Silvertone 4710 to Magnatone 410

[johnnyreece]

Hey, all! I recently picked up a Silvertone 4710 organ chassis that had the exact tube complement required to make a Magnatone 410. I figured, what the hell...why not? Well, after trying to figure out a layout, I may have answered that question. It's kind of a cluster-f@#k in there, but I think I've got it all. Using pins 1 and 6 of the 6V6, as well as some other non-intended tie-off points, I think I can make it happen with the hardware included. I've attached the layout and schematic. There are some interesting things going on in the 410...

(edit 11/17/14 - please note my layout has a couple errors...notably the cathode bypass cap is backwards; same in the schematic. Should be positive side towards the speaker positive. Creative NFB. When I get a new and improved version I'll replace it).

[Stevem]

The use of those two veristers is interesting, but not really needed, one thing I would change would be to kick the first filter node up to 40uf and the second to 30uf since the amp being single ended will have more filament hum than a PP output stage.
And make sure the 6v6s cathode resistor is a 5 watter!

[johnnyreece]

Interesting...it was my understanding that the varistors were required components for the vibrato. Yeah, I should have show the cathode resistor as being larger. Just lazy. I may take your advice on the filtering. I'm really debating on whether to keep it stock, cc resistors and all, or modernize it a bit.

[martin manning]

Varistors are absolutely necessary for a Magnatone vibrato. NOS are getting pricy, and while there are some modern ones that will work, I don't think anyone has found a source from which to obtain small quantities.

Looks like a cool project!

[johnnyreece]

Yeah, this came with two of them. The original circuit is very similar. If this project turns out well, it could very well be a source for others to convert. That's what I'm hoping! I think these chassis are pretty cheap when they are available.

[Stevem]

Oh crap yea, if you are doing the whole amp you need those for the Vibrato, sorry I mis read!
I am totally not for the so called mojo of using carbon comp resistors for the factor of there 1% added vintage distortion in the plate circuits as that added mojo is not worth the noise they add, nor the temp drift they can have!
My 2 cents is to use metal film types!

[johnnyreece]

Okay, moving forward with this, and in trying to strip the old chassis, I broke a pin off a noval socket (Mojo ruined! ). So, instead of shoehorning this into my previous chassis, I'm building it to an old solid state combo. Tube socket and transformer holes are ready. Need to drill for controls, paint the chassis, and decide if I'm still going PTP or full circuit board...I guess we'll see!

[johnnyreece]

So, I muscled through this over the weekend, and when I flipped the switch, squeal city. Just to be sure I didn't screw something up, I disconnected my power tube cathode from the OT lead and tacked it to ground temporarily. Success (kind of)! It works, and that's a plus. However, there's a significant amount of hum. I'll try chopsticking after I sort out the power tube cathode. The chassis is also open for all the wonderful interference to find its way in. Do you guys thing a simple swap of primaries will sort out the squeal? I'm pretty stoked about getting this up and running!

[martin manning]

I would definitely try the OT lead swap. Your test says that is very likely the cause of the squeal.

[johnnyreece]

Okay, it works. I'm quite happy.

First impressions...the speed and intensity controls leave a little to be desired. Below about 4 on the speed knob, you can no longer hear the vibrato. At full-on on the intensity knob, it's not really crazy intense. Also, at full intensity, I get some of the clicking I think others have mentioned. Also, even with the volume turned down and no instruments plugged in, I still get a faint "whub-whub-whub" sound that I assume is normal.

I put this in a gutted Rogue solid state amp with a crappy no-brand 10" speaker. It works, but I think I'd like to get a better speaker with the correct impedance (it's a 4 and the OT says it should be . Still, works well enough to give me a general idea. Real grindy little fella! Plugged into the Hi input was quite hard to get any kind of clean tone with my Les Paul (which I didn't really expect). The Lo input was up to the task of my LP, but not nearly as much fun.

