Maggie- Another amp is born

[DougH]

Magnatone 107. Electronics are finished, now on to finishing the box.

[Pigyboy]

Looks great. No tone?

[DougH]

No tone control. One volume knob. One switch. Rock.

[Pigyboy]

I Love it!

[DougH]

Thanks.;-)

With the filter on the output transformer primary and the bright cap (I added) on the volume control, it sounds really good with no tone control. Sounds fine with all my cabs (with some radically different speakers). It's a simple & elegant design and I'm amazed how much headroom it has at only 120v B+. Crunches up pretty good at max but there's a lot of clean available too. Gave it the pedal test last night and it passed. Really loves boosters for heavier sounds.

[sst4270]

Nice Doug.

[John Lyons]

So with the heater voltage adjust it's just a set and forget right?
Looks cool, surely very simple.
If you get around to clips I'm all ears. 

[liquids]

Why only 220R on the screens?  I'd beef that up, though I'm not expert nor familiar with the 50L6....also, you'll get considerably longer tube life (and allegedly a different, some say 'better' tone if you lower the screen voltage to 80% or as low at 50% of B+

[DougH]

So with the heater voltage adjust it's just a set and forget right?

Yup. I didn't feel like subbing out fixed resistors until I got the right value. (Actually, I didn't feel like taking another trip to the store to pick them up. I had the variable resistor already, so I thought why not use it. The amp is built from mostly scrap parts including the tube sockets.) This way I could just test the heater voltage as I adjust the resistor.

Why only 220R on the screens?

Because I didn't have any 100R's in my parts bin, so I went with the bigger 220. It's not as big of an issue with a beam tube as it is with a pentode. The tube is biased to about 70% max dissip. so it's not getting stressed anyway.

[MikeH]

So how does the heater wiring work?  One set has 50v on it and one set has 12v... how is that possible?

[phector2004]

So with the heater voltage adjust it's just a set and forget right?
Looks cool, surely very simple.
If you get around to clips I'm all ears. 

+1

But can't you measure a "good" trimmer resistance and replace it with one of those giant heat-sinked resistors?
Or are there more sounds to get from a hotter/colder tube? 

[DougH]

But can't you measure a "good" trimmer resistance and replace it with one of those giant heat-sinked resistors?

I could, but I had plenty of room to mount it so why bother. Part of the fun of this build was using a bunch of stuff I had that was not getting used anywhere else. I've had that resistor lying around for 6-7 years and it wasn't getting used. Same with the ridiculously oversized 15W Heyboer output xformer, and the iso xformer for that matter. It was fun putting idle parts to use. I kind of had my own "Junkyard Wars" at my workbench.;-)

[DougH]

So how does the heater wiring work?  One set has 50v on it and one set has 12v... how is that possible?

It's a series circuit. See series and parallel circuits, my friend.

[MikeH]

So how does the heater wiring work?  One set has 50v on it and one set has 12v... how is that possible?

It's a series circuit. See series and parallel circuits, my friend.

Hardy har har.  Seriously though; The resistance of the first heater filament steps down the voltage into the second?  I'm surprised that you get 12 volts that nicely.  And I'd think it would also fluctuate?  Not the case?

Edit:  I guess instead of saying "possible" I should have said "practical". 

[DougH]

Both tube heaters draw 150ma, that's the key. So the "make up" resistor is set to draw 150ma at the remaining voltage drop (approx 55-60v) across the 117v secondary of the transformer. This series arrangement was pretty typical of "plastic radios" of the 50s-60s which incidentally also used the 50L6 output tube. In fact, these circuits (and the original guitar amp this was based on) were designed to run directly off of line voltage/current. (That's not recommended- please use an iso xformer if you're going to do this.) I believe the high heater voltage tubes like the 50L6 and the 35V rectifier (that was in the original amp) were built specifically with this in mind. With a series heater circuit and a resistor (if needed) you could run the heaters off the same set of windings you are rectifying into B+ for the audio portion of the amp. No filament transformer or special dual winding transformer required, which makes it a lot cheaper to build and sell. It's especially cheaper if you don't use a transformer at all, but again I don't recommend that. You don't want to be attached directly to your wall outlet if there's a fault that puts AC on your guitar cable.

[liquids]

This is very cool info all around, makes more sense to me now that you put it all that way!  Thanks for sharing.  Seems highly doable...would love to hear it, though it wouldn't be anything hard to mod however one darn well pleases.  I'm curious how it compares to champs...probably closer to a tweed than a blackface since it has no EQ... Looks like RCA 50L6s can be had for $10.  Neat.

[DougH]

Here's the youtube vid that inspired me to try it. Mine sounds pretty similar with slightly more crunch and less overall noise. He doesn't really demo the clean sounds but with the bright cap mine sounds pretty good clean too.

