The AM Forum

Well, I believe I have asked questions about this problem before, but I couldn't find info in previous posts.  So I will ask again.

To make a long story short....I have replaced the 75THs with 100THs in the modulator in my Collins 30K-4 transmitter.  The 100THs require considerably less bias voltage on them than the 75THs and....even after some modification to the voltage divider network in the bias supply,  I'm still having problems getting the proper bias voltage.  The 75THs required about -90 volts of bias and the 100THs want to see around -35 volts.  The modulator B+ is around 2000 volts.

I would like to come up with a simple regulator circuit to provide an adjustable bias supply that might provide enough range to cover both tube types....say
 -20 to -100 volts.  I would like to, perhaps, utilize one of those variable voltage regulator chips driving a power transistor, or perhaps some zener diodes.  My concern, however, is how well something like that will perform in a strong RF field inside the 30K-4 cabinet.  Any special considerations to prevent RFI problems....like bypassing leads, adding RF chokes, etc. ??
Any suggestions for a bias supply circuit would certainly be appreciated!

73,  Jack, W9GT

Hi Jack, sound too high tech to me.
Why not just a simple resistor voltage divider connected to the existing?
I'm thinking that you could hang one of those robust Ohmite sliding contact resistors under the chassis.
Or you could make it a little more convenient with a "Dial A Bias" pot.
I've got a few spare 75ths, but that was my plan when my stock runs out in 75 years or so.


(http://media.digikey.com/photos/Huntington%20Electric%20Photos/AVT200-1.0K.jpg) (http://www.machmat.com/sales/images/3r5ohmite2.jpg)

Hi Bruce,

I already relocated the pot in the voltage divider stack.....doesn't seem to work properly.  Bias is still too high.  I think a simple regulator might be a good idea and shouldn't be that hard to add.

73,  Jack, W9GT

put the 75 th's back in and dont worry b happy.  :D

75th heap mighty scarce there pale face.
Big Chief in the sky not more cause 75th to rain down on little injuns.

Translaton, good 75ths are as scarce as hen's teeth Derb.  :-[


Jack OM, is the problem that you can't get the bias voltage down, or that the voltage is unstable?
How about a VR tube stack?

Is the range you need really -20 to -100, or did you pick that so you could fine tune the bias voltage?
I didn't think the 100th were that picky about bias, get it close and it should work.

If you really want to go silicon state, what about a zener stack, you could series up several, pick off the voltages at the appropriate place.

Big surprise, Jack. Bias circuits seem to be one of the areas where Collins chose to cut costs, probably figuring the cost/benefit to an end user for a set-and-forget circuit wasn't worthwhile. You may recall my posts here with bias problems in the KW-1. They had two circuits, one for the 810s and one for the 6B4G drivers. Take a look at the schematic on the CCA site, it might give you some ideas.

http://www.collinsradio.org/archives/manuals/KW-1-(01-52)Sec7,ant-ref.pdf

Somewhere in the 30K data I have info on the 100TH conversion. Finding it will be the trick. If you need a spare set of 75THs, email me. Contrary to popular belief, there are a number of reasonable sources for these. TLs will work too, with...(wait for it)....an adjustment to the bias. 

Derb, you shoulda grabbed that bad boy out of the basement before it disappeared! At least the photos survive. Where did the 77 go?

Todd, did you find the 75ths you mentioned you had for Jack?

I thought I had one or two spares, but the one which I had at the barn has a broken grid pin connector on the tube. It is broken right at the glass, so impossible to connect to it. Thought of a solder gob, but I won't be able to get the broken pin hot enough to solder on to.

I was going to check at the shop, but if you've got two at hand, perhaps that will do the trick for him.