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Has anyone tried the Emission labs 2A3-Mesh V4 with center tap heater? Here is some text from the Emission Labs site about the tube. I am very curious about using this tube, as it appears (based upon what EML says) that the cathode resistor and bypass cap can be eliminated. If so, this should be a very nice enhancement, as the signal path no longer has to go thru the cathode bypass cap, which would result in a more pure sound.
From EML web-site.
"With the center tap connected tubes, we enter a new field of applications. This has not been done before with Audio tubes. When biasing a DHT tube there is no cathode to connect something to. The cathode must be artificially constructed, by a resistor network. Since many years, all EML tubes already feature a center tapped heater, which is a way to enforce better electrical field geometry. Only, to this center tap there was never an electrical connection. So to the tube itself we had to make no changes. To the center tap of the heater, is now connected a wire, which becomes the third connection to the heater. Now the tube has five wire connections, and for this reason we can not use the 4 Pin UX4 base any more. We have chosen for the Octal base. It must be said, the V4 is a very special tube, which fulfills the needs of Circuit Design purists the best possible way."
Follow Ups:
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
as it appears (based upon what EML says) that the cathode resistor and bypass cap can be eliminated.
I don't see where EML suggests that in your quoted text and this connection does nothing to eliminate either of those parts.
simply put, the cathode connection now sits at 1.25V and if referenced to AC ground you will now have +- 1.25V on either side of this point that are 180º out of phase with each other which in a perfect world will sum to a potential of 0vac showing up at the cathode from the filament heating circuit.
dave
Yes?
The main upside is the lack of need for balancing resistors and/or a balancing pot. But, depending on how exact the "center" tap really is, you might still gain something with a pot on the fils (also possibly since aging may affect this too.)
IMNSHO, the pain in the ass part is needing a different socket and non-backwards compatibility with other tubes of the type, as a result.
Cool idea though.
-Ed
Does this make it an indirectly heated triode, like for example 1626? If so, doesn't this open the way for DC heaters? I don't know the answers.
The 6A5G looks like a 6B4G but actually has a filament center tap connection, as well as an internal cathode wired to this "extra" pin ! Whether a DHT or IDHT, these or the new 2.5V mesh plate versions are still going to be great sounding tubes and are easier to balance out any hum issues. DC heaters should not be needed. Film cap or multi-cap bypass can still be tried, but may not be necessary.
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