HI Friends , i finished one d3a-300b amplifier. 300b would work with 420v 80ma aprox. D3a with 185-190v 16-19ma. When i turn on the amp, i can measure 520v high tension , an then in five minutes go down to 420v.. In cathode d3a i can measure 3.4v with 140ohm resistor ( 24ma), ..in anode 125v In 300b cathode 200v with 2kohm resistor ( 100ma ).. But sometimes some voltage change..i dont know what's going on.. Someone could help ??
Thanks you
Thanks you
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Yeah, cathode resistor should be 1K and are you sure the grid resistor is grounded and/or that you have not swapped grid and plate pins? I have done this very thing and it's not too cool.
What brand output tubes are you using? I have found in the past that some Chinese made 300B malfunctioned at plate voltages above 350V. (They basically ran away and drew lots more plate current than they should)
Umm, where are the grid resistors for the 300B? And it appears grid and cathode are tied together on one of the 300Bs.
Build quality needs a lot of work. Components should be anchored to terminal strips. Caps held in place with glue is not a good practice although I have done it.
What brand output tubes are you using? I have found in the past that some Chinese made 300B malfunctioned at plate voltages above 350V. (They basically ran away and drew lots more plate current than they should)
Umm, where are the grid resistors for the 300B? And it appears grid and cathode are tied together on one of the 300Bs.
Build quality needs a lot of work. Components should be anchored to terminal strips. Caps held in place with glue is not a good practice although I have done it.
Checking D3a triode curves, these are impossible operating points.
Try to get operating points correct first with the appropriate cathode resistances.
Seems to me that you confuse grid resistor with grid stopper, the schematics clearly shows a 200K grid resistor - from grid to ground - not shown in pictures.
No matter you use a stepped attenuator, you must use a grid resistor for the D3A too, 470K should be fine.
Yes, you need to add a 200K resistor from the 300B grid to ground otherwise the tubes will self destruct, there needs to be a DC path from the grid to ground. Normally I recommend 100K for 300B, but since this is cathode biased 200K will be OK.
The treasure series are OK, don't have the problem I referenced last time I knew.
The treasure series are OK, don't have the problem I referenced last time I knew.
No, this is a grid-stopper, you must add another 470K resistor from the other extreme of those 1K to ground, in the schematics from input to ground, maybe parallel with your stepped attenuator.
Yes, as we are trying to say, this is the major problem and cause the voltage go sky high, and as Kevin pointed up this can destroy your 300B.
Just send me a PM.
Yes! I have tried to tell you this...
Popilin please help this guy..
Hello Kevin, sorry for the late response, I am home alone and dealing with a riot on board, 3 dogs, 2 parrots and a sick pigeon...
Your wife is dedicated. I feed my cats store bought cat food, but they are fussy, but oddly they don't like most gourmet cat food. (They hate anything advertised as "clean" - they have good taste.. lol) The bunnies of course are vegan.
Now back on topic, Santitrucco, let us know when you get the amp running. For 300B you want quiescent current of 60 - 70mA, I favor the higher end of the range, also the cathode resistor value for the D3A should be around 100 - 120 ohms.. I would try not to go too much over 20mA due to plate dissipation concerns. I would increase the plate load resistor (10K) slightly if you have excessive plate current/dissipation. I use D3A as drivers in my amplifiers (six channels, four are 300B and two are GM70) so I am pretty familiar with them over about 10 years now.
Now back on topic, Santitrucco, let us know when you get the amp running. For 300B you want quiescent current of 60 - 70mA, I favor the higher end of the range, also the cathode resistor value for the D3A should be around 100 - 120 ohms.. I would try not to go too much over 20mA due to plate dissipation concerns. I would increase the plate load resistor (10K) slightly if you have excessive plate current/dissipation. I use D3A as drivers in my amplifiers (six channels, four are 300B and two are GM70) so I am pretty familiar with them over about 10 years now.
Your bunnies are vegans, my parrots should also be vegans, but they always want to eat like us, so during lunch (noon here) if you do not feed them with the same from us is a guarantee for a mutiny, and parrots are extremely noisy... very low transconductance birds, I guess. Lol.
As for the Santi amplifier, I think the key is just the absence of grid resistors, and a more neat construction should be great.
Your advice is absolutely correct, as always, but be satisfied with the very basic first.
As for the Santi amplifier, I think the key is just the absence of grid resistors, and a more neat construction should be great.
Your advice is absolutely correct, as always, but be satisfied with the very basic first.
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