speaker choice, it's purely subjective. But for this project we went with a 20 watt Weber 12A125A the one that says "woody and reedy".

https://www.tedweber.com/12a125-a

I love this speaker. My favorite amp that I built uses a 30 watt version. A bit on the darker/warmer side of the spectrum, nice smooth breakup.

The second thing is the power supply. Stock it uses 2 1N4007 diodes loading into a 100uf 450V capacitor. I decided if it's an easy fix to add a tube rectifier let's go for it. I like my little amps to compress! You can see diodes right under that blue power resistor.

So once I looked around I noticed 2 things right away that made me happy. Chassis is pre-punched for more octal sockets and there are 2 yellow wires coming off the power transformer. That means there is a 5 volt filament tap! YES! This job just got easier.

So I took out the plastic socket plug and installed a socket. Removed the 2 diodes, put the red wires to pins 4&6, the yellow to pins 2&8 and replaced the 100uf with a 47uf cap. Why replace that cap? The 5 volt filament line is at risk with anything too high. With diodes the sky is the limit, but with a tube rectifier you don't want to go too much above 50uf.

I opted to use a 5V4 for the rectifier. For squishiest results go with a 5Y3. I figured this would be a dramatic shift as is and didn't care to go too dramatic. Fun thing is, they are interchangeable and my client likes to fiddle with tubes.

The circuit itself:

I won't go too much into detail on the circuit mods cause I encourage you to use your own creativity. What I did start with was the cathode resistor on the output tube. It's a 270 ohm. I replaced it with a 500 ohm. It's just more loose. I also replaced all those horrible brick drop polypropylene caps with warmer sounding Mojo Dijon caps and any resistors in the signal path with carbon composition. Also the first gain stage cathode cap is a 6.8k. I just copied Fender and used a 1.5k bypassed by a 22uf cap. The whole amp seemed to come to life in a good vintage fun way.

So now this amp sings at any volume. It has a master volume which didn't really serve too much of a purpose before but now you can bring it down and blast the preamp. With my S.G. it's like a mini AC/DC concert.

I think these are good amps even stock. Very well made and built to last. The reverb is transistor driven but that doesn't offend me, it's quiet and sounds nice. But if you aren't opposed to making your Bad Cat a Badder Cat, try some of these fun mods, use your creativity and you can tailor it to suit your needs.

Disclaimer: if you are a novice, don't try this at home. Pay someone like me to do this kind of work. Tube amps are no joke inside, they can kill you with the amount of juice running around in there! So tell your favorite tech what you hear in your head and let the magician work his or her magic.

Thanks! JB

8/16/17 Magnatone Model 108 Varsity Deluxe

I love Magnatone amps. Just a great, warm sound all around. I've worked on dozens of Varsity amps, but not the Varsity Deluxe. Completely different circuit, more like an early tweed Princeton. I do own a Magnatone Model A646 that is similar to this amp circuit wise except it uses a 6SL7 rather than a 12AX7 for the preamp. This amp loses the Hawaiian theme, a move from the 40's to the 50's. Classic Jet Age style.

Typical single 6V6 tube with a 5Y3 rectifier and 12AX7 preamp. 5 watts, 8" speaker....

Grounded it, re-capped the power supply, fired it up for the first time in probably more than 2 decades (pulled from an old studio that had 100 amplifiers in various states of neglect 17 years ago!).

Sounded like crap.......

A little secret. The ceramic caps in these can sound terrible. Just change them. These amps aren't worth much so why have 'originality anxiety'? I replaced them with some Mallory 150 caps and the amp came to life. It sounds fantastic. Punchy, good volume, lots of warmth and harmonics.  Great jazz tone with mild breakup. The earlier Varsity amps had great signal caps. If they are still working I leave them alone. The later ones use those lovely Ajax blue pill caps that Fender used. But these? Pure crap. Trash them and enjoy your amp! These can hold their own with any other small amp from the era. Transformers are excellent and the speaker is a nice little Jensen. Worth re-coning if it's fried.


-JB

8/15/2017 Three Ampeg B-15 amps, one week!

Simply put, the Ampeg B15 is a legend of the studio. This plus a Fender Precision Bass plus massive amounts of talent and persistence equals the legendary Motown sound. But, any passive old school bass sounds great on one of these. I've had the pleasure of working on many of these. This week offered a nice journey into 3 versions of this lovely amp.

First a 1963 model:

This one came from MSR studios in New York City. Countless artist probably recorded through this amp including the Stones, Talking Heads, Roberta Flack.....

