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Monday, June 5, 2017

GM70 Output Transformer

For most of users out there, with usual modern loudspeakers, the efficiency normally falls around 90dB +/- 6dB. Such speakers require considerable power to drive and control the loudspeaker to a satisfactory level. Tube power of ~20W and above are usually preferred, and in order to get such power for tubes, one would need to go for push-pull or PSE design, if not, to turn to high power tubes such as 805, 211, 212, 845, 833 and such.

NOS high power triode, and/or even pentode are hard to find nowadays. Even if they are available, prices are most probably sky high, such as RCA 211, RCA 845, or United 211, not to even mention the exotic ones such as Western Electric 211 tubes. GE VT4C or GE 211 can still be found, but even so, their prices have soared since. 

What other inexpensive alternatives do we have? Well, we still have one option left, the GM70! It will only cost you about U$60-100 a pair, depending on the year made, and also the type of plates, either graphite, or copper plates. GM70 copper plate versions are rumored to be warmer than the GM70 graphite plate variety, but, YMMY. GM70 copper pate version is almost 2-3x more expensive, and are getting rare as collectors start to hog those tubes.

One can even design a tube amplifier that's 845 and GM70 compatible. Both tubes accepts load resistance of ~5K to 10K, with similar plate current. Output power varies slightly but is not of utmost importance here. GM70 has higher plate dissipation, and therefore the amplifier should be designed as such that it does not exceed 100W at all time.

845 Data


GM70 Data

The most ideal load resistance for both could be ~7K - 8K to get decent power and distortion figure. One just need to ensure a few things, no biggie, simple stuff (if you dare to build >1KV stuff, this is nothing to you!):

* Have separate filament supply to cater for different requirements
* Different tube sockets side by side for GM70 and 845
* Switch on cathode, or CCS cathode load to regulate the plate current


Which GM70 output transformer should we use? Of course, none other than from J&K Audio Design! We have recently designed one for our customer for his GM70 single ended amplifier. 

Here are the GM70 output transformer details:

* Power: 37W (20Hz)
* Measured frequency response: 20Hz - 20kHz +/- 0.23dB
(5Hz - 45kHz +/- 3dB - tuned to have higher/better loading at low frequency)
* Primary: 7.5k Ohm, Secondary: 0-4-8 Ohm (other primary/secondary - please request)
* Core: NanoCrystalline (aka Finemet) double C core
* Primary DCR: 235 Ohm (B-P, 20C)
* Secondary DCR: 0-4 Ohm (0.2 Ohm); 0-8 Ohm (0.29 Ohm)
* Primary inductance: 61H
* Max Primary Current: 120mA
* Efficiency: 0-4 Ohm (>93%), 0-8 Ohm (>94%)

(In circuit measured with Shuguang 211 at 1W. Full power response will be similar. Customer that wishes to have more high frequency response - please request at ordering or we will tune based on our house sound/voicing.)

 
* Matching: primary DCR +/-0.1 Ohm, inductance +/- 1.5H (<5%)

The above are made with NanoCrystalline double C core, which are a difficult material to work with on such power level. Such glass cores are usually for high frequency usages, and for 400Hz power supplies. NanoCrystalline core excels at high frequency, and significant compensation is required to augment the low frequency response. Using a larger core helps but that increase the magnetic length significantly (causing high frequency loss), and various other problems. Using them at full range applications is super challenging. Try asking other winders for it.

The rewards, after overcoming such obstacles, are of course very fruitful - micro details, dynamism, real sense, directness, purity, tonality are outstanding. If one can consider using OCC (Ohno Continuous Cast) copper cable with it, the results will be even more significant. The purity of sound with OCC + NanoC is out of this world.

If one is looking for purely at specification perspective, EI Z11 would be a better choice where we can get a much wider bandwidth than NanoC. The voicing of EI Z11 will be different than the NanoC. 

Enjoy life, enjoy music.

J&K Audio Design
6/5/2017

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