Check the filament feed through caps. They are famous for being leaky to
ground and shorting to ground.
73
Jim W7RY
-----Original Message-----
From: Paul Kraemer
Sent: Thursday, December 08, 2016 8:52 PM
To: amps@contesting.com
Subject: [Amps] L7 problems
Group
I have an L7 that was sent to me for repair and I am getting nowhere with
it.
The problem is the grid meter has a small negative reading in standby even
with NO tubes installed.
If the antenna relay is keyed the reading goes to zero. The amp does work
if you ignore that pesky off scale negative reading in standby. The amp
does same thing on three different supplies so it is NOT the power supply.
I have an L7 of my own that is fine on the supplies so it is the L7 at
fault.
Here is an A/B comparison of the bad amp (customers) and a good one (mine)
The ONLY things I have been able to observe between bad and good is:
(1) Bad the center tap of the filament winding is off center by about
0.2vac. In other words, it should be the center and instead measures like
2.6vac one side to center and 2.8vac other side to center whereas good will
measure EXACTLY the same either side to center
(2) Bad there is a measureable 25mv (dc plus 60hz)to ground on the negative
connection
of the HV supply. Since that voltage is across a 1 ohm resistor it makes
the amount of negative grid current. If the amp is keyed the voltage goes
nearly to zero as it should be all the time. Good amp is ZERO all the time.
Same supply, same location, just swap the linear.
I hope someone can give some insight to this.
Thanks
Paul K0YTA
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