Discussion in 'Amateur Radio Amplifiers' started by KJ4CVY, Sep 22, 2013.
amp will note tune ans screen current about 60 when turned to linear or cw
Is this a new problem that just occurred, and it was fine before?
Is it showing any screen current in standby (that is, when not keyed)? What's the plate current read when keyed (idling current, relays keyed but no signal applied)?
Is it showing any grid current? If so, how much, both unkeyed and keyed but idling (no drive)?
Is the amp "new" to you, or have you been using it a long time?
thank you for your reply yes it is new to me it don't show and plate or grid keyed or unkeyed yes it show about 35 to 40 in the tune mode
as far as driving the amp i have not tryed to drive it at all i have my radio is a icom 251a power is turned down to about 5 watts as far as the relay setup it has 2 dpdt relays 120volts thats about all i know about it
My first guess is the tubes are shot.
Luckily, 4CX250Bs aren't terribly expensive.
But in normal operation their screen current is close to "zero," and so is grid current. In fact, watching screen current is a great way to "load" that amp: Adjust the loading for zero screen current, and that's just about the right point.
What does the high voltage indicate?? One great way to "blow" the screens is to drive the amp with the high voltage turned off or not working. If that happens, all the drive power gets dissipated by the grids and screens, and "poof" go the tubes.
Another way to blow the screens is to drive the amp without a proper load. Make sure the T-R antenna relays are working properly and really "connecting" when you transmit. They must be "outboard," since the Thunderbolt did not have a built-in antenna relay of any kind.
This is not a "plug and play" amplifier, at all.
well i guess it is time to look for new tubes s it hard to to retube it when i got the amp it didn't have any plate or grid current maybe that is why he wanted to get rid of it well thank you you helped me alot
Remember, this old amp (last manufactured in the early 1960s) is not nearly "plug and play," at all.
Are the relays (external) keying? What kind of relays are they? Does the amp go into "transmit" when the relays key? You won't have any plate current unless this happens.
You have to turn the PLATE switch "on" and be sure the plate voltmeter is reading (about 2200Vdc or so) in order to have any plate current.
What's the meter indicate?
Have you worked with 50 year-old amplifiers before?
yes they are external as to what kind i have not yet looked i did try them yes it does go into transmit but still no plate current the first meter when switch to volts reads about 2200vdc but the funny thing is it also drops my drive power back on my radio it is set for 4-5 watts my radio only does 10 watts so don't think it was overdriven the person i got it from said it was sitting for over a year so may when i got it it was not working the relay click in yes i can hear them and it has a red light on the box it comes on
the 2nd meter which has ma on the top and watts on the bottum read very little but the first meter when on volts does read about 2200 and when i put the meter switch on screen it goes all the way over
That's a problem, of course. Could be bad tubes (likely) but could be something else, like something damaged in the metering circuit.
You have not described the system though -- really, at all. What kind of relays are being used? Do they "key" properly when you transmit? Does the Ip (plate current) rise to about 120-160mA when you key the amp without applying RF drive? What does grid current indicate? I need a LOT more information on all of this to try to diagnose a problem remotely...