Hi Richard,
Because I am going to be doing the same thing soon, would it be better to
bring the SB-221 up on a Powerstat without the tubes fitted. Also would it
be better to wire the amp for 220v and wind the powerstat up to 110v?
I was thinking that if the tubes were not fitted and it started to smoke, I
would at least know that the tubes were not at fault.
My situation is a little different as my SB-221 has never been used and I
have just finished building it (great fun by the way).
Any other tips will be greatly appreciated.
Mike K6BR
On May 7, 2006, at 1:52 PM, Dino Darling wrote:
> After cleaning the dust out of an old SB-221, I pulled out the trusty
> POWERSTAT to slowly bring up the amp.
>
> I checked the POWERSTAT with the VOM so I knew where 55 volts were
> and especially where 110 volts was. (Amp was received wired for
> 120V. I have not done anything to the amp other than to dust it
> out. It has been sitting for probably 20 years.)
>
> I'm not saying I did this right, but I started at ZERO, turned on the
> amp, and slowly brought up the POWERSTAT until the light came on, the
> tubes began to glow, and the fan was spinning. I let it sit there
> for a couple of minutes and brought it up a little further. The
> lights got brighter, the fan spun faster and the tubes glowed nicely.
>
> At about 75%, I got smoke! A whole lot of smoke! POWERSTAT to zero
> and I flipped the switch off!
>
> After removing the outer cover, I see where the 1mH choke on pin 4 of
> the inside tube (socket N) smoked.
>
> #1 Where can I get a replacement?
You need a new tube because that one that ate the choke has a grid-
fil short which caused the +110V PS - that is applied on Rx to the
fil xfmr CT- to short to gnd through the RFC, and that opened the
choke. Also, it is safer to replace the grid-to-gnd RFC with a 1/4A
fuse or a fusing R because if the RFC fails to open, the unfused fil
xfmr will melt down unless the amplifier is promptly switched off. .
> #2 What caused the failure (or where do I start looking)?
Such shorts are caused by the perpendicular EMF created during the
large pulse of cathode-current concurrent with an intermittant
parasite c. 110MHz. Basically, the 1830ºK tungsten filament can be
bent laterally when it is hot.
>
> Dino - K6RIX
> dino@k6rix.com
>
>
> --
> No virus found in this outgoing message.
> Checked by AVG Anti-Virus.
> Version: 7.1.392 / Virus Database: 268.5.5/333 - Release Date:
> 05/05/2006
>
>
> _______________________________________________
> Amps mailing list
> Amps@contesting.com
> http://lists.contesting.com/mailman/listinfo/amps
>
R L MEASURES, AG6K. 805-386-3734
r@somis.org
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
--
----------------------------------------
I am using the free version of SPAMfighter for private users.
It has removed 43 spam emails to date.
Paying users do not have this message in their emails.
Try www.SPAMfighter.com for free now!
_______________________________________________
Amps mailing list
Amps@contesting.com
http://lists.contesting.com/mailman/listinfo/amps
|