[MikeH]

Both tube heaters draw 150ma, that's the key. So the "make up" resistor is set to draw 150ma at the remaining voltage drop (approx 55-60v) across the 117v secondary of the transformer. This series arrangement was pretty typical of "plastic radios" of the 50s-60s which incidentally also used the 50L6 output tube. In fact, these circuits (and the original guitar amp this was based on) were designed to run directly off of line voltage/current. (That's not recommended- please use an iso xformer if you're going to do this.) In fact, I believe the high heater voltage tubes like the 50L6 and the 35V rectifier (that was in the original amp) were built specifically with this in mind. With a series heater circuit and a resistor (if needed) you could run the heaters off the same set of windings you are rectifying into B+ for the audio portion of the amp. No filament transformer or special dual winding transformer required, which makes it a lot cheaper to build and sell. It's especially cheaper if you don't use a transformer at all, but again I don't recommend that. You don't want to be attached directly to your wall outlet if there's a fault that puts AC on your guitar cable.

I see I see...  Very cool

[Brymus]

Thats a sweet little build Doug.
Looking forward to the clips.
I played an old Magnatone from 93 till 97 with an old Silvertone guitar from the same era.
They came out of my friend's father's attic after he passed.
He bought both new in the early/mid 60s.
It did have really nice vibrato, I miss that set up but gave it back when my friend wanted it back (he was a drummer) to give to his son.

[PRR]

> No tone?

It's a ten-buck starter amp. You want more knobs, pay more. Anyway, isn't the EPI Valve Junior toneless?

> the heater voltage adjust it's just a set and forget right?

You could calculate it, no set. That's how it would be done if you ordered parts.

Doug's "bunch of stuff I had"/"Junkyard Wars" set-on-test works too.

The original, radio, plan was something like 12BE6 12BA6 12AT6 50L6 35Y4. This adds up to 121V, and nobody sweated the difference from 117V nominal wall.

Magnatone, KAY, et al omitted the two radio tubes, changed the diode-triode to a (pentode or) twin-triode. Most would keep the 35V rectifier because it was still cheaper than non-vacuum rectifiers AND it is 35V heater drop.

Doug's plan is stripped of the 12BE6 12BA6 and 35Y4. The twelve-watt resistor takes up the slack. It should drop 12.6+12.6+35V= 60V at 150mA. 60V/0.15A= 400 ohms at 60V*0.15A= 9 Watts. Using less than half of a tapped 1K 12W, one end isn't doing much and the other end is working too hard.... hey, it's a copy of a ten-buck amp, it wasn't supposed to live forever, and if that resistor ever does open-up Doug will find another.

Using just 12+50= only 62V of heater from a nominal 117V supply, he's dumping a lot of useless heat in the box. But that's very much in the spirit of the times.

> Why only 220R on the screens? ...considerably longer tube life (and allegedly a different, some say 'better' tone if you lower the screen voltage to 80% or as low at 50% of B+

Bah. Nah. You are thinking of HIGH voltage amps. You rarely want to starve screens. Especially at only 110V!! In this case you would actually want the screen to be some higher than plate (and it probably is). Bench power supplies did that. Cheap radios (or amps) couldn't afford it. The screen leverage is _designed_ to work well very-near main (plate) voltage.

And 50L6 hardly ever die unless they were made bad. And they are cheap. And there's a lifetime supply.

And I think the 50L6 is in fact a power pentode, not a beam-power tube, despite the suggestive "L6" in the name. But at this scale, who cares? That's like asking if the codec in my LG cellphone is a Cyrus or a KFC..... I'm not going to change a codec or a screen-grid/beam.

Remember that while many power tubes were designed for multiple uses, the AC/DC radio tubes were MADE to play well and sometimes LOUD with these specific conditions. Radios were by far the largest tube market. A lot of thought went into these tubes.

Doug: your B+ is probably well over 110V, or is that measured? And as a matter of style, you have way more filtering than a cheap amp ought to have. 50uFd 30uFd 10uFd is more typical, though at that point you may need a "hum-buck": some supply ripple fed into the amp anti-phase, a very cheezy trick which maybe should not be reproduced. And the twin-triode dropper could be a lot larger than 1K considering there's only a half-mA flowing. 22K would be fine and allow 5uFd here, penny saved.

The OT loading "should probably" include a resistor. The naked cap causes a ringiness. The speaker impedance rises above ~~1KHz.... inductance! About 0.5H on the primary. With the 0.01u cap, resonance is ~~2.2KHz with Q near 2. 0.05uFd+3K is a more "ideal" network. The $13 radio would have 0.05u and a 5K tone-pot which trims from rising-highs to a steep cut suitable for bad AM/phono use. If horribly abused (normal for guitar) you may smoke a common pot or even a 1/2W resistor, which is why we don't do this on bigger stuff. And putting the R-C across the primary (instead of to ground) is the same to audio but less DC stress on the cap, another penny saved.