300B based preamp....looking for schematic

jmillerdoc · 16th September 2010, 07:12 AM

Hello,
I was recently reading about a 300B based preamp and it got me interested. I was wondering if anybody had some schematics for something like this or a link to some. I have done a cursory search with Google but come up empty handed.
Thanks,
Jeff

CLS · 16th September 2010, 07:21 AM

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?

jmillerdoc · 16th September 2010, 07:23 AM

For now the cheaper the better, I would opt for a RC load. Any thoughts?

GTHICM · 16th September 2010, 07:24 AM

30/300B TRANSFORMER COUPLED PREAMP

CLS · 16th September 2010, 07:34 AM

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.

CLS · 16th September 2010, 07:35 AM

to GTHICM, no picture is visiable in the links

CLS · 08:17 AM

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

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.

slowmotion · 16th September 2010, 09:52 AM

There has been threads about this here before.
I'd do a search if I were you.

GTHICM · 10:35 AM

Quote:

Originally Posted by CLS

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

CLS · 17th September 2010, 01:07 AM

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:
You may laugh..but don't waste your time trying to dissuade me. 300B preamp.

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

A friend of mine uses a pre of 6V6 p-p with OPT. He is happy with it.

Fun of course

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

16th September 2010, 07:12 AM	#1
jmillerdoc diyAudio Member Join Date: Mar 2009 Location: Lawton, OK, USA	300B based preamp....looking for schematic Hello, I was recently reading about a 300B based preamp and it got me interested. I was wondering if anybody had some schematics for something like this or a link to some. I have done a cursory search with Google but come up empty handed. Thanks, Jeff

16th September 2010, 07:24 AM	#4
GTHICM diyAudio Member Join Date: Sep 2008	Link to 300b Preamp Schematics 30/300B TRANSFORMER COUPLED PREAMP

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16th September 2010, 07:21 AM	#2
CLS diyAudio Member Join Date: Jun 2005 Location: Taiwan	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?

16th September 2010, 07:23 AM	#3
jmillerdoc diyAudio Member Join Date: Mar 2009 Location: Lawton, OK, USA	For now the cheaper the better, I would opt for a RC load. Any thoughts?

16th September 2010, 07:34 AM	#5
CLS diyAudio Member Join Date: Jun 2005 Location: Taiwan	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.

16th September 2010, 07:35 AM	#6
CLS diyAudio Member Join Date: Jun 2005 Location: Taiwan	to GTHICM, no picture is visiable in the links

16th September 2010, 09:52 AM	#8
slowmotion diyAudio Member Join Date: Nov 2002 Location: Norge	There has been threads about this here before. I'd do a search if I were you.

17th September 2010, 01:07 AM	#10
CLS diyAudio Member Join Date: Jun 2005 Location: Taiwan	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: You may laugh..but don't waste your time trying to dissuade me. 300B preamp. 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 A friend of mine uses a pre of 6V6 p-p with OPT. He is happy with it. Fun of course I'd like to see where people stop. Maybe never. (There have been already 300B pp line stage concept, or build.... just WOW! )

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