[Boatanchors] HP-606 tube failure

[Ian Wilson]

Very interesting. I did notice that on power up, several of the filaments
glow immediately and
are (in my experience) excessively bright. Will see if I can locate and
replace C12.

73, ian K3IMW

On Thu, Sep 27, 2018 at 10:36 AM, Jacques Fortin <jacques.f at videotron.ca>
wrote:

> Hello Ian,
>
>
>
> This is good news because the 606A is completely different, power supply
> wise.
>
> Which 12AT7 filament is failing, and on which side? There is many so I
> just want to provide a good advice….
>
> On the 606B, the filament supply is regulated at 25.6V, have you checked
> that is working OK, btw ?
>
> A shorted pass transistor (Q3) will apply much more voltage to the
> filament stings that it is supposed.
>
> But if the regulator output is OK, let’s check C12 (a 6.8µF) in the power
> supply section.
>
> If needed, the filaments power turn-on can be delayed more by increasing
> the value of C12.
>
> The designed time constant is there is 462mS.
>
> If ever C12 is dried up, and have way less capacitance than it is
> supposed, the power “ramp” to the filaments will be faster, almost like
> applying the full voltage instantly, which is not good.
>
> Using a 100µF for C12 (for example) will increase the time constant to 6.8
> seconds, making the filaments powering “ramp” a lot slower.
>
> The 25.6V supply current limit is about 3A, but I do not think that this
> was designed to limit the filament string current at power up.
>
>
>
> *73, Jacques, VE2JFE*
>