The AM Forum

OK- I'm taking up stamp collecting..

I wanted a simple 160 to 75 meter converter to use with the Drake 2B. (I have tried some of the suggested 2B mods with so so results)

I found this one in 5/69 QST. It is based on a series of QST converters by Deane that seemed to have been very popular.

It converts.... sort of.
It is impossible to peak anything correctly and the band is full of birdies, images and the entire mess is unstable. Changing the RF capacitor tuning changes the output signal to a different frequency. Trying to peak up the tuned circuit in the converter changes the output frequency. (The xtal is happily oscillating away on its frequency) I found a number of circuits almost identical to this one in QST and the schematic seems to be generally the same except for the L/C values.

I have followed the circuit to the letter and checked, rechecked wiring parts and values.I have tried different tubes out of the junk box. I have made sure the tuned circuits all dip correctly with a GDO, installed the shield as specified and no joy. Tube shields are installed and all grounds check out. I did a search for indications in  QST that there were errors but did not find anything. Voltages all look OK.

I have walked away for a day and come back to look it all over with no errors to be found.

Any thoughts on how to stabilize this beast?

Its only 160 meters. God forbid I go UHF up to 75 mtrs and try to build something!

Carl,

Have you tried building a copy of the Drake 2LF?  I believe the info is on BAMA; otherwise I will be happy to email it to you.

My 2B came with the 2LF and although it isn't my main 160 meter receiver it does a credible job.  The way it is set up is to convert 1.8 - 3.5 Mhz. up to the 10 meter ranges using a 26.2 megahertz crystal although a better choice might be to use a somewhat higher frequency crystal so you could also cover AM broadcast if you are interested.  My 2LF came with both crystals so it converts the .1 to 3.5 range up to the 10 meter ranges.

Rodger WQ9E

Are you sure the xtal is oscillating?  Peaking the tuned circuit should have no effect on the frequency, which is determined by the xtal.  The xtal oscillator stage is untuned, so it should start right up if it's going to oscillate at all, and not be affected by the tuned circuit.  I don't see how that circuit would work, since there is no coupling between the xtal oscillator and the mixer stage.  It looks like the only way the oscillator signal is injected into the mixer is through stray coupling between the sections of the 6J6.  Shouldn't there be a cathode resistor or rf choke or something to introduce some impedance at the common cathode for both stages in order to inject enough signal from the xtal stage to modulate the incoming signal at the mixer stage?

Hi Don,
Thanks for the help


Yep. Can hear it loud and clear on 5.575mhz. Stops when I pull the xtal out so it is me!


That seems to be the plan. there are at least 3 construction articles based on this design in QST so it must have worked. However all of those were converting down to 3.5 mhz.

However this is the only one that calls for a shield between the grid/cathode and screen /plate sides of the 6AK5 so there must have been something going on when the initial one was built.

Even if I remove the RF amp and feed the converter from the plate of the 6AK5, via a small cap and a sig generator the problem still exists, just weaker.

W1JZ emailed me and said he had the same problems. He said "I had N2ZAB and W1VZR look it over and they said Put it in the trash".

It looks like the original author may still be around as K3DQ, but QRZ has changed their policy so I cant see his email without registering.

Here is the original circuit for 10 meters to 3.5 from Nov 52 and thir is another similer one in 1954 so it must have had some merit...


73

Carl
/KPD

I have tried a lot of 80/160 converters and none of them repeat NONE of them perform as well as any commercial RX that was designed to operate on those bands.....Anyone else have the same experience?...

Carl ... the  6J6 interelectrode spacings are tiny ... I am curious how much current it is drawing with/without the osc side working ... the circuit seems to suggest 'wide open' operation ... the RCA tube manual (as I recall) specs 17 mA plate current .... interesting problem ...73 ... John

Hi John,
I'll take a look this weekend and let you know. Something is taking off thats for sure.

Hi all,
Just because it was in several forms in QST doesn't mean it ever worked. I remember vividly my building a p-p 4 - 125A amp for 2 meters based on a handbook design (it was in the ARRL handbook for a number of years around 1950+) and spending maybe 40 or so hours trying to get it to work in any usable fashion. Some years later after I had pretty much given up and put the thing on a shelf  I met the ARRL VHF columnist Ed Tilton, W1HDQ, who had authored the article and questioned him about my problems. He told me much of what was in QST and the handbook was never tested, the criteria was - it had to look professionally built, not much else mattered. Working as described was another subject. Ed told me he never got the pp 4-125A amp to be stable on 2 m, but never bothered to tell anyone else that and it was in the handbook for at least 8 - 10 years! The ARRL at that time featured a lot of lab-built devices, from what I have heard over the years from those who should know maybe 50% of all designs ever worked as promised, and maybe 25% were never tested at all. But, they all looked very nice.

