Mod tube change in DX-60

[W6ZKH]

I ended up buying a DX-60 at a AMI-West BBQ afew weeks back, and the speech/Modulator section has been quite modified to the fact that it now uses a 6CL6 instead of the 6DE7 mod tube.  Seems to work fine and sounds good.  Just wondered if anyone else has seen this modification?  I am using a Shure 444 mike with it.

John W6ZKH

[DMOD]

Never heard of that mod.

Maybe the 6CL6 is wired as a triode? By itself, the 6CL6 in pentode configuration has too high a plate resistance to be used as a high current cathode follower, IMHO.

What does the speech amp/modulator circuitry look like?

The best performance can be had by simply substituting the 6DE7 for a 6EW7 and using the first triode as an RC coupled amplifier feeding the second stage.

Phil - AC0OB

[W6ZKH]

Here is a schematic of that modified DX-60 that was used in this one I just picked up, using the 6CL6 instead of the original.  Seems to work fine and has plenty of audio punch..  This is not my design, as I am no electronic genius, but if it works, dont mess with it, my dad use to tell me.

[W6ZKH]

Well put this DX-60 with modified modulator on the air yesterday; had 4 Q's and each station said the audio sounded "great"... so that is a relief...  :-)  Only problem is that darn HG-10 seems to drift quite abit, so think I will go just xtal control with it, being it is only used on 2 80M Freq's and 1 40M freq.  (3865, 3870 & 7293 here on the west coast).. but the price of xtals is more than I paid for the vfo...figgers..  :-(

John W6ZKH

[DMOD]

Here are some schematics:

The first one is your circuit and the second is a similar design of an Ameco TX-86 CC modulator stage using a 6AQ5.

Your 6CL6 is triode connected and the voltage developed at its plate modulates the 6146 screen grid..

I question the value of Rps, since not enough voltage can be developed across it (at that value) to modulate the 6146. Compare that to the Ameco R28.

You might want to check the resistor connections again around the plate and screen grid.

Phil - AC0OB

[WD5JKO]

Phil,

   Looking at that image again, the resistors appear to be 10K and 27K in series, and then a 33K in parallel with that, making a 17.44 K resistor.

That drawing although pretty good sure tripped me up the first time I saw it.

Jim
Wd5JKO

[DMOD]

Phil,

   Looking at that image again, the resistors appear to be 10K and 27K in series, and then a 33K in parallel with that, making a 17.44 K resistor.

That drawing although pretty good sure tripped me up the first time I saw it.

Jim
Wd5JKO

Thanks Jim.

If the plate load resistance is a 12.7k (10k in series with a 2.7k), and in parallel with a 33k, then the equivalent plate load resistance would be 9.2k.

Assuming a 7 mA current through the equivalent 9.2k, the non-modulated (static) voltage would be 64.4 Volts, which would be about average for the voltage presented to the SG of a 6146 for a CC circuit.

I think the Time Stamp may have hid some critical info.

Awaiting for John to re-examine his circuit and clarify.  

Phil - AC0OB

[WD5JKO]

  Phil,

   Yes we should wait for clarification. That said, looking at the image, I blew it up, and cropped that section. There does not appear to be any decimals in the three resistor values. My allergy tearing eyes see:

10K 1W
27K 2W
33K 2W

Jim
Wd5JKO

[DMOD]

Phil,

   Looking at that image again, the resistors appear to be 10K and 27K in series, and then a 33K in parallel with that, making a 17.44 K resistor.

That drawing although pretty good sure tripped me up the first time I saw it.

Jim
Wd5JKO

Thanks Jim.

If the plate load resistance is a 12.7k (10k in series with a 2.7k), and in parallel with a 33k, then the equivalent plate load resistance would be 9.2k.

Assuming a 7 mA current through the equivalent 9.2k, the non-modulated (static) voltage would be 64.4 Volts, which would be about average for the voltage presented to the SG of a 6146 for a CC circuit.

I think the Time Stamp may have hid some critical info.

Awaiting for John to re-examine his circuit and clarify.  

Phil - AC0OB

Another question, John:

What is the non-modulated voltage across the 470 ohm resistor at pin 1 of the 6CL6, the plate supply voltage, and at the plate?

Phil - AC0OB

[W6ZKH]

Jim and Phil.... yes those are the correct resistor and capacitor values that is being used in this circuit.  Sorry my drawing was poor, but couldnt find my CAD program to make it nicer.  Again, I did not design or build this circuit, I was just showing what had come with it, which is different than others I have seen, but seems to work fine.  I am not going to take the covers off again to make any voltage checks... you'll just have to take my word for it, that it works with a 6146... 

