drain R max, DC heater Q

[kvb]

What is the maximum drain value for a 12ax7 and el84 without creating an overload.
It's the Epi VJ: a clean shot from the cap to the next stage with one resistor to ground.


Also, I have a MKI with a strange heater situation. When I looked up heater supplies they all said 6.3 AC or DC with one wire on either side of the heater (4+5 to 9 and 4 to 5). My VJ was converted to DC using a bridge rectifier with the cap in-between the + and -. So, the heater voltage is + 3.2 and -3.2. Is this unusual?

[markh]

Drain value? Sounds like a FET. Could you please restate your question?

Any time you CT the heaters, you end up with the voltage split. Even AC heaters show 3.1...3.2 on each side with respect to ground.

--mark h

[kvb]

Sorry, I am referring to the resistor in a voltage divider that goes to ground. I have removed the series resistor to increase gain.

I just wanted to ask, in general, what is the max value that one can use in the instance that there is no voltage divider?

I read inhttp://tone-lizard.com/Mods_and_Odds.htm that with respect to the preamp 2.2M seems to be the limit.
So what's the limit for the EL84 ?

[Michael]

Sorry, I am referring to the resistor in a voltage divider that goes to ground. I have removed the series resistor to increase gain.

I just wanted to ask, in general, what is the max value that one can use in the instance that there is no voltage divider?

I read inhttp://tone-lizard.com/Mods_and_Odds.htm that with respect to the preamp 2.2M seems to be the limit.
So what's the limit for the EL84 ?

It depends on whether you are using cathode bias or grid bias. The Grid Leak limit for grid bias is normally about 10 times smaller than cathode bias. The 2.2M limit you quote is definitely correct for a grid-biased 12AX7 (22M limit for cathode bias).

Just found a data sheet for an EL84 that has Design Center Maximums as follows:
Cathode Bias: 1M
Grid Bias: 300k

See it here: http://www.wps.com/archives/tube-datash ... F656/5.JPG

May be other specs out there, but this is at least one reference.

Michael