Grounding the grids isnt any help.
With that idle it appears that both tubes are working to some degree.
The grid chokes are 3 pie's and should be equally seperated. It they are
touching it is a sign of a tube gas arc. Also carefully inspect all 6 grid
capacitors for cracking. If you have a good analog VOM or VTVM check for
leakage.
Carefully look at the input and output switch for melted contacts.
If the power falls off as frequency goes up that is often a sign of weak tubes.
Do you know the history of the amp? When did it last work 100%?
Carl
KM1H
----- Original Message -----
From: Rahul Sathe
To: Carl ; amps@contesting.com
Sent: Tuesday, September 08, 2009 1:39 PM
Subject: Re: [Amps] SB221 Low Power Help
Carl,
Plate Idle Current in SSB position is ~ 120 to 125 mA. I have to check the
grid chokes as suggested by you, how abt grounding the grids directly without
chokes?
Checked the tubes... there is no short.
The coaxial cables looks ok, as the VSWR is far better on other bands like 40
and 80 input can be tuned on 40 and 80.
Will run the amp with individual tubes as suggested by dan.
I am not sure if it is a European build, however it is wired for 230 V no way
to find out if it has 50Hz transformer.
Thanks for all the help.
Regards /// Rahul
On Tue, Sep 8, 2009 at 10:50 PM, Carl <km1h@jeremy.mv.com> wrote:
A couple of things to check Rahul:
1. Plate idle current should be 120-150 ma in the SSB position.
2. One of the grid chokes is open from a tube arc or short.
3. If it has always been this way since built the coax cables going to the
input bandswitch may be reversed.
Input VSWR is a function of plate current & voltage so with one defective
tube it will be quite high and out of the networks tuning range.
Dont go ripping things out, that 100W should be more than enough at 1000W
out.
Also, unless it is a European built/kit amp from Heath UK, or supplied with
a 50 Hz transformer it will have to be run at reduced power to keep the
transformer from burning up.
Carl
KM1H
----- Original Message ----- From: "Rahul Sathe" <vu3raz@gmail.com>
To: <amps@contesting.com>
Sent: Tuesday, September 08, 2009 11:32 AM
Subject: [Amps] SB221 Low Power Help
Hello Experts,
My name is Rahul call VU3RAZ. This is my first post to the list. I am
seeking help on fixing my old SB221.
as such the amp works on all bands, no arc or parastatics, tuning and
loading is smooth and easy.. I am not able to load the amp to full rated
plate current even with a 100 W drive with my 756 Pro3. Input VSWR is
high,
I tried tuning the input tuned circuit but no luck on few bands (20, 15).
With the 756 ATU the things improve but I can not get plate current
beyond ~
400 mA. Even tried disconnecting HT from each tube to see if one if the
3-500 is dead... both work but even with single tube I am not able to load
the amp fully. Am I looking at dead (weak) tubes ... can both of them show
exact same behavior? Or I am missing something I am not sure if I need to
re
build the PSU (replace caps)
Please help...
73
DE VU3RAZ
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