[Johnson] Valiant Grid Drive Problem

[Dennis Petrich]

Hi Harry,

First off, you need the -70v to put the 6146's into class C opearation.  At -35v you are starting to approach class A which is not good.  (-50v is class AB1 linear)

As far as drive goes, without looking at how your Valiant was constructed it would be hard for me to say.  You might look at the coupling caps to make sure they are not leaky or changed value....

My Valiant gets enough drive on all bands so I have never had to fix a problem like that.  I do know that "how the driver/amplifier section is wired can have a big effect on grid current on the higher bands.  So if that was a kit it may have been wired wrong?

Ciao, Dennis, k0eoo



-----Original Message-----
From: johnson-bounces at mailman.qth.net
[mailto:johnson-bounces at mailman.qth.net]On Behalf Of Harry Williams
Sent: Thursday, January 03, 2008 12:15 PM
To: johnson
Subject: [Johnson] Valiant Grid Drive Problem


My Valiant has adequate grid drive through 40M. On 20 I have to run the drive control wide open to get 8 MA. On 15 and 10 I can not get enough. I have checked the tubes and replaced the 5763 as it showed an intermittent short cathode to filament but that did not fix the low drive issue. I checked the bias on the 6146's and it was
-70V as the manual specifies. Just for the heck of it I turned this back to -35V and my grid drive immediately came up to more normal levels. So first question is am I just compensating for another issue by doing this which is what I am thinking is the case. After reducing the bias I did some playing and it now appears that to load the final fully I need to set the grid current in the range of 10 - 12 MA to get 320 MA of plate current. So my next question is what the heck is going on?

Thanks and 73's,
Harry
W0LS
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