Most GM70 have Graphite Plates, as well as 845 tubes.
Many 845 users have experienced, or heard from others, the 845 can have some whisker noise after bad transport conditions. This comes from the electrical connection to the anode, which is (and only can be between hard metal and graphite parts) a shrink connection. This connection stands in the output signal path, and is listenable at high quality system.
With the copper GM70 version, that problem was overcome with the copper anode, to which you can normally weld something. As a result, sound becomes more clear and vivid, compared to graphite plate GM70.
This is quite reasonable question, since, among copper plate GM70, sound is varying depending on measured anode current and Ri at the same operation point.
Many people say, that, with copper plate, sound is more detailed, but bass is better with graphite plate. As I have found out, this is because graphite tube examples usually have higher Ia and lower Ri than copper ones, and this affects bass reproduction.
However, if one manages to find a pair of copper plate GM70 with Ia and Ri better than those of typical graphite examples, than one gets a genuine gem of output tubes.
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Around a year ago I received a pair of copper plate GM70, for 200 USD, and was disappointed by weak bass compared to graphite tubes. However, recently I got another copper pair, with better measured parameters, and sound has improved essentially, to absolute superiority over any graphite GM70 examples.
I my case, I use two power supplies for rough GM70 testing.
With one (DC) supply I set 3A heater current. Connect grid to cathode (positive heater contact). With another supply apply 200V and 300V anode-cathode voltage, observe the Ia values. The Ia at 300V can vary from 70 to 120mA, for NOS and used tubes. Frequently used tubes perform better than NOS.
Those copper plate examples with Ia from 105mA to 115mA - are real treasure.
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