EF85 substitute for EF89

EF89 R.F. pentode tube with variable transconductance are very common use as I.F. stage amplifier and tunning R.F. SW stage amplifier.

When I directly substitute EF89 by EF85 (also RF pentode tube with variable transconductance) either in I.F.  stage or tunning R.F. amplifier stages for some Philips and other manufactures radio models, the output signal will suddently increasing ! When I fixed AVC and adjust AVC voltage level, EF85 appears more amplification than EF89.

So I think  EF85 may be better than EF89 for these stage applications.

According to EF85 tube data shows:

Transconductance S = 6.0 mA / V ;  Amplifacation factor : ug2g1 = 26 

@+250 Vp,  +100Vg2  &  - 2.0 Vg1

 And EF89 tube data shows:

Transconductance S = 3.5 mA / V: Amplification factor : ug2g1 = ~19

@+250 Vp, +100 Vg2  & - 1.95 Vg1

C anode-g1  and Cg1-heater  of EF89 is 0.002 pf  and 0.05 pf (max.) both  are less than EF85's 0.007 pf and 0.15 pf  (max.), It seems better use EF89 for higher ferquency design, but SW band range  beyond 22MHz max. EF85 also can handle it easily and output more amplification of signal .

Need anyone give more suggestion to me, Thanks!

 

To thank the Author because you find the post helpful or well done.

Hello Don,

there were several reasons why the EF89 was created. For example in IF-amplification stages in combined AM- and FM-receivers: for stability reasons and for keeping the filter-courves constant while using AGC/AVC, the transconductance and the capacities were reduced compared to the earlier EF85 model. But that does not mean that the EF89 always is the better tube. Maybe that the EF85 is better for some RF amplification stages.
Additional important notes:
- Direct replacement is not always possible, because the tubes have different pinouts (pin 1)!
- ug2g1 is not important for the stage amplification as long as the g2-connection is grounded by a capacitor. In first order the stage amplification is depending on the transconductance, the inner resistance, the quality of the connected resonant circuit (or working resistor), the capacitive loads and the feedback conditions.
- Normally it is highly recommended not to change any tube types. However, if you do, then a higher amplification factor could cause overmodulation of the following stages or degrade the AGC/AVC-characteristics or the S/N-ratio.

Andreas

To thank the Author because you find the post helpful or well done.

May be I forget the impedance matching problems ,  from previous stage like ECH-81 coupling via IFTransformer to IF stage then to 2nd IFT and demodulator detecting stage, higher gain will also decrease bandwidth of IF amplifier, and causing distortion audio output.

Thanks for your advice!

Best Regards

Don Chuang

 

To thank the Author because you find the post helpful or well done.