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
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
Hi, I came across your blog looking for a schematic for my newly acquired 1954 Varsity model 108 (tan leatherette cabinet). My amp has a 5497 labeled tube compliment but someone along the line rewired it for a 12ax7. In your opinion should I go to the trouble of rewiring back to 5497 or just leave it as 12ax7? I am still in the process of replacing a couple failed electrolytics, so i really havent heard the amp much.
ReplyDeleteHi Ava!
DeleteThanks for your question. I can't seem to find any tubes labeled 5497 anywhere. Are you certain that's the number??
I'm always a fan of putting something back to stock and seeing what it sounds like then proceeding to experiment from there. However, the way this amp sounds as it is with the 12AX7, it's one of my clients favorite recording amps. So if it ain't broke......
So start with the electrolytic caps. If it has the ceramic disc caps for the signal path like this one did I say replace those too. Stock this amp was dull as dishwater! I was considering giving up until I replaced the signal caps. Really came to life.
Best, Jef
I'm sorry I mispoke, I meant 5879. I couldnt find a schematic with that preamp tube. I guess if the 12ax7 sounds great why mess with it... I'll go ahead and replace those weird looking ceramic caps also.
ReplyDeleteOne other thing that I noticed was that your schematic doesnt have a cathode bias coupling capacitor on the second half of the 12ax7 like the one on the magnatone website does. Thanks for posting that schematic, I cant read the cap and resistor values on the magnatone website but yours comes through clear.
ReplyDeleteAh yes, the 5879. I do like those tubes. Very interesting to see how this amp evolved.
ReplyDeleteThe 5879 is a pentode, not too unlike the octal 6SJ7 which is my favorite tube of all time. Your amp could be quite interesting wired with that tube in there.
Does the wiring look factory? Can you send photos? It's quite possible they simply used the 5879 tube chart but were already moved on to the 12AX7. I've seen this in numerous tweed Fender amps when they moved on.
The 5879 would make your amp sound more like my favorite earlier Varsity amps. Maybe a bit brighter which isn't a bad thing. And of course with a tone control which the early ones lack.
Here's a bit of info from a great site: http://magnatoneamps.com/100series.html
Also, feel free to experiment with cathode bypass caps on both halves. I'm pretty sure I did that with this amp. Brings the gain up. It may overload but worth a try. 22@25V or better is a good place to start.
-Jef
Here is a link to a picture of the chassis.
Deletehttps://drive.google.com/file/d/0B8k53z45uhVUUFhNS29WbzlFNnljWEpZcGxYMDZBeDVYMy1B/view?usp=sharing
I added the blue 20uf Sprague Atom, I tested the condenser and found that one of the electrolitic capacitor was bad. The old Orange 16uf sprague looks like a modification... it came to me wired to a 12ax7. I would rather put it back to original but cant find a schematic with the 5879, that magnatone website doesnt show a 5879 schematic either. I may have a rare amp...
Nice. I would replace that 16uf cap as well. Also the big bumble bee cap at the top of the picture. Your amp is grounded and that's the 'death cap'. You can remove it.
DeleteHard to tell from the photograph whether it's the original circuit or not. The 2 I have seen had ceramic signal caps, not the 'molded' one and the red Tiny Chief. Those are better than the ones I have seen.
If I had to guess, your amp was intended to have a 5879 tube in it but was wired from the factory for a 12ax7 instead. My Magnatone A646 amp is like the Varsity 108 in terms of circuit but has a 6SL7 instead, which is octal and similar to a dual triode 12AY7 electrically. That amp pre-dates these.
So yeah, you may have a rare amp for sure. I wish I had taken a photo of the inside of this one to compare. Next time I'm in Vermont perhaps I will.
Thanks! Jef
Here ya go, the A646, my favorite lil' amp!
