Date   
Re: Using Vacuum Tube heathkit as Linear Amplifier for QRP

Robert D. Bowers
 

I've played around with a Kenwood TS520... using a softrock connected in through the filter board.  The combination was in a word AWESOME... if a different radio even thought on frequency, I probably would have heard it (voice QRM became the limiting factor - static and hiss less of a barrier).  I plan on eventually using the softrock as an IF and the filters (and so on) for transmission.  The good thing is that the tuned circuits in the old radio are going to greatly improve the signal, one way or another.  In fact, they should be at least as clean as the best modern unit.

There are more powerful old tube rigs 'out there' - I can think of a KW rig right now that would be awesome as an amp (plus great filtering) for a BitX (or softrock).

Right now I'm trying to get something on the air with 2m SSB/FT8.  Then other projects will follow...

Bob

On 11/9/19 5:01 PM, Gordon Gibby wrote:


Finally got a chance to try my idea of using a heathkit ssb rig as a power amplifier for a QRP rig.   I've seen "amplifiers" for these rigs going for many hundreds of $$$.  I happen to be swimming in ancient heathkits with 2x6146 linear finals and tuned plate circuits on the driver as well (all good for getting rid of unwanted harmonics.....)

Allison gave some much needed advice on this crazy idea months back, and I finally got the chance to try it:

1 watt output from an exciter [I used an icom 718 that was readily availalbe] -- my only "power measurements" are ancient SWR meters with built in diode detectors so these measurements may be fairly INaccurate.

coax over to a 49:1 homebrew balun our group made for matching resonant half-wave antennas:   ours is a simple autotuner with 14 turns total of #18 teflon wire around a FT-240-43 (or FT-140-43) tapped at turn 2 to give us 7:1 turns ratio, 49:1 impedance ratio, and that checked out roughly with an antenna analyzer and some resistors

I loaded the output with 2500 ohms (temporarily made out of 4 10K resistors in parallel because that is all i had)
output of that via 6 inches of small coax to a 0.01 1kv ceramic capacitor connecting to the PLATE circuit of the removed 6CL6 driver, and to a nearby ground.

Output tested only with very brief bursts of CW -- on 80 meters and 20 meters, got approx 40-50 watts output measured also with an ancient swr meter/wattmeter into a 50 ohm load, everything on the Heathkit peaked for output.   Plate current rose appropriately.   Tuning circuit all appeared appropriately.

No obvious signs of any instability in this pilot test.    
Did not make any measurements with spectrum analyzer.   (In due time......)

That heathkit has other problems elsewhere but the finals appeared in fine shape.   Earlier VERY CRUDE measuresments with a siglent oscilloscope and some math suggested that the drive required at the INPUT to the 6CL6 stage would be less than 100 milliwatts, and that the drive required at the output of 6CL6 would be in the single digit watts range.    The above test appears to confirm those crude measurements.   Since it appeared the two 6146's could be driven adequately with single digit watts I opted to temporarily just remove the 6CL6 driver and connect to its (DC powered!!!!!!!  High voltage!)  plate circucit with a 1kv .01 blocking capacitor.   

Quite pleased with this very very initial test.   
At some point I'll repeat with a uBitx, but that's all for right now.
Gordon Gibby

Attachments:


Re: Wiring question - Nextion case

Ian Reeve
 

The pcb does need a little talc to make everything fit.The connectors I suggest are not soldered in until the jacks are fitted and a assembly dry run done.on mine the edge of the pcb needed to be filed back to ensure jacks sufficiently poke through to be fixed with the round threaded fixings.Once that is done assemble the connectors and make sure there is sufficient room for the main pcb.Again on mine the holes for some connector pins was not in perfect alignment.Hope this helps


From: BITX20@groups.io <BITX20@groups.io> on behalf of Justin Phillips K5AXL <dolmdemon@...>
Sent: Sunday, November 10, 2019 9:41:13 PM
To: BITX20@groups.io <BITX20@groups.io>
Subject: Re: [BITX20] Wiring question - Nextion case
 
On an aside, if you haven't assembled the front panel board yet make sure you solder the digital and analog sockets pulled back towards the edge as much as you can. And when you solder the jacks on, make sure they're pulled forward as much as you can, however much play in the holes you have. Mine is so tight i have to press the harnesses in and a bit of an angle to squeeze them in behind the jacks. The jacks are so far back i couldnt get the nuts to get any thread, I wound up using the old ones from the stock ubitx kit, they seemed to get more thread to bite.

Re: Wiring question - Nextion case

Justin Phillips K5AXL
 

On an aside, if you haven't assembled the front panel board yet make sure you solder the digital and analog sockets pulled back towards the edge as much as you can. And when you solder the jacks on, make sure they're pulled forward as much as you can, however much play in the holes you have. Mine is so tight i have to press the harnesses in and a bit of an angle to squeeze them in behind the jacks. The jacks are so far back i couldnt get the nuts to get any thread, I wound up using the old ones from the stock ubitx kit, they seemed to get more thread to bite.

