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In Reply to: RE: If your speakers are 100dB you'll appreciate the extra power posted by Tre' on April 01, 2025 at 08:00:39
Two 300Bs in class A. PP.
To get 26.5 watts from two 300Bs in PP Class A you would have to be running those tubes very hard.
Normally a single 300B in Class A is good for about 8 watts so a pair in PP (or PSE) Class A would give you 16 watts.
There are people, like Jack Elliano of the transformer company Electra Print, who run direct heated output tubes really hard.
I think the HD would be a lot higher running them that hard.
But I could be wrong.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
If class A2, which that tube does easily, 27 Watts is no problem. To do it you'd have to transformer couple the driver or direct couple it since the driver needs good linearity when grid current is present.
We made an OTL using 4 300bs about 27 years ago or so and showed it at CES. The single driver tube (12AU7) had no problem driving all 4 power tubes into grid current. The amp was more of a demonstration than anything practical as it only made about 15 Watts and needed 16 Ohms to do that.
Ok but I have never seen a data sheet showing Class A2 operation of a 300B.
I don't think the tube was designed for that but I could be wrong.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
What you do is make sure you're not exceeding the max dissipation; actually you can design the circuit so you're well within parameters.
I thought in a tube intended of Class A2 operation the grid had to be designed to stand up to the current that is not present in Class A1?Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/03/25 04/03/25
Some tubes are rated for maximum grid dissipation, or max grid driving power, or max DC grid current. The 211 is an example. Some tube data sheets even include curves for positive grid voltage. I presume these indicate suitability for A2/AB2/B2/C2 operation.The Type 45 is explicitly rated for AB2 operation.
Some other tubes specify a max grid voltage of zero, presumably they are not suited to such operation.
Edits: 04/04/25
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Have Fun and Enjoy the Music
"Still Working the Problem"
Its likely that some tubes are better at it than others. But I don't know which they might be. I do know from having done it that 300bs have no problems with it at all.
If I use 5VDC for 300B Filament, should I use separate 5V for each 300B. Or I can use the same 5VDC for both 300B?
That will make it trickier to have a hum balance setup that is only one control.
I have a PP type 45 power amplifier. I don't use a hum balance, instead I was careful about the filament connection so that the hum waveform cancels in the output section. That required a minor amount of matching but since the mu of the tube is so low that was not difficult.
In my 300B SE mono block amplifiers I use critical inductance LCRCL filters and replaced my hum pots with fixed resistors.By the way the hum will only cancel in a PP design. And only if it's in phase and equal amplitude in both tubes.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
Edits: 04/09/25 04/10/25
VAC was getting 32 watts from their VAC 30/30 amps...and yes, they were driving the tubes very hard to get that and I think the lifespan of the 300bs in that amp was pretty short for less robust tubes. JJ300Bs, whatever you think of them sonically, are very robust and could survive a pretty long time in those amps.
I had a JJ322 PSE 300B that delivered around 20 watts...15 of which was actually quite clean.
I currently consider the following configuration.1. A1 not A2. So output about 20W
2. Mono blocks.
3. Separate power supply box because I can't fit everything into that nice chassis. Also power supply is noisy.
Edits: 04/02/25
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I did my best digging more watts.
I ordered these Chassis but not sure everything can fit inside.
That looks like a nice chassis.
Tre'
Have Fun and Enjoy the Music
"Still Working the Problem"
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