Just a quick question,
Does any one know the effect of using LED's in the cathode of all pre stages of a tube amp? Is there any noticable effect good/bad or indifferent?
I have this in one stage now and was wondering if its worth trying in the other stages before I actually do it..
Is there any colouring if used in all stages? I know it might cast some light on the subject..LOL
I guess I should add its been quite promising so far!
Also what is the effect if used with CCS or gyrator<<<for and against or preference and why?
Regards
M. Gregg
Does any one know the effect of using LED's in the cathode of all pre stages of a tube amp? Is there any noticable effect good/bad or indifferent?
I have this in one stage now and was wondering if its worth trying in the other stages before I actually do it..
Is there any colouring if used in all stages? I know it might cast some light on the subject..LOL
I guess I should add its been quite promising so far!
Also what is the effect if used with CCS or gyrator<<<for and against or preference and why?
Regards
M. Gregg
Last edited:
Just a quick question,
Does any one know the effect of using LED's in the cathode of all pre stages of a tube amp? Is there any noticable effect good/bad or indifferent?
I have this in one stage now and was wondering if its worth trying in the other stages before I actually do it..
I believe it degrades audio quality adding semiconductor alinearities and destroying tube sound. Also, I strongly dislikes mixing tubes and semiconductors in a set. Bit it is only personal taste.
The RLD uses them in the output stage and has been reported to be very good by several people.
I've only used LED cathode bias in the first stage, but of several amps and find it to be quite acceptable. It's non-linearity does not come into play in a fixed bias mode such as a cathode bias position since the voltage variation is minimal.
What other stages were you thinking about?
I've only used LED cathode bias in the first stage, but of several amps and find it to be quite acceptable. It's non-linearity does not come into play in a fixed bias mode such as a cathode bias position since the voltage variation is minimal.
What other stages were you thinking about?
The LED acts as a constant voltage source. Or close to it, depending on the particular LED's dynamic impedance.
Whether it's appropriate and of benefit for any given stage depends on the design. It's just another component available to the designer and must be considered in context.
It is typically not used in a second stage because the second stage is often a concertina or LTP splitter. But if your second stage (or any other stage) is a simple common-cathode voltage amplifier, then I see no reason you can't use it.
As noted, it is also fine to use for cathode bias in a final stage.
Whether it's appropriate and of benefit for any given stage depends on the design. It's just another component available to the designer and must be considered in context.
It is typically not used in a second stage because the second stage is often a concertina or LTP splitter. But if your second stage (or any other stage) is a simple common-cathode voltage amplifier, then I see no reason you can't use it.
As noted, it is also fine to use for cathode bias in a final stage.
OK, voltage variations don´t care, but really import are current variations, plate of a triode you found escentailly current variations translated to voltage thanks to the plate to +B resistor.
ANODE CURRENT IS THE SAME AS CATHODE CURRENT (except in grid leak), so if the cathode current is distorted, the anode also will be, and there, the anode AC voltage to ground will be also distorted.
And yet, experimentally the distortion of circuits using LED bias is quite low (assuming competent design). My phono preamp, for example, has 0.03% distortion at full output, almost entirely second harmonic, with no harmonics detectable past third, and it uses LED bias for two of the three amplification stages.
If you think about the actual AC impedance, how it varies over the current swing of the stage, and the effect on circuit operation, it becomes more evident why the cathode LED is not a significant source of non-linearity.
SY,
Do you have any thoughts on the effect of LED cathode bias in preamp tubes with CCS or Gyrator anode load?
Just an observation OK the circuit I describe should have a cap across the cathode load to prevent drift however here is a tail...I used resistive load in place of a ccs with no cathode bypass cap.. the volume of the amp altered with the increase in temp over about an hour (on its own). Replaced the resistor on the cathode with an LED and 1N4148 for the set point..Guess what no drift..and better drive from the stage..
Regards
M. Gregg
Last edited:
Yes the anode and cathode currents are essentially equal, otherwise the split load PI wouldn't work. The difference is that a tiny voltage change on the anode wouldn't be noticed. A tiny voltage change on the cathode will affect the bias on the tube causing a bigger than normal change in current through the tube.
I have also noticed some very low distortion readings in LED biased stages. Replacing the LED with a resistor and cap adjusted for the same operating conditions did not yield the same low distortion. Adding a bypass cap across the LED actually increased the distortion. The LED improvement varies with the LED and the tube being used, even among batches of inentical LEDs and tubes. The tubes were CCS loaded 12AT7's and the LED's were from a bag of 1000 surplus green LED's.
it is my belief that the nonlinearities in the 12AT7's and the LED's were cancelling to some extent, but I have not tried to prove this theory yet.
For the lowest THD in a triode input stage, I use a constant current source on the anode and LEDs in the cathode. For best results use LEDs with a low dynamic impedance. The red and green ones seem to be the best with a dynamic resistance under 10 ohm in most cases.
I have verified my claims that the lowest THD is obtainable with a CCS and LED biasing using an HP 8903A distortion analyzer. I have verified this with several triodes - 6J5, 5842, d3A, to name a few.
I realize that for some, this may be too much sand in the glass, but ultimately it comes down to your design goals. For me, the goal is to achieve the best sound quality.
~Tom
I have verified my claims that the lowest THD is obtainable with a CCS and LED biasing using an HP 8903A distortion analyzer. I have verified this with several triodes - 6J5, 5842, d3A, to name a few.
I realize that for some, this may be too much sand in the glass, but ultimately it comes down to your design goals. For me, the goal is to achieve the best sound quality.
~Tom
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.