Re: Wiring question - Nextion case

Justin Phillips K5AXL
 

Yes, which is normally connected to pins 5/2 of the phones jack when no plug is inserted, completing the connection to pin 7 on analog J1 connection. Inserting a plug disconnects J4 (if im observing it correctly). Mine is wired this way and I have audio to the amp with no plug in the jack and no audio to the amp/speaker with phones plugged in.

Re: uBITX Wireup Diagrams on the HF SIGNALS WEBSITE ARE WRONG!! #ubitx #ubitx-help

kg5mg@...
 

Scott, did you ever get a response from Al, N1AW regarding the CW jack adapter for the uBitx you posted to the group 12/3102018?  I am just starting on my kit and want to use paddles.

Thanks
73
Hamp KG5MG

Re: Wiring question - Nextion case

mgassman@...
 

Looking at the schematic for the front panel board, I’m confused.  Pin 1 of J4 goes to pin 4 of J8 - is that also NC to pin 5 - so its using the speaker output from the ubitx board?

Re: Reducing power

Gordon Gibby
 

I would suggest to just get a non-inductive potentiometer,  put it on the panel, and run tiny coax to it so you have a panel mounted drive controller.  


On Nov 10, 2019, at 14:42, Sascha Bohnet | DL5SMB via Groups.Io <saschabohnet@...> wrote:

There is also another approach by John, VK2ETA, who tried to control SSB and CW power output via software/firmware by changing the clock frequency of the raduino (40 Mhz IF).
I do not understand it fully, but details can be found here.

Re: Using Vacuum Tube heathkit as Linear Amplifier for QRP

Gordon Gibby
 

Thanks very much! I think we’ve got a pretty cool project developing here. 


On Nov 10, 2019, at 13:36, ajparent1/KB1GMX <kb1gmx@...> wrote:

Gordon,

Yes don't.  Those vacuum fats have plat to grid capcitance and if not neutralized
(or even if it is) it will take off and get unhappy.

If anything lower might be the thing.  The key is you need enough RF grid voltage
to drive the tube but likley less than the total bias needed to get plate current.
Some tubes gt really unhappy or terribly non linear if they see grid current flow.

Grounded grid has some of the same issues but offers a naturally lower input
impedance and a bit less gain.

Allison

Re: Reducing power

Sascha Bohnet | DL5SMB
 

There is also another approach by John, VK2ETA, who tried to control SSB and CW power output via software/firmware by changing the clock frequency of the raduino (40 Mhz IF).
I do not understand it fully, but details can be found here.

Re: Reducing power

ajparent1/KB1GMX
 

Please observe the schematic of the radio board.

Resistor RV1  is there to serve that function.  

NOTE; if you turn it down (clockwise, yes its odd) for 5W on 80M
you will get less power on successive bands.  

Allison

Re: Wiring question - Nextion case

Justin Phillips K5AXL
 

It's coming from J4 on the front panel mic/key/phones board.

Reducing power

R. Tyson
 

Hi,
I have a uBitX to build over the Christmas period. I will be using the KD8CEC firmware and Nextion 2.8" screen
Is there an easy way of altering the output power, short of dropping the supply voltage ? If I could make this easily adjustable....

I like to run QRP CW so that means no more than 5 watts output.
Any ideas appreciated to save me re-inventing the wheel.

Thanks,

Reg

Wiring question - Nextion case

mgassman@...
 

Folks,

I have the AmatuerRadioKits 3.5 Nextion case kit and a recent v5 ubitx.  I’m trying to determine where to get the audio input for the audio amp board that comes with the kit.  The picture in the downloads shows a connection on an earlier version of the ubitx.

Thanks.

Mike
WA0YCN

Re: Using Vacuum Tube heathkit as Linear Amplifier for QRP

ajparent1/KB1GMX
 

Gordon,

Yes don't.  Those vacuum fats have plat to grid capcitance and if not neutralized
(or even if it is) it will take off and get unhappy.

If anything lower might be the thing.  The key is you need enough RF grid voltage
to drive the tube but likley less than the total bias needed to get plate current.
Some tubes gt really unhappy or terribly non linear if they see grid current flow.

Grounded grid has some of the same issues but offers a naturally lower input
impedance and a bit less gain.

Allison

Re: Using Vacuum Tube heathkit as Linear Amplifier for QRP

Gordon Gibby
 

Thanks very much for sharing that schematic!   Even with the resistive loading it looked like the grid given PA of heathkit transceiver was doing about 16 dB gain.  I wasn’t brave enough to try it higher than 20 m 

Do you have any advice on whether we should try resistive loading higher impedance than 2500 ohms???

