300B's for the BeePre

[johnsonad]

As this will come up eventually

I am intending to use the EML 300B Mesh with my BeePre.  Go big or go home right   These tubes draw 1.4 to 1.5 amps and my question is, well the BeePre have any problem in meeting 5v/4% tolerance/1.4 amp on the heater?

Thanks,

Aaron

[Paul Joppa]

Yes, the BeePre will have a severe problem with non-standard current draw. At 1.5 amps, the 300B will see only 4.4 volts, well below what is required for the full 60mA current draw. I doubt Jac would honor a warranty under such conditions, and he would be completely correct in my opinion.

Of course the circuit can be modified, but it would require more current from the filament power supply, causing it to run hotter and thus be in danger of overheating.

The BeePre is not designed to accommodate non-standard "300B" types which do not meet the specification for 300Bs.

The only non-standard tube that will work correctly is the 6A3, or a 6B4 with a different socket. That is not so much by design as a fortuitous accident!

[johnsonad]

Thanks Paul, I was worried about this and Jac brought up the warrenty in our emails.  The kit isn't even out yet and people are already trying to modify it!  If and when you work out a potential circuit mod, please let us know.

Regards,

Aaron

[Paul Birkeland]

If and when you work out a potential circuit mod, please let us know.

You can change a couple resistors, then put the BeePre in a small refrigerator.

[johnsonad]

If and when you work out a potential circuit mod, please let us know.

You can change a couple resistors, then put the BeePre in a small refrigerator.

Point taken    Too bad as the EML tubes sound amazing.........

[braubeat]

Well, you can always buy some big beefy filament transformer, if you're dead set on eml tubes.

Michael

[Paul Birkeland]

Well, you can always buy some big beefy filament transformer, if you're dead set on eml tubes.

Michael

You could, but then you'd have a preamp with a lot of hum.  A big part of the research for the Bee Pre was how to get the circuit as quiet as possible (remember, there is no 3k:8 step-down transformer).  AC was out of the question, DC was pretty noisy, then quieter with a regulator, then quieter yet with some active noise cancellation, and finally acceptable with one more filter at the output.  Bumping up the current flowing through all this circuitry will increase the heat generated, and the voltage dropped.  The current is actually available from the transformer winding itself, but everything else is optimized for the 1.25A filaments, and there is no space for additional heatsinking (literally, the heatsinks in question are flush with the bottom of the wood base, one has to use the rubber feet for proper operation).

-PB

[Paul Joppa]

PB's summary is pretty accurate. That said, I hope we will continue to explore filament power schemes. I feel that further research will pay off.

Now don't start asking "when?!?!?" - this will be a long term project. I think if this preamp is successful, there will be a revision in maybe 5 years. That's roughly the schedule for Foreplay - a new version every 5 years or so.

[islanddave]

Hello,

What brand of tubes are used in the compete kit? If I order I am debating which el 84 and 300 B tubes to use? But $176 for two 300B's seems very reasonable! By the way earlier here it was mentioned that "non-standard" 300B tubes won't be supported with this amp. What parameters dictate a "standard" 300B?

Thanks Dave

[Paul Joppa]

Standard in this case means the filament draws 1.2 amps at 5.0 volts. Some "super" 300Bs draw more current.

[islanddave]

What brand of tubes are used in the compete kit?

[Paul Birkeland]

All BeePres will ship with Russian Military EL84's (I think they are early 80's), and those who ordered the kit with 300B's will receive EH 300B's.

-PB

[caffeinator]

Cool...really looking forward to this kit.

I'm going to finally break out the TJ Meshplate 300B's I got way back when with my Parabees...having aged them to perfection in a box in the basement all these years.

I presume that being used in the shunt reg, rolling different EL84's will have relatively little or no effect on sound - is that a valid presumption?

[Paul Birkeland]

Eh, the EL84's are in one of those positions that "shouldn't" matter, but ultimately everything at this level matters at varying levels of importance.

[Doc B.]

Yeah, it shouldn't matter. But I have changed shunt reg tubes and heard a difference. I don't have any definitive advice on what tube to use. The Russian tubes we supply sound pretty great in a Tode amp. I ended up liking one better than an old Mullard EL84 in that app. Whether that data point applies to the BeePre I can't say.