Teach me diode bias preamp?

[Reeltarded]

I have a pile of 4000 series diodes, some unused 5v zeners, and a couple hours to be silly. I would like to know about biasing preamp sections with 4001/7s or the zeners on a common split-cath Marshall instead of 820/250 on V1a cathode.

Merlin has a schematic showing some other diodes in a series pair bypassed with a .1 cap, anodes to ground, so if nobody has anything important to say, I am going to guess at math..

(I have a dart board with numbers and a new package of lucky darts)

I am guessing that I will be ok with a pair of 4007s, and if it leads to an unexpected outcome, still won't be anything catastophic.

EDIT: Datasheet says 4007 passes 1.1v, Merlin's schem has a pair of 4148 @ 1.2v. So, one 4007 anode to ground it is. I can't be too wrong! I have faith.

[diagrammatiks]

average voltage drop is .7-.8

put 2 or 3 diodes in series cathode to ground.

small value cap in parallel. it doesn't need to be .1n.

i usually use 1000pf to 4700pf.

[marcoloco961]

I have a pile of 4000 series diodes, some unused 5v zeners, and a couple hours to be silly. I would like to know about biasing preamp sections with 4001/7s or the zeners on a common split-cath Marshall instead of 820/250 on V1a cathode.

Merlin has a schematic showing some other diodes in a series pair bypassed with a .1 cap, anodes to ground, so if nobody has anything important to say, I am going to guess at math..

(I have a dart board with numbers and a new package of lucky darts)

I am guessing that I will be ok with a pair of 4007s, and if it leads to an unexpected outcome, still won't be anything catastophic.

EDIT: Datasheet says 4007 passes 1.1v, Merlin's schem has a pair of 4148 @ 1.2v. So, one 4007 anode to ground it is. I can't be too wrong! I have faith.

Mess around with it and see what you think. I tried it with some led's. The different colors led's pass different voltages. I didn't notice a big difference in tone from a normal resistor to a diode. I believe the diode was supposed to be quieter but my ears didn't hear a big difference, however, it was a low gain amp I was playing with. It might make a bigger difference on a higher gain amplifier.

[roberto]

It might make a bigger difference on a higher gain amplifier.

I've only used them to cathode biasing. A led with a 10n ceramic cap in parallel to obtain a more clear top end. they work better in clean stages, I don't like them in highly overdriven stages.
They can work in hi gain stages with engl-like configurations (High Ck - Low Co), because impedance to ground is too small to obtain an useful shelving filter with a 100n-1µF Ck.

[Reeltarded]

average voltage drop is .7-.8

put 2 or 3 diodes in series cathode to ground.

small value cap in parallel. it doesn't need to be .1n.

i usually use 1000pf to 4700pf.

Very good, so as it is written, so shall it be done... and then it WILL get loud, bet on it!

Hoping for a bit of noise suppression, but moreso for flatter response in that stage.

[Reeltarded]

They can work in hi gain stages with engl-like configurations (High Ck - Low Co), because impedance to ground is too small to obtain an useful shelving filter with a 100n-1µF Ck.

This amp is cascaded every stage cathodes 820r. I have tried every reasonable cap value there, and just want to experiment some without being tied to the huge cap values.

[roberto]

So you simply like the 100k 820R polarization, but want to raise the gain.
Use a led/diode with the same voltage you obtain from 820R Rk, and use a small ceramic cap in parallel. I suggest you to use them in early stages, but try them also on overdriven stages.

[Reeltarded]

Phone rang so I had to drop it for a minute.

Ok, here's the dumbest question I hope I will ever ask; What is the voltage drop on the 820R.. don't answer, I need to learn this stuff!

Ya know, I haven't toyed with the plates at all on this amp. Hrrrmm..

[diagrammatiks]

that stage is usually biased around -1 to -1.5 on the marshalls if your talking about the first stage.

sometimes a bit more.

one or two diodes will put you there.

3 diodes will put you closer to mid bias territory.

all you have to do is look at the loadline bit a bias point and then pick a diode or led that drops that voltage.

much easier then calculating the proper resistor size.

[roberto]

Well, it depends on what's the current flowing into the tube:

a 100k 820R with a 380V supply has 1.6mA current flowing inside,
so the cathode will be at 820R*0.0016=1.3V

if you use 300V supply for the same stage, you'll have 1.25mA and so
820R*0.0012=1V

if you use 250V supply you'll have 1mA and 0.82V

So look at every stage's cathode voltage and use a diode (or more in series) with a similar voltage.

[Reeltarded]

Excellent. 820R*0.0012=1V.. I see the light.

Another piece of the puzzle I have that I can relate to now. Having the keys keep you from being locked out.



I do thank you fellas, lots!

[Reeltarded]

Flat, hard, sterile. Feels like watching a 7 year old play Guitar Hero.

This might work better for a stage later down the line.. eehhhh maybe not for this amp.

[diagrammatiks]

Flat, hard, sterile. Feels like watching a 7 year old play Guitar Hero.

This might work better for a stage later down the line.. eehhhh maybe not for this amp.

i really wouldn't do it on any stage other then the first.

It also really depends on how big the cathode bypass cap was on the stage that is getting diode biased.

There really should not be that much difference between the shared cathode 820ohm/250uf bypassed first stage and 2 1n400x diodes in series.

[Reeltarded]

It was very similar in the middle, but the lows tucked in like that stage was faking it, and the highs were extended enough to notice in a bad way, mostly feel though. This would be the way to go on a bass amp, huh?

There is no soft landing on the fretboard, by comparison. It broke my fingers!