[Hammarlund] NOISY FRONT END TUBES

[Roy Morgan]

On Jun 26, 2008, at 10:41 AM, Magoo wrote:

> Anyone contemplating changing front end and mixer tubes in a  
> receiver would do well to have a look at the mods suggested for the  
> 75A-4 in the "Collins Compendium".

But beware.

A number of the mods included in the ER are NOT now recommended, for  
one reason or another. One example is the sheet beam tube mixer mod.   
The tube (is it the 7360?  I can't remember the number), is quite  
expensive now, and other mods work better.

Here are the mods done by Howard Mills to the 75A-4's he rebuilds.

Briefly, they are:
  1) The AGC mod
  2) New 1st RF amplifier tube (6GM6 replaces the 6DC6)
  3) New 1ST  mixer tube (6DJ8 replaces the 6BA7 -note: N0DMS uses the  
6ES8 instead)
  4) Replace 2nd mixer (6EA8 replaces the 6BA7)
  5) B+ removed from mechanical filters
  6) Audio amplifier feedback change.

Other things included in Howard's standard rebuilding include  
replacing many capacitors, complete PTO rebuild and alignment, Case  
and panel work/repaint, new dial scale if needed, and polishing of  
knobs and internal metalwork.  Replacement of the 2nd IF amplifier  
tank circuit with a mechanical filter is an optional mod.

> The tubes KM1H suggests are good choices; I changed the 1st RF and  
> both mixers in my 75A4 and the result was astounding.  My receiver  
> outperforms any boatanchor I have ever used.  The Compendium  
> articles give desired voltages to optimize tube operation and so the  
> front end change, at any rate, is not simply a drop-in.

Most or maybe all of the mods Howard does are found in "The  
Compendium".  It's safe to say that he has done more complete  
overhauls of 75A-4's than any one else.

Roy

Roy Morgan
k1lky at earthlink.net
Lovettsville, VA 20180