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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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Tubes Asylum Questions about tubes and gear that glows. FAQ |
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In Reply to: Emission Labs 2A3-Mesh V4 posted by banpuku on November 4, 2016 at 14:29:05:
I was asked by one of the writers here, to comment on this subject as well. I can, but this is diffult to put in a few lines.
Of course we all know the subject of the "humming tubes", but tubes produce by themself only colored noise, not hum. For hum, when it comes out, it came in somewhere first.
Some of the details, I have put together in an application note (AN6) but that is "live" document right now. I need to change it here an there, so it becomes easier to read.
Center tapped tubes help us to reduce hum at the output. (We call those V4 Version at EML). As additional advantage, the two centering resistors become obsolete. Moreover, my own experiments show, distortion at high hsignal was reduced slightly. These advantages are small, but all little bits help.
Here is an important thing I would like to point out. Unlike for 20 years, I see today many amplifiers are DC heated. People tend to ground the heater with one end, and put the DC voltage on the other. This is essentially wrong, but I understand why they do it. With center tapped tubes, this can be done in a better way. (read AN6 for this)
The idea of center tapped tubes was brought to me by a gentleman from Japan, using a radio frequency tube with center tapped heater. With that tube, the intention is to loose less RF signal in the heater. So two short paths are less of a problem than one large. This sparked the idea of center tapped tubes with Emission Labs. I have to admit I wasn't aware of the existance of the 6A5G at that time, but what we read in the Sylvania dataheet confirms the observation of higher output power.
When looking at 6B4G this is just a 2A3 with 6.3V heater. However 2A3 is specified at 3.5 Watt output power and 6B4G only 3.2 Watt. The higher heater voltage of the 6B4G may be responsible for that, as it's the only difference I can see. Going in the other direction, 6A5G is a center tapped 6B4G, and it brings 3.75 Watt. Perhaps this 3.75 Watt is a little bit "tuned" by Sylvania, to prove their point, but the observation of slightly higher output power, I can confirm from my own experiments with the EML 1605 vs. 1605-V4, and also 520B-V3 vs. 520B-V4.
For systems with very high effiency loudspeakers, low hum becomes the first issue, not output power. It's a major difference when an amplifier has 0.5mV residual hum, and it's connected to a 94dB speaker or a 106dB speaker. Wheras 0.5mV hum is not easy to achieve. Many amplifiers are above 1mV.
As such very high efficiency speakers are useually connected to a 45 or 2A3 amplifier, I would expect, the V4 versions are good idea. But the final word is with the users.
Sylvania write in their datasheets 6A5G doesn't need a hum balance potmeter. That seems like a good hint to me.
6AG5 is a double plate tube, so I expect they constructed the center tap just from the series connection of the two tubes in the bulb. So each tube in the bulb is 3.15V. That is legitimate, and it works.
A better effect with respect to hum, you will get with a single plate tube, and a physical center tap in the middle.
P.s. the 6A5G is a bit rare. You find it at www.4tubes.com under manufacturer scans.
-Jac
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- RE: Emission Labs 2A3-Mesh V4 - EML 11:26:32 11/06/16 (0)
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