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300B based preamp....looking for schematic

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For line stage (and such big tubes), the freedom in choosing operation point is huge. And the load can be anything. So I think there won't be a 'standard' circuit around. You might have to try it yourself.

Any preferences for load? Simple RC, or choke, transformer, or even CCS?
I've read somewhere that 300B (or such power tube) would need some more voltage drive to make it come alive. So you might need at least a pair of step up input transformer or an input stage. (I'd prefer transformer)

And the filament supplies! A pair of absolutely quiet supplies are needed.

All these, sigh~ means no cheap build. Unless you got all major parts on hand already.
OK, this is a real quick 'mockup' on paper (screen):

5K ohm load, 180Va, -35Vg, 40mA

So you'll need 415V on top:
40mA * 5k = 200V across the plate load
+ 180V across the 300B
+ 35V on cathode

Under 450V you'd be easier and cheaper.

In case you have 1:4 step up input transformer and with source can drive it.

Then 2Vrms = 5.6Vp-p

5.6 *4 = 22.6Vp-p into 300B

then you got 73Vp-p output (pretty much the mu of the tube)

Enough to drive a, say, 6L6 spud :D

Lower voltage, you might be too close to induce the grid current due to not negative enough on the grid. Lower current, you're facing worse and worse linearity.


Endless tradeoffs.


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300b Link DIYHIFI

to GTHICM, no picture is visiable in the links :(
It will not load at the moment. To be honest, I would consider building a preamp based on other DHTs, such as the 26, 01A or 10Y. There is plenty of information and experience here on the forum with these preamplifiers. I have a 26 preamp and like the sound very much. Plus, the tubes are much less expensive!

But if you are hell bent on 300b, have a look at the Tram 2 schematics here: http://www.diyhifisupply.com/files/Tram2R1.32.pdf
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Searching for "300B pre" or "300B line stage" among the old threads is not as easy as intuitive expectation. I think it's because "300B", "pre amp" and "line stage" are too commonly seen as to too many of them show up in the search:( I myself is very intereted in this topic, so I did some homeworks a while ago and found this.

This is the only one I found, which is buried under several pages back in the search results:

Somewhere in my vauge memory, it seems there's another one or two posts in the 12B4 related thread...

After the 12B4, there's a 6V6 line stage thread. I saw a bigger and bigger trend here:D A friend of mine uses a pre of 6V6 p-p with OPT. He is happy with it.

Fun of course:D I'd like to see where people stop. Maybe never. (There have been already 300B pp line stage concept, or build.... just WOW! )

Sometimes, DIY is more about fun, instead of making sense;)

Barely any chance for myself doing it, but I'd love to see people do things like this - hot rodded mower, buggy with jet engine, wheel chair with rockets... etc :D It's a combination of wealth (in time and money), passions, motivations, imaginations, and creativities. A lot of fun to watch :D
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Joined 2005
The idea of using a 300B for a pre-amp make about as much sense to me as repowering a lawn mower with a 5.7L Chrysler Hemi. Can it be done sure. Will someone do it, most likely. Make any sense, not much.


seems like many ethusiasts around the world build lineamps with a single DHT power tube
its not hard to find info
but 300B doesnt seem to the first choise
I wouldn't use the word "many". 12A*7, 6922, ECC**... or the likes, are many. 300B as pre is not.

I think the key point is not the circuit diagram but the 'experiences' in the whole implementation. Drawing some load lines and find a few operating points is easy, but finishing the whole thing and getting the sound right are not.
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Joined 2005


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I would suggest taking a look at this --> ecp.cc

Its well written and most of the design thoughts and choices seem good. The only thing I would change is I'd go with filament bias instead of fixed bias and also ditch the input transformer.

An externally hosted image should be here but it was not working when we last tested it.

The 801 in the below schmatic is using a 33 ohm resistor as filament bais. Filament bias uses DC filament current to get a small value cathode resistor so you don't have a large bypass cap. The below example also makes use of the Ultrapath concept.

An externally hosted image should be here but it was not working when we last tested it.

Rgs, JLH
Ah, this bias method is new to me.

With higher filament current rating - like 2.5A of 2A3, the resistor can be even smaller. With 18 Ohm, there'd be 45V on the cathode. However this is very inefficient. In this case 2.5A * 47.5V = 118.75W for filament and bias (alone)!! -- For a triode which can produce 3.5W max. Wow!

How is the sonic benefit of this "small cathode resistor/no bypass cap" method justifying the overall power consumption?
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