SB-102 mic level

[Emil Dular]

 Already tested the 6EA8 that came out and it tested strong and dropping the
filament voltage didn't make any difference at all in emission.  Looks like
I will have to just be systematic and look at everything you and others
mentioned.

> Set the filament voltage in your tester to 6.3 v, test the tube, then drop
> the filament voltage to 5.0 v and test again. If it drops precipitously,
> toss
> it.
>
 Then I would check and verify alignment.
>
> If that didn't fix it, I would check the other things I mentioned.
>
> Of course, the problem MAY be related to a resistor drastically changing
> value as it heats up...
>
> Or some critical voltage dropping as the unit heats up...
>


----- Original Message ----- 
From: "Kenneth G. Gordon" <kgordon2006 at verizon.net>
To: "Emil Dular" <wd4scz at AOL.COM>; <Heathkit at yahoogroups.com>
Sent: Wednesday, 25 February, 2009 17:06
Subject: Re: SB-102 mic level


> On 25 Feb 2009 at 11:06, Emil Dular wrote:
>
>> I have been running some 80 meter phone lately and the amount of mic
>> gain needed to peak the ALC has slowly gone up. 80 always did take
>> more than the higher bands. I swapped microphones from an HM-12, to
>> EV-638 to a D-104 and negligble difference. Swapped out the 6EA8 at V1
>> for a 6CQ8 which was always the old trick for low mic drive and no
>> difference.
>
> Try a 6GH8A. Works better than the 6CQ8 in that service.
>
>> In tune, you can turn mic level to about 3 o'clock and the plate
>> current will top out at 210 ma.
>>
>> Anyone else been down this road that has a list of suspects or likely
>> places to start troubleshooting?
>
> Yes. I have. With both an SB-102 and an HW-101.
>
> There are a considerable number of tubes between the mic and the
> balanced modulator, or parallel to that circuitry. Surprisingly, 
> practically
> any one of those can cause the problems you are mentioning. Even
> those concerned with the VOX circuitry can effect this.
>
> However, a few other areas to investigate would be, 1) alignment (tends
> to drift as tubes age), 2) driver (6CL6) going flat, 3) 6CB6 going flat 
> (2nd
> transmitter mixer), 4) finals going flat, 5) neutralization drifting off, 
> 6) your
> "isolation amp" (6AU6) going flat.
>
> I think if I had that problem, I would start with testing all the tubes 
> and
> looking for weak ones, replacing those.
>
> Set the filament voltage in your tester to 6.3 v, test the tube, then drop
> the filament voltage to 5.0 v and test again. If it drops precipitously, 
> toss
> it.
>
> Then I would check and verify alignment.
>
> If that didn't fix it, I would check the other things I mentioned.
>
> Of course, the problem MAY be related to a resistor drastically changing
> value as it heats up...
>
> Or some critical voltage dropping as the unit heats up...
>
> Ken Gordon W7EKB
> 

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