The wonderfully simple 300B directly heated triode is virtually Lee De Forest's very first evolutionary 3-element piece, comprising only filament, input grid and anode output. Brought to reliable perfection by Western Electric USA and later by Standard Telephones UK as the 4300B, the 300B was very much intended and widely used for telephony amplification. But audio enthusiasts of older times knew a good thing when they saw (heard!) it and the great love affair between the 300B and music began... and dwindled many years later as the American Western Electric 300B was taken out of production in the late 1980's making the genuine ones extremely valuable and, accordingly, very highly prized as collectors' pieces. With the Asian-driven passion and resultant revival of SINGLE ENDED topology the call for the 300B to be put back into production was too overwhelming for the Chinese factories to resist. The Russian and the re-start of USA manufacture of the genuine Western Electric original have further expanded the selection. Basic thermionic simplicity... directly-heated triodes... it is a drug for some, a religion for others...

With its space-efficient chassis size, this amplifier shared a chassis design with our then-current models, the Manley 50 watt monoblocks and the 120 watt monos. The input tube was a 12AT7 Ei Yugoslav tube and the driver was the beefy 7044 or 5687 dual triode. This amplifier used solid state rectification. A full wave bridge converted the AC to DC for the B+ rail.

When several other manufacturers started selling single-ended amps with 5 to 9 watts of power, we understood subjectively and objectively the good part of the story: the superb mid range, the delicate detail, the inherent simplicity. The bad news was that there were almost no speakers efficient enough to help form a dynamically viable system. The worst news was that either these single-ended amps cost more than the condo and or they had specs that could only be read as a joke. As the triode fans increased, more efficient speakers became available and several fanatics began to dismiss push-pull as a bad thing to be avoided at all costs. We objected to such blanket generalities and stressed that the push-pull done right with truly phase-balanced transformers, quality parts throughout and well designed circuitry has proven to be a superb technique. We also enjoy single-ended topology but only when it is done right, mostly because we expect flat frequency response and low distortion out of an amplifier. We knew that we could achieve this in a properly conceived single-ended design. We chose to rise to the challenge of designing special circuitry that could be switched "on the fly" From 12 watts in parallel SINGLE-ENDED to 25 watts in PUSH-PULL which, we believe is unique in the industry.

The Output transformer (hand-wound in our own factory) is of complex design employing inordinately high primary inductance to compensate for the mandatory magnetic "gap" required for the uni-direction current and voltage flow of the single-ended output stage. Another factor in the single ended triode saga (currently enjoying a wide spread popularity) is demand for little (or sometimes zero) negative feedback. We at Manley Labs believe in the judicious or sensible amount of feedback to refine and maximize performance. Our Neo-Classic SE/PP300B gives you the choice in precision switched 1 dB steps to go from ZERO dB's to 10 dB's of negative feedback. Even with zero negative feedback, this amp is cleaner and flatter than any single-ended amp we know of. For more information about variable feedback, read Notes on Negative Feedback.

One interesting aspect of negative feedback is it gives a higher damping factor. Conventional wisdom would have us think that the higher the damping factor, the better. Our experience is that the optimum damping factor is not infinity and that it depends on the speaker, the room. and the taste of the listener. In other words, the optimum is variability, and this we provide. This amp satisfies us as long time amplifier builders, amazes the critics in several well known magazines and is bound to satisfy you with the ability to fit into your system and your expectations. BUT REMEMBER PLEASE the overall facts of power ratings versus loudspeaker sensitivity and room size when electing to venture into lower power amplifiers (less than 50 watts)and especially when contemplating single-ended topology. Please read Considerations of Amplifier Power for more info.

Serial codes for the COMPACT SEPP 300B amps ran from MSEPP### to MSEPP128




  • Vacuum tubes:
  • Output 2 x 300B
  • Driver 1 x 7044 or 5687 GE NOS USA
  • Input 1 x 12AT7 Ei
  • Output Power @ 1kHz with 0 dB FeedBack in SINGLE-ENDED: 18 Watts @ 3% THD
  • Output Power @ 1kHz with 10 dB FeedBack in SINGLE-ENDED: 25 Watts @ 3% THD
  • Output Power @ 1kHz with 0 dB FeedBack in PUSH-PULL: 36 Watts @ 1.5% THD
  • Output Power @ 1kHz with 10 dB FeedBack in PUSH-PULL: 42 Watts @ 1.5% THD
  • Input Sensitivity 1 V nominal, depends on feedback setting
  • Input Impedance: 40 kOhm
  • Output Impedance: 5 Ohm (fixed) appropriate for 4 & 8 Ohm speakers
  • Freq. Response SINGLE-ENDED: 0 dB FB 15 Hz - 6 kHz ±0.5 dB (measured at 5 watts)
  • Freq. Response SINGLE-ENDED: 10 dB FB 15 Hz - 28 kHz ±0.5 dB (measured at 5 watts)
  • Freq. Response PUSH-PULL: 0 dB FB 10 Hz - 12 kHz (measured at 5 watts)
  • Freq. Response PUSH-PULL: 0 dB FB 10 Hz - 60 kHz (measured at 5 watts)
  • S/N Ratio: Typically 100 dB or a -76 db noise floor 10Hz-30K
  • Power consumption: 216 Watts 
  • Factory set for 100V, 120V or 220-240VAC operation for original destination country's mains voltage.
  • Operating Mains Voltage changeable with power transformer re-wiring and fuse value change.
  • Mains Fuse 100, 110, 120VAC operation: MDA 2 Amp SLO-BLO Ceramic Time-Delay fuse
  • Mains Fuse 220, 240VAC operation: MDA 1 Amp SLO-BLO Ceramic Time-Delay fuse
  • B+ Fuse: 4/10 A (250V) SLO-BLO Ceramic Time-Delay fuse
  • Mains Voltage Frequency: 
  • Dimensions: W=12", D=11", H=8" 
  • Minimum Mounting Surface Dimensions: Feet Footprint = W= 7.5" D= 18"
  • Shipping weight: 25 lbs. each 
  • Badge Illumination: 8V, 0.3A "Fuse-Lamp" 1/4" X 1 1/4"  

Specifications subject to change because they just might.



FAQ 1: May I use the KR 300BXLS in the 300B SE/PP power amplifiers?

NO. Not recommended unless the amp is modified.

First, KR filament is 1.9 amps, not 1.2 as is a standard 300B. This will drag the filament down below good...unless filament pi filter resistor values are you've got an amp that will only work with the KR XLS tubes, and the ripple on the filaments will go up a little....

It would be possible to install a filament voltage switch to change the filament voltage, and the filament rectifiers would probably have to be beefed up to the next bigger current-handling part. This should allow auditioning of the KR tube operating at regular 300B operating stress levels. But the output power would be about the same, according to what I see on the KR data sheet.

To get more power:

The XLS tube's plate resistance, transconductance and gain(mu) are really close to the WE book values. Plate dissipation is bigger though, but to take full advantage of it, the plate voltage and OPT transformer primary impedance would need to be shuffled...plate voltage would have to go up a little.

(Now you've got a modded amp that is "tuned" for the KR tube, and no other lesser tube had better be fitted, or there'll be trouble.)

So, NO. Do not use the KR 300B XLS in these amplifiers. Stick to normal 300B tubes.