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]

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

An externally hosted image should be here but it was not working when we last tested it.

(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.

 

[fdegrove]

Hi,

I recently heard it driven by a 2A3

What is driving what here?
Is this a 2A3 driving a 300B or a 12AX7A driving???

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

I'm using a 12AX7A driving a PP of 2A3s with a load of 6K, bass certainly isn't a problem but I also have my doubt about the highs.

I tried 6N1Ps which work fine as well...Highs are much better here.
These are the weirdest triodes I've ever seen, BTW...They can be driven into gridcurrent without as much of a hint of distortion...Strange beasts...

Next in cue is the 12DW7/ECC832 which I hope will kill two birds with one stone; gain and drive.
Note this is a budget amp...nothing fancy. Still it sounds promising.

Cheers,

 

[Sch3mat1c]

These are the weirdest triodes I've ever seen, BTW...They can be driven into gridcurrent without as much of a hint of distortion...Strange beasts...

Try it with 6V6, I dare you! 150V supply, +/-15V grid drive (zero biased), 20mA peak grid current so a 6DJ8 CF can provide ample drive. The loadline looks awesome. Same for pentode mode, +/-10V drive instead (adjusting RL as required of course). A little cutoff nonlinearity is noted in pentode mode with +/-15V grid drive. Oh if only I weren't so lazy I'd build and test it.

Tim

 

[fdegrove]

Hi,

Tim,

Try it with 6V6, I dare you!

No need to dare me...I've used ETM 6V6s to drive a VT4-C and it works great.

I probably didn't express myself clearly: it's the 6N1Ps that I can hit with up to 6V peak with Vg=0.
You can't do that with a 6DJ8 and get away with it, I assure you.

I also suspect the Chinese ECC88s to be redone USSR 6N1Ps, BTW.
They more or less give me the same curves on the plotter.

Oh if only I weren't so lazy I'd build and test it.

Have you looked at the curves of a trioded 6BQ5?
Pretty nice too...
I was (still am, actually) thinking of using a 6BQ5 in penthode mode as a load for a ETM one in a mu-follower for a headphone/line preamp combo...

As you say, so much to do so little time....sigh.

Cheers,

 

[G]

I suppose I could just use the 7044 in the traditional anode follower into a cathode follower into the 300B circuit. Or I could just build the one out of the Fi Primer to see what a "true" power triode sounds like in a SE circuit. THanks for all of the replies.

 

[Sch3mat1c]

No need to dare me...I've used ETM 6V6s to drive a VT4-C and it works great.

But have you tried 6V6 class A2 (which was my point)?

I probably didn't express myself clearly: it's the 6N1Ps that I can hit with up to 6V peak with Vg=0.
You can't do that with a 6DJ8 and get away with it, I assure you.

Why, (grid) dissipation limits or nonlinearity?

Until you get to the saturation point (diminishing returns), positive grid voltage is just the same as any other point, although some tubes have a probably inconsequential kink in the curves. But a 6DJ8 (apparenly 6N1P is similar enough so I'll stick with what I know) is a signal tube, not much use for +Vg anyway...?

Have you looked at the curves of a trioded 6BQ5?
Pretty nice too...

Yeah. Too bad current is so low on all these types, the cathodes are all so small. Doesn't feel right using a 6V6 (trioded) as a driver for like 6L6, unless it's a massive dual sextet of them or something.

Anyway, I'll go look up something on the 7044 to get back on topic...
Looks like a 6DJ8 or 5687. Basing is same as latter. The high gain (erm what was the mu..) is more like a 'DJ. At any rate, the current is high enough that it'd work great as an anode follower for some heavy tube loads.

Tim

 

[fdegrove]

Hi,

But have you tried 6V6 class A2 (which was my point)?

Ah...nope...Not yet...To be put in the cue...

Why, (grid) dissipation limits or nonlinearity?

Nonlinearity. The ECC88/6DJ8 is one of those rare animals where the curve are shown when the grid is driven positive. (Don't really remember why this was)
While the 6DJ8 remains quite linear when driven that way upto say 0.5V, the 6N1P can really be drive way into the "redzone" that way.
Which kinda proves that it's more/less than just the 6DJ8 clone some manufacturers would have you believe. Plotting the curves shows that up quite clearly, tranconductance isn't in the 6DJ8 ballpark either.
Not saying it's bad or anything, just too different to pass as equivalent.

not much use for +Vg anyway...?

Depends on what you want to do with it, think pro audio gear...

Doesn't feel right using a 6V6 (trioded) as a driver for like 6L6, unless it's a massive dual sextet of them or something.

True but worth a try nonetheless. It never hurts to have some headroom.

Anyway, I'll go look up something on the 7044 to get back on topic...

Yeah...make me feel guilty while you're at it...

The 7044 is another of those on-off analog computer switchers but it has potential and could replace a 6SN7 for similar duties so it should be fine for a classic plate follower-RC or DC coupled- cathode follower cascade stage amongst others.

It's similar to the 6350/6463/E182CC etc but at least the 7044 has low microphony built in according to the datasheets.

So, Gavin...Put them to service if you like.

Cheers,

 

[grataku]

Has anyone tried the good ol' ongaku?

 

[Bas Horneman]

That is Reichtert's flesh and blood you mean?

A mini Ongaku he calls it?

Cheers,
Bas

 

[Bojan Hajdinjak]

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.

We don't understand each other. I told about 12AX7 in SRPP configuration. 12AX7 and 12AX7 isn't the same (depend on amplifier configuration). SRPP is capable to drive very complicated load.... also with 12AX7 (far from anemich....)

 

[Sch3mat1c]

Has anyone tried the good ol' ongaku?

Fine with me, though a little heavy on the filtering (20uF would be enough for me on the SN's, if any at all.. in fact... knowing me I'd run them from the same rail ) and without circles around those tube symbols the vacuum is going to get let out!

Tim