SB-400 Question: Power Output

[Stu Lyon]

Dave,
The problem may be that you are trusting the SB-400 meter indications too
much. Scaling resistors used in the plate current circuit are known to age
poorly and usually drift high. And even if all components were perfect, 250
on the PLATE meter would correspond to 255 mA cathode current of which 26 mA
is from the screen.

I calibrated the current meter in my SB-401 by placing a Calibrated 500 mA
meter in the positive plate lead** of the 6146s, tuning for 250 mA Ip on the
500 meter then adjusting the scaling resistor (R6 in -401) for 250 on the
panel meter. The plate current now reads correctly at 250 mA. A convenient
place to splice in the meter is at point G, Pictorial 4-3 -- the blue wire.
Also you should first change the six 10 ohm resistors in the 6146s cathode
circuit.

** BE CAREFUL -- you can get killed in here.

Next measure the plate volts with a DVM or some other accurate meter. The
plate HV is connected to the bleeder centertap (junction of the two100K/2
watt resistors) and is somewhat dependent on the match of the two resistors.
There are not many options for adjusting the meter reading when it reads low
except just taking note of correct vs. observed.

My rig is down at the moment (making another mod to the SB-301) but I know
it outputs at least 120 watts on 40 and 110 watts on 20 as read by a SA-2060
tuner. It is normal for the output to decrease as the band goes up.

----- Original Message -----
From: "Kechkaylo Dave (ATLINKS)" <kechkaylod at ATLINKS.COM>
To: <HEATH at LISTSERV.TEMPE.GOV>
Sent: Friday, December 12, 2003 6:10 AM
Subject: SB-400 Question: Power Output


To All:

I have an SB-400 that's in excellent shape, except, the problem is lower
than expected RF output power. The output I have on a trusted wattmeter is:

80M: 95W
40M: 80W
20M: 72W
15M: 60W
10M: 50W @ 28MHz, 43W @ 29.5MHz

So the RF output drops as I go up in frequency. The RF voltage at the grids
(pins 5) of the finals V10 and V11 is more than 35VRF on all bands. The
transmitter loads and tunes into a dummy load like normal.

The key-up HV is about 750VDC, but the key-down HV is about 650 to 680VDC,
depending on the band. On the 10M band, particularly, the cathode current in
the finals approach 295mA; more like 250 to 270mA on the other bands.

My questions are: what is the typical key-up AND key-down (properly loaded)
HV on an SB-400 or SB-401? Also, has anyone an idea why my RF drops like it
does as I go up in frequency even though there appears to be adequate drive
to the final amplifiers? This RF output scenario is the same whether or not
I use neutralized 6146Ws (new, probably 6146Bs) or my good old standby GE or
Tung-Sol  6146s that work in another piece of gear. The cathode currents are
lower (around 240 to 250mA) using the GE or Tung-Sol 6146 tubes, but the RF
power output is still the same as shown above.

Any suggestions would be appreciated.

David Kechkaylo, W8QIZ

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