Fw: [HEATH-TEMPE] SB400 - Low Grid Drive on 40 Meters

[Mark Johnson]

> ----- Original Message ----- 
> From: "Mark Johnson" <mvjohn at sympatico.ca>
> To: <G3OOU at AOL.COM>; <HEATH at LISTS.TEMPE.GOV>
> Sent: Thursday, August 13, 2009 10:20 AM
> Subject: Re: [HEATH-TEMPE] SB400 - Low Grid Drive on 40 Meters
>
>
>> Hello Bob, thanks for your note and thoughts on this interesting problem.
>>
>> In my case, the SB300 and SB400 set up, the problem occurs when in split 
>> mode; separate transmitter and receiver and in transceive mode.
>>
>> I went through the circuits and can find nothing. I spent a lot of time 
>> on the HEt Osc circuits as I was sure that was the source, but found no 
>> problems.
>>
>> Last night using an HP-410a VTVM, I measured the grid drive into the 
>> 6CL6, the RF Amp driver, and measured just under 2 volts RF into the 6CL6 
>> on 40 and over 2.4 volts RF on  80 and 20 meters. I have an old document 
>> that says the RF drive into the 6CL6 should be 1.5 to 2.5 volts. I 
>> measured the RF drive into the 6146's and get about 40 volts RF on 40 
>> meters and slightly higher on the other bands.
>>
>> I'm beginning to think that as there is full output on 40 meters, this 
>> grid drive meter problem is a "working as designed problem."
>>
>> I have discovered during this journey, the following;
>> 1) RG-62 interconnecting cable, cut to the recommended 24 inches, is 
>> important.
>> 2) Alignment as per the manual is relevant, but a final tweak when the 
>> SB300 and SB400 are interconnected is required with an eye on tuning for 
>> full output.
>> 3) Ten meters is a compromise as its so broad. I tuned up the transmitter 
>> driver and plate coils for max on the 28.5 band in the middle; that is 
>> around 28.8. That gave me fairly good output across the band. About 60- 
>> to 70 watts out on the lower part of the band, as measured by my watt 
>> meter and dummy load, and about 40 to 50 watts on the higher end of the 
>> band.
>> 4) That darn 8.65 mhz trap adjustment is tricky and has to be done right 
>> or you will loss output on one or more bands. The adjustment procedure in 
>> the manual did not work well for me so I fed the transmitter directly 
>> into my spectrum analyzer, using proper RF attenuators, and got an nice 
>> null adjustment.
>> 5) Soft finals can affect low grid drive. A new pair of 6146A's did a lot 
>> to improve output and grid drive.
>> 6) Interesting, I could never get a good balance adjustment on the SB400 
>> between the USB and LSB output as per the alignment instructions. I 
>> played with that for some time and realized that there was very very 
>> little relationship between that adjustment and what you saw on the final 
>> RF output side. I tried different USB and LSB generator crystals etc. but 
>> could not get a balance as per the manual even after changing that Pot, 
>> (The balance pot), some of the resistors in that circuit, and other 
>> related components. Funny, the SB401 manual makes no reference to that 
>> adjustment!!
>>
>> Thanks again Bob, for taking the time to reply.
>>
>> Mark V Johnson VE3DJU/VE3DDI
>> >> From: "Bob Burns" <G3OOU at AOL.COM>

>>  Hi All
>>>
>>> If you are intending to run the SB400/401 PA in class AB1 on SSB then 
>>> there should not be any grid current and if there is then the ALC system 
>>> should
>>> kick  in as that is how it detects excess drive
>
>>> Does your transmitter behave in the same way if you run it standalone -
>>> disconnected from the receiver? 73
>>> Bob
>>> G3OOU
>>>
>>> Subscription control - 
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>>>
>>
> 

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