[Triode Pete]

I have a 71A Stereo Single Triode Linestage Preamplifier with separate dual regulated tube power supplies. Extremely sweet sounding with just enough gain. Toughest part is getting non-microphonic 71A tubes. Currently using a pair of RCA Radiotron UX-171A globe triodes. The ST-shape ones were too microphonic.

The one pair of globes I have is more microphonic than my ST bottles, but they all vary quite a bit and all are more microphonic than a 300b for sure.

Is the preamp transformer or RC coupled at the output?

I use the tube in both a headphone amp (single stage xfrmr output) and a power amp (ECC99 xfmr coupled to the 71). The amp will run out of gas much sooner than  300b's on the same speaker, but I never actually get there in normal listening - I would guess my average listening level is in the mid 80's which translates to 50, maybe 100,  milliwatts on my 101dB speakers. This is also a reason why a low power operating point worked so well with the 300b in my set up.

BTW, I agree with your profile comments on power conditioning - it's never improved a system I've had and made things worse more often than not.

[Niteshade]

WOW! Such great enthusiasm for the 300B!

Do we have anybody for the 572B? Lots of triode power available here and it's a reasonable priced tube....

[Rocket_Ronny]

O.K. boys, read up.

Here's the shinny on the 300B.


To get suburb performance from an amplifier the following is required;

- low AC plate resistance on the output tubes. The 300B has a very high
plate resistance that requires an output transformer with a large turns
ratio and thousands of primary turns. This large amount of primary wire and
source impedance match gives poor high frequency response and large amounts
of overshoot in the transformer.

- Fixed bias for the output tube- 300B's are direct filament and have to be
cathode biased. This means that a very large cathode resistor ( typically
600 ohms ) is connected between the cathode and ground and is directly in
series with the output transformer - like putting a resistor in series with
your speakers. This severely reduces the slew rate - speed. No 300B can have
a fast slew rate.

- Most 300B tube amplifiers use tube rectifiers- 5U4 etc. No tube rectifier
can handle more that 185Ma's of current ( less that 1/4 of an amp!) Due to
this low current you cannot put more than 100uF of capacitance on the power
supply. Large amounts of power supply capacitance is absolutely
essential for fast transient response among other criteria.

-So to sum up 300B amplifiers have very poor slew rates, rise time- speed,
have insufficient power supply capacitance, average at best frequency
response, poor bass due to the cathode biasing and also electrolytic caps
are used in the cathode circuit as a AC ground return and are directly in
the signal path. Also most 300B designs do not even have proper drive stages
for the output and further contribute to average at best transient response.

-A good amplifier will have lots of power supply capacitance and low
impedance, well designed input stages with driver stages to proper drive the
outputs, fixed bias for fast transients and good output iron. Sadly the 300B
and designs using this tube are lacking most of the above.


There's some good info regarding the 300B. No wonder I got bored with mine. Still, there has been so much hype on the amp that I would welcome a second go at it. Only this time not with my money on the line. Just to listen to someone else's instead.

Rocket_snoop dog_Ronny

[jrebman]

Ronny,

The Wright Mono 7s I'm listening to right now don't sound anything like what you describe.  I know what you're saying because I've heard it all too often with other 300b amps, but this one seems to be an exception, and a really nice one at that.

-- Jim

[jon_010101]

O.K. boys, read up.

Here's the shinny on the 300B.

Where did you find this?  Basically every point is wrong 

[jon_010101]

- low AC plate resistance on the output tubes.

AC plate resistance is actually pretty reasonable.  The tube works great into a 5k load, or even smaller.  For a push-pull, I'd pick a Williamson amp transformer, at 10k anode-anode.

- Fixed bias for the output tube- 300B's are direct filament and have to be
cathode biased.

You can absolutely use a 300B in fixed bias configuration, no problems.

- Most 300B tube amplifiers use tube rectifiers- 5U4 etc. No tube rectifier
can handle more that 185Ma's of current ( less that 1/4 of an amp!)

You can use a choke input supply and load up on capacitance, not that this is even necessary.  Nothing wrong with paralleling tube rectifiers, or using damper diodes, or even solid state.  If I built a 300B amp, it would have a solid state voltage doubler supply, just due to personal preference.

-So to sum up 300B amplifiers have very poor slew rates, rise time- speed,
have insufficient power supply capacitance, average at best frequency
response, poor bass due to the cathode biasing and also electrolytic caps
are used in the cathode circuit as a AC ground return and are directly in
the signal path. Also most 300B designs do not even have proper drive stages
for the output and further contribute to average at best transient response.

The only valid point here is that most 300B designs do not have proper driver stages, and it's a red herring argument since it is quite possible to design a 300B amp with a great driver stage.  Throw in a high-transconductance pentode, or a beefy triode, and you can drive the snot out of a 300B.  It's just that most designers underestimate the difficulty of driving the large voltage swing necessary.

-A good amplifier will have lots of power supply capacitance and low
impedance, well designed input stages with driver stages to proper drive the
outputs, fixed bias for fast transients and good output iron. Sadly the 300B
and designs using this tube are lacking most of the above.

This is impugning the archetypal 300B amplifiers, not the tube, or the potential amplifiers that could be built by an engineer who knows what he's doing.  And a good amplifier doesn't need a whole lot of capacitance if enough iron is used and the current demand is sufficiently constant (for example, in push-pull class A). 

Don't let dogma scare you away from the 300B.  Just be aware that the vast majority of 300B amplifiers have wimpy driver stages and mediocre circuits. 

[Rocket_Ronny]

Interesting points guys.

This is a educational thread.


Rocket_Ronny

[bricktop]

Joshk:

Now I am going from memory,  but seems that what was said is that the 300B rise time was slow, as in the tube itself, and that it was a poor tube choice to design an amp around. Everyone has their opinions I guess.

I don't think that is right.  But I am going to defer since I can't refute that 100% technically, I'll have to come back when I've verified my facts.  Maybe some engineer can help out.

Right.  Tubes don't have 'risetime' or 'falltime.'  There are no storage effects like BJTs.  Tubes have capacitances, inductances, and gain.  In audio, what is done with them in the circuit will determine their bandwidth performance.

[Niteshade]

Either Single Ended Parallel or Push-Pull is the way to go with a 300B. It's a great tube and I'd like to see them move ALLOT of air! At least 16-40 watts per channel running into 95db efficient speakers would be quite nice.

My thoughts: If it costs hundreds more to make a dream high power 300B amplifier vs a 6L6 or KT88 type, but the added quality is justifies the price, then it's worth it! The jump in price has to justify a jump in quality. I believe such an amplifier, properly made, would be well worth the high price.