By using the tuned plate circuit of the 6cl6 driver, I guess we are already dealing with any grid capacitance in the process

I never did anything like this before (since I home brewed a 3x 811 a amplifier in high school ) it’s pretty cool for me to get back to this


On Nov 10, 2019, at 11:59, ajparent1/KB1GMX <kb1gmx@...> wrote:

The attached image was the solution of the 60 and 70s for hams (and CBers)
for 100W amp. it was cheap 10-13DB of gain.  Alas sweep tubes got expensive 
as people burnt them all up pushing those.  There were generations of them with
up to 6 (SIX!) for a KW input peak.  Not all were clean.

Some did grid driven and they worked save for over drive the grids and the
tubes protest by going away.  At 25$ each that is a bad thing.

For laughs a decade ago I took a 4CX250  and ran it grounded grid like that
and it was good for about 80W (on 6M) out in a simple circuit without forced
air cooling. The tube with screen bias (its a ceramic metal power tetrode)
added did an easy 300w out at 2000V (a very lethal power supply).  The
amp was terminally simple save for a typical tube amp tapped load coil
Pi network for multi-bands.  There are a long list of tubes that can can
be used for that that..

Allison
<courier_bl100_sch.pdf>

Re: Power output tests at 24v PA

Chris [N7CCX]
 

@mIguel, it is not designed to do 160, this was just a test.

7/10 ill check..


@mark variable multi meter

Re: Using Vacuum Tube heathkit as Linear Amplifier for QRP

John Faivre
 

I have a HW104 with solid state finals and was thinking of doing the same thing. 
--
John Faivre WA9SGD

Re: Using Vacuum Tube heathkit as Linear Amplifier for QRP

ajparent1/KB1GMX
 

The attached image was the solution of the 60 and 70s for hams (and CBers)
for 100W amp. it was cheap 10-13DB of gain.  Alas sweep tubes got expensive 
as people burnt them all up pushing those.  There were generations of them with
up to 6 (SIX!) for a KW input peak.  Not all were clean.

Some did grid driven and they worked save for over drive the grids and the
tubes protest by going away.  At 25$ each that is a bad thing.

For laughs a decade ago I took a 4CX250  and ran it grounded grid like that
and it was good for about 80W (on 6M) out in a simple circuit without forced
air cooling. The tube with screen bias (its a ceramic metal power tetrode)
added did an easy 300w out at 2000V (a very lethal power supply).  The
amp was terminally simple save for a typical tube amp tapped load coil
Pi network for multi-bands.  There are a long list of tubes that can can
be used for that that..

Allison

Re: Power output tests at 24v PA

ajparent1/KB1GMX
 

A note on 24V operating...

If you do that there will only be a bit more power is the result.
Since the gain of the finals does not increase with greater
applied voltage you still need more drive to take advantage
of it.  It only enables it to produce more power.

That goes back the the initial problem, not enough drive on
the higher bands from the decaying gain from the 3904s.

As to dummy loads and detectors,  Diodes are terrible load
and when connected to dummy load they change the
effective impedance presented back to the radio.
They also have a hard to predict offset (threshold)
that at best is approximated and untested.  Above a
few watts its not a big deal unless some one can show
a significant difference for 15V vs 15.5V (peak)
across a 50 ohm load. 

Then we get into peak vs p-p and RMS power.
Note most diode detectors are usually PEAK so
the actual is .707(give or take considering diode
offset) times the peak voltage (correct only for
sine wave). 

Also detected waveform makes a difference and assuming
a sine wave is not always valid.

Its those details that impact read power and why many people
get sometimes wildly different numbers. 

the simplest test is pit a fresh 1.5V battery across hte dummy load.
That should cause 30mA to flow and the diode detector if perfect
(and DC connected) will read 1.56V (fresh alkaline cell).  Likely
it will be a diode drop lower and that depends on the diode and
meter load presented to the diode.  Typical for 1n34 is about
1.45V,  1n5711 Schottky maybe 1.35V, and 1n4148 silicon
about .96.   FYI the resistor in the load if 50 ohms is dissipating
.045W (45 milliwatts) DC.  If it was 1.56V AC P-P its a lot less
(about 6mW).

Allison

Re: Power output tests at 24v PA

 

Hi Chris,

In your first column of power calculations, the formula is correct but you must have made a mistake in the spreadsheet calculation.  Either that or you didn't take into account the the probe gives Vpeak not Vrms so it must be converted.

This is shown correctly in the second column formula:  your reading squared divided by 100.  In converting Vpeak to Vrms you divide by the square root of 2.  However, since this is subsequently being squared, you can just put that in the divisor with the 50 and that makes it 100.

Of course both voltage measurements should have the diode voltage drop correction added back in before and any calculations are done.

Be aware that dummy loads/detectors can be built in different configurations so the formulas may vary (e.g. Elecraft's DL1 where the probe is measuring across only half the load).

73,


Mark

PS:  What is a VMM?