[Boatanchors] HP-606 tube failure

[Jacques Fortin]

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