[DMOD]

Jim and Phil.... yes those are the correct resistor and capacitor values that is being used in this circuit.  Sorry my drawing was poor, but couldnt find my CAD program to make it nicer.  Again, I did not design or build this circuit, I was just showing what had come with it, which is different than others I have seen, but seems to work fine.  I am not going to take the covers off again to make any voltage checks... you'll just have to take my word for it, that it works with a 6146... 

John,

You submitted the post and asked the question. No one is criticizing anything.   

Being radio circuit geeks, we were simply curious about the circuit operation and circuit values since it was somewhat different from previously submitted posts and circuits.

Just to clarify, is that a 2.7k or a 27K up there on the upper right corner?

Thanks,

Phil - AC0OB

[w4bfs]

the old goose tube shunt modulator ...it would be interesting to see how well it compares to a cathode follower screen modulator in technical testing for distortion and spectral occupancy

[W6ZKH]

Yes Phil, that is a 27k resistor, not a 2.7k.  that is in series with a 10k to B+... what caught my eye was the plate and screen tied together using a 33k to B+ also... when I get a chance, I will disconnect the radio and take it out to the shop and take off the covers and verify that drawing I made....heck who knows with my eyesight (!) and then will also make those voltage checks you asked about....

[DMOD]

Yes Phil, that is a 27k resistor, not a 2.7k.  that is in series with a 10k to B+... what caught my eye was the plate and screen tied together using a 33k to B+ also... when I get a chance, I will disconnect the radio and take it out to the shop and take off the covers and verify that drawing I made....heck who knows with my eyesight (!) and then will also make those voltage checks you asked about....

Thanks John, much appreciated.

Phil - AC0OB

[W6ZKH]

Hi Phil.... took the DX-60 out to the shop and retraced that schematic again, and it is correct... what has me curious as to why they would tie the plate and screen together?  To me, what is the purpose of the 10k and 27k resistors, as they are in parallel with the 33k, except maybe they didnt have a correct value resistor to use?   Anyway here at the voltages you asked for:

Pin 1 Cathode +3.7v
pin 3 Screen +118v
Pin 6 Plate +118v

Supplied B+ is +323v

Voltage to 6146 screen is +70v

These were all taken in AM mode activated, non-modulated

Hope this helps..

73 de John

[DMOD]

...what has me curious as to why they would tie the plate and screen together?  To me, what is the purpose of the 10k and 27k resistors, as they are in parallel with the 33k, except maybe they didnt have a correct value resistor to use?...

Thanks John,

The voltages you supplied now makes sense of the circuit.

They tied the screen grid and plate together to make the 6CL6 operate as a Triode, as I suspected in a previous post. Nothing wrong with that as the 6CL6 can sink up to 30 mA.

They tied the resistors together as drawn to make an equivalent 17.4k resistor to get a static plate voltage of 118V. As you suspected, this was probably done because they did not have a 17.5k resistor.

I do wonder how much distortion and bandwidth this circuit is producing since there is no feedback indicated and since the cathode voltage is so low.

But hey, if you're getting decent audio reports that is good.

Regards,

Phil - AC0OB

[WD8BIL]

Is there an actual wire between pins 3 and 6?

[W6ZKH]

Is there an actual wire between pins 3 and 6?

Yes.....I know, it is strange looking, but Phil explained it to me that config makes it into a Triode instead of a Pentode.

John W6ZKH

[WD8BIL]

Yes. And the plate resistors can be simplified to one 18K/3W resistor.

[DMOD]

...what has me curious as to why they would tie the plate and screen together?  To me, what is the purpose of the 10k and 27k resistors, as they are in parallel with the 33k, except maybe they didnt have a correct value resistor to use?...

Thanks John,

The voltages you supplied now makes sense of the circuit.

They tied the screen grid and plate together to make the 6CL6 operate as a Triode, as I suspected in a previous post. Nothing wrong with that as the 6CL6 can sink up to 30 mA.

They tied the resistors together as drawn to make an equivalent 17.4k resistor to get a static plate voltage of 118V. As you suspected, this was probably done because they did not have a 17.5k resistor.

I do wonder how much distortion and bandwidth this circuit is producing since there is no feedback indicated and since the cathode voltage is so low.

But hey, if you're getting decent audio reports that is good.

Regards,

Phil - AC0OB

[WD8BIL]

Was just pointing out a simplified SCHEMATIC change.

[W6ZKH]

Yes that is true it could have been more simplified in the drawing, but I drew what was ACTUALLY installed.  I did not design, build or install this mod, as it was that way when I bought the rig afew weeks back... seem to get good audio reports with it, no matter what...

[WD8BIL]

Cool John! As the saying goes; If it works don't fix it!