• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

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

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Bas Horneman

Bas Horneman

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Joined 2002
basaudio.wordpress.com/
#1
I want to build a preamp with a 300B. Why?

Simple..I want to build a preamp with a DHT tube and don't want to buy another tube for a while. (Cash and too many unused tubes in cupboard already)...at first I was thinking...gonna build a nice choke loaded TJ 205d preamp or 101d...But that's a lot of bucks to fork out...then I thought...but I already have some DHT's "in stock" for years that I have not used (that is where the 300B's come into the picture). Plus I have a Hybrid Gary Pimm CCS built for 60mA (Just need some EL34's..but they are cheap)

I may or may not use a 7n7 SRPP stage or so before...to get some gain first.

Any suggestions...should I parafeed because I have a CCS already?

Manley Labs have done a 300B preamp and so have a company in Singapore (any idea about the topologies of these companies?)
Pete Millet also has a low mu preamp where he uses say a 6AS7 with EL34 current source...I might do something like this as well...substituting the 300B in place of the tubes Pete has suggested. (Anybody built this baby?) http://www.pmillett.com/lowmu_preamp.htm



I have this idea that there are more folks that have built a 300B preamp but are afraid of the Environmental lobby groups...and won't admit to it?

Experiences and suggestions are very welcome.
 
#2
Hi Bas, all

I used a 300b linestage for a while,
it sounded very "organic" , dynamic and musical,
but I wouldn't really call it "accurate", whatever that means 😉
I used the 300b chokeloaded, with a cap on the output.
Regulated DC filaments.

cheers 😉
 
#4
It's been a while,
but if I remember correctly
( fat chance 😉 )
probably 300V and 60-80 mA
or something like that.
It's nice to put a bit of current through it.
Regulated DC on the filament with catode return taken from the negative side of the filament only.
Choke was a Tamura output transformer with the secondarys left floating, output cap was a Mundorf I think.

hope this helps

cheers 😉
 
#8
Bas,

Use current regulated DC on the 300Bs' filaments. There are NUMEROUS remarks around about "washed out" sonics when voltage regulated DC is used with DHTs.

FWIW, I would float the filament supply and rig a 4PDT switch to periodically reverse the DC polarity applied to the filaments. The 300B was designed to be heated with AC. The potential gradient associated with DC heating can shorten service life. Flip/flopping the DC polarity about once a month (more often, if you like) will compensate for the problem.

Consider using unbypassed cathode resistors. The 300Bs will be linearized without significantly altering their overall character.
 
#9
Hi Eli,

Will do...

I've got a voltage reg followed by a current reg...so I might take out the voltage reg part.

The thing I don't really understand about a current reg is this :
With what voltage do you need to supply it? AFAI understand it ...it does not really matter..since the filament will only be supplied with 1,2A? So I can supply 8V or 12V does not matter?

I usually leave out the cathode resistors. But this time round I want to also add a switch so that I can hear myself the advantages/disadvantages.

Thanks and regards,
Bas
 
#10
Well, I had suggested a phono preamp using a 6528 some time back, so a 300B linestage is only the second most ridiculous idea I've heard of.

You might think about incorporating that into this preamp. If you're going to make your electricity provider rich, may as well go all the way.
 
#12
Bas Horneman said:
[snip]The thing I don't really understand about a current reg is this :
With what voltage do you need to supply it? AFAI understand it ...it does not really matter..since the filament will only be supplied with 1,2A? So I can supply 8V or 12V does not matter?
[snip]Bas
Click to expand...


Hi Bas,

I don't know your particular current reg, but there will be a minimum voltage it needs to function - depending on implementation. And then there's the voltage across the load (heater). So, if the nominal heater voltage is 5V, and the current reg needs 6V to function, you need to supply it with at the minimum 11V. Obviously you should stay above that, but anything higher will lead to more dissipation in the current reg, that's just V x I, so you'd want to minimize that. (The V being the voltage between the two terminals of the current reg of course).

Jan Didden
 
#13
Obviously you should stay above that, but anything higher will lead to more dissipation in the current reg
Click to expand...
Thanks Jan,

That makes it crystal clear.. Thanks!

By the way it is a simple current reg on Doede's boards.
http://www.dddac.de/at05.htm
lm317pcb1.jpg
 
#14
Hi Bas,
Same thought came to me several months ago for a 300B line pre in order to drive my 845 monoblocks.The idea was to built a whole DHT system,from the beginning all way to the end.I wonder,what would be the sound of a projeckt like this after building preamplifiers with ECL-86,5687 and currently 6BX7.
300B is one of the most linear tubes has been ever made,it's a DHT,and as they say, the sound or the colouration of a stage is infected by the earlier stage.
An interesting approach is here,
http://www10.big.or.jp/~dh/work/93072.html ,which I thought to modify.
Anyway,I'm looking forward to seen the outcome of your search
 
#19
In my case the best result i had with C3m tubes (I used transformer input 845 power amps).I use local feedback in gane stage.



This is best preamp for my Power amps.

Iam very sorry for my Inglish🙄
 

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