My First Tube Preamp

[tony]

@fantom, pm mo requirements mo baka meron ako......

[fantom_mayonaise]

Sir Tony :

Uubra kaya kung gamitin yung first stage ng preamp nyo (using a single tube), followed by an Aikido Follower by JB? Bale three tubes.

Nakakalito yung common-cathode ni John Broskie. http://www.tubecad.com/2005/April/blog0042.htm Iba sya sa common-cathode nyo.

[tony]

medyo complicated na yan....differential amplifier ang topology.....

yung sa akin ay very basic ang design kaya madali para sa newbies......

remember that as the circuit becomes complicated, nadadagdagan din ang requirements sa power traffo, not to mention cost.....

if you want to do an Aikido, you can just download one of Broskie's guides.....

[Tukkar]

i did the aikido octal version by JB...using 6N9s and 6N8S tubes...sounds good....

[fantom_mayonaise]

Sir Tony :

I rebuilt my tube preamp so I can locate the tube outside the chassis. The four hot resistors were located together between the two tubes and a vent cut above the resistors for cooling. Then I added a relay so the input jacks are connected to the output jacks when power is off. This allows mo to switch between the original signal and "tubed signal".

 

The original signal has wider soundstage than the "tubed signal". It's very noticable. I did not have this in my previous layout although I was not able to make a direct A/B comparison then.

I 'll post some pictures later tonight. I think I located the CF stage cathode resistor very close to the 1.5k and 1m resistors of the first stage. Could that be a problem?

[tony]

as long as there is clearance between them, should not be a problem...

[fantom_mayonaise]

I think I found the problem.



Nakadikit yung 1.5K input resistor sa wire papunta sa 10K plate resistor. Sa parehong channels ito . When I re-routed the wire high over the resistor, bumalik yung lapad ng soundstage.

[fantom_mayonaise]

The current PCB







Assembled Preamp Chassis






Another round of modifications and this should be done.  

1. Remove the relay and replace with toggle switch
2. Separate switch for the heater, still dependent on the main power switch
3. Replace pcb-mount neon bulbs with chassis mount neon bulbs. Handa na yung butas sa front panel
4. Optional Selector switch for cathode bias : Diode Bias, Different LEDs Bias, Battery Bias  
5. Optional Ulit : Light an LED under the tube.

[s2kov]

I used LED biased on my headphone/hybrid GC amp with good results!

[tony]

to use the led with this preamp, you have to increase B+ to about 180volts and plate resitors to about 12k ohms.....

[Tukkar]

looking at the tube curves will greatly help in determining the bias....
Sir tony d ba kung ilang volts ang bias mo sa cathode will also be the input signal capacity of the tube.....if for a red led with about 1.6v drop will allow until about the same magnitude (Vpk-pk) of input signal.....tama ba sir?

[tony]

^that's right....

[efren]

@ fantom-we must always put in mind that tubes are high impedance, so that whenever we build a tubed circuit, we must take into consideration the spacing of signal paths to ground & to other component, I read this in the 1962 edition of RCA receiving tube manual, i have yet to read it in other books about tubes...

Glad you discovered it, makes you a lot more confident in your next project about tubes, there will be more to discover a plus for you.

[Tukkar]

^that's right....

thanks sir for the confirmation....+1 ka ulit at unti unti eh medyo naiintindihan ko na nagbasa ng curves..

[efren]

i also noticed your shielded wire is of low quality, this will have a big impact on your signal especially the highs, i also recommend replacing it with a single conductor, that current set up can cause interaction between left & right channels, affecting your "soundstage", again be reminded tubes are high impedance devices

there are Teflon insulated cables, the conductor does not have to be silver, just make sure there is substantial shielding

[Tukkar]

i also noticed your shielded wire is of low quality, this will have a big impact on your signal especially the highs, i also recommend replacing it with a single conductor, that current set up can cause interaction between left & right channels, affecting your "soundstage", again be reminded tubes are high impedance devices

there are Teflon insulated cables, the conductor does not have to be silver, just make sure there is substantial shielding

pwede rin gamitin mo yung mono sheilded wire kesa dyan...pero para di naman sayang yan isa lang ang gamitin mo at magdagdag ka na lang ulit ng isa pa para sa isang channel....kung me mas maganda ka pa ayon ke sir efren di yun na lang...

[efren]

don't get me wrong, that shielded cable is low quality when used on tube circuits, for SS however that is just right, i also use that type on my SS projects.

I use that type on my tube projects before, that's why i am sharing my experience on such cables when used on high impedance tube circuits, they attenuate high frequencies...

[fantom_mayonaise]

Thanks for the tips Sir Efren.

I'm still not happy with the layout. Kulang pa din yung lapad ng tunog. One more round before I scrap the board. Naiintindihan ko na ngayon kung bakit iba ang layout ng mga DIY project. Halimbawa yung volume controls eh nasa ibabaw halos katabi ng mga tubes and yung mga input/output jacks magkakalayo tulad ng layout ni Sir Tony. Performance first before aesthetics.

That black cable is actually non-shielded. May nabasa ako na mataas daw ang capacitance ng shielded wires. Di daw maganda effect sa sound.

[Disabled]

ganito po ba yung laman niyan sir fantom?

[efren]

That black cable is actually non-shielded. May nabasa ako na mataas daw ang capacitance ng shielded wires. Di daw maganda effect sa sound.

yes high capacitance depending on the type of insulating material used, Teflon on the other hand exhibit low capacitance which has a negligible effect on sound quality if any, high end Tube & SS audio products commercial & DIY'ed use Teflon insulated cables on signal paths, usually the conductor is high grade silver, another alternative is the shielded cable used on high precision instruments such as oscilloscope.