SB-400 Question: Power Output

[Kechkaylo Dave (ATLINKS)]

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 at 28MHz, 43W at 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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