[nikongod]

So I have LED grid biased the bottom triode in a WCF, and its pretty sweet but requires a dedicated bias supply. I have a different amp with LED cathode bias for A WCF, but I want to convert it.

has anyone tried with a AA cell? what did you think?

 

[digger945]

I'de like to know what the change in conductance of the ? tube would be with a 1.5V AA on the grid of the lower triode, and what it would be as you trim the voltage down, or the battery discharges.
Is this a HV project?

 

[nikongod]

yes, this is HV. Im working on a xiang-sheng 708B which is essentially a morgan jones/Earmax clone BUT with a 2c51 gain stage. I dont think that battery drifting is too much of an issue when battery grid biasing, as there is (hopefully) no load on the cell. A few sources I have found indicate the lifespan of a grid-biasing battery as its shelf life.

Anyways, I am pretty sure that grid biasing the output stage wont work in this amp without some serious tweaking of the overall operating points, or stumbling upon some 2 to 2.2V lithium AA size batteries. The negative supply is an option but I have a budget on this, and everything must fit in the case (which as room, but not worth it..)

The gain stage also has LED cathode bias, and runs like a champ, with a 1.7V led. A perfect fit for a lithium AA.

 

[pinkfloyd4ever]

doug did it with 9V's ecp.cc

I'm sure he'll be around sometime to give you some real advice on it since I don't know jack about it

 

[nikongod]

Yeppo, I like doug's work even though he (with valid reasons) dosnt seem to like OTL circuits. I have wanted to build a variant of that amp for a while now.

I wonder what would happen if a "smaller" resistor were used between cathode and ground to generate say 0.3 or 0.4V, and a 1.8V cell were used to grid bias simultaneously... Vgk is then -2.1v or so which is what I need, and I have gotten closer to the ideal of Rk=0ohms.

 

[dsavitsk]

I'm not sure I'm following what you are doing. Can you draw a schematic? Or, just say how it's different from this?

If you are replacing Rk with a battery, you need something rechargeable -- an alkaline will not be happy. I do prefer a battery or LED to a cap in a grounded cathode, but just a plain old resistor seems reasonable here. Admittedly, I don't know anything about WCFs, but Rk doesn't enter into either Z out or gain formulas.

 

[nikongod]

The output stage is DC coupled to the gain stage. The voltage on the plate of the bottom tube is 97V
My idea was to have:
Rk is very low: about 50-60 ohms to generate 0.5V with 10mA target idle current.
Rather than grounding the 1M grid leak resistor directly it goes to a 1.5V lithium battery (measures 1.8V, positive to ground) for bias.

Considering the rest of the amp, im not sure its worth it. Probably worth a try on a different amp: perhaps one with 5687/ecc99 output tubes though.

 

[nikongod]

So a small update.

I converted the amp from this setup:
gain tube: CCS plate load, LED cathode, DC coupled to input.
Output: 6dj8 in WCF, with LED cathode bias

to this:
Gain: CCS plate load, Grid biased with battery, added input cap.
output 6dj8 in WCF with cap bypassed resistor cathode bias.

Nice stuff

 

[digger945]

You like cap bypassing over led bypassing?
You wanna share with me exactly how you go about led bypassing?

 

[nikongod]

I think I tried to get parts which didn't fit together right to fit together here.

The first LED I used for this was a 2V red one, which is the right voltage but I think it has a higher dynamic impedance than the resistor&cap. Something was just way off compared to the resistor&cap.

Then I tried a 1.7V red, which has a very low dynamic impedance, but forced the circuit to put about 12mA through a 6DJ8. apparently they didn't like that much.

When things work together properly its a whole different game.

Anyways, if you have a circuit that this works with, it is basically a matter of replacing the cap&resistor with the LED. Its helpful to design around Vgk that makes swapping parts easy as I have learned today.

section of valve amplifiers by morgan jones (I hope)

 

[digger945]

Thanks. I wanted to try the led with the Aikido, it's supposed to have a cap bypassing the cathode resistor and I would like to try it both ways and see what the difference is.
It isn't the low volt 24V amp I'm listening to now, but the HV 6FQ7/6H30 one that I'll start on/finish after the meet later this month.