My first preamp with tubes

[Shoog]

Hi,

I'm very tempted the suggest biasing the bottom triode with a nice led.
Just as little icing on the cake. After that

Cheers,

LEDs are a choice of taste, not everyone likes the change in tone. Worth a try but may not be a keeper.

Shoog

 

[Max Martin]

What would be better, and difference between a filmcap vs led in this position?

What is negative with a led?

I somewhat liked bypassed cathode, sounstage seems a bit more narrow without it but also little blurry.

 

[fdegrove]

Hi,

The LED serves to bias the grid. It replaces the cathode R and bypass C

Connect the anode to the tube's cathode and the led's cathode to ground.

Have a good listen and report back if you like.

Ciao,

 

[Max Martin]

So are you talking about leaving out the resistor and bypass and only use a red led?
It's a bit unclear to me- i thought 330R is gonna be bypass this led instead of a cap.

 

[fdegrove]

Hi,

So are you talking about leaving out the resistor and bypass and only use a red led?

Precisely.

Ciao,

 

[cjkpkg]

Yes try 1 red led as only connection from cathode to ground. No resistor, no capacitor.

It will light up like a Christmas tree.

I use led bias on a 6n1p front end to an el84 pp amp and prefer this setup over resistor and cap.

 

[Max Martin]

ok cool. Nice pic

Can i still measure voltage across this as with a resistor?

And is the other 330R then setting the total bias or how does it work? I mean how can i then decide current draw from tube, if i want "hotter" biased if not?

OK but i will for sure try today (within couple of hours or so)

My DIY classD amp for bass is broken in one channel little bit so hard time to give it a serious listening. (Iraud550)


EDIT: Is'nt possible to use a blue led instead?

 

[Shoog]

ok cool. Nice pic

Can i still measure voltage across this as with a resistor?

And is the other 330R then setting the total bias or how does it work? I mean how can i then decide current draw from tube, if i want "hotter" biased if not?

OK but i will for sure try today (within couple of hours or so)

My DIY classD amp for bass is broken in one channel little bit so hard time to give it a serious listening. (Iraud550)


EDIT: Is'nt possible to use a blue led instead?

Blue LEDs have a voltage drop of about 3.2V, RED has a drop of 1.6V. These voltages represent the bias point the circuit will use which in this case will represent about half the current for a Blue LED.

LED bias works on the principle that a LED will produce a fixed voltage drop but with a very low static resistance, typically a few ohms. Since the triode multiplies the cathode resistance by its mu to create its own dynamic resistance, the difference between an unbypassed cathode resistor and an LED would be substantially different output impedance.

As an example, given a 12 mu and 1K plate resistance. the output resistance would be;

LED bias = 1000R +(2R x 12) = 1024R
Unbypassed cathode resistor = 1000R + (330R x 12) = 4960R
Bypassed cathode resistor = 1000R + (0.1R x12) =1001.2R

As can be seen, the LED is roughly comparable to a bypassed cathode resistor.

An unbypassed cathode resistor represents a form of degenerative local feedback - so should be more linear in operation. The absence of this local feedback will be the primary reason why the two biasing systems sound different in tone - and some feel the more clinical presentation of the LED bias not to be to their liking.


Shoog

 

[popilin]

Can i still measure voltage across this as with a resistor?

And is the other 330R then setting the total bias or how does it work? I mean how can i then decide current draw from tube, if i want "hotter" biased if not?

Measure the voltage drop on the LED and on cathode resistance, and adjust the value of that resistance, for a cheap red LED should be something around 270 Ω.

EDIT: Is'nt possible to use a blue led instead?

Green LEDs and blue LEDs are more noisy than red LEDs.
Try a cheap red LED (red body) instead of high efficiency red LED (transparent body)

An unbypassed cathode resistor represents a form of degenerative local feedback - so should be more linear in operation.

Totally agree.
This is an ideal situation here, because of high μ of the triode, you will reach lower distortion, lower noise and lower gain with unbypassed version.

 

[Max Martin]

ok soldered leds in place, nice red shine now. Didn't have time to listen yet. Warming up

Voltage drop over led is 1.89V (cheap chinese i guess,red coloured case) and section two (triod 2) with resistor is 1,6V. that makes 4,7mA vs 6.7mA.

Should i even this two out so both are about the same or?


Thx for good info above.

 

[Max Martin]

Listening 3 hours now. Sound is very "naked" and open with lots of details. Soundstage is tight n big.

Gain is higher, atleast feels that way.

I feel biggest difference is to be found in the treble. Almost electrostatic sounding.. Led seems to favor good recordings, on bad/worse treble can be little harsh.

 

[Max Martin]

I will listen couple days more n make up my mind after that if it's a "keeper".

 

[Magz]

If the color/brightness of the LEDs bothers you, you can also use other types of diodes for bias. I use Cree SiC Schottky diodes to bias the input tube in my Midlife Crisis amp and in another preamp I've made; each diode has a forward voltage (Vf) of 0.8 - 0.9V, so two in series should give ~1.7V. I find the sound to be very clean and clear.

I plotted the Vf vs current a while ago, attached is the plot. Dynamic resistance is very low, lower than most LEDs, actually.

 

[Max Martin]

OK looks/sounds interesting. Where can i get this diodes? Ebay search gave nothing.

 

[Magz]

These are the ones I've used, widely available (in the US, at least).

CSD01060A Cree, Inc. | Mouser

 

[Max Martin]

These are the ones I've used, widely available (in the US, at least).

CSD01060A Cree, Inc. | Mouser

This is the right one yes?
http://www.ebay.com/itm/171175669975?ssPageName=STRK:MEWAX:IT&_trksid=p3984.m1423.l2649

Connect them In series right? I don't see any marking. Don't know where minus is? Bad pic?

 

[Max Martin]

Today something very interesting happened.....

After playing music for about 3-4 hours and enjoying it it suddenly came a small "poowwff" and i saw my red leds die out slowly... hmmmm i thought.

I thought maybe my beloved rectifier died and i thought, *nice that my transformer did'nt blow up then* Checked fuse on 230V line secondary to rectifier. Looked ok.

Anyway, replaced that russian rectifier with the old chinese 6z4 and turned it on - dead... oops forgot to check fuse in the back. AND that one was broken (0.5A)

Replaced it and fired it up again.... And out came a stinky fart smell similar to that if you ever burnt a voice coil. I instantly turned it off and located this bad smell to my BELOVED CHINESE CRAP TRANSFORMER.

So i guess some of you were right, it was a "bad" one.

So i'm looking for a replacement. A hammond seems to be easy to get from Helsinki for me. But which one, same as my finnish friend, Hammond 370BX?

Hammond Mfg. - Universal Primary - "Classic" Power Transformers

 

[Shoog]

Transformers generally go down for a reason (even poor ones)- check every component before installing a replacement.

Shoog

 

[Max Martin]

Before this incident i measured temp with new "gun"
Room temp 24.

rectifier 60 degres
6n3p: 75 degress (165F)
Xform: 44 degress (music)

 

[Max Martin]

I took away xformer and measured little...

Both 6.3V shows 0.05ohm (as before)

230-0-230 =
First 230 to centertap = 107ohm
Second 230 to centertap shows 157(?)
and from 230 to 230 i get 57 ohms.... that was before i think 477ohms.

And primary i'm not sure if i measured before but now i get 55 ohms.

I can't figure out what has caused this... been checking around but everything seems to be fine.