C3m/C3g 300b Amp

I have been reading positive reviews on the net in using C3m pentode to drive the 300b. 

I am wondering if there is anyone in the community that has a C3m-based system that I can audition before I embark on a DIY project.

Pls contact me on 012 2236157 or mengtaihoi@gmail.com if you can share your experience in using these pentodes.

Hi Hoi,
  Came across this thread in an overseas forum that touches on what you might be interested in. Check it out... 

 http://www.audio-talk.co.uk/phpBB2/viewtopic.php?t=3958

Hi Vt4c,  thank you for the URL.  I have earlier read the postings and has included in my current design.  I have also trolled several other sites (diyaudio, audioasylum, etc.) for ideas and designs/schematics.  There are all positive comments by DIYers using the C3m tubes to driver the 300b.  They seem to remark the C3m as a true gem.

I have not heard from anyone local that has a c3m/300b system for auditioning, but I have decided to proceed with the DIY project.

I am in the midst of accumulating parts and awaiting their delivery.  I have ordered a pair of NOS Siemens C3o (a 6.3V version of C3m), a pair each of of Telefunken C3m and ITT C3m, and a pair of siemens C3g.  

Most likely, I would use the C3o to replace the 6SJ7 for my existing WE91a, for a listen before I embark on building a new c3m/300b amp.

Reg.

Hi Hoi,
   I like to drive the 300B a bit harder using a relatively low impedance driver....A small power tube.

    This give the 300B a different taste...From purr to a  beast.

I am inclined to agree - after listening to Noodle88's pair of 300b monoblocks driven by a 45 tube. It do sound excellent. Clarity of a 45 and yummy 300b sound.  

I have a similar arrangement in one of my set-ups, but a 45 driving a 845 tube.  I am quite happy with the 45/845 after the recent upgrade on the power supply, PIO bypass (now operating at 1100+V DC).

I also realize that as I age (diminishing hearing), I look forward to a bit more higher frequency extensions in my music.  The DIY WE91 seems to be missing  some HF (miller effect and old ears) that hopefully the C3o/C3m would address. 

My C3m/C3g are being cleared through customs.

Looks like you're getting your soldering iron all hot again!

45 driving the 845....Nice. I bet it sounded very dynamic putting sweet music into the VOTT.

I have converted my We91 - 6SJ7/300b to a C3o driver tube (6.3V version of C3m) in triode mode, over the weekend.

Initial listening (10 hours) has been positive, and in many ways an improvement over the 6SJ7 pentode. The C3o has a more "modern" sound (more hifi-ish and less warm) which has better clarity and soundstaging.  Better bass control. Overall musical PRAT seems to improve. 

For old-ears like mine, the additional higher frequency extension is a definite plus point.

Waiting for additional parts for a pentode mode connection.

Some pics to follow.

Hi Mhoi,
The way you talk about tubes is like a master chef--mix and match for the occasion and music.
Ha ha ----an iron chef !
I must take some lessons about tubes from the master !

cheers

kp93300

Hi Dr, It is just an interest in doing something in idle time.  I guess DIY is a pastime hobby that keep me occupied.



If you decide to build a tube amp, I am very willing to share my experience.

Re.

Oops.. upside down.

I have about 20+ hours listening to the c3o-300b and it has been improving.

I have made changes to the schematics and operating points for the C3o tubes :-
i. Ia to 18ma
ii. regulated the 150V to g2 with obbligato film caps.

Having regulated grid supply further improved the clarity and soundstage. Its like peering through a cleaned window. If you have a pentode driver, seriously consider regulating the g2 supply!

The latest upgrade involves changing the OPT from Sophia Electric 91-03 to a made-in-Malaysia Nano-C Core OPT from Promitheusaudio.com.



After 10hr of burn-in since Sat, I like the overall presentation - better clarity in mids and highs.

Hi Mthoi, 

I am in the progress to rebuilding my 300B amp too. Very intereted in your 300B amp in detail if you mind to share. Like what's the 300B biasing voltage/ current, fixed biased or auto biased, direct coupling? 

My current 300B amp is using E180F pentode. I am using Nic OPT trans too. Plan to try out the James trans from Taiwan. I have the same finding as urs on the pentode G2 and G3 with regulated supply open up the sound a lot.   
Did u use film cap on ur 300B B+ too?

I read that you have 845 SE amp too. How the 845 sound different from 300B?

Hi Phfs98,

I am currently using EML 300b meshplates which has a max dissipation of 25Watts (WE spec for 300b is upto 40 watts).  The Ia current is around 55ma/18Watt dissipation, autobias for the 300b, and obbligato film oil for the PSU.

I recently upgraded the B+ with a mosfet B+ regulator (at 400v DC).   There are further refinements in sound quality - clarity and bass control.  If you have the opportunity, do consider putting in a regulated B+ supply.  I am planning for a regulated 225V for the B++ to feed the c3o pentode.

I have tried various amps - 45 (1.5 watts output), 2A3 (3W), 300b (6W and 8W) and a 45/845 (12W and 18W).  All things considered, power still matters - the higher the output power, the more control.  It is akin to a car with a low capacity engine vs high capacity engine.  Both get you there, but slightly different joy.

I used a 45 tube to drive the 845 amp - it sounds like a souped-up "45 amp".  The overall dynamics are better compared to the 300b.  However, with my choice of music (vocals, jazz), I listen more to the 300b.

Which version of Nick's OPT are you using?  Is it an EI or a C-core?

Hi Mthoi, 

I am using the EI core opt, ported one. For 300B i have old shuguang 300BSC which one of it has started developed arching problem and EH300B. 400V @ 65-80mA fixed biased. Driver is E180F (philips gold pin, Telefunken and Mullard) with shunt regulator on the driver B+ supply. 300B supply is unregulated. What kind of regulator are u using on the 300B? Agreed that the regulator benefit on the SE amp. Especially on the driver stage where the stage gain is huge (amp factor over 50?) so the benefit will be much larger  . 


I am using another small power EL84 amp while waiting to complete the 300B rebuilt. EL84 triode connected just less than 2Watt of power which drive my coral flat8 to quite a impressive sound level. One things good about the EL84 sound is the clearer and more high freq detail than the 300B amp, transient response also faster. However it lack the midrange body of the higher power one. Sometimes I think this sound more balance than the midrange rich 300B sound and sometimes i do miss the 300B sound richness.

I have heard a lot of good comment on 45 but never have a chance to listen it.

Hi Phsf98,

If you have the opportunity, try out a pentode driver for your 300b.  A C3g, C3o uses a 6.3v/0.3A filament supply. The clarity and HF extension of these "modern" pentodes are quite refreshing. I find triode driver tubes a bit laid with the 300b.

I build my own B+ regulators using IRFP450 mosfets. The cost of the parts are less than RM 50 to build 2 set of regulators.  

If you are in PJ/KL, contact me and we can catch up.

The schematics for the 400v B+ regulator.
Pls note that I used 2 x 200v 5W zener diodes (for the 400v reference), the 100R and 6R resistors are 5W, all other resistors are metal film 1%, 1W.

The build regulator looked as such.  I bolted the heatsink to the chassis. It dont generate much heat, you can use the amp chassis as the heatsink.

Thanks for sharing. I have similar regulator for the fixed biased power supply. One things on the zener diode rather than using high voltage one, I use multiple smaller value one in series give lower impedance and noise. 


1N5378B (100V) =  90 ohm
1N5360BG (25V 4ohm/ea) x4 = 16 ohm 

In fact E180F is a high gain pentode just like C3g   . The design is inspired by this group built:  http://audioroundtable.com/forum/index.php?t=msg&th=6046&start=0&