Collins 30K-4 Bias Problems

[W9GT]

Hello all,

Anyone have any experience with the Collins 30K series transmitters....changing modulator tubes from 75THs to 100THs??  I put 100THs in mine and reconfigured the voltage divider in the bias supply to apply less bias for the 100THs,  Problem I'm noticing now is some pronounced fluctuation of the RF final grid current with modulation.  I did not notice this before the conversion.  The bias supply is common to the modulator, final and driver stages, with different taps on the voltage divider.  All that I did was relocate the mod bias adj pot in the string to provide less bias.  Problem might be...when the pot is all the way down it effectively puts the mod bias tap and the driver bias tap at the same point.  Any suggestions other than adding a separate bias supply?

Thanks and 73,

Jack, W9GT

[c. mac neill w8znx]

Hello Jack

  Have 30K-5
changed mod tubes to 100TH's
did nothing to bias supply

went down and dirty
hung two wire wound resistors
on the back of the mod deck

works fb om

yours truly Mac

[W9GT]

Hello Jack

  Have 30K-5
changed mod tubes to 100TH's
did nothing to bias supply

went down and dirty
hung two wire wound resistors
on the back of the mod deck

works fb om

yours truly Mac

Hi Mac,

Did you just put resistors in series with bias lead to mod grids?  What values did you use?  Have you measured the bias voltage?  Where does your mod current idle with the 100THs?  I was shooting for 50 mA.

73,  Jack, W9GT