35V is NOT correct for the 4X150 screen, go back to the way it was after you
get the parasitics settled down. The screen should be fed from a series of 2
or 3 VR tubes and be around 300V. A 4X150 shouldnt require neutralization
on 2M and I see no mention of it in the rather poor BAMA manual.
Be sure you have proper bias on the control grid otherwise the tube will
take off.
Carl
KM1H
.
----- Original Message -----
From: <TexasRF@aol.com>
To: <n1miw@cox.net>; <Amps@contesting.com>
Sent: Sunday, March 30, 2008 10:47 PM
Subject: Re: [Amps] Gonset 903 2 meter amplifier question
>
> Larry, your descriptions sounds like a classic example of self
> oscillation.
> The usual fix for that is adjustment of the grid/plate neutralization
> circuit.
>
> There is plenty of info about this in any ARRL handbook printed in the
> last
> 50 or 60 years.
>
> 73,
> Gerald K5GW
>
>
>
>
> In a message dated 3/30/2008 9:15:31 P.M. Central Daylight Time,
> n1miw@cox.net writes:
>
> Hello guys, trying to get one of these amps back on the air, and I am
> having a little bit of difficulty. I hope this is an easy question for
> some
> of you, and would appreciate the knowledge feedback! Here's the history
> of
> my amp... purchased on ebay with the hopes of learning about tube amps
> from
> restoring it, but the electrical problems were more elaborate than I was
> told (said it needed the HV transformer, and would be including the spare
> with the amp). I found the diode stack blown, the capacitors were moldy,
> so
> I knew they needed to be replaced. The HV xformer is fine, so I have a
> spare
> now. Also needed to replace the tune resistor since it was broken (4k 100
> watt ceramic). The screen supply xformer is reading 140V instead of ~35V,
> so
> I installed a small xformer for that circuit. Now, she powers up fine
> with
> no sparks or blown fuses, so I think I'm getting somewhere with it. My
> problem is this... when I turn the HV on, I see an output on the
> wattmeter -
> about 80 watts or so, with my plate current at ~70ma (70ma is spec for
> the
> 4x150a). I am NOT transmitting into the amp when this happens! If I
> provide
> a 5-10 watt input at this point, nothing happens on the bird. The dummy
> load
> IS heating up also. Now, if I try to tune the amp with an RF input
> present,
> I get the bird to INCREASE in what I'll call "stray RF", and the meter
> will
> also DECREASE more and more as I tune the amp. As an example, if the bird
> reads 50 watts - with 10 watts input, no change. Tuning the amp more, if
> the
> bird reads 150 watts and I apply an input of 10 watts, the needle drops
> to
> 130 watts. With 200 watts with no input, after I apply the 10 watt input,
> the meter drops to ~75 watts, but at this point, my plate current is WAY
> over 350ma, and the fuse blows. It almost seems like something is
> backwards,
> but I don't know what. Where is this RF coming from if I'm not giving the
> amp an input? Is there a voltage somewhere that's too high, which is
> causing
> the "stray RF"? I've also heard a carrier on my receiver which "comes &
> goes" while I'm tuning this thing. Again, you experienced guys might have
> fixed something like this before, but I'm too new to understand what's
> happening. Any help will be appreciated. Oh, I did try 3 different
> tubes -
> all with the same result. They are known to be working from my FAA
> amplifier. Thanks... Larry
>
>
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>
>
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