73
Don, W2DL   

I'll bet the RF amplifier is oscillating. Replace the cathode resistor with a 5K pot and run the resistance up and down. The gain should vary smoothly, but I suspect you will reach a critical point where the amplifier takes off.

73,
Darrell

FYI, Jim , K3DQ don't have an email address listed in QRZ...

Try reducing the "Q" of the plate circuit of the 6AK5...IE,
use a slug tuned coil and eliminate the compression cap.

Or, eliminate the tuned circuit completely, stick an RF choke
in there...

The 6AK5 is acting as a TPTG osc.  What do you do to make
an amplifier NOT become an osc?  You remove the feedback !
Or, you can work up a neut. circuit between the grid and plate...

Just my 2 cents worth...

Don,
Sorry to read that as I always thought it was a gold standard of sorts when it got to QST or the Handbook. Seeing it more then once to me was a sign that it was a popular and successful circuit.


Darrell
Thanks, I'll give that a try. I think both tubes are taking off as it is finiky even with the RF amp out of the circuit- that is feeding the antenna  through a cap directly to the tuned circuit.


Ralph,
Thanks for checking

Both it and the 6J6 are not playing nice.  I'll get to this- How hard can it be?

Thanks again all for the ideas. I'll keep you posted.

That's a pisspoor design.

Couple of suggestions:  both sections of the 6J6 (osc and output of the mixer) could used tuned circuits.  I'm betting that the 6J6 has all sorts of parasitics.  Yeah, it's oscillating at the xtal freq but it's probably also oscillating a bunch of other places.  Instead of those stupid RF chokes on both sides of the 6J6 go with resonant circuits, the B+ side bypassed to ground.

Bypass each leg of the filaments to ground to eliminate coupling thru the fil lines between the 6AK5 and 6J6

Put a 50-100 ohm resistor in the cathode of the 6J6.  This oughta quiet things down but could also kill the oscillator.

I suspect this will STILL perform poorly due to no AVC action and hardly any input filtering (you're gonna get BCB IMD).  I wonder why the designer chose a 6AK5 for this????

John,

At that time there must have been millions of those tubes
available on the surplus market @ .05/ea or less along with
the 6J6's as well...

I still have a case of both stashed out in the shop from AF
MARS issue from 1948 or so... Oh yeah, there's also a case
of 814's as I recall...

wow Don ... I didn't know that ... I wonder if the other handbooks are equally guilty of these errors of omission ...

I think JN has the right idea ... would the modifications mentioned be "pullin' the mixer to a Pullen style circuit? ... sorry about that ...I like corny puns ...73...JOhn

 :-\It's good to find out that all my frustrations with building circuits from the Handbook or QST aren't unique. I thought it was just me! hi

Nothing new.  I have heard the exact same story about the kw final described in the 1936 ARRL handbook that used a single 204-A tube.

That's funny because most of the stuff in QST and the ARRL handbooks looked very amateur compared to the stuff in the Radio (West Coast) handbooks.

I agree - why the UHF tubes in an MF circuit? OSCILLATION

Today's broadcast band TEEERASH may require a dual front end filter. Those stations could be overloading the front end or mixing with the oscillator or both. That is another good reason to convert up further. You do not have to go as far as 10M but you get the idea - move the trash out of band. 


Take that 6AK5 out. Go Old School, look under your bench and you will see a 6BA6 and maybe a nice 6BE6 rolling around on the floor.

Mike WU2D

Done...

Thanks for the help all!

In the morning I tried some of the suggestions, but could not tame the _(*&@$(_@#_($_  beast.

However, last night I found a different circuit in Nov 65 QST. It is a 160, 80 and 40 meter converter to .2 to .55 Mc. It uses a 6BZ6 as a RF amp and a 6U8 as an Osc./Conv. It has some of the ideas and suggestions that were recommended here.

So I after lunch I did some L/C calculations, drilled and blasted and used just the Osc/Conv part of the new circuit. I decided to initially forgo the 6BZ6 for the trial run

However to my suprise it works nicely as is, certainly good enough for what I want. Since I am looking only for some casual listening to 160 AM on my Drake 2B I think I will leave the RF amp out for now. It seems to have very little in the way of images and bleed through. Simple afternoon project if anyone wants to copy it.

Attached is a Photshoped copy of the original schematic, reflecting my changes and component values.

Thanks to all for the help and ideas. This is a great group.

73
Carl
/KPD