I was able to get rid of the hum by adjusting some grid wires (it's still a clusterf@#k, though). It still has a little, but well within my "acceptable" range. Switching the primaries did solve my positive feedback issue. I also didn't have a couple resistor values I needed, so I borrowed the carbon comp ones (power tube 330 cathode resistor and a 1M5). All in all, a pretty satisfying project!

[andyfromdenver]

Hey JR!
I just finished my Magnatone 310 (hehe it looks like a 210, but discreetly has the "mellow-bright" on a pull pot of the 410".
So, I've also played the real deal and mine came out just like it.
I don't get a clicking noise. I can confirm the depth of the wobble isn't super great and my speed adjust (used a log pot but thinking Lin will "feel" better) is pretty lame to start til about halfway.

I got the "squeal of life" too and was surprised cause I thouuuught there was no (-) feedback. But I guess cause of the cathode cap and resistor going to the speaker +??? There is some type of feedback? *calling all gurus*

Anyway, 20mfd was typically AC ripple hummy and unacceptable, went to 40mfd.

Also, on the actual amp did you use a .1 in the tone control even though the side to cathode scheme says .05? The real deal had .1 there.
Also also, did you keep the input scheme original? I went to a more fender with a 1meg grid ref to not rely on the guit pot and cause I want to use kybds in a true hi lo on the lo.
Anyway, I'll goof around today and make a video and post the guts cause my layout seemed to work .
PS, it sure sounds vintage

PPS, how were your voltages? The 210 scheme on the second triode plate is like 78vDC which is weird and the 410 is like 160-ish?? Mine was 160-ish.

This thing is just too ripe for modding and re biasing. Neat tone control though. There are several little circuit aspects I don't understand and need to find some president in the waaay back books etc.

Take care!
Andy

[johnnyreece]

Yes, the cathode connected to speaker + is NFB. It had to be done this way because of the way the earlier triodes are being used. My 410 is actually slightly different, as well; it has a footswitch for the vibrato. It was an easy enough mod; two parts and a hole.

As far as the tone controls, yes, I used a .05 (well, .047). I may switch that out, as it's not really useful as-is. I wired it backwards, too...so that will have to get fixed. When I get done, I'll update my layout to reflect what actually happened (since it's no longer in the 4710 chassis). I wired the sockets as shown in the schematic. I plan to use this for guitar only. I may change it someday, but I try to keep my changes on original designs minimal (build it stock, then modify later).

I think my clicking noise is a volume issue; it's when I plug my LP into the Hi input and crank it. It's not too distracting, but I tend to pay attention to minute things when trying an amp the first time. I don't really think that setting is particularly useful, but just wanted to throw out that it happens, in case of the replacement varistor designers peek in.

I've not taken steady voltages yet other than filament, which is high at 6.8-ish. My B+ also starts out high before the tubes start conducting. I may replace the caps with a 500V rated version, as my initial voltages are close to that. They eventually drop, but I worry about longevity.

[andyfromdenver]

Oh. Thank you for that bit re: the nfb, I hadn't seen it before but your explanation helps.
I am a pretty dedicated Tele man so even cranking everything max doesn't probably get to the click threshold you mentioned.

I'll start a new thread and post about it.
There's a build thread on another site which is easier for me to post iPhone pics and vids on. I don't know if it violates the tos to say where.

Congrats on your new amp!

Plus, yeah try doubling that tone cap and seeing if that helps.
It's a neat tone control cause it's active (I guess right?)
Did you use linear for the tone? Log for the speed? Thanks.

[johnnyreece]

Hah, yeah, I found your other thread at the other site (thanks for using the same screen name ). I used an audio for the tone, as it's all I had handy. I actually used the speed pot that came from the Silvertone chassis. so, I've got 3 Alphas, and this giant Allen-Bradley (with extra long post) for the speed...one of these things is not like the other! Eventually it (along with some other parts) will be replaced, but not today!

[Kagliostro]

Johnnyreece

Many thanks for the link

I've read all with interest

K