Please could anyone familiar with building 3-500zg linears help or advise me as I am getting very confused over blower output measurements.I am cooling my tubes identical to the Ameritron AL82. The blower is mounted above chassis and air is directed through a hole to the Eimac tube bases.I am using the correct Eimac bakelite bases and Eimac glass chimneys.Using a portable anemometer the air flow through the bases is approx 75 ft/minute but if I measure in the cfm r ange I get a very lowreading below 1.I am just not understanding cfm and the maths.!
While testing recently one of the 4-400A tubes (old tubes picked up at rally) overheated and filament burnt out.As the tube was old could have been due to age but this has made me wary as I don't want to risk my new 3-500zg tubes. Unfortunately I can't get info on my Blower to find air specifications but does 75.0 ft/min sound reasonable ?
Any help or advice would be really appreciated Many thanks,
73, Mike G3XLB
A simple piece of U shaped (clear) plastic tubing connected to the chassis rear apron ( away from the socket) with water in the U will suffice. With blower on, measure the height difference between the 2 x water levels. Compare that measurement to the Eimac data. It's just a hb manometer. The commercial version is called a magenhelic gauge, made by dwyer and other's...and used in the HVAC industry. Mine reads 0-2" pressure. I use it on my hb 3CX-3000A7 amp, 1.2" is the spec They come in various ranges. Some versions have an alarm built in, if the pressure drops below a pre-set threshold.
I have had several 4-1000's, when the fil was 1st turned on, the fil went super bright..and burned out...with a ton of smoke swirling around inside. These tubes had...'gone up to air'. They had leaked over the years.
As far as fil inrush, as other's stated, the cold resistance is typ 1/10 the hot resistance. Eimac states to limit the inrush to no more than double the normal operating current. This is where you have to be careful if using a fil xfmr with greater ratings. Larry, W7IUV, used a 30 amp fil xfmr on a single 4-400 amp. That xmfr was capable of a lot of current, when used with a single tube...right at turn on. He destroyed a few tubes before installing step start in the primary of the fil xfmr.
I added a hb step start, externally for my drake L4B ( 2 x 3-500Z) amps. Installed in the 240 vac mains. This step starts both the HV and the fils..at the same time. 20-25 ohm resistor used. In normal operation, the tubes will come up to aprx 2/3 to 3/4 normal brilliance. Once resistor shunted a few secs later, B+ jumps up the last bit to max, and tubes go to full brilliance.
On the bigger hb amps, either a manual or full auto delay is used for the blower. IE: B+ is shut off, then the fil xfmr.... but the blower stays on for XXX mins. Called blower over run. Easiest method on a hb amp is to use 2 x toggle switches, 1st one for the blower, and 2nd one for the fil. Output of 1st toggle feeds the blower and also the input to the 2nd toggle. Simple mechanical interlock. IE: blower can be turned on, with or without the fil on. But no way to turn on just the fil.
Be careful with blower specs..esp the curves for 50 hz and 60 hz. When used with 50 hz, the blower speed is reduced a bunch.
I bought 4 of the EBM papst blowers as used in the AL-82, except I bought the 230 vac version. For some reason the 230 vac version was not much more than 1/2 the price of the 115 vac version. ( mouser / digikey). EBM provides graphs and specs for both 50 hz..and also 60 hz.
I replaced the 4 x oem drake blowers in my 4 x drake amps. The ebm blower is aprx 2 1/2 times more airflow. I can easily vary the speed for different applications. I can also use a reduced speed on RX...after a 1-6 second delay. IE: vox drops out, and a few secs later, blower drops in speed. Useful when tuning around and extended RX periods. Very little air required when on RX. Less noise with the lower rpm. The ebm papst blowers I bought are 'quiet' to begin with at max rpm.