The 300B tube is probably the most popular Directly Heated Triode tube. Its been used as a premium hifi tube for nearly one hundred years now, so its been well and truly validated.
Its famous for its natural sound , with its reputation beginning with the Western Electric 91A amplifier., way back around late 1930.
This was initially manufactured for theatre sound , but over time gained a reputation for its hi fi capabilities .
Many versions have been built over the years , using different driver tubes, but the "real" WE91A used a 310A pentode tube driving the 300B tube in Single Ended configuration .
This tube is quite rare these days and no longer available , so other different pentodes have been substituted . (Actually PSVane make a replica)
The 6SJ7 is quite close and the easiest substitution to find these days . There is a Russian version widely available that is reasonably well made.
Pentodes have high gain , so can be used as a single driver stage, but pentodes dont have low output impedance (power) so struggle to overcome the 300B's Miller capacitance, and usually have a falling frequency response.
This isnt necessarily a bad thing, they can still be flat out to around 10Khz , and this is still considered to be "hi-fi" by many.
This results in a warmish sound , and is sometimes given as the reason for the 300b tube having midrange warmth .
As an aside, I recently serviced a parallel 2A3 SE amp for a friend. This was a very famous Japanese brand , with a very good reputation for its sound and expense!
I brought it back to as new condition , and was quite surprised to see it had a roll-off beginning at 9Khz , which contributed to a warm sound .
This however did not negate the sound quality , it sounded very good , and did everything right, except for the highest levels of detail.
(Interestingly, the design and engineering of the amp was very good, only let down by an output transformers bandwidth)
In the case of the WE91 , the pentode driver is the cause of a high frequency droop, although in the original WE91 , the designers used local feedback to lower the output impedance of the 310A , and restore most of the bandwidth.
Most of the later day subsequent rebuilds did not use feedback, as the sound of the no feedback design was considered more than adequate. And a No-Feedback mentality was gaining traction in the hi fi world.
I've had a pair of AVVT 302B tubes (higher rated 300B) for a long time , which needed a new home , and a preamp chassis which had some minor flaws in it , so I decided it was time to build myself a new 300B SE amp for my smaller 3 way horn system.
As I only had sockets for a single driver tube and the 300B tubes , I decided to build a WE91 clone, partly because these horns have an extended response and I was seeking a slightly warmer sound to compliment them.
The amp is nothing unusual , except for the fact that the driver 6SJ7's power supply is separate and tube shunt regulated. This gives a bit more solidity to the sound, and transperancy is enhanced over the WE91A sound.
I also used a pair of Polish made Toroidy output transformers. These are different in that they are toroid wound instead of the usual IE configuration , and have very good bandwidth , which is somewhat negated by the pentode drivers response curve.
A bit of local feedback tightens up the bass, and makes the test equipment happier, with better top end response but I found the warmth and immediacy of the pentode/300B still there.
I put a switch in, so I could use either feedback or no feedback , depending on the requirements of the speaker.
Its a lovely sounding amplifier , with that 300B "sound" that is very pleasing to the ear, and has good imaging and soundstaging capabilities .
I used a 5K:8 ohm output transformer, which is a bit high for maximum power, but gives low distortion , with a soft clean clipping function. It outputs 6 watts clean.
It really compliments this horn system, and when I want to listen to a less than critical sound I'm very happy with it.
6SJ7 -300B above .
EF86-12B4A-300B below
However, it was apparent that the Toroidy Output transformers have very good bandwidth , and I wanted to build a SE amp that would exploit the excellent frequency response of these toroid outputs.
I found another preamp chassis with 2 sets of driver sockets , along with another set for a pair of 300B tubes.
I wanted to use tubes that I have good stocks of (its getting hard to get good tubes now in these Covid times) so I chose to use 12B4A tubes as both input and driver tubes for the 300B .
However , after a week of trying many circuit designs and implementations I settled on a EF86 as the input tube and 12B4A as the driver tube for the 300B.
Detail and dynamics is greatly magnified is this amp, there's nothing missed and the complete recording is unveiled and presented with holographic imaging and sound staging.
Obviously this amp is quite different from the WE91A clone , it has extended response up to 30Khz and bass extends to below 10Hz .
This is exceptional performance for a no feedback SE amp , and it sounds very accurate and revealing of the input source.
It does still retain the 300B "magic" and is still very enjoyable to listen to . You dont feel like you are missing out on any aspect of the recording , and I would think its just about as good as a low power SE amp can get.
It puts out full power for a 300B - 8 watts , so an efficient speaker of at least 90dB , with a crossover that is not difficult to drive is needed to get the full potential of this amplifier.
So two quite different 300B designs , that do sound different, but both still have that wonderful 300B sound .
Both amps are typical Suprateks - over-built power supplies. Regulated driver tubes . Engineered to be maintenance free, long lived and excellent sound.
I think everyone needs to experience the delight of the legendary 300B tube combined with high efficiency speakers . Its an essential part of the hi fi journey.
I built my first 300B SE amp nearly 40 years ago, and have been through so many variations and combinations of amplifiers and speakers since then , but the 300B /HE speaker still deserves a place among the best.
Warning. As good as the 300B SE amp can be, they fail to work well into any speaker that has less then 90dB efficiency.
Over my 40+ years of building audio tube equipment Ive built tube amps using these tubes :
300B (famous sound, and rightly so)
45 (low power, only for 100dB+ speakers)
2A3 ( a bit more power than 45 , but still low, nice sound)
10Y(another low power DHT, makes a great driver )
211 (very sweet , high voltage (1000V!)
845 More power than 211 , heavier bass , again high voltage.)
6B4G (Octal version of 2A3 , difficult to work with (hum) but it can be very good sound)
6C33C-B (Famous Russian MIG fighter jet tube. It also amplifies room temperature , but makes a very good audio tube - in winter!
810, 811, 805 (Class A2 tubes. Obscure used in Shishido type designs)
807 (Ancient British beam tetrode , mostly used in push pull designs.)
6L6GC (Beam Pentode)
EL34 (Pentode, sweet sound)
EL84 Lower power than EL34, but even sweeter sound)
Plus many more obscure tubes , largely unobtainable these days.
I've used these in Single Ended, Push Pull and OTL designs .
Of course the total design, including the driver tubes used, and the power supplies have much more effect than just the output tubes.
I dont really have a favorite tube or design, they can all work very well into the right system.
I was very fond of, and still am, of the 6C33C-B . But it will be hard to get good stocks soon, and they do get very hot in operation.
If you have the speakers its very hard to beat the 300B, or if you need the power then push pull EL34 never fail to impress.
Currently I'm using in my systems:
2 x 300B Single Ended (SE)
2 x 6C33C-B SE
6B4G Push Pull (PP)
2x EL34 PP
6C33C-B PP
A bunch of solid state amps, used for low bass, we dont need to talk about them.
When I find the time I do want to re-explore my 211 SE designs , and the much simpler , but gorgeous push pull EL84.
Also, in the next couple of months an integrated amplifier, with 300B output tubes and an inbuilt LCR phono stage.
