These 2.5V/2.5A filament 300Bs from EML might be of interest

[Deke609]

Just noticed that Jac of Emission Labs makes a 300B mesh tube with a 2.5V filament requiring 2.5A current - same as a regular 2A3. Perhaps they'd be drop-ins for the 2A3 in the MA, at one  or both of the MA's plate current settings. Perhaps PJ or PB could weigh in.

It'd be pretty sweet if they could work without having to make any changes. Of course, the JJ-2A3 40 is reported to be the same thing: a 300B with a 2A3 filament. I have both the EML 2A3 mesh and JJ-2A3 40 and used them in a Stereomour II. I like both, but I much prefer the 2A3 mesh - YMMV, IMHO, etc, etc. So, subject to what PJ or PJ says, for those inclined to tube roll these might be worth a try.

Fair warning: I've found that EML tubes take a long time to "burn in" -- 100+ hours in my experience. I have two pairs of EML's regular 300Bs, a pair of 45Bs and the 2A3 mesh - all took along time to really come into their own.

Here's a link: http://www.emissionlabs.com/datasheets/EML300B-25V.html  (no affiliation - I just like EML tubes)

cheers, Derek

 

[Paul Birkeland]

They should work nicely in a MonAmour.

 

[triode]

Derek

Emission Labs also has 2A3-S which is 2A3 with higher maximum dissipation.
Do you have a chance to compare between these 300B-25 vs 2A3-S ?

Which one would give more power in MonAmour?
Maybe Paul could estimate with limited data from Emission Labs site?

Thanks

 

[Tom-s]

Both EML 2A3S and 300B2.5 (normal and Mesh) will work perfectly with MonAmour on high power setting.
The 300B2.5V should give a higher output power due to it's lower plate resistance (compared to 2A3).

Beware, the "normal" EML 2A3 Mesh (18W dissipation) is too stressed by the MonAmour at high power operating point, for Mesh tubes i'd go with the 300B 2.5V as you can use both settings on MonAmour.

 

[Paul Joppa]

Technically, the EML 2A3-mesh is rated for 18 watts maximum, which is the MonAmour high-current operating point.

Admittedly, it would be prudent to stay below the maximum dissipation for such an expensive tube, and the filament voltage is not as tightly regulated as EML prefers, so they would probably not recommend it.

 

[Deke609]

Do you have a chance to compare between these 300B-25 vs 2A3-S ?

Hi triode - Unfortunately, I've never hear either of these tubes. I've only heard EML's 2A3-mesh in a mostly stock Stereomour II, 45B in a Stereomour II converted for 45 tubes, and "regular" 300B ("regular" b/c it requires 1.3A filament current rather than the standard 1.2A) in a Beepre (modifed specifically for this tube) and Kaiju.  So I've never heard different EML versions of the 45/2A3/300B family in the same amp. That would be interesting.

On that note, I wonder whether it would also be possible to run the EML 45B (note the "B", the regular 45 wouldn't work) in the MA. Assuming the low plate current setting of the MA is 50mA, perhaps it could be tweaked slightly downwards to 48mA to make the EML 45B work. Or maybe no tweaking would be needed -- the 45B appears to have more than double the plate resistance of the 2A3 at Jac's recommended 48mA plate current operating points.  Perhaps this additional Rp would do the job of shaving a bit more current? ... And to better approximate Jac's recommended higher plate loads for the EML 45B, maybe one could install an impedance switch on the MA output transformers so that, for example, if using 8 ohm speakers, one could select 4 ohms on the impedance switch, thereby doubling the reflected load on the primary (Hmmm ... I may be talking myself into getting the MA amps).

cheers, Derek

 

[Paul Birkeland]

I would not run a tube with twice the Rp of a 2A3 in the MonAmour.

 

[Deke609]

Because the tube would be under-loaded? I guess proper plate loading would be a big issue.

 

[Paul Birkeland]

Damping would be a big issue.  Trying to make the 3K output transformers act like 6K output transformers isn't the best plan either.