What are the requirements for driving a 300B?

[G]

How much gain and current is needed to drive a 300B? I would like to try it with a 7044 in one stage if possible.

[SY]

Depends on your operating point. You want to be able to swing something more than the standing cathode-to-grid voltage for class A or AB1. You want a source impedance low enough to keep the rolloff from the time constant (pole) formed by the driver source impedance and the input capacitance of the 300B at least a decade higher than the dominant pole. If you're running the 300B in a greater-than-unity-gain output stage, be sure to account for the Miller input capacitance, too.

Bottom line: the 7044 will do just fine in an appropriate circuit.

[Sch3mat1c]

I don't know what a 7044 is but a 300B takes about 40 (actually I haven't seen a circuit this low) to maybe 80V peak drive. To run from a .25Vp signal source you'll need as much as 160 gain. This can only be done in one stage with a pentode. Typical circuits use a 12AX7 (I wouldn't, not much drive capacity), an 'AX and a low-gain, high output driver, or two 6SN7 stages.

Tim

[SY]

To run from a .25Vp signal source you'll need as much as 160 gain.

That's an unusually high sensitivity. 1.5 to 2.5 volts rms is more common.

[Sch3mat1c]

Ya... just overestimating to give the maximum amount needed. Of course, double that once or twice if you want NFB (typically not done to triode amps, certainly not 300B's )

Tim

[Bojan Hajdinjak]

Originally posted by G
How much gain and current is needed to drive a 300B? I would like to try it with a 7044 in one stage if possible.

300B power triode (not European version of this) has very low input capacitance. If you'll made some (one) driver stage in SRPP - something like this

(see only the input/driver stage) with the valves lika 12AX7, 12AT7 or 12AU7 (sonding are these a lot of different options), you'll get a good sound, enough gain,....

It isn't neccessary to find some exotic drivers. Some of good 12Ax7 like Sylvania (5751) or Mullard,... are enough exotic choice..

A lot of informations you can see on www.hinedfi.com (USA/ENG site) in the next days.

Sound of 300B design often depent a lot on primary/secunary relation of output transformer....

[analog_sa]

It isn't neccessary to find some exotic drivers. Some of good 12Ax7 like Sylvania (5751) or Mullard,... are enough exotic choice

I would respectfully disagree but i can't, driving a 300B with a 12AX7 is outright hilarious! A lot of the undeserved bad rap the 300B gets is due to inadequate, anaemic drivers.

I recently heard it driven by a 2A3 - it had unusual power, dynamics and surprisingly - bass.

[Eli Duttman]

A lot of the undeserved bad rap the 300B gets is due to inadequate, anaemic drivers.

Mu followers built from 6BM8s as the 300B driver have some popularity. It's not difficult to see why. Plenty of gain combined with good drive capability is never bad. The triode in the 6BM8 is essentially 1/2 a 12AT7.

A Gary Pimm style CCS load, with its built in mu follower, allows you latitude in the choice of gain triode. The 6n1p, the 12AV7, and the 6J6 are possible candidates. IIRC, GP indicated that the pentode (6AU6)/FET variation of his CCS is particularly good sounding in front of a power triode "final".

[Layberinthius]

A lot of money for one.

[Bas Horneman]

I recently heard it driven by a 2A3 - it had unusual power, dynamics and surprisingly - bass.

I've heard it being driven by a 12ax7 ....bass was best I ever heard....EVER.

Just the high's were a bit ragged.