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previous: Lars Yeterday I changed Powertubes in my... -- 11/11/1999 11:39 AM

Re: Powertube change and Rectifier Q...

11/11/1999 7:35 PM
Ken Gilbert	Re: Powertube change and Rectifier Q... Lars, as to your question of why the idle current went up when you switched rectifier tubes, the replacement gz34 was new and had less internal resistance than the old one. Perhaps it is a different tube entirely, a rebranded substitute... Who knows? With less internal resistance, the normal current draw on the B+ rail causes less voltage drop across the rectifier. That means you end up with a higher voltage rail. This higher voltage rail will cause more current to flow through the output tubes, since the plate/screen voltage will be higher. The bias supply is a completely separate supply is was unaffected. Thus, with more B+, and the same bias voltage, the plate current went up. This is to be expected. As long as you made the bias voltage more negative, and kept the plate currents in check, there is nothing wrong. You will make more power with the higher B+, but probably not enough to notice. With 32 mA @ 388V, each 5881 is dissipating 12W or so. That's fine and they should last a long time. They are tough tubes. KG

11/11/1999 7:35 PM

Ken Gilbert

Re: Powertube change and Rectifier Q...

Lars, as to your question of why the idle current went up when you switched rectifier tubes, the replacement gz34 was new and had less internal resistance than the old one. Perhaps it is a different tube entirely, a rebranded substitute... Who knows?

With less internal resistance, the normal current draw on the B+ rail causes less voltage drop across the rectifier. That means you end up with a higher voltage rail. This higher voltage rail will cause more current to flow through the output tubes, since the plate/screen voltage will be higher.

The bias supply is a completely separate supply is was unaffected. Thus, with more B+, and the same bias voltage, the plate current went up. This is to be expected.

As long as you made the bias voltage more negative, and kept the plate currents in check, there is nothing wrong. You will make more power with the higher B+, but probably not enough to notice.

With 32 mA @ 388V, each 5881 is dissipating 12W or so. That's fine and they should last a long time. They are tough tubes.

KG

Replies:

Lars Thank you for the info. -- 11/12/1999 7:48 AM