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...
1 of the reays is a dow key the other has no name that i can see yes the key when i key them i use a foot switch because my radio doen't have a keying out on the back of it no the plate does not go up at all when you key it i see no grid current the relays are 120 vac wired in parallel they are dpdt relays
the screen current when plate switch is turned on goes to about 2 on the first meter in the tune spot but when you put in cw or linear spot it goes all the way over what are the 4 small tubes for they don't lite up at all hope this helps you
Not much, you need some local help from someone who is very familiar with 50 year-old amplifiers.
The 4 "small tubes" that you're referring to are gasseous voltage regulators used in the grid bias and screen supplies. They should "light up," but not with a filament (they don't have any), but with the glow of the gas inside them, which is purple or blue.
I owned a Johnson 6n2 Thunderbolt for years and am very familiar with it. But for someone who is not, this is not a great choice. It's from 1960 or so. It was designed to be used by "old fashioned" hams who had a lot of familiarity with vacuum tube amplifiers. I cannot envision any kind of "new op" who isn't a degreed electrical engineer figuring out how to actually use this amp. It's not simple, and one wrong connection, or overdriving it, or not properly loading it, can destroy the tubes.
it has to be bad tubes can you tell me the right way to tune it the manual don't tell you much well the 1 he gave me don't kinda faded i have reworked a sb200 and a clipperton-l i should see some plate current when i key it up
the only thing i can think of is the tubes where bad when i got it because i never put any power into it and when i did power it up never did see any plate or grid current i priced the tubes and found some for 100 for 1 i never put these kinda tubes in before i have reworked a sb200 and clipperton-l
also i tryed to put power into it with the radio 3 watts and for some reason the power on the radio dropped down but when i turned the amp off the radio power was back at 3 watts can't figure that out
If my bad memory serves me those 4 tubes are the voltage regulators for the screen supply, and if screen is there but no HV it will destroy the 250b's. I can't remember if it cooked the VR tubes the one time I had it happen, glitch resistor in the HV line went open after a flash over.
Since the pair of RF tubes appear to be shot I would consider purchasing a pair of 4CX350A instead 4CX250B, this will give you a higher Plate disappaition 700 W versus 500 W for the same size tube. I remember years ago, many hams would do this for more output on 2 m SSB.
Also you might contact ICOM as they have a Molex type plug that you can wire up for an external PTT circuit using a NPN transistor, a 12 VDC Relay and 1N4001 Diode across the relay coil. This way you could eliminate the Floor mounted foot PTT switch and just use your IC-251A.
Since the amp has external T-R relays anyway, installing an attenuator in series with its input is really easy and much better protection for the tubes against possible overdrive. The IC-251A (and many rigs) "adjust power output" just fine on FM but not so well on SSB and a voice peak may exceed the power you think it should be.
If you exceed the grid dissipation (or screen dissipation) rating of those tubes even briefly, they can become doorstops. With my 6n2 T-bolt I always used a "pad" (resistive attenuator good for VHF, about 6 dB attenuation) in series with the input (between the external Dow Key relay and the "input" port of the amplifier) to help prevent accidental overdrive. Those tubes are not forgiving at all.
Ditto re the B+. If the high voltage doesn't reach the tube anodes and screen voltage is still present, and cutoff bias is removed from the grids even very momentarily, that will take out the screens in the tubes and again: Doorstops.
Be careful changing the tubes. Before installing new ones, I'd thoroughly check "everything" about the amplifier, including assuring the 2200V is actually getting to the tube clamps. The tubes must be inserted by pushing them gently "straight in" to their sockets without any tilting, or you can break the spring fingers at the circumference of the sockets. The sockets are special. While you have the tubes "out," I'd recommend cleaning the ceramic chimneys (two of them) and also the blower cage (fins) to make sure they're free of dust. Make sure the blower spins freely, and when power is applied, it turns fast without wobbling or making any mechanical noise.
what kinda 12vdc relay you talking about and when you say across the relay coil sounds like something i would like to try and where can i find a 6db resistive anttenuator at