You need to understand the current misbehavior, and because a red LED provides fixed bias, the feedback effect of the cathode resistance will be gone and the current will be determined entirely by the tube's transconductance and the voltage on the plate. I use LEDs cascode CCS or some other means of adjusting the operating point. You'll probably have some further interesting behavior, although if the inductance in that bypass cap is part of the problem and the driver is oscillating it might fix it.
Using an LED is just a band-aide, there is a problem somewhere else - most likely with your build technique. The design is not the best either, but I see no reason why it would not work as designed if built correctly..
You are casting about for solutions without understanding the actual problem. Do you know anyone local with a scope and a good understanding of tube amps to help you?
I don't get the sense I am getting through.. I have 25 years of experience designing amps with the 300B and 15 years with the D3A.
Using an LED is just a band-aide, there is a problem somewhere else - most likely with your build technique. The design is not the best either, but I see no reason why it would not work as designed if built correctly..
You are casting about for solutions without understanding the actual problem. Do you know anyone local with a scope and a good understanding of tube amps to help you?
I don't get the sense I am getting through.. I have 25 years of experience designing amps with the 300B and 15 years with the D3A.
Friends, I have news ... yesterday I listened the amplifier with values outside the ideal for about 4 hours, with 30ma in d3a, I went to sleep depressed ..
And today !!!!!!!
Today in the afternoon , I measured it again and it has 415v in anode 300B and 77ma, and 175v 20ma in d3a tube !!
how can it be??
And today !!!!!!!
Today in the afternoon , I measured it again and it has 415v in anode 300B and 77ma, and 175v 20ma in d3a tube !!
how can it be??
santitrucco,
How much hum do you have? _____ mV?
Are you using the schematic in post #1?
I will give suggestions based on that schematic.
With the AC filaments on the 300B, even with a hum pot, I would expect some hum.
It will vary from tube to tube, even after you adjust the pot.
Check your B+, I expect that you have about 20mVrms hum there.
With the 8uF and 10Henry choke, it is only followed by 100uF of filtering, and there is no resistor cap filter stage to feed B+ to the 10k D3A plate load.
If you ground the grid of the 300B and the hum reduces significantly, use a 1k Ohm resistor in series with the B+ to the 10k plate resistor, and put a 47 or 50uF cap to ground at the junction of the 1k and 10k resistors.
Let us know how well that works.
How much hum do you have? _____ mV?
Are you using the schematic in post #1?
I will give suggestions based on that schematic.
With the AC filaments on the 300B, even with a hum pot, I would expect some hum.
It will vary from tube to tube, even after you adjust the pot.
Check your B+, I expect that you have about 20mVrms hum there.
With the 8uF and 10Henry choke, it is only followed by 100uF of filtering, and there is no resistor cap filter stage to feed B+ to the 10k D3A plate load.
If you ground the grid of the 300B and the hum reduces significantly, use a 1k Ohm resistor in series with the B+ to the 10k plate resistor, and put a 47 or 50uF cap to ground at the junction of the 1k and 10k resistors.
Let us know how well that works.
Ok , the HUM is almost inaudible , i am agreed..
I am enjoying now , i have to say THANKS YOU friends..
You are a great team.
PD: I was listening with Russian K40y-9 capacitor, that I always liked a lot ..
Now I put an AudynCap True cooper .. I think it's a Big positive leap, and not very expensive (u$s 24)..
I am enjoying now , i have to say THANKS YOU friends..
You are a great team.
PD: I was listening with Russian K40y-9 capacitor, that I always liked a lot ..
Now I put an AudynCap True cooper .. I think it's a Big positive leap, and not very expensive (u$s 24)..
While there are lots of choices, it partly depends on how you want to couple the driver to the grid of the 300B.
Both Rod Coleman and Yves have posted circuits to try on this thread. Thorsten Loesch liked the C3M as pentode.
Both Rod Coleman and Yves have posted circuits to try on this thread. Thorsten Loesch liked the C3M as pentode.
The russian tetrodes , could be good drivers working like triodes. I have a d3a 300b amplifier , but i was thinking in one russian tetrode working like triode to compare.. some friends tell that are excellent choice like 6E6P-DR
And 6E6P DR like triode have the same pinout that D3a , i am not sure.
See
And 6E6P DR like triode have the same pinout that D3a , i am not sure.See
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The russian tetrodes , could be good drivers working like triodes. I have a d3a 300b amplifier , but i was thinking in one russian tetrode working like triode to compare.. some friends tell that are excellent choice like 6E6P-DR
See
See here: http://www.bartola.co.uk/valves/tag/6e6p-e/
They are one next the other. 6e6 requires 2x heater current. It has about 1/2 gain mainly because of the lower plate resistance and a bit lower gm. Still high gm device though. Pinout is almost identical except the fact that pin n.6 in the 6e6 is connected to the cathode while it's free in the D3a and then pin n.8 is g3 + screening in the D3a and just screening for the 6e6p (which is a tetrode).
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