[Amps] Cetron 811a.

[Eddy Swynar]

Hi Ulf,

Well, I can only speak of my own personal experiences here with a 4 x 811A amplifier that I built here some 31 years ago...FWIW!

First-off, I used surplus tubes in mine---so take what I say with a grain of salt, or ignore it altogether. 

(1) One of the  811As that I purchased had, unknown to me, an internal short from the grid to to the filament: the moment I applied power to it, the grid meter pegged itself, & I had to get a new one. Lesson learned: check all of the electrodes for shorts BEFORE installing the tube.

(2) When I initially attempted to run all 4 tubes under load in the amplifier, the very second I hit the key I noticed that the plate of one tube would appear redder than the rest---then in seconds it would turn yellow, then white, and then a small hole the size of your smallest fingernail would appear right in the middle...and a fuse would blow. Lesson learned: MATCH each individual tube for resting current. Failure to do so will cause a "discrepant" tube to "hog" the load, & it'll self-destruct in short order.

I can't even recall now how many 4-pin coil forms I ended-up getting out of that exercise, before I learned the importance of matching tubes for resting current! But again, these were all surplus jugs that cost me probably two bucks a pop ("pop"---literally!), so the lesson---though still rather expensive back then---didn't exactly break the bank...

One more step to do as a precaution might be to "bake" the tubes for an hour, or so, with just their filaments on---no plate voltage. I've never done this personally, but have read that it's a good idea with surplus stock...

There you go, Ulf---at least ONE taker to your query, so far...

~73!~ de Eddy VE3CUI - VE3XZ



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On 2011-12-05, at 5:36 PM, Ulf (SM0NOR) wrote:

> No takers?
> This ought to be a very simple question :)
> 
> 
> 73's
> 
> Ulf (SM0NOR)
> ulf at sm0nor.com
> 
> 
> 
> On 4 dec 2011, at 11:23, Ulf (SM0NOR) wrote:
> 
>> Hi all,
>> 
>> I recently bought 4 Cetron 811a from Nebraska Surplus, date coded 1988.
>> I intend to put them into a home brewed amp that is real close to smoke testing.
>> Is there any start up procedure that is recommended for NOS 811:a? I have not come across any recommendation regarding this so far. But before, I put them in and apply filament and HV, I thought i'd ask here.
>> 
>> Thanks!
>> 
>> 
>> 73's
>> 
>> Ulf (SM0NOR)
>> ulf at sm0nor.com
>> 
>> 
>> 
> 
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