[AMRadio] BC-610 Modulator feedback - and notes


Rbethman rbethman at comcast.net
Sat Mar 18 09:14:32 EST 2006


Rick,

Additionally, there should be NO shield on the 2A3s.  The entire 
transmitter was designed to function with CONVECTION or OPEN air cooling.

There is only ONE tube that has a "shield".  It is the oscillator deck 
6L6.  It SHOULD be a metal jacketed tube.

The 1/4 X 20 bolts on top of BC-610(X)s were intended for the mounting 
of the BC-939(X) antenna tuners.  Since the BC-614(X) speech amp is 
fully enclosed w/o vents, I, for one, would be very hesitant to use it 
on top of the BC-610(X).  All the heat from below would add to the heat 
inside the speech amp.  The components "probably" weren't designed for 
THAT additional heat.  The BC-614(X) has a long enough connecting cable 
to place it on the operating bench.

Pay close attention to WB2FCN, Jim!  He restored my first one.  He has 
"carnal" knowledge of these BEASTS and has helped MANY get them up and 
going.

Bob - N0DGN

James M. Walker wrote:

> I'll bet you a nickel the bias supply has a problem, the plate supply
> for the 2A3s is not normal the plate supply is achieved by putting a
> negative voltage on the 2A3s cathode to raise hte plate to 360 volts.
>
> Sounds like the bias supply voltage is off. Check the manual and then
> the components that provide voltage to the 2A3s
> Jim
> WB2FCN
> YMMV
>
> ----- Original Message ----- From: "Rick Brashear" <rickbras at airmail.net>
> To: "Discussion of AM Radio" <amradio at mailman.qth.net>
> Sent: Friday, March 17, 2006 6:10 PM
> Subject: Re: [AMRadio] Modulator feedback
>
>
>> Thanks Mike.  I just made a discovery about the 2A3 A-F drivers.  I 
>> had a set of matched NOS RCA tubes in there.  The feedback seemed to 
>> be coming from the area of those tubes more so than anywhere else.  
>> So, I exchanged them for another pair I have and it seemed to help 
>> quiet the feedback. Obviously, microphonics in the 2A3's is a 
>> factor.  Next I looked for shields for the tubes, but have none, so I 
>> used a "poor man's shield", a couple of pieces of aluminum foil.  It 
>> helped considerably!  As a matter of fact, I could then advance the 
>> audio of the speech amp enough to achieve good modulation.  So, I am 
>> now on a quest to find some way of shielding the 2A3's.  I'm afraid 
>> my temporary foil shields would cause the tubes to over heat, plus 
>> it's not the best way of doing it.  Have you had any experience with 
>> this?  I wonder if the 100th's are also part of the problem.  I 
>> really think the "talk back" is coming from the modulation 
>> transformer, which I can live with.  I just have to get the feedback 
>> out of it.
>>
>> Thanks for your assistance and advice,
>> Rick/K5IZ
>>
>>
>>
>> Mike Sawyer wrote:
>>
>>> Rick,
>>>    The UTC may work so don't discount it. Those I and H model 
>>> transformers were only about half the size as the D and E models. I 
>>> can't think of the company that made them but they were 
>>> substantially different then the earlier models. As always, your 
>>> mileage may vary.
>>> Mod-U-Lator,
>>> Mike(y)
>>> W3SLK
>>>
>>>
>>
>>
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>>
>
>
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-- 
Bob - NØDGN
                       Bob Bethman - NØDGN
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