By JAMES R FORGIONE K6JRF
Operate a Linear Amp w/ an 8877 Safely . . .
I’ve operated an linear amp for 35+ years that uses an 3CX1500 (8877)
tube and I’ve followed some simple guidelines.
These are some limits which you should observe and that
will guarantee that it operates w/o destroying” the 8877 tube.
1)
Never run more than 50ma pulsed grid current. This would be the observed
meter deflection during CW or SSB operation.
2)
Do not exceed 75ma grid current even for very brief periods during key down
tune-up. Lower the drive signal
input since the tube is now in (or close) to a compressed operating point.
3)
Do not apply HV or draw cathode current unless the heater has been within the
factory specified range for 3 - 4 minutes.
I’ve added this into my homebrew 8877 amplifier as the ckt schematic below
details.
The ckt configuration prevents a high potential to exist between the cathode and
filament. The tube spec is 150V peak.
The filament is at DC ground by virtue of the center tap ground
connection on the filament transformer but follows the RF potential due to the
bifilar wound choke.
Also the heater warm-up has been increased to 4 – 5 minutes to ensure a properly
“heated” filament.
The following summarizes the 8877 testing that I’ve done using the Henry 4K as
the tube’s test bed.
Powered the tube for 4 hours total. Hi-voltage "soak" for 10mins.
Tube: 9129 date code; Eimac YU-158 -
Test Results:
Quiescent current: 200ma . . .
Drv
Pwr Output Grid
25W
1000W < 5ma
35W 1500W
< 5ma
45W
2000W 20ma
55W
2250W 25ma
65W
2400W 40ma
[Tests @ 14.180Mhz with the power measurement made by
transfer-calibrated Nye/Viking wattmeter into a 50 ohm dummy load]
8877 Tube Test: 5/25/09
Tube:
7612 date code; Eimac 3CX1500A7 - [Tube in
"Henry 4K" floor amp]
Test Results:
Quiescent
current: 200ma . . .
Drv Pwr Output Grid
20W
1000W < 5ma
30W
1500W < 10ma
40W
2000W 20ma
50W
2200W 25ma
60W
2400W 40ma
8877 Tube Test: 4/9/09
Powered the
tube for another 1.5 hrs for a total of 6.5 hours. Sat w/ Hi-Voltage
(4200V) on for 30 mins.
Tube: date not recorded
Test Results:
Quiescent
current: 200ma . . .
Drv Pwr Output Grid
40W
1400W < 5ma
50W
1700W < 5ma
55W
1850W 5ma
70W
2200W 10ma
75W 2500W
25ma
[Tests @
14.180Mhz with the power measurement made by transfer-calibrated Nye/Viking
wattmeter into a 50 ohm dummy load
