Magnatone Amp SE 6V6GTA Model A-646

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CJ

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this time we are going to get after another small Magnatone git amp,

this one actually has a pwr trans,  :D

single 6V6 with a 6SL7 preamp tube and a 5Y3 in the pwr supply,

 

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chassis separation complete, 

Model A-646,

no schematic on the web, but this simple circuit takes just minutes to trace,
 

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we have a cap can with 20/450, 20/450 and 20@25 for the 6V6 cathode bypass,

and another 8/450 for the preamp B+ and pwr tube screen,
 

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easy structure, pri-sec, more turns than the 50L6 version and bigger core, 625 EI , one lam size up from the 21 EI,

3 layer sec,
 

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bigger core and more turns = more Henries, 12 vs 3.5 for the 50L6 amp,

frequency rolled off to protect the small speaker from bass,

inductance more linear with the paper gap in the core,

 

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19K loading sounds very odd for a classic SE 6V6.

5K is traditional. 8K is jazzed-up. 7K was the classic Champ loading. 10K would save some DC current but give less Power.

I'm suspecting the design is 9.5K to 4 (or nominal 7.5K:3.2), and that 8 Ohm speaker is a sneak: factory fumble or later replacement.
______________________

> more Henries, 12 vs 3.5 for the 50L6 amp

50L6 is a high current low voltage tube. It has MUCH fatter cathode than 6V6, 7.5 Watts heater instead of 2.8 Watts, so can flow more current. OTOH the 50L6 is rated 200V max but generally found with a 110V (AC/DC transformerless) B+, while 6V6 is often found with 250V-350V B+.

The V/I ratio leads to the optimum load impedance.

Well, if 50L6 loads in ~~2.5K, and 6V6 loads in ~~10K, then 4X the inductance is the same audio response.


 
good catch PRR!

took the speaker out and found P8T C47 printed on the bottom of the magnet, (speaker is mounted upside down)

no info online for that p/n, however, DCR is 3.2 ohms so we are going to call it a 4 ohm speaker, probably what the 4 is in the C47 p/n,

so that makes the Z ratio 9.5 K to 4 Ohms, probably would have been 10K:4 if the lady got on all the turns, but the wire might have been running thick that day,

engineer probably had 10K/4=2500 Z ratio, so turns ratio is root 2500 = 50,
nice round numbers for the slide rule,

so turns s/b 65 x 50 = 3,250 instead of 3,160, but it was a nice layered paper job with insl between each layer so we will not fire the winder,

print has been updated,

 
> if the lady got on all the turns, but the wire might have been running thick that day

Second shot in your image a1.jpg shows the winding is not over-packed. It's maybe 80% full, so another 10% could be wound-on and still leave lots of clearance for assembly.

Depending how the 6V6 is run, I'd leave the primary alone and throw a few more secondary turns, to get primary Z down.

But this isn't aircraft design. A w-i-d-e range of impedances will work with similar power outputs and somewhat different distortion trends.

Multiply that by speaker impedance lumps and precision impedance is a folly.

Then maybe factor-in the Model Range. If the next-up model with two 6V6 is a little weak, you might mis-load the one-6V6 model so the two-6V6 does not sound so tame and belie its higher price-tag.

> DCR is 3.2 ohms

AFAICT.... speakers with no long lines were normally aimed at 3.2 Ohms DCR. Combos, car radio, anything with short wires to the VC. Also distributed systems with 70V transformer nailed to the speaker.

Speakers with long lines were often wound higher, 15 or 32 Ohms. Theater systems with hundred-foot wires.

Higher VC impedance is thinner wire and more trouble. Justifiable when line-ohms is high, budget is large, and VC is large (not so fine wire). But for small cheap stuff, fewer turns of fatter wire is the production department's dream.

Lower than a couple Ohms is bad because the tinsel leads have significant resistance and will eat your lunch. Fatter tinsel is more mass which again eats your lunch. While 2 Ohm windings are found in car woofers, 3 Ohms is probably a better limit for speakers which speak, not just thud.

The switch to 8 Ohms "seems" to come with early transistor amps.
 
here is the schematic, never seen this this tone/volume control setup, maybe add another cap and make it like the old Princeton tone control, this amp is very similar to an old Fender,

also notice there is a .005 cap on the input like a grid leak circuit, but the grid resistor is 470K not 10 meg, maybe a mic compatible input?

voltage measurements are included, modified orig circuit by using 15K pwr supply resistor instead of 22K after the second filter cap and also a 150K plate resistor on the first stage
instead of 100K,

6V6GT running a 250 ohm cathode resistor, used to seeing 470 here,

built a component library from a Fender schematic, cropped out the parts and text,
that is why the schemo looks familiar,

edit-missing 270K grid resistor on 6V6GT-will fix on rev 2 schemo,
 

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did a rewind on the transformer and put the coil on new lams, .014" instead of .018"

used #26 wire on the sec instead of #24 but used two strands instead of 1, (bi-filar)

freq plot comparing the two coils shown below,

voltage ratio is 41:1, which works out to 41^2 = 1681 Z ratio,

so pri Z is 4 ohms x 1681 =6.7K,  so the leakage inductance is dropping the pri Z down close to 7K which PRR said was normal for a Champ type circuit,

this much difference between the voltage ratio and the turns ratio is due to the simple pri-sec winding structure,

getting better coupling with the new coil, random wound on a bobbin as opposed to layered paper, and also the bi-fi sec,

the first layer of #38 pri wire was breaking during removal, very fragile, so it was ready to fail so glad we did the rewind,
 

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new lams increased the pri inductance, 65 year old lams lose a little perm over time,
and less core loss with thinner steel,
 

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here is a graph comparing "static" Henries (xfmr out of circuit) with "Apparent Henries" (in circuit test with 8 ohm wolfer attached)

there is a resonance going on in the speaker which causes ac current in pri to go down, which raises reactance which would indicate more Henries, but it is really just an impedance rise due to the interaction of the load with the transformer,

 

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here is a graph showing Henries vs pri signal level at 20 Hz,
so the  core is saturated at about 70 volts AC on the primary, 47 ma DC.
this was an in circuit test,
looks like the DC reduces L from 16 H to 14 H
 

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> here is the schematic

Missing grid resistor on 6V6.

> this this tone/volume control setup

Odd, so odd that it may be mis-built, mis-fixed, or mis-transcribed.

0.005u coupling into 100K pot across the signal cuts-off everything below 350Hz, all the time. Huh?

0.01u cap, wiper turned to top, works against ~~30K plate node impedance to cut-off everything above 530hz.

Strongly suspect tone pot should be 250K-1Meg.

The intent is a simple treble shelf. This is useful with no-NFB pentode outputs and most speakers.... the speaker impedance rises above 500hz, pentode output rises, response is shrill. At the 1W level you can put a cap across the OT primary. At ~~4W level that's a sturdy cap, top-shelf in a low-level stage is cheaper.

Maybe compare to other Magnatone plans? I'd hate to see this become "just another Fender copy", the world has plenty of that.

This *appears* to be a Varsity, which changed with every batch built, but the later 12AX7 job shows 1Meg tone pot (and topology similar to some Fenders; also a double-oh-one tonecap instead of oh-one.) (However with the higher-gain AX7 they took NFB from around the OT, so the tone-goal may have been different.)
http://magnatoneamps.com/schematics/magnatone_108.png

Both input jacks ground-switched? So if you only plug one guitar, you have a 2:1 cut-back at the input? Fender has only one finger-switch grounded so there is a 1:1 input or a 2:1 input. (The AX7 model does have all-grounds.)

The Melodier's tonestack is also of interest. http://magnatoneamps.com/schematics/magnatone_109_melodier.pdf
 
tried 3 different tone circuits, used the one in the red box, 100K pot reduces gain a bit, but there is still enough watts to pop the 1953 Jensen 8" so no problem there,
 

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dropped that 6V6 grid resistor twice now, previous time on the Princeton Reverb i believe,

here is the final schematic with all the changes,

both inputs are self shorting so the signal does get cut in half,

looks like the tone/pwr sw pot has been replaced, sw probably went bad, should probably be a 1 meg tone pot,

input circuit looks like it was originally for a 6SJ7 grid leaker, they changed to 6SL7 and then 12AX7a, but they left the grid leak cap in there during the transition, the 6SL7 socket looks like it was original with crusty carbon comp cathode and plate resistors that have not been re-soldered,

 

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