MrCurwen, When I will get more time, I will realy be interested ! My first amp is what it is : a first amp, a starting point, a learning machine
Now I'm confident in myself to be able to build an amp
That is exactly as it should be.
Do you plan to mod your existing amp or build a completely new one?
Sorry I don't understant what you want to say / explain
OT means output transformer. Series feed OT means a topology where the OT primary is in series with the output tube. So it is between B+, the tube, and ground, having the same DC current pass thru it that passes thru the tube.
Loudness filter means a frequency response with some degree of added highs and lows. Anti-loudness filter is the opposite of that, some degree of less highs and lows.
Series feed OT amps almost always have some anti-loudness filter going on, that is because of the properties of the OT. With DC bias the OT has difficulty reproducing low frequencies. The DC "drowns out" any nearby (like 50 or 100 Hz) action. Also the windings have a lot of parasitic capacitance; think about it, lots and lots of wire stacked up right next to each other. This capacitance - if not satisfied with current - forms a low pass filter, depressing high frequency response.
These problems can be satisfied by
1) not having DC current thru the OT (fixes the bass issue completely)
2) driving the OT primary from a low output impedance source (fixes the high frequency issue).
Grid current is the umbrella term for parasitic properties of tube control grids. They generally require some current to satisfy. Imagine there is a capacitor inside the tube between control grid and cathode, and also control grid and anode. These capacitors in conjuction with other circuitry around the control grid act as low pass filters, dampening transients and often dampening high frequency response.
The most common comments people give upon fixing this grid current insufficiency problem are that the sound is "more airy", "more 3D" and "faster". So, high frequency response and transient response are improved in technical terms.
RC cathode bias does a similar thing but on the bass side of the frequency response. Point of the cathode C is that it is supposed to keep the cathode node at a steady DC and drown out all AC by charging and discharging accordingly.
Now once the C has discharged some, it can only be recharged by current that arrives thru the tube from B+. This doesn't happen instantly; think about Ohm's law. There is a tube with impedance of X ohms between B+ and the cathode node. This impedance limits the amount of current that can be transported.
So it doesn't charge instantly. While the C is not charged, the DC bias point of the tube (cathode node voltage) is different. What happens is the cathode node voltage floats up and down with the signal, a little bit. Now since the tube only amplifies the VOLTAGE DIFFERENTIAL between cathode and the control grid, this means that if the cathode node voltage is floating in sync with the signal, there is less amplification. This effect happens most on the biggest amplitude frequencies, because they resemble DC the most.
If the cathode C has to discharge a long long time, it takes a long long time to recharge it, allowing the cathode node voltage lag.
Why does the cathode node voltage fluctuate in the first place? Well, same reason as the anode node voltage fluctuates. The tube doesn't know which resistor - anode resistor or cathode resistor - is supposed to be the output.
Yep I know. But it's to have exactly 6,3V whatever my local voltage could be
That's understandable. I'd go for 6.1V instead; doesn't hurt operation in any way and doubles lifespan of the tube.
I really want to heard thoses golden age tubes. I'm open to other stuff. 47 could be an option, in triode strapped modes. Indeed that's not the "good" or "best" approche, but ... well .... it's a pleasure, and, at the end pleasure is the objectif (if you see my point ?)
So what I'm hearing is you like the way the ST envelope tubes look? That's fine. Go for the 47 then, it's every bit as linear as any 45 or similar.
But I'd suggest testing new circuit topologies with ugly TV sweep tubes first. After you've got them working, then bring in the pretty tubes with difficult PSUs.
Did you have some triode strapped curves for the 47 ?
Please note 0V is not at the origo, but rather at the point where the first curve begins. Individual tubes will differ +- 15 % on anode resistance.
And for the Western Electric 32A, you'v got the "grid to cathode coupling" trick
Yes it's basically the "Ultrapath solution" applied to the grid. You can trade a large (and in those times very poor quality) electrolytic for a nice high quality film cap (or paper in oil in those days).
Right thanks for the information, beginning to understand the functionality of the CCS.
The FET needs about 4.5V positive on the gate to start conducting. The higher the gate voltage, the more it conducts.
The Darlington pair needs a certain positive base voltage to start conducting.
Remember Ohm's law. Look at the resistor above the Darlington pair, let's call it the R. At rest the FET gate is at B+; since there is no current thru the R (the Darlington pair is basically 'open', not conducting) there is no voltage drop across the R.
Once there is current thru the pair, and thus, thru the R, there is a voltage drop across the R.
So, the gate voltage lowers. FET conducts less current. There is a smaller voltage drop across the sense R underneath the FET.
This means the base voltage for the pair is lower, meaning it conducts less current. Less current thru the R means FET gate voltage rises -> FET conducts more -> more current and thus voltage across the sense R -> higher Darlington pair base voltage -> Darlington pair conducts more -> more current thru the R -> FET gate voltage drops -> FET conducts less current etc etc ad infinitum.
It doesn't actually oscillate like this (best case scenario...), but rather it stabilizes so that any change in the feedback loop is very strongly resisted.
Every feedback loop contains a time paradox, so it's rather difficult to fully imagine. Don't think about how the loop is first set up, at power on. You'll hurt your brain; I know I cannot imagine it. Which component needs to conduct first?
The Darlington pair and the FET work in a see-saw. If one begins to conduct more, the other conducts less. But each has their input at the output of the other (the pair is set up like a normal gain stage, with 'anode' output, the FET is a source follower).
This is inside the loop; looking in from outside all you can see is the FET changing the voltage across itself. When the FET gate voltage changes, this also changes the voltage across the FET as well as how much current it is conducting. This effect is what makes the CCS useful in a LTP circuit. It floats the cathode node voltage to achieve constant current. Cathode node voltage changes tubes' biases. Tube biases change the voltage conditions inside the tube.
Ah, interesting. In that case gug42 should notice the missing frequencies. Strange that he said that the treble was almost too much.
Possibly some high frequency distortion.
