I just gutted and redid a single 4cx250b amp. I have 1200vdc on plate ( old amp had 2400Vdc for a single 250B) 300vDC on screen and about -55v for bias. I am getting about 300-325W pep for a single 250b. I noticed that as soon as plate/screen switch is turned on, tube starts to heat up. I have a decent blower on it. Bias is only applied when amp is activated. Is it usual for tube to run hot? This is my first tetrode and i am not familar with these. Any ideas, suggestions? any and all comments are welcome.
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4cx250B
- Thread starter Crusher
- Start date
I swapped to another tube i have and it tuned up to around 400W pep. Just concerned why tube starts to get hot as soon as plate and then screen applied. Wonder, would it be better to just run main supply wire for screen/bias transformer thru relay so that both are only active when amp is activated. Would this allow more time to cool and lessen dissipation? Just learning and trying to figure it out.
I have -55V on grid. This is for linear service. I do not have a plate meter installed on amp. This amp I redid. Before it was ugly. Being polite. I have a seperate screen and bias transformer. Completely seperate from plate transformer. Screen supply, used a 250Vac transformer rectified. Gives just over 300 with 10K resistor from + to ground. Bias is 50vac rectified. Will give me about 70V max of - bias. + is attached to chassis and - goes thru relay to grid. Is only applied when amplifier keyed.
Crusher,
is this for AM?
what does the word 'activated' mean.
there needs to be -bias on the grid,
if there is +volts applied to the plate.
Ii am not sure if you
can do that with a grid-leak-resister.
generally, there needs to be active negative bias
on the grid,all the time. please correct me if I am wrong,
as I do not have the tube's plate-current characteristic
in front of me.
is this for AM?
what does the word 'activated' mean.
there needs to be -bias on the grid,
if there is +volts applied to the plate.
Ii am not sure if you
can do that with a grid-leak-resister.
generally, there needs to be active negative bias
on the grid,all the time. please correct me if I am wrong,
as I do not have the tube's plate-current characteristic
in front of me.
Yes, this is for AM. When I mean activated, I mean when amp is keyed. That makes sense that the grid bias should be on all the time. When I got amp originally they had 1 side lifted but I think that might have been screen. So bias is on all the time and screen applied only when amplifier is keyed. That actually makes sense in order to keep dissipation down while in standby. Guess I have some reworking to do when I get home tonight and see what happens. I appreciate the help. The top 2/3 of case is the perforated metal case. So I am not able to install meters. I would like too. But the cabinet is so compact. I literally only have about 3" on bottom to work with. And 10"'s wide.
op:
Pride DX300 Manual
Crusher, I have been through many of the Pride DX 300, not a bad design, few flaws are the zener for bias, under rated component, I do not like how they use the connectors to jump around, I always soldered them on, the HV board runs are to close together but NOMAD has an after market board to fiux that problem.
Schematic is decent and the basic design is good.
Might give you a few ideas of how that tetrode plays.
Good luck.
Crusher, I have been through many of the Pride DX 300, not a bad design, few flaws are the zener for bias, under rated component, I do not like how they use the connectors to jump around, I always soldered them on, the HV board runs are to close together but NOMAD has an after market board to fiux that problem.
Schematic is decent and the basic design is good.
Might give you a few ideas of how that tetrode plays.
Good luck.
Bias on a tetrode is different than a triode. With the triode it is typical to just open the bias connection on the cathode and let this negative voltage rise to the cutoff point across a resistor in standby.
The 4CX250B may be in class AB1 with -55 volts on the control grid but when you remove this voltage the plate dissipation will jump up because the tube conducts more heavily as the negative bias gets closer to zero volts.
The proper way to bias a tetrode into cutoff would be through the control grid by increasing the negative bias voltage in standby. One easy way to do this is to open the connection to the bias regulator in standby. Providing there is additional voltage held back from the regulator on the bias supply, opening the regulator connection should put the tube in cutoff.
Some people just cut the screen supply off in standby and that can make the tube draw zero current in standby but is not the preferred method. In any event, a current meter should be used to insure the tube is indeed in cutoff when in the standby mode. Even if you just temporarily install one in the HV return line for setting the bias.
The 4CX250B may be in class AB1 with -55 volts on the control grid but when you remove this voltage the plate dissipation will jump up because the tube conducts more heavily as the negative bias gets closer to zero volts.
The proper way to bias a tetrode into cutoff would be through the control grid by increasing the negative bias voltage in standby. One easy way to do this is to open the connection to the bias regulator in standby. Providing there is additional voltage held back from the regulator on the bias supply, opening the regulator connection should put the tube in cutoff.
Some people just cut the screen supply off in standby and that can make the tube draw zero current in standby but is not the preferred method. In any event, a current meter should be used to insure the tube is indeed in cutoff when in the standby mode. Even if you just temporarily install one in the HV return line for setting the bias.
Thanks everyone for the replies. Gonna change the wiring when I get time later tonight. I appreiacte everyone input. I am as new as they come when it comes to tetrodes. So this is a new learning experience for me.
Now im getting somewhere. I changed bias, so that i have -68V at all times. The bias circuit turns on when plate V is turned on now. When i first turn it on, amp stays nice and cool. But when i keyit up and unkey, amp stays in full conduction. I am thinking i need more bias? Probably around -100V for closer to full cut off. -68V just aint gonna cut it i guess. Part of the learning curve. So looks like i will have to install a second bias transformer and run them in series to give me about -140v. Then switch in a zener regulator or something more suitable to handle regulation so that amp will conduct when amp is keyed and drop it down to a more workable voltage.
It would be easier to leave grid bias on permanently.
Plate and screen voltage can also be left on.
Higher bias voltage will turn the tube off, or you could possibly lift the cathode return. No need to turn the power supplies on and off.
There are several sockets available for the 4CX250. You need to know which is being used to determine proper circuit configuration.
Plate and screen voltage can also be left on.
Higher bias voltage will turn the tube off, or you could possibly lift the cathode return. No need to turn the power supplies on and off.
There are several sockets available for the 4CX250. You need to know which is being used to determine proper circuit configuration.
Yeah zeners are the way to go or with a pass transistor configuration. The socket I am using is from rf parts. Its similar to the sk600, but its the tiny metal case with a ceramic insulator and little sockets for each of the pins. Each one is isolated from the other. I believe the pride dx300 used something like a 82V zener, so that's telling me I don't have enough bias to start with.