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300B alternatives

crazyfrog

Member
2009-05-31 2:35 am
Hi, since my last moving the 211 project is still sleeping in boxes (started building 211 stage prototype but noting more)...At the time I had in mind to use 300b as a driver or even 2A3 (but not enought headroom apparently). I bought interstage transformers meant for those tubes (1:1, 40-80 mA, 600-900R). I'm looking for alternatives. Can I use those IT for other tubes not corresponding exactly to specs. Suggestions would be very appreciated.
 
This may not be what you are looking for, but you could use pretty much any beam power tube or power pentode configured like the output tubes in this amp that I built: Tube Amps with a Twist: A Push-Pull Amplifier with Simple Plate-Grid Voltage Feedback Driven by a P-Channel FET

I'd probably use an EL34 in to do the job. You would get higher linearity and more gain than a 300B (and more swing because you could drive it to saturation without grid current).

The drawback is that you need a negative rail and a few extra parts, but the money savings would more than make up for it in my opinion.
 

crazyfrog

Member
2009-05-31 2:35 am

crazyfrog

Member
2009-05-31 2:35 am
The 300B at a Va-k of 380V running at 60mA into say a 5K:5K IT will be able to swing 280V one way or 200 VRMS. Shouldn't that be enough to drive the 211 well into positive grid territory? Just curious, what are your swing requirements from the driver?
Not sure I can push in A2. This was not my goal (my power transfo is a Hammond 724 for 211 only). The driver should have enough headroom to bring
the 211 "on the edge of A2" to get 17-20W.
 

crazyfrog

Member
2009-05-31 2:35 am
Would be good for driving the 845 as well. The 845 is significantly more linear than the 211.

An EL34 should match that voltage drive capability easily when triode strapped into these transformers.

As much as I love the 300B a good quality EL34 is a better choice than an inexpensive mediocre 300B.

What is an "inexpensive mediocre 300B?" Something like EH golden grid I have? :D (BTW they're brand new in the box. I bought them when there was a sale)
 
Not sure I can push in A2. This was not my goal (my power transfo is a Hammond 724 for 211 only). The driver should have enough headroom to bring
the 211 "on the edge of A2" to get 17-20W.

Okay, fair enough. I am still a bit perplexed though with your original comment on "not enough headroom" with the 300B/2A3 as the driver. In the case of the 300B, it will provide about 4 times your maximum required swing. You haven't stated your chosen operating point for the 211, but for A1 operation you'll most likely need less than 70V one way from the driver. Heck, you could even skip the driver altogether and built a two stage amp with just a mid / high mu input tube. There's a lot to be said about the simplicity and performance of a two-stage amp IMO - just sayin.
 
Yes an el34 strapped in triode mode could be have a Rp a little under 1K. Thanks for the link.

Actually, that isn't strapped as a triode. It is running as a pentode with plate-to-grid feedback which is connected to the source of a p-channel fet. It is simple but a bit exotic and may not be what you are looking for. It is definitely not a tube purist approach, but it will give much less distortion than a 300B, especially if you choose a tube with a good bit of transconductance like an EL34 or KT88 if you plan on running at higher currents.

The feedback gives the overall circuit the characteristics of a very linear triode. You set mu by the resistor ratio. I used this circuit as the output stage for an amp I built and got excellent performance.
 

crazyfrog

Member
2009-05-31 2:35 am
Okay, fair enough. I am still a bit perplexed though with your original comment on "not enough headroom" with the 300B/2A3 as the driver. In the case of the 300B, it will provide about 4 times your maximum required swing. You haven't stated your chosen operating point for the 211, but for A1 operation you'll most likely need less than 70V one way from the driver. Heck, you could even skip the driver altogether and built a two stage amp with just a mid / high mu input tube. There's a lot to be said about the simplicity and performance of a two-stage amp IMO - just sayin.

My comment concerning limited headroom was about 2A3, not 300B (sorry for the confusion). The exact operating point is not clear. The answer is " around 1200V, 60mA" but the problem is...Capacitors! (I'll have to make up my mind on this). The transformer is able to give a lot of current so I could push the 211, no problem.
 
This may not be what you are looking for, but you could use pretty much any beam power tube or power pentode configured like the output tubes in this amp that I built: Tube Amps with a Twist: A Push-Pull Amplifier with Simple Plate-Grid Voltage Feedback Driven by a P-Channel FET

I'd probably use an EL34 in to do the job. You would get higher linearity and more gain than a 300B (and more swing because you could drive it to saturation without grid current).

The drawback is that you need a negative rail and a few extra parts, but the money savings would more than make up for it in my opinion.

Sure, feedback to the first grid in pentode mode is the way to go, instead of loosing precious properties of the tube turning it into a dumb triode.

Here is another example attached. Choice of N-type FETs is much wider. Here the pentode is connected as a "Right handed triode", but it will work similarly in pentode mode, just higher negative voltage & swing would be needed. Here feedback goes before the source follower (not after like on the site you referring to, that I also like).
 

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Sure, feedback to the first grid in pentode mode is the way to go, instead of loosing precious properties of the tube turning it into a dumb triode.

Here is another example attached. Choice of N-type FETs is much wider. Here the pentode is connected as a "Right handed triode", but it will work similarly in pentode mode, just higher negative voltage & swing would be needed. Here feedback goes before the source follower (not after like on the site you referring to, that I also like).

Very cool. I have thought about using the high-impedance, high-mu triode connection before to make a cathode-follower SE amp. Many people here told me it would be a terrible idea and would not work well but I suspect it would be just fine as long as I drove it properly.

Of course, then I just started thinking that a Mosfet would be much better in several respects...
 
What is an "inexpensive mediocre 300B?" Something like EH golden grid I have? :D (BTW they're brand new in the box. I bought them when there was a sale)

Yes, that would fit my definition.. :rofl:

The best bang for the buck I've found has still got to be the JJ 300B, not boutique priced and pretty consistent and good sounding. Quality control in modern production tubes leaves a lot to be desired and the JJ DHTs have proven fairly good in this regard. (Their octals are another story, why?)
 
Sure, feedback to the first grid in pentode mode is the way to go, instead of loosing precious properties of the tube turning it into a dumb triode.

Here is another example attached. Choice of N-type FETs is much wider. Here the pentode is connected as a "Right handed triode", but it will work similarly in pentode mode, just higher negative voltage & swing would be needed. Here feedback goes before the source follower (not after like on the site you referring to, that I also like).

zero bias triode+schade fet drive
thinking about811a... "spud"
 
JJ DHTs have proven fairly good in this regard. (Their octals are another story, why?)

Their EL34's and 5AR4's have been hit or miss. This seems to be batch related. Sometimes they all suck, sometimes they are all good.

Their EL84's have been good to me, although it's been a few years since I bought any.....why? I got a dozen several years ago, and they are all still very much alive, despite extracting about 30 watts per pair on 425 volts. The JJ's and old stock RCA and Sylvania 6BQ5's are the only ones that will eat this abuse and keep on working.

Note.....425 Plate, 320 screen, 6600 ohm load in an SPP board.