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[Amps] L7 problems

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Subject: [Amps] L7 problems
From: "Paul Kraemer" <elespe@lisco.com>
Date: Thu, 8 Dec 2016 20:52:20 -0600
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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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