[Central-Electronics] 6550

[Jim Candela]

Gordon,

   While waiting for your new tubes you might make sure there isn't some other problem. Perhaps you have a tube tester, or can borrow one. An emission tester will verify if those big tubes are flat. In the 100V, a lack of screen voltage would bring the output way down.

  I have a tube audio amplifier here that uses the 7868 type tubes. These (and the 7591) are the baby brothers to a 6550. A quad of Sylvania 7868's have been in use since 1981 where they get used about 5 hours a week. Every 3-5 years I check the bias settings, and these tubes just don't die. 

  Back in 2003 I decided to use 7591's in my QRO Central Electronics project, and here again, and with lots of abuse, these old tubes from the early sixties will not die as the bias point remains stable.

  I did have a recent failure of a 6CM6 (9 pin version of a 6AQ5) in my Gonset G-76 transceiver. It went to zero emission one day to the next. In the rig where it was set up as a class A audio amplifier/driver, there was no cathode current. Testing it on my old Mercury emission tester showed zero emission. Replacing the tube fixed the issue.

  Some big power tubes with a indirectly heated cathode do have an issue where flakes of oxide from the cathode comes off and gets caught on the control grid. Here it gets hot, and acts as an emitter of electrons which in effect reduces the grid drive, and therefore the power output suffers. The effect is more pronounced on the higher frequencies. A tube like the 4D32 will do this.  I asked a wise physicist about the 4D32 problem, and he told me in the early stages it was beneficial to drop the filament voltage about 1/2 volt below 6.3 to lower the cathode temperature.  

   Maybe your 6550's are NOT flat??

Jim
WD5JKO

________________________________
 From: Gordon <gordon at n6wk.com>
To: w7fe <w7fe at cox.net>; central-electronics at mailman.qth.net 
Sent: Tuesday, July 23, 2013 12:51 AM
Subject: Re: [Central-Electronics] 6550
 

Thanks Stu for the info,
I just ordered a matched pair of the Russian Tung-Sol tubes. They are shouldered as you suggested.
I should have them in a couple of days.
I'll report back once I receive and install them.

73,
Gordon

On 7/22/2013 10:16 PM, w7fe wrote:
> When buying replacement 6550's for the 100V/200V, be sure to get ones with the shouldered glass envelope.
> I have found that the ones with straight-sided envelopes turn out to be too tall and will interfere with the RF cage cover.
> 
> I bought a pair with the shouldered envelopes a few years ago from the Russian guys who claim to be manufacturing per the original Tung-Sol specs and use the original box colors and artwork.  They have been working fine.
> 
> 73 de Stu, W7FE
> Henderson, NV
> 
> ----- Original Message ----- From: "Gordon" <gordon at n6wk.com>
> To: <central-electronics at mailman.qth.net>
> Sent: Monday, July 22, 2013 7:01 PM
> Subject: [Central-Electronics] 6550
> 
> 
>> I am almost finished with my CE-100V Restoration.
>> Today I discovered that my 6550 finals are bad.
>> I have about 8 watts going into the grids and less then 1 watt output.
>> Voltages check fine on the Finals. Where is the best place to buy a new pair? Or, is there a better tube that can be used?
>> Also, I notice that my meter action seems sticky. Sometimes it drops back past Zero, other times it stops above zero.
>> Anyone have a meter or movement to sell?
>> 
>> Thanks,
>> Gordon
>> N6WK
>> 
>> 
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