SX-101 Mark 3 RF amp substitution

[WA2ROC]

A really nice 101 Mark 3 followed me home from our club ham fest this past weekend.  Other than needing a 3 wire cord and minor alignment, the RF amp, a 6CB6, tests marginally on my equally as marginal Knight Kit tube tester.

Rather than order another 6CB6, is there a drop-in replacement suggestion for a better RF amp tube?


(Yes, I do have the 8 bolts holding the chassis into the cabinet)

TNX  
Dick WA2ROC

[KA2DZT]

There are a number of replacements, but the 6CB6 is probably the most common.  They were used as IF amps in TVs and should be easy to come by.  The fact that it test marginal doesn't mean much, use it and look for some 6CB6s later.

Fred

[Jim, W5JO]

What is the sensitivity of the receiver in with the current tube?  If it meets specs, then use it until you conveniently find a spare tube. 

[N8ETQ]

Hey Dick,

       Not sure if it would be "Better" but a 6DC6 or
even a 6AU6 would "Drop right in".  Other subs
include 6DE6, 6DK6, 6HQ6, 6CF6.

GL

/Dan

[Pete, WA2CWA]

The 6DE6 and 6DK6 have higher gain then the 6CB6. Using either as a sub would probably require re-alignment of the front end stage to take advantage of the gain increase.

[DMOD]

Hi Dick,

I vote for the 6KD6.

I used the 6DK6 as a replacement for the 6BZ6 in the Knight R-100 upgrade and found it to have more gain and less noise than any other Sharp-Cutoff pentode.

I tried many tubes and found the 6HQ6 to have too much shot noise for my taste.

Phil - AC0OB

[WD5JKO]

The 6KD6 is a sweep tube....perhaps you meant 6DK6? 

Jim
Wd5JKO

[DMOD]

The 6KD6 is a sweep tube....perhaps you meant 6DK6? 

Jim
Wd5JKO

Good catch.

Yep, can't believe I typoed twice. It should be the 6DK6.

http://frank.pocnet.net/sheets/049/6/6DK6.pdf

[WA5VGO]

This isn't as straight forward as it may sound. If you change to a tube with different cut off characteristics it can upset the AVC. Personally, I'd leave it alone.

Darrell

[N8ETQ]

      Play with the value of the screen resistor.. You can dial it in..
If the RF section is near dead your not going to get much AGC
action anyway...


/Dan

This isn't as straight forward as it may sound. If you change to a tube with different cut off characteristics it can upset the AVC. Personally, I'd leave it alone.

Darrell

[DMOD]

Looking at the SX-101 schematic, the screen voltage is set by a voltage divider.

As with most class A RF voltage amplification stages, the delayed AVC is filtered and isolated through a 1 Meg resistor and cap to ground.

The more important parameters are the inter-electrode capacitances and gain.

Comparing the 6CB6 with the 6DK6 we find.

Input capacitance: 6CB6 - 6.5 pF; 6DK6 - 6.3 pF
Output capacitance: 6CB6 - 2.0 pF; 6DK6 - 1.9 pF

Gm: 6CB6 - 8000 umhos; 6DK6 - 9800 umhos.

Rp: 6CB6 - 280k; 6DK6 - 350k.

Since Mu(Voltage Gain) =  Gm X Rp, the higher the Gm and Rp the better the voltage gain.

As far as the average transfer plate characteristic curves, I see little difference between the two tubes.


Phi - AC0OB

[WA2ROC]

I appreciate all the suggestions and comments regarding low RF gain in my SX-101.  But last night I did a little more digging.

Seems that 15 meters is the only band with low gain, meaning it's either the band switch contacts or the components associated with the front end of 15 meters.  The RF amp and mixer tubes seem to be doing their jobs just fine.

Interestingly enough, all the other bands have an adjustable coil and a trimmer cap that are adjusted to peak the RF and mixer stages, but 15 meters only have 2 trimmers.  There's gotta be a coil in there somewhere, so next time I'm in there exploring, that'll be the things I'll look for.

Thanks again!

[WA2ROC]

I just discovered something that should have been done earlier.

1- You gotta read the manual. (Yes, I actually read it)

2- You gotta read it in the CORRECT ORDER! (OOPS!)

The manual says you start adjustments with 160, then 80, 40, 20, 15, no, wait!

The front end adjustment procedure explains which L and C to adjust for the RF amp and mixer, and at what frequency you provide to do these adjustments.  

But you must adjust the 10 meter band before you adjust the 15 meter band.  They share the same inductors, so if the 10 meter band isn't adjusted right, there's no way to get the 15 meter band adjusted correctly.

Out comes the BC-221 again tonight and we'll try it again, this time in the suggested order.

No, the 221 is only used to provide a signal NEAR the alignment frequency, but not for absolute frequency adjustments.  But, it's close enough for this process.

[kt4ae]

You may have discovered this by now, but of the eight screws on the front, only the four corner screws hold the chassis to the cabinet.  The other four hold the front panel to the chassis assembly.  Be careful.

Only the corner four screws need to be removed to pull the chassis out along with the three on the underside near the back. 

Harry, KT4AE
Watkinsville, Georgia

[WA2ROC]

Yep, found that out last night. 

But the three on the bottom are not there.  Gotta replace them next time.

Thanks!

[kt4ae]

You're welcome.

Harry, KT4AE

[WA2ROC]

WHOOPEE!!!

I made the alignment on the 10 meter band first and then did the 15 meter band, just like the book says.

Such a difference! 

I used the original 6CB6 and now I can hear signals on 15 meters, (DX European stations too) and 10 meters also.  Not bad for 6 PM local time.  I tried all the other bands, finding they are fine, just to be sure.

I may replace the RF tube after a few sessions, but that can be done with only the top swung open.  Nothing too radical that may upset the alignment as it sits now.

Thanks to all for your help.

[WA2ROC]

Round 2 of updates:

There are a bunch of "Black Beauty" caps (BTW, they're not that beautiful, are they?) and several pinkish red caps that are of the paper and wax variety that everyone says to replace.  Well, I ordered replacements for all of them, using polystyrene metal film replacements of the same working voltage, from Digikey.  Using my company discount, they cost me about $13.00 for 24 caps, including free shipping.

Here's a few pictures of the before and after.  There are still a few BB's left but none in bad condition like some of the ones in the pictures.

[N8CMQ]

Hi, it's been a while, have you replaced the paper caps yet?

I am rebuilding an SX-101 Mark 3 and making improvements along the way.

You can modify the RF amp to a 6DC6 and mod the circuit to the SX-101A level. Without the
mod, just swapping the tube will not make much difference.

The mod is easy to do, and even Collins did it with some of their receivers back in the day.

You need to add two resistors from the SX-101A circuit. R1 and R6, which are 22 Ohm 1/2 Watt.
R1 connects to pin 1 of the RF amp, and R6 connects to pin 5.

While you are at it, the AVC can be improved by changing the AVC to the mixer. Lift R5 from
ground and connect it to the AVC line. R5 is on the mixer grid pin 7.

When you get ready to align the unit, follow the manual to the T. Failure to follow the directions
will cause the sensitivity to be very low on some bands.

Good luck with your SX-101!