This was the best of the 3. It's an early cathode bias version. Probably in the 25 watt range. What do I like about it? Punchy, great low mids, clear sound with a nice bit of growl.

This particular amp was sitting neglected for over a decade. It was a hot mess when I got into it. Random wires clipped, some components flopping about and the tag board burnt through where the cathode resistor resides. Typical, that resistor gets hot. Something blew up in this amp and the power transformer was also fried. This is typical too. I replace power transformers in about one of these a year. This year it's been 2 so far. Weak part I suppose. I didn't need to replace any of the Cornel Dublier caps, they are almost always still good. Surprisingly the circuit is dead quiet with all of the original resistors as well. They didn't mess around with making things cheap back then.

After replacing the power transformer and re-capping the power supply it roared back to life. Glad to say it's making music again in another studio. I'm stunned anyone would let this one just sit unused for that long. The tone is simply perfect. Loud enough for a small combo live too. But best suited for studio tracking.

The next one is a 1966 model:

Similar cosmetics, the differences are it's now a printed circuit board rather than the old school tag board. Ampeg did these so well that it's still fun and easy to get around in. It's also fixed/grid bias rather than self/cathode bias. Result is more power, more headroom, more 'clinical' sound. Haven't dug into this one yet but will be re-capping the power supply.

It doesn't hum so why re-cap it? Isn't it best to leave it original?

Short answer: no way. Just do it. Why?

First is safety. This amp is 51 years old. Caps fail. You don't want this happening.

Second is sound. I can always tell and feel the difference. Stronger low end and more even across the range. My clients agree. Most of them spend years listening carefully in the studio and they say the same thing. Amp got 'bigger' sounding.

Right now the amp sounds good, but we're going for great.

Amplified Parts (CE Distribution/Antique electronic Supply) makes brilliant caps for these. The grid bias amps require a 525 volt can and they also have a beautifully made 25uf@800V "firecracker" cap for the first stage.

Here's your can:  https://www.amplifiedparts.com/products/capacitor-ce-mfg-525v-404040uf

And your firecracker:

https://www.amplifiedparts.com/products/capacitor-ce-mfg-800v-900v-surge-25-f-electrolytic

The firecracker cap saves a good amount of time on installation. I used to use the Sprague Blue Atom 20@600V cap, but you need to monkey around with stuff to make that work. These come with a wire tie and easy mounting. Takes a whopping 60 seconds to install.

Oh yeah, order this too:

https://www.amplifiedparts.com/products/insulator-4-section-fp-cap

Spend the extra $1.95. It's worth it.

Next photo is one I worked on last winter. It's a 1968 model. Similar 'drip edge' cosmetics to the Fender amps of that year. Pretty much the same amp as the '66 but with a new sexy look. Great amp.

I needed to replace the power transformer on this one as well. Full re-cap of the power supply. Powerful. Great live or studio rig.

The 3 amps above all have the original CTS speakers loaded into them. Still making music!


Last but not least is an early 70's model. These are great live amps and are a bargain:

This one also came from a studio that let it rot away. Didn't take much, power supply re-cap, new preamp tubes (6SL7) which were all dead. I left the signal caps alone in this one as well. 1/2 are the Cornel Dublier green pill types, the preamp is loaded with the turquoise little fish caps. 2 of the input jacks were opening up so with nothing plugged into the amp it squealed like a electric hog. I've seen people re-cap amps and go to all sorts of trouble when all that was needed was a good closed circuit Switchcraft 12 jack.

These late model amps have nice but not really necessary filter switches for 'ultra low' and 'ultra high'. These share the most similar sound to the SVT out of all of these, probably due to the preamp. More of an analytical sound than the amps above, less warm but still plenty of warmth. Not nearly as attractive, which is typical of the 70's designs over the 60's. But, this is the best live amp choice. Lots of power! This particular amp was loaded with an Eminence speaker and is now slated to be a live workhorse.

On all of the grid bias amps above, I installed a bias trimpot. Why not do this? They do tend to run quite hot. And hey, you may save the life of that very expensive power transformer:

http://www.mojotone.com/amp-parts/Transformers_1/Ampeg-B-15-Power-Single-Primary-Transformer

So if you have a serious recording studio, don't go without a real flip top. Even the B12SB amp is a fine choice for your bass tracks. I wouldn't buy a new one, there are so many good old ones around and they can be had for not too much. A late model can sell for less than a grand, middle period between $1.5k-$2k. Early ones can be a lot more but if you have the dough, a good tech or some good skills on the bench, you will not regret it.

JB