The AM Forum

I am working on a SX-25.  Most of the tubes are metal shell and the audio tubes are 6l6s instead of 6f6s.  Some of the tubes are weak.  2 questions:

1) Should I replace the metal tubes with metal tubes (6sk7) or can I use the glass equivalents (6sk7GT)?
2) Should I replace the 6L6s with 6F6s?

I know the metal tubes are original, but they don't glow  :(


Tom - AB3FL

Metal ones work fine, no reason to switch.

Yes, replace the 6L6s with the proper tube - they put a much bigger drain on the power tranny (both fil and B+ current).

JN provided solid advice on both counts.  These power transformers don't have excess capacity to start with so you don't want to stress them further. 

Make sure to replace C36, C38 (coupling caps that keep DC off the grids of the output tubes) and C39 (output stage cathode bypass) as leakage/failure in any of these will greatly increase B+ current draw potentially damaging the power and output transformers.  This general advice applies to any vintage gear.

Since the S meters are difficult to find putting a pair of diodes across the S meter to help protect it from damage due to circuit failure is a good idea.

Equipment naming was much better back then, with a name like Super Defiant it just has to perform better  :)

In some cases either the metal or glass will interchange, they are electrically the same. But.......................... Some appilcations relied on he metal shell for shielding purposes. Look at the sockets UNDER the chassis and see if pin one is slobbered to the chassis or grounded period. (The metal shell is just about always tied to pin 1 on metal types) If so, you may need to stay with the metal ones, or put external shields on the GT styled ones to prevent oscilations or other wierd problems.

                                                   The Slab Bacon

Thanks guys!   I will just stick with the metal ones then and replace the 6l6s with the proper 6f6s

Tom - AB3FL

Another question.  The dial is a bit faded and I am not sure where the bandspread marks on the mail dial are supposed to be for 10-20-40-80.  Can someone please tell me?

thanks

Tom

btw it sounds great!

Tom,

For 80 it is just above 4.0 (4.02 as close as I can interpolate)
40 7.5 plus "a hair"
20 14.4
10  30.5

What kind of hair?  ;D   

Thanks.   Now I have to carefully remove the dial and clean it very carefully and put the lines back on it

Tom,

Here is a site with a scan of the SX-24 scale (it is the same as the 25):  http://www.ppinyot.com/Dialscales/dial_scales.htm

He also has other scans and provides information on using them to make replacement scales.

Here is the SX-25

Needs a lot of cabinet work.  What can I use to clean it without damaging the lettering/labels?


Tom - AB3FL

looks like someone oversprayed the cab  after taking some lacquer to the lettering. SX 25 cabs are like dirt, I'd just find a non working example with a good cab. SX 24's and 25'5 are still pretty cheap.

OK....6F6s are not the easiest toob to find.  Can I sub in 6V6s, they seem to run cooler than the 6F6s and just drop in.  If so, does anyone have any 6V6G's?


Tom - AB3FL

I have plenty of 6F6's and 6V6's. Email me direct.
Regards,
Gary

Thats not a radio Id put a lot of effort and money into. It is a moderate PITA to recap and check resistors and is a good training ground for a royal PITA such as a SX-28, 42, and 62. :o  When done it will still drift and have only moderate to poor  performance from 20M and up.  I have the SX-24 and 25 and they do make pretty shelf queens.

Carl
KM1H

OK......Where is the band 2 padder?   Manual says top of chassis, but I cannot find it


thanks

Tom - AB3FL

you just described most pre ww2 rxers with some notable exceptions pretty well.  :D

Id take a SX-17 over the 25, at least I cycle mine in with some other sets into one of the vintage positions where a SX-28 also resides.  Naturally there are some good prewar Nationals and the HQ-120 is no slouch with a few tube swaps and the SP-200 is a given even with its drift which at least can be tempered a lot.

Carl

Locate the 6K8 and you will find it between the 6K8 and the front panel.  It is almost directly in line with the main tuning knob.

thanks, I would have never found it

Tom - AB3FL

my sp200 never drifted enough that i could ever tell it was moving.  or my 400, my 110 did a bit though.  :D

Carl,

My SX-25 receivers came as a pair and one had the BFO converted to crystal controlled for USB/LSB.  It would also need a crystal controlled HFO to go with that so I converted it back to original and used the crystals in an SX-16 and SX-24 that were missing their optional single crystal filters.

I agree with your choice of an SX-17 (or 16) over the SX-25.  They perform well and have the cool spiral bandspread dial.

Read thru the WW2 QST's and see that there are several comments from servicemen complaining about Super Pro drift. It got so bad the AAF developed a crystal oscillator kit. Years ago I had a BC-779 and 1004 and they never stopped drifting on 20M or the SW stations on that band range, even when left on for several days.

Carl
KM1H

SX-25was my first restoration and I liked it. Had the same problem finding that padder. I was lucky, mine came out of a non-ham estate and it had sat in an basement with a bad oil burner> Net result was it has a fine coat of oil on it. It was a bear to clean up but well worth it. With that pp audio it sounds great.
Some tricky caps to replace and not the best 15 mhz+ performer but good for great on BC, 160, 80 and 40
73
/KPD