Bias of Emission Labs 300B Mesh

One has to wonder why you would even consider violating the recommendations of the manufacturer knowing it will reduce the life of the tube. Instead of attempting to wring the last few mW out of the tube, why not follow the Western Electric recommended operating points for least 2H and 3H distortion? Two of the operating points, in particular, with the lowest distortion are:

Ep= 250V, Ip= 60mA, Eg= -48V, Rl= 2.7kohms, Po= 4.1W, Plate dissipation: 15W

Ep= 350V, Ip= 50mA, Eg= -76V, Rl= 5kohms, Po= 6.2W, Plate dissipation: 17.5W

This is not just theoretical for me as every day I listen to a pair of EML 2A3 V4 mesh plate tubes. I adhere to the recommended operating point and never felt like it was a challenge to achieve the best sound from this tube. The EML 2A3 V4 mesh plate has been completely trouble free for a couple of years with hundreds of hours use.

RE: Bias of Emission Labs 300B Mesh, posted on October 25, 2019 at 15:44:35
Tube747 Audiophile Posts: 419 Joined: May 11, 2004	I'm thinking to replace the 300B with EML 300B mesh for the DRD 300B amplifier. I can replace the opt with any primary impedance but not quite sure what B+ voltage should I go for.

RE: Bias of Emission Labs 300B Mesh, posted on October 26, 2019 at 09:19:30

Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008

First you need to determine which driver tube you will use. This gives you the voltage you will subtract from the total B+ giving you the 300B plate to cathode voltage. For instance if we choose a driver tube requiring 150V - 200V (which is typical for a triode connected pentode like the D3a or E180F), we can then choose our operating point for the 300B. For practical and safety reasons I keep my total plate supply for a DRD amp at less than 550V. This allows one to use the WIMA DC MKP 4 Link capacitors with a 600V rating in the power supply. So, 550V - 150V leaves us 400V for the plate supply if we need it. However, a classic operating point for the Western Electric 300B is Ep= 350V, Ip= 60mA, Eg= -74 into a 3k ohm load for 21W plate dissipation and about 8W power output.

This operating point plus the 150V - 200V for the driver tube means that our total plate supply voltage is less than 550V making for a safer and easier to implement power supply.

For a driver tube, you are constrained by the AC8037 plate choke to <20mA. So the driver can be any high transconductance tube with sufficient MU to drive the 300B to full output with around 1Vrms input. No preamp is required nor is it going to improve the sound if one is used. The 2Vrms from the DAC gives you almost 6dB of headroom already. Fortunately, if our plate supply is closer to 550V we have 200V @ <20mA available for the driver. So with Jack's example of the 6AN4 we have a MU of 70 for an Ip of 13mA Ep= 200V Eg=-1.3V. I would try to find an LED with the requisite voltage drop of about 1.3V - 1.4V and eliminate the cathode resistor and 1000uF bypass cap. The grid resistor would be a 100k ohm volume control. Use Ohm's law to determine the two series cathode resistors for the 300B. The resistor closer to ground determines the driver plate voltage and the upper resistor determines the bias on the 300B.

RE: Bias of Emission Labs 300B Mesh, posted on October 27, 2019 at 07:16:23
deafbykhorns Audiophile Posts: 1067 Location: Florida Joined: October 17, 2003	I have an amp with JJ2A3 tubes that can run 40w I have them biased at 24w I tried an emission labs a few years back in the circuit and they didnt do so well at 24w, from what i remember, they red plated and arced.

RE: Bias of Emission Labs 300B Mesh, posted on October 27, 2019 at 08:34:30
Tube747 Audiophile Posts: 419 Joined: May 11, 2004	My friend just made a suggestion by having the circuit with EML 300B Mesh and adjust the B+ supply for the tube. What do you think??

RE: Bias of Emission Labs 300B Mesh, posted on October 28, 2019 at 11:54:40
Palustris Audiophile Posts: 2408 Location: Cape Cod Joined: September 12, 2008	I made a very similar amp using a single 6K5G. Give it a try.

RE: Bias of Emission Labs 300B Mesh, posted on November 2, 2019 at 12:06:16
Tube747 Audiophile Posts: 419 Joined: May 11, 2004	To play safe, I decided to run the tube at 350v @63 mA and loaded to the 4K output transformer. This is because I wan to operate the 300B Mesh within 22W power dissipation. Any thoughts??

RE: Bias of Emission Labs 300B Mesh, posted on November 2, 2019 at 13:27:53
Palustris Audiophile Posts: 2408 Location: Cape Cod Joined: September 12, 2008	I think it will sound great.

RE: Bias of Emission Labs 300B Mesh, posted on November 2, 2019 at 13:45:17
Tube747 Audiophile Posts: 419 Joined: May 11, 2004	Can I lower the B+ voltage and current of DRD 300B circuit to adopt the 300B Mesh? This means 700v down to lower voltage as the operating point (350v @63mA) of the 300B Mesh is much lower than the conventional 300B. Possible??

