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Can anyone offer any advice on whether the Mullard ECC32 can be used in the driver stage of the ATMA-SPHERE MA-2's? Any experience using this tube out there?Thanks.
John.
Follow Ups:
I don't know about current production, but historically the tubes in Atma-sphere amps were run very conservatively to maximize tube life and reliability. This is understandable, because of the erroneous notion that OTLs are "unstable" or "unreliable", and Ralph had to combat that belief among the ill-informed. However, in my own view the sonic performance of the amp can be improved noticeably by increasing the current used in the input voltage amplifier stage. This stage consists of a dual-differential/cascode with another tube used as a CCS in the cathode. To determine the amount of current, measure the voltage across the cathode resistor (Rk) of the CCS and divide the result in volts by the value of Rk in ohms. This will give the current in amperes. Then convert the answer into milli-amps. If the result is 4mA or less (meaning the current thru each half of the dual-differential circuit is 2mA or less), then you might benefit from doubling the current, by halving the value of Rk. Remember to select a resistor that can handle the power (P = {voltage dropped across Rk} X current). This was a tweak originally suggested by Paul Speltz, and I found it to work better than tube rolling to improve the sound of my amps. Even doubling the current still leaves plenty of headroom to assure long tube life.
There are a few notable differences between ECC32 and 6SN7 types:- ECC32 has 950mA filament vs. 600mA for the 6SN7
- ECC32 is rated with a Mu of 32 vs. 20 for 6SN7
- ECC32 is rated for a max plate voltage of 300V vs. 450 for the 6SN7.The difference in gain is substantial (over 50% more voltage gain) and will certainly make the amplifier's input more sensitive - which may have a negative impact on total distortion. Considering the high absolute voltage differential in the MA-2 driver section (over 800V, though during operation the device sees half of this) I'd be a tad shy about plugging in one or more of these spendy NOS devices. This said, the 6SN7s are bulletproof in this amp.
All in all, I think this is a good tube to _sell_ ;-)
If I remember correctly, the ECC32 is not electrically identical to a 6SN7, so ask the factory. The ECC32 may have a slightly higher mu or Gm compared to the 6SN7, either of which properties could make the ECC32 sound louder, because the input sensitivity of the amp will increase slightly. Don't confuse "louder" for "better".
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