If someone were to just give the age-old, very simple UL connection, with minimal gNFB a try, they may be surprised at how good these 6HB6 tubes (as well as many others) can sound.
At one time it may have been, but it's no longer rocket science.
All the best.
Scott
At one time it may have been, but it's no longer rocket science.
All the best.
Scott
when you move the bias, would you also have to move the plate and screen voltage's to the left.
skal
I forgot to mention that I was referring to the actual prototype: making the adjustable bias with a potentiometer in order to compensate the valves that are a little out of the calculated parameters (at least in quiescent current).
For balancing AC gain we can adjust the predecessor stage.
This will make possible to use valves that are not perfectly matched or tubes with large spreading of values.
This schematic of the RCA was one of the first tube amp schematic I've seen in my life, many years ago (many years for those with 31 years old ...), and I've done this mixed feedback system in small SE amplificators, always resulting in a "boring" sound... coincidence or not, I came to prefer to use Schade alone plus some extra normal fdbk if necessary.
By the way, we not found everyday an cheap triode with mu 31 and 10W at the anode ... some maybe can imagine an great driver for big amps.
The screen grid seems not to have high gain enough to use it as input (screen grid drive), at least with the typical supply voltages of using this valve, right (I only analysed them for ten seconds...😀)? Since it have the linearity you quoted it would be tempting ...
Speaking of screen drive, I use screen drive on my plasma tweeter, and one day I might try this with my PL509 ...
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Ok , 1 ? the -5 bias to the grids was intended to induce a current flow of 33ma yes , is this for 1 tube at idle or 2
That is why I suggested to adjust the grid 1 voltage (bias) in the prototype. If you got 23mA and want to get 33mA, reduce a little the g1 voltage; will be less than -5V (I believe around -4.5V). You do not have to worry about what you plotted on the graph for bias effects.
Remember, because the valve is delivering a different current value than you calculated in the load line, means that you can no longer use the manual curves to calculate the bias. -5V in the "real/prototype" valve does not correspond to -5V from valve manual curves, so you can freely adjust the bias to obtain the desired current.
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ok did that my results are
plate current 0.33ma
screen current 3.5
bias voltage -4.22
both grids at bias @ -4.12
cheers
skal
plate current 0.33ma
screen current 3.5
bias voltage -4.22
both grids at bias @ -4.12
cheers
skal
so 2 bottles 66ma in Class A? so Really i should be seeing 66ma draw at the plate ?
cheers
skal
What's the difference between these two numbers? If both tubes are biased equally, that would suggest they are perfectly matched. Is that true? Because smoking-amp and I are seeing a lot of variations from tube to tube.
33mA each, so 66mA total drawn from the PS at idle, the peak current at full power of course will be much higher.
So i have to push up the the bias then to get both tubes to idle at 66ma.
Yes both grids have the same voltage on there grid's so good match then using two Rca etch glass.
cheers
skal1
Hello smoking-amp,
Thank you for posting the 6HB6 triode curves. A couple of questions remain.
1. Are the grid voltage steps 1 volt per step; 0, 1, 2, 3, 4, 5, 6, 7, 8, 9, and 10 ?
2. Is the y axis incremented at 10ma per box on the graph; 0, 10, 20, 30, 40, 50, 60, …..?
3. Is the x axis incremented at 50 volts per box on the graph; 0, 50, 100, 150, 200, 250, 300….?
I am just guessing
Thanks DT
Edit add 1 more question? I assume that the beam plates are connected to the cathode and the screen is connected to the anode. Is this correct for your plotted curves?
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No, why would you want to do that? At 33mA and 250V, they are already at 82.5% of the 10W maximum plate dissipation rating. You are not making a class A push-pull amp, are you?
Hmmm, 66mA is for both tubes, I presume, right?
Skal, how you measured the individual valve currents? With an small cathode resistor for each valve, for monitoring current? Or with series multimeter? Or removing one to measuring the another?
Skal, how you measured the individual valve currents? With an small cathode resistor for each valve, for monitoring current? Or with series multimeter? Or removing one to measuring the another?
No Class AB1
measuring B+in series with multimeter and out to the center tap of out tx
"measuring B+in series with multimeter and out to the center tap of out tx"
Perfect, when we know the internal wires resistance (rdc).
Is good to check if one side resistance differs to other side, since most trannies differ rdc between 1/2 primary windings.
Perfect, when we know the internal wires resistance (rdc).
Is good to check if one side resistance differs to other side, since most trannies differ rdc between 1/2 primary windings.
DIYBras
so i just want to clarify this v1 current should be 0.033ma and v2 current should be 0.033ma at my chosen bias point?
thanks again for everyones support on this
cheers
skal
so i just want to clarify this v1 current should be 0.033ma and v2 current should be 0.033ma at my chosen bias point?
thanks again for everyones support on this
cheers
skal
Skal,
Yes, for better output trafo performance, the two valves need to be with same anode current, so V1=33mA and V2 also 33mA. This will balance the two valves current and helps to bass response (less core saturation).
Yes, for better output trafo performance, the two valves need to be with same anode current, so V1=33mA and V2 also 33mA. This will balance the two valves current and helps to bass response (less core saturation).
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