RE: Bias of Emission Labs 300B Mesh, posted on November 3, 2019 at 07:25:13
Palustris Audiophile Posts: 2408 Location: Cape Cod Joined: September 12, 2008	A schematic of the amp with power supply would be a help, but in general, if you have a capacitor input supply and a decent sized choke in the PS, you can loose about 100V by converting to choke input from cap input.

RE: Bias of Emission Labs 300B Mesh, posted on November 3, 2019 at 09:48:36
Tube747 Audiophile Posts: 419 Joined: May 11, 2004	FYI Thanks!

RE: Bias of Emission Labs 300B Mesh, posted on November 3, 2019 at 19:12:51

Tube747
Audiophile

Posts: 419
Joined: May 11, 2004

This link is including the power supply circuit.

DRD 300B

RE: Bias of Emission Labs 300B Mesh, posted on November 4, 2019 at 10:39:43

Palustris
Audiophile

Posts: 2408
Location: Cape Cod
Joined: September 12, 2008

Are you confident that your amp is exactly the same as this schematic?

My Electra-Print power transformers are closer to 500V-0-500V. Does your chassis have a socket for a rectifier?

While I can offer suggestions to get your supply down to a safer and more reasonable level, unless you built the unit yourself and are comfortable working with these high voltages, I am hesitant to give detailed instructions.

Model the entire supply in PSDU II first.

Then go to a cap input filter with your model by eliminating the first two caps in series along with their resistors. The specs. for the choke are sufficient at 10H - 15H to meet critical inductance; but you will need to measure the DCR for the modeling software. That should take the output at the first cap from the choke down to about 550V - 560V. You may find that you need to add another LC filter to get your ripple down to where it was originally. An Edcor XC87-2H-350mA might fit under the chassis.

If you still have a bit too much voltage adding a tube rectifier can drop another 25V - 50V depending on the rectifier. To reduce P - K voltage on the output tube you can increase the plate voltage of the driver since it subtracts from the output tube plate voltage. The driver will then need to be re-biased by increasing the cathode resistor or adding another diode in series if you use LED bias.

So, these are the basic ideas to reduce the output tube plate voltage, but as I say above, unless you are confident working with these voltages and have the testing skills necessary to ensure it is safe, I am reluctant to be more specific.

Thank you! (nt), posted on November 4, 2019 at 14:40:36
Tube747 Audiophile Posts: 419 Joined: May 11, 2004	:-)

Wait a minute here ..., posted on November 20, 2019 at 23:14:38

Paul Joppa
Industry Professional

Posts: 7296
Location: Seattle, WA
Joined: April 23, 2001

So I looked at the schematic, and it looks like all you need is to augment the 50-watt resistor with an additional 1000 ohms or so.

Even without any change, you should be able to use that circuit to get the current down to 70mA (0.7v across the TP1 test points).

The B instruction is to adjust the plate voltage of the driver. OK, a 300B at 63mA is about 74v bias, so its grid would be 424v below the plate. If the output transformer drops 16 volts, that brings it to 440v below 700v, or 260v. The instruction is to adjust to 220v, only 18% off. You might increase the driver cathode resistor from 100 to 107 ohms but it's not likely to make much difference.

RE: Wait a minute here ..., posted on November 22, 2019 at 16:33:23
banpuku Audiophile Posts: 1008 Joined: January 19, 2006	Paul, Hope all is well. Wondering if I can run something by you offline related to Hexfreds. It will make sense if I send an email, but prefer not to post. Thanks, Pat banpuku@mac.com

RE: Wait a minute here ..., posted on November 24, 2019 at 21:43:13
Paul Joppa Industry Professional Posts: 7296 Location: Seattle, WA Joined: April 23, 2001	Can't say I know a lot about hexfreds, but no problem with email - my Bottlehead address should work.

RE: Bias of Emission Labs 300B Mesh, posted on December 5, 2019 at 07:03:43

Gordon Rankin
Manufacturer

Posts: 2928
Joined: June 9, 2000

747,

More so the problem with the mesh EM 300B's is not the overall power but the arcing of the plate to the grid.

What ever your amp is if it does not have a slow turn on or is direct coupled then you may have an issue as others have.

Personally I don't know why companies keep making tubes that do not at least adhere to the specs of the originals. Why call this a 300B, instead call it a Mesh WE275 or something totally different.

Thanks,
Gordon
J. Gordon Rankin

RE: Bias of Emission Labs 300B Mesh, posted on January 15, 2020 at 03:59:23

EML
Manufacturer

Posts: 57
Joined: March 19, 2002

Oh my God.

"Should I bias the Emission Labs 300B Mesh limited to maximum 22W plate dissipation as their website recommended?"

What will you do, if somebody writes here, that doesn't matter?

SET Asylum

Bias of Emission Labs 300B Mesh

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