ampage Tube Amps / Music Electronics	For current discussions, please visit Music Electronics Forum.

ampage archive

Vintage threads from the first ten years

Powertube change and Rectifier Q...

:

11/11/1999 11:39 AM

Powertube change and Rectifier Q...

Yeterday I changed Powertubes in my Marshall Bluesbreaker combo from Groove Tubes 6L6 to Sovtek 5881. I messaured the bias before I switch tubes, it was 28 mA. Something went wrong when I moved the amp and I accidently pushed the rectifier tube (Some Chines GZ34),something come loose inside the tube. I was lucky and found a new one (Ruby Tybes GZ34) at a local store in an hour. After the switch of rectifier tube I messaured the bias (with the old power tubes) and now it was 36 mA and the anod voltage 388V. Why ?????????????
I forget to messaure the anod voltage with the old rectifier tube.
After the rectifier switch I went back to do what I actually was going to do.
I switch the output tubes and set the bias to 32mA, the sound was improved in my opinion more compressed and with an earlyer breakup. I change the feedback resistor from 27Kohm to 78Kohm. First I put a 100Kohm pot instead of the 27 Kohm resistor, played and change the possition of the pot, when I was satisfied I messaured the value, it was 78Kohm. After the feedback change the amp got more distortion and maybe a looser feeling in the lower notes.

Lars

11/11/1999 7:35 PM

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

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

Page 1 of 1