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LED cathode bias - how to pick LED?

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EC8010

EC8010

Ex-Moderator
Joined 2003
In general, old technology red LEDs are best, avoid high brightness LEDs, and go for the dimmest LED you can find - they will have the lowest slope resistance. A particularly nice LED is the HLMP6000 series that produces a forward drop of about 1.6V with about 5 Ohms of slope resistance.
 
rdf

rdf

Member
Joined 2004
Much of the past week I've been working on a LED-biased, choke loaded 6C45-2A3 SE amp. The LEDs are two stacked IR of unknown pedigree from a gutted remote control, force fed extra current from B+ with an R-C to ground-R string. The amount of noise on the 6C45 plate was shocking, that is until the M-Audio card feeding it was replaced with 150 ohms to ground. Noise dropped below the scope's lower resolution limit. When a 24-bit sound card injects more noise than cathode LEDs into the grid circuit of a high mu tube running flat out, I don't much sweat it.
 
rdf

rdf

Member
Joined 2004
The plate chokes in use are limited to 8ma, fortunately my preferred ballpark for the 6c45 but not enough to get the LED completely out of the high curvature section of the VI curve. The extra current runs them in a more linear region, especially important when plate load isn't a CCS. The cap in the middle smooths the current tap off B+ and provides extra noise filtering. It's also a convenient place to tie the centre tap of the 6C45 filament windings.
 
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sharpi31

Member
Joined 2005
I replaced the caps & resistors in my Spud amp with the OPE5594A IR LEDs. The LEDs must have pretty tight tolerances as the two series pairs gave 2.48V and 2.49V bias (this was with the first 4 LEDs picked at random).

The sound seems to have changed from top to bottom. Bass seems to go deeper and tighter and a degree of mid-bass warmth has reduced. Mid range seems cleaner (could be due to the reduction in mid-bass flab) and HF seems to go higher and is better resolved. All in all a step forward - the sound seems cleaner and more balanced.
 
yagoolar

yagoolar

Member
Joined 2004
6C4C SE parafeed amp LED bias mod

I have recently been working on Red Light District amp and thought if LED bias for 6C4C SE parafeed amp is a good idea. I have built 2 LED arrays 4 x 26 pcs. for each. Replaced the RC network in cathodes and fired the amp. And my friend's amp is a new amp! Detailed with more bass control and bandwidth.

Lots of soldering yet worth the time (about 2,5 hrs)

LEDs measured:
Uf=1,869V @ 29 mA
 
I

Ian444

Member
Joined 2008
sharpi31 said:
The sound seems to have changed from top to bottom. Bass seems to go deeper and tighter and a degree of mid-bass warmth has reduced. Mid range seems cleaner (could be due to the reduction in mid-bass flab) and HF seems to go higher and is better resolved. All in all a step forward - the sound seems cleaner and more balanced.
Click to expand...

I found similar results on three PP amps. I haven't had an opportunity to directly compare LED's to fixed bias yet. Now that I think about it, in one amp I have, I could short out the LED's and increase the negative (fixed) grid bias and see if there is a difference in sound. The 5mm Fairchild MV50152/MV50154 LED's are quite capable of handling a constant 100mA but are hard to get, I have run them with 6L6, 6V6 and EL84 as a single series string for each output tube cathode and have had no problems. I've also used MV50640 3mm but they handle less current, so these need two or three parallel strings depending on the current required.
 
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