|
Audio Asylum Thread Printer Get a view of an entire thread on one page |
For Sale Ads |
63.246.183.223
In Reply to: RE: It will be used as a current source. nt. posted by Triode_Kingdom on November 23, 2014 at 17:41:24
For example, let's say you have a SET with a 1626 in the output. You're applying 250v to the anode through the output transformer, and you want it to idle at 25mA. Curves for the 1626 indicate those operating conditions occur with 32v grid bias. So, with 250v on the anode, the LM317 set to 25mA and the grid grounded through a resistor, the cathode pin will have +32v, give or take.
I have to say, I don't consider this particular example a very good use for the LM317. Anything external that contributes to tube conduction (such as a leaky coupling cap) could blow the chip. If the chip opens up, that's OK, they're cheap. However, if it shorts, it will probably take the tube with it.
--------------------------
Buy Chinese. Bury freedom.
Follow Ups:
Hi Tk, In my case I am using it with an el84 so I wanted to check what the bias current is. I am using a 26 ohm resistor to set the current and I want to calculate the actual current under operating conditions to see how "hot" I am running the tubes. cheers, Dak
If you are using a 26 ohm resistor to set the current, it comes down to the equation I = 1.25/r where r is your resistor.With 26 ohms current is set to 48mA approximately. I usually advice 27 ohms, for a current draw of 46.3mA, and 27 is a standard value (never heard of a 26 ohm resistor, unless that's the measured value: try to stick with metal film 1% and less precision resistors, since 250mW is more than enough for that position).
Now about running your tubes hot - or not:
1) You should measure the voltage from cathode to anode thus getting the "across tube" value.
2) You should keep in mind that if the tube (EL84) is used in triode mode, the set current can be multiplied with the voltage across the tube for a static anode dissipation.
But if the tube is used in pentode mode (which is what it was designed and built for), you must reduce the screen grid current from the set current, and while there are ways to find out exactly how much current is that, we can assume 4-5mA for EL84 at the "usual" operating conditions, where the voltage across the tube is just slightly higher than cathode to g2 voltage.Thus if it's pentode mode, your anode current is probably 43mA. If the voltage across the tube is slightly lower than 300V, your anode dissipation is just above 12W (which is another reason why the current setting resistor in the RH84 rev2 is 27ohms, to keep dissipation at still acceptable levels).
******
http://rh-amps.blogspot.com/
Edits: 11/27/14
Hi, Alex, thank you for the detailed explanation. The reason I came up with 26 ohm for the current setting resistor is I did not have any 27 ohm but I had 52ohm so I paralleled 2. This seems to work out ok since the plates are not glowing but I will take the measurements that you have suggested just to be sure. Happy holidays, Dak
In order to "meet" those operating conditions for the 1626 you would need to have 282V on the anode, 32V on the cathode for the required 250Vpk.
dave
I don't know what you mean by "peak." I was referring only to static conditions, and the numbers are from the curves published by RCA. Anyway, it's strictly an example for expedience, intended to provide more detail regarding my earlier comments.
--------------------------
Buy Chinese. Bury freedom.
Edits: 11/24/14
The red dot on your curves assumes the cathode is at 0V, the anode 250V and the grid -32V. You stated the Anode was at 250V and the cathode at 32V which requires the grid at 0V giving you 218V from anode to cathode.
dave
Yes, an oversight on my part. Hopefully, it's close enough to make the point.
--------------------------
Buy Chinese. Bury freedom.
Post a Followup:
FAQ |
Post a Message! |
Forgot Password? |
|
||||||||||||||
|
This post is made possible by the generous support of people like you and our sponsors: