Debunking An Audio Myth

[Curious_George]

If you have ever read about a 300B or 300B amp design, you've read that the 300B is "so hard to drive" compared to a 2A3. As PWK would say... Bullshit!

 

Parameters	300B	2A3
Filament Voltage	5V	2.5V
Filament Current	1.2A	2.5A
Plate Voltage	400	250
Electrode Capacitance
Grid to Plate (Cgp) pF	15	16.5
Grid to Cathode (Cgk/Cgf) pF	8.5	7.5
Plate to Cathode (Cpk/Cpf) pF	4.1	5.5
Total Cin (pF)	82	93.3
Mu	3.9	4.2
Plate Resistance	750-800	800

 

Above are the specs for the 300 B & 2A3. They are very similar. So similar that the driver circuit for a 2A3 could easily be used for a 300B and the driver would not know the difference. I don't see anything special about the 300B parameters that would make it hard to drive. Now a 211 or 845, those I would agree need a good driver circuit. It's all above in black & white. I guess people read it or hear it and they think it is true. Unfortunately, there is so much of that non-sense in the audio world. 

 

If you Google search 300B schematic, almost every circuit that comes back is a 3 stage design. Most 2A3 circuits are 2 stage, with some exceptions that are 3 stage. If you are going to drive your 300B or 2A3 amp directly from a source (something I never do), then you may need the extra gain of the 3rd stage depending on your speakers. When I design amps, I always try and have the amp input driven to 1 to 2VRMS for maximum output. Not that there is an amp input voltage standard, but most amps have an input sensitivity in this range for full power output unless there are specialized amps for particular purposes. 

 

In my experience the 300B and 2A3 are somewhat forgiving of input tube used and sound good with even a basic input tube circuit such as a 6SL7. In addition, the minimum transformer impedance I would use for either tube is 5k. Anything more than 6 or 7k requires more drive from the input tube. Even though the recommended transformer impedance for the 2A3 is 2.5k, that is for maximum power output. You can still get max output using 5k, you just have to drive it a bit harder. 

 

[tube fanatic]

Those who can live with less power can achieve 2A3 performance with a triode strapped 6Y6 which can usually be found NOS for around $4.00. The curves are very similar.  And, of course, a pair could be used in parallel for a little more “oomph.”  
 

Maynard

[Curious_George]

Very true Maynard. There are a lot of tubes that can be strapped for triode operation and be used for a good SET amp. 

 

I missed the whole tube craze and only started building tube stuff in the mid to late 90's. After reading about DHT's, I just had to try one out to see what all the hype was about, so I built a pair of mono 2A3's. I have to admit, their performance astounded me, especially with the right speakers. Since prices have gone up considerably, I don't think I will be buying more DHT's anytime soon. I'll just strap my 1625's and use them as triodes. 

[seti]

The 1625 in triode mode is superb.

 

I have a two stage DC 2A3/45 amp I have wanted to up the schematic to use a 300b. The driver is a 7F8 loctal. This should be fun and there are plenty of 3/00B clones out there and no not the aleged WE. If I were to go modern production Emission Labs are second to none.

 

 

 

 

 

[EMRR]

On 8/7/2021 at 4:11 PM, tube fanatic said:

Those who can live with less power can achieve 2A3 performance with a triode strapped 6Y6 which can usually be found NOS for around $4.00. The curves are very similar.  And, of course, a pair could be used in parallel for a little more “oomph.”  
 

Maynard

 

Yep there's a GE regulated power supply that started as 6B4G pass tubes, switched to 6Y6 with no other changes.