Double checking bias results 500B

[Sam08861]

I have the unit open so thought I'd double check the bias. I installed an IBBA and got the following before and after.

Pre adjustment

----------V13------V14------V15-----V16
PIN 3-- 404.7------404.4------404.7------404.6
PIN 4-- 377.1------377.3------377.2------376.9
PIN 5-- .398-------.398-------.385-------.383
PIN 6-- -17.56,,,,,-16.99,,,,,-16.47,,,,,,-17.18
WATTS-- 16.1------16.11------15.6------15.5

Post Adjustment

----------V13------V14------V15-----V16
PIN 3-- 403.4------403.5------403.5------403.0
PIN 5-- .393-------.394-------.394--------.395
WATTS- 15.85----15.90-----15.90------15.92

2 questions.

Is my math correct?
Are the wattages at a reasonable range?

Sorry if this has been asked too many times, but I've read a variety of conflicting suggestions, mostly for 500C instead of B so thought I'd take the opportunity to double check while I had the unit open.

Pin 5 reading was taken both at the tube and at the IBBA test point and did agree.

Thanks!

Sam

 

[dcgillespie]

Sam -- Everything looks very good. For the best accuracy however, you should subtract about 3 ma from each each cathode current reading figure to arrive at the true current flow to the plate. The 3 mA being subtracted out is accounting for the current flowing to the screen gird, and therefore does not figure into determining the dissipation of the plate element. So, assuming that your cathode resistors are 10 Ohms, it means that current flow (post adjustment) to each plate is actually 36.3 mA, 36.4 mA, 36.4 mA, and 36.5 mA for V13, V14, V15, and V16 respectively. If you now use these numbers to determine the plate dissipation, you get 14.64, 14.69, 14.69, and 14.71 watts respectively. Against a rated plate dissipation limit of 19 watts for 7591 class tubes, it means that your tubes are operating at about 77% of their rating, which is a very good setting for producing good sound, and good tube life.

Super job!

Dave

 

[Sam08861]

Thanks Dave, always a huge help!

Cathode resistors are indeed 10 ohms and thank you for explaining about the true current flow. My IBBA pot for the level (as opposed to balance) of V15 and V16 was at the bottom of range and couldn't be adjusted lower, so that's a relief, not having to change resistors or add another pot!