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
Hi Jef, congrats on the successful restoration!
ReplyDeleteWhat a thing, I didn't know you like Maggies.
I love my Varsity Model 108, the early version (a TV front MOTS cover ver. with 2 inputs, not 3 like the Varsity Deluxe) the tone quality is exceptional, the very first 8" amp that didn't make me wish it came with 10" or a bigger speaker. The stock Permoflux 8” has an incredibly valanced voice.
Oh yes, may I ask you a couple of questions?
1. The previous owner has changed the rectifier tube configuration from the original 5Y3GT to the 6X5GT. I don’t know why. What do you think? Should be reverted to the 5Y3?
2. And do you think the amp should be used with a variac to get the 117v input?
Sorry, I think know that I’ve asked you too many questions lately…
I'm a big fan of the Magnatone. Great amps.
ReplyDeleteI actually have a post about a 108. Check it out!
They made a version with a 6X5 rectifier. I suspect yours is this model so I would just leave it alone. In order to convert it you would need to make sure you have a separate 5V filament winding on your power transformer. But really there would be no benefit.
I never bother with setting an amp to 117 with a Variac. Unless it happens to run far too hot and the voltages are exceeded on your filter caps, there is no real need.
Here is a copy and paste Link to that schematic:
http://www.magnatoneamps.com/schematics/magnatone_108.png
Keep those questions coming! I don't mind at all. -Jef
Thank you, it’s very helpful!
ReplyDeleteMy amp doesn’t have that specific configuration chart, but has a smaller/simpler one inside of the cab:
- without the schematic for the circuit design
- with just the tube type/location (similar illustration to the one shown at lower left in your picture), serial # (with 1 digit more, the speaker code says it’s made in 1954, btw) and the manufacturer name and address.
- There is no “6X5” originally printed on it but someone wrote it by hand afterward with awkward pencil letters.
So I thought it’s been modded by someone, but now with your info I assume that it can be an original manufacturer option or it’s a transition example…
Still, I don’t think the 2 rectifier tube types are compatible or share the same character other than the GT socket design. So I still wonder why and am curious about the tonal/functional difference which might be made by changing to 5Y3.
p.s. Yes, I went the page for the Varsity Deluxe before I came to here. And you taught me the reason why I like the older no Deluxe version better. Thanks.
I've seen many tweed Fenders where they went from using 6SL7 or 6SC7 tubes to the modern 12AX7 and they simply crossed it out on the chart and did rather goofy mounting to go from the octal preamp tubes to 9 pin mini.
ReplyDeleteThey aren't compatible tubes, the pinout is different. The 5Y3 is a directly heated rectifier whereas the 6X5 is indirectly heated meaning it has a separate cathode element. In a bigger amp that would typically spell less sag/compression with the indirectly heated one such as a 5AR4 or your 6X5. But in an amp that is only 5 watts maximum you're splitting hairs. I've had amps that use a 6X5GT that sounded lovely. So the old adage goes: "If it ain't broke....!"
Also the 6X5GT has a 6.3v heater whereas the 5Y3 has a 5 volt. To change that would mean changing the power transformer or adding a 5 volt filament transformer then changing the pinout on the tube itself.
I'm a fan of both the Varsity and the Varsity Deluxe. I have found with the Deluxe on some they used those ceramic signal caps. The last one I worked on sounded terrible. Dead. Low volume and thin tone. I changed those 2 caps to good old Mallory 150, kept the same values and the amp roared to life. It could keep up with my Gibson GA5.
Good stuff! Love the small amps, especially in NYC!
-J
Hi Jef, thanks.
DeleteToday I got a 6X5 rectifier tube (Hytron, NOS, 100%) to replace the noisy (almost dying) Tung-Sol, and the noise went away. BUT, what a thing, I miss the tone with the Tung-Sol, which was a lot warmer and musical with a little more sound space. The same type rectifier tube from the different brand can make this difference, or simply because the older the better!? Don’t remember that I felt this much on a rectifier tube.
Unfortunately, it’s not very easy to get a 6X5 from a certain brand of your choice though it’s more inexpensive than a 5Y3…
Thanks for the explanation about the 2 types of rectifier tubes. I haven’t found any better one on internet than you did here. I really appreciate and will go with this 6X5GT, don’t want to change the circuit with the transformer which looks like an original. I now feel better with your convincing explanation.
I love Gibson amps especially from 50s and early 60s. Regret that I sold my 1959 GA18 whose overdrive sound was more musical than tweed Fenders that I have owned. Yeah, I thought it’s way too big to enjoy here in NY (I didn’t know you were in Brooklyn!) and no headroom (vol “1” started to distort), BUT even within that small headroom it was producing great jazz tone… lol
Oh yeah rectifier tubes can change the behavior of your amp quite a bit. But good thing is the 6X5 is cheap as chips so you can buy a bunch and experiment till you find what you want. That's one of the joys of having these small amps. Keep searching, you'll find a Tung Sol or RCA.
DeleteI once built a hi fi amp for someone and we auditioned rectifier tubes. I had your Sovtek 5AR4 and Sylvania etc. My client paid the extra $120 for my Mullard (RCA labeled) tube. We noticed an instant improvement in the bass and overall musicality. It was worth the extra expense for him.
Jazz needs more tube amps. When I hear a JC120 I want to barf. Glad to see joints like Smalls has a house Deluxe Reverb! Jazz guitarist are sounding warm and toasty again. I lived through the 80's and it was rough! -J