Upgrading my 300B Fi Primer

[Pedro]

Hi,
I've built a few tube amps over the last 5 or 6 years with help from very generous people ( some of whom I see are here on this forum regularly )
My main amp is currently a 300B SET based on the Fi Primer with a slightly improved power supply, but basically a 6SN7/300B set up. It drives DIY folded TQWT with Fostex FE167E's in them. Occasionally I use an OTL pre amp to drive the main amp ( just for a slightly different character )

Now then , to my question.....

I recently asked on the SET Forum what good upgrades to my DIY 300B Fi Primer would be, and had lots of great feedback - partic on using the type 76 tube. I cant get availability of 76 triodes to try yet and have been reading round on other circuits. The C3m as a 300B driver gets lots of good coverage. So although I know i will have to set up 20V heater supply etc I wondered how easy a conversion this would be ? I want the simplest ( purest ) C3m / 300B circuit I can find. any ideas ?
I can get hold of some NOS siemens C3m's so looks like a quick project.
The 6sn7/300b can be driven Ok direct from CD source but I also have an OTL headphone/line stage designed by Kurt Strain that I built some years ago which drives the existing 300B amp really nicely - so I'm guessing I could use that to drive the C3m/300B set up given the lowish gain of the C3m's.
Does all that make sense ?

Although I can get my mind around the datasheets, and basic maths, I like to build direct from a well-liked schem and then check and tweak operating points once the amp is up and running.

Any views very much welcomed. In meantime still looking for the 76's as well here in the UK.

Stop Press - being an impatient soul I just went and ordered some C3M and C3g on ebay too !!

Rgds
Pete

[pre65]

Hi Pete-in "The best of Audion" there is a circuit and build article for a 300b amp using the PT49 (C3m) driver tube run as a pentode.

this article runs to 6 sides of A4 and if you want to PM me with your e-mail address i can scan it for you.

Philip

[Ianm2]

here ya go, do some reading, great site...

http://members.myactv.net/~je2a3/se300b.htm

[powertriode]

300b amp with C3M.

Currently got this breadboarded. Although it's being driven by an El84 at the mo.
IMHO it sounds better than the Reichert/Fi.

http://www.fortunecity.com/rivendell...cy/Legacy.html

[pre65]

Hi-i have scanned the 300b + c3m amp details for Pedro (Peter) as a PDF file so if anyone else want a copy just let me have your e-mail.

Philip

[Pedro]

Thanks to Phil the article arrived (by email) today !... and so did my matched Siemens C3M's and C3g's yesterday !! Excellent !!

Will let you know how I get on modding my Fi Primer 300B.

Powertriode - how is your sonding on the bench ?
Did you modify a 6SN7/300B set up ?

cheers
Pedro

[powertriode]

Hi Pedro,

Basically, mine has been lashed onto an old bookshelf for two years or more!
The bookshelf is on the bench in the workshop; you understand; not still fixed to the wall!
It's one of a number of 'multiplexed' projects I fiddle with, when I get the chance.

The C3M and the EL84 do it for me. I think the C3M has a *slight* edge.

Don't forget, the C3M and the C3G are not the same animal.

You could use a 6.3 mains TX and a voltage doubler and drive the C3M with DC.

[Pedro]

Thats interesting - thanks

I have plenty of EL84's from building guitar amps so I'd like to try both C3M or el84 and see whats best

what schematic are you using for the C3M/EL84 pentode driver ?
Ive never used pentodes in an input stage before so could do with some guidance on how to set up the operating point etc.

cheers
Pete

[Pedro]

Hi all,

I'm going to try and replace the 6SN7 dual triode with the C3M pentode input stage - with the C3m set up as per the Audion dual 300B SET input stage.

Could I just leave the 300B stage and associated compinenets as it is on the Fi Primer or do i need to make any changes ? Maybe I should just experiment ?

I'm very interersted in seeing any schematics for pentode input stages for a 300B SET. especially anyone who has a C3m which has replaced a standard 6SN7 set up.

Any thoughts welcomed
cheers
Pete

[Paul Barker]

That circuit Ian linked you to would work. Nice find Ian.