Deletehttps://leightonamplifiers.blogspot.com/2017/08/early-50s-magnatone-646-perfect-tiny.html
Hey Jeff! I added a 25uf/25v cathode bypass cap to the second half of the 12ax7. The amp has a new brilliance to it, I really dig it. However when I turn the volume past about 7 the speaker starts to do like a 120 cycle (or fast) flutter. Any clue as to what may be going on? I played with this amp for almost two weeks on full volume (controlling mainly with guitar knob) and didnt have this issue.
ReplyDeleteHere is a picture of my circuit
https://drive.google.com/open?id=1lrqi3SJWV3mIygIR3m1FEMU10VKkDTV4
Hmmmm. First thought is remove that 25uf@25v cap or maybe experiment with a lower value like 2.2uf? Sounds like something is overloading.
DeleteAdding that cap will add quite a bit of gain. Sometimes when gain is added you need to compensate for it elsewhere.
Remind me, how many stages of filtration are there? 2 filter caps or 3? Some Magnatone amps use only 2 like in my old Varsity and I believe my A646 stock only had 2. I added a third on both amps. One right at the rectifier where the output transformer tap goes, then a 1K-10K 5 watt resistor going to the second stage and screen grid followed by a 22k-47k going to the preamp stages.
Typically flutter is also called 'motor boating' which is caused by under-filtering the amp. Early brown and black Princeton amps sometimes suffer this which is why they later added a stage.
I used my A646 on a blues gig tonight. Good god that's a loud little box!
-J
Ah I see on the schematic you have 2 stages of filtering. Yeah, you can do better and I bet the fluttering will go away.
DeleteYou can use the existing 20k going to the screen grid, just add another 22k-47k resistor after that and a smaller value filter cap for the preamp stage, like 10uf or so. That should clean that up.
I don't remember if I did this on my friends Varsity Deluxe. I fixed probably 50 of his amps by now! I'll have a look next time I'm in Vermont. -J
Hi Jef , I have the same Varsity Deluxe 108 . I'm replacing the speaker , do you know what Ohms the amp is rated for?
ReplyDeleteI don't know but, every Magnatone Varsity I've owned had a 4 ohm speaker in it. I currently own a model A646 which is like the predecessor to this amp (single 6V6, 12AX7, 5Y3 into an 8" speaker) and that speaker is 4 ohms. So that's most likely it. Thanks, Jef
DeleteHi, I believe I have the same amp that Jonathan has. It's a Magnatone Varsity 108 with a 5879 preamp tube. The tube chart on mine also shows it as having the 5879.
ReplyDeleteI've put some pictures of my amp in this folder (below)
https://drive.google.com/drive/folders/1eEauZjRog2CkJRLt3I23uCbKqskFuxHG?usp=sharing
You might need to copy and paste the address into your address bar.
My amp does not break up, but instead remains clean. The tone is nice. The volume is low. I haven't measured it, but it is not loud. From the looks of things, it is 100% stock and untouched. I wonder if it has lost some of it's juice over the years. Do you think it would benefit from a servicing of the caps and some of the resistors?
That is a very cool amp. The missing link perhaps between the Varsity and the Varsity Deluxe perhaps. I've never seen one of these before. A real beauty!!!!
DeleteI would not expect to get much gain out of it. The reason is it's electrically similar to the older Varsity amp with the 6SJ7 pentode driver. The Deluxe has a dual triode, 12AX7 which provides 2 gain stages before hitting the power tube.
I would definitely change all of the electrolytic caps right away. They're old, they're risky. I would also check to see if any of the signal caps are leaking DC.
If it were my amp I would bypass that .005 cap going into the preamp tube, or replace it with a .01 or .02. That will open the sound up, provide better bass response.
I wouldn't worry about the resistors unless they are really out of whack or the amp is noisy. If they've drifted way high, replace as needed. The caps are more important.
Every Magnatone Varsity I've owned or played had a decent amount of volume when serviced. Not very high gain like a tweed Champ, but a beautiful, clear sound. I've never had the early circuit with a tone control like yours. I bet it will be a fantastic amp when serviced up. J