by EWBrown » Fri Jun 07, 2013 7:16 pm
The bypass cap, especially if it is an electrolytic will have inductive properties, and can create weird undesirable resonances, and uneven frequency response.
The LED should not nullify the inherent degenerative feedback that an unbypassed cathode resistor would cause, though the self-generated negative feedback effect might be a less.
The use of an LED is dependent on the expected cathode bias, different colors will have different forward voltage drops, the "old style" cheap red, orange,
amber, yellow and "slime green" LEDs are best, the newer 525 nm "true green", blue, white, pink, UV, etc, might not perform as well - I have not tried these newer generation LEDs for cathode biasing.
Though I have read of using 3 larger (5050) SMD white LEDs, connected in series for EL84 cathode biasing, with a B+ supply between 300 and 325VDC, the LEDs will place the EL84 cathode at 10-11 VDC above ground, which is correct for this B+ supply voltage range, I have not yet tried this method either..
IR LEDs have forward voltage drops between 1 and 1.2V, but will not emit visible light, except for some of the 880 NM IR LEDs which may appear as a "dark cherry red" to some people - my vision can detect these in a darkened room. 880 nm is the longest wavelength for most human vision.
The LEDs I use most often are the small HP (now Avago) HLMP6000 which have a forward voltage drop of 1.56 to 1.6VDC, these are fairly inexpensive, and are halfway between being a "leaded" LED and a surface mount, the leads are very thin and flat, and can break off rather easily, if excessive tension is placed upon them..
I bought a "lifetime supply" of these from Jameco several years ago, and the voltage drops have been very close tolerance, they were from the same manufacturing batch. I've also used the cheap 3mm red LEDs from Radio shack, they fall between 1.6 and 1.8 VFD. Amber and yellow LEDs fall between 1.8 and 2 VDC, and the lime green ones, 2 to 2.2 VDC. If higher voltages are necessary, just connect two in series (LED cathode towards 0 volts / signal ground, anode to the tube's cathode.
I would suggest, start with the cathode resistor, then try the LED, and determine which approach sounds better.
The "ultimate" application would be to use the LED for cathode bias, and an appropriate constant current source in place of the B+ resistor, this gets the most gain,
and generally lower noise, out of a voltage gain stage, than the standard plate and cathode resistor approach.
I have used this method in a 6DJ8-based phono stage, which is otherwise quite similar to Shannon's "Budgie" design, the main difference being that I operated the triodes at around 3.5 mA, with a 150VDC B+ voltage. Otherwise, 18K plate resistors, and 430 ohm cathode resistors would be used.
I had some confusion with the R/C coupling to the 1626s' grids, until I re-read your explanation about using it as an HF compression horn driver, with an LF roll-off
of 2 KHz.
HTH
/edB
Real Radios Glow in the Dark