Download Magnatone 210 Details

Magnatone 210 Details
Doug Ahern, 2014. Special thanks to Andrew Gonzales for providing the scans of this original schematic. For more Magnatone Schematics, see http://www.magnatoneamps.com/ Also see the schematic for the 410 which succeeded the 210 in 1961 with mostly cosmetic updates. Amplifier The Magnatone 210 was introduced sometime in 1959 and was produced until mid­1961 when the 410 was introduced. It was a basic 5W 6V6 driven single­ended student or practice amplifier that featured Magnatone’s patented full modulation vibrato. It was the most basic, economically priced amplifier in Magnatone’s lineup to have this feature. The successor 410 was nearly identical to the 210. The feature of a “mellow­brite” two position switch was added to the 410. (“mellow’ was normal, and “brite” was a treble­bleed cap across the volume pot). The 410 was produced for a few years, possibly as late mid­1964. Pre-amp and Oscillator Sections:
Vibrato, Power Amp, and Power Supply Sections:
Power Transformer and filament center-tap note:
In the above schematic photo, the T­1 transformer is torn away. Below is an excerpt from the 410 amplifier (210’s 1961 successor) which shows this part of the schematic. Most notably important is that the filament 6.3V winding has a center­tap which is connected to the 6V6’s cathode. ⇐The filament centertap connects to the 6V6 cathode. Notes:
1. The cathode of the 6V6 is grounded through the T­2 Output transformer’s secondary speaker winding. Furthermore the 25uf/50v electrolytic by­pass cap is wired with its negative terminal to the cathode. See the section “Negative Feedback” below for more information. 2. The original speaker was an 8Ω Oxford alnico 8EV41. 3. There is no fuse on the original amplifier. It is highly recommended that a fuse be added, no larger than a 2A­SLOBLO, possibly as low as a 1A­SLOBLO. 4. On the 210 documented by CJ Poulos, T1 was a Woodward­Schumacher P­50­6201 and T2 was a Woodward­Schumacher #8102. 5. The Schematic differs from what was produced on the .05uf tone cap that connects the Tone 1M pot to the cathode of the next triode. Both CJ Poulos and Andrew Gonzales found .1uf caps originally installed in the 210’s they’ve documented. Adding a Footswitch:
Magna left the footswitch off of the 210 as well as the 410. If you want to add one, use this Magnatone M2 schematic as a reference. The M2 circuit is similar to the 210 and 410, not only in the oscillator section, but in its entirety of the vibrato circuit. Negative Feedback
The cathode circuit of the 210’s single­ended 6V6 is very unique among guitar amplifiers. Rather than connecting the 330Ω cathode resistor to ground, Magna engineers connected it to the positive side of the T­2’s secondary speaker winding. Furthermore, the 25uf/50v is installed backwards, with its (+) positive lead connected to the speaker winding (+) side, and its (­) negative lead connected to the cathode. This was done to provide negative feedback to the amplifier. In audio amplifiers, Negative feedback (or NFB) is the re­application of some portion of the output audio signal to the input of the amplifier in opposite phase potential such that part of the input signal is cancelled out due to the opposite phase relationship of the two signals (the two signals being the input signal and the feedback signal). This serves the purpose of reducing non­linear harmonic distortion. It results in an amplifier that is more stable (in terms of clean audio signal amplification), more clean headroom, and less distortion. On similar Magna single­ended amplifiers that preceded the 210, namely the Magnatone 108, NFB was provided by connecting the output transformer’s (T­2) (+) positive speaker to the cathode of the preamp gain stage that preceded the 6V6. This cathode is a convenient place to inject NFB because of the out of phase relationship of the NFB signal and the input signal as it appears on the plate of that triode (This is the same NFB design used on most single­ended guitar amps, such as the Fender Champ). So when it came to designing the 210, why didn’t Magna engineers use the more common 108­style NFB circuit? For one, they couldn’t use the preceding triode’s cathode because it was already in use in the phase­splitting duty of the vibrato circuit (See below) schematic. For many reasons (all vibrato related), this would not work as a place to attempt to inject NFB. So, if NFB was to be applied, it had to be done elsewhere. Magna engineers chose to do it at the 6V6’s cathode. The signal from the speaker can be directly applied to the 6V6’s cathode (without attenuation) and still maintain a desirable NFB ratio of 2% to 4%. But why the backwards installation of the electrolytic? Instead of thinking of this cap as the cathode by­pass cap, it must be thought of as a coupling cap between the speaker and the cathode. The potential of the AC signal at the speaker is positive in respect to the AC potential at the cathode. Remove NFB?
You can (and many 210/410 owners do) disconnect the 330Ω and 25uf cap from the speaker’s (+) positive connection, and connect them directly to circuit ground. Note, that you will have to flip the 25uf cap so that the (­) negative lead connects to ground and the (+) connects to the cathode. When you do this, you will remove NFB from the amp. It will be slightly louder, and slightly more prone to distortion (This might be exactly what you are looking for!). The original intent of adding NFB to Magnatones was to lower distortion levels and add stability to the amplifier (This was long before distortion was popularized). You should, however, leave the center­tap of the filaments connected to the 6V6’s cathode for maximum hum­cancellation. Awknowledgements
Special thanks is due to the following: ● Andrew Gonzales for providing the images and other details. ● CJ Poulos for his hand­drawn schematic and documentation. ● Terry Dobbs (and CJ) for 210 related posts on Yahoo Magnatone­Valco mailinglist. Further Reading
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http://www.magnatoneamps.com/210.html http://www.magnatoneamps.com/410.html (page contains a 410 schematic). Patent: 2,286,337 (Publication US2286337 A) Negative Feedback Circuit, Lothar Brunk, 1937.