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Screen Modulation - 6DQ6 or 6146 for best linearity?




 
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Author Topic: Screen Modulation - 6DQ6 or 6146 for best linearity?  (Read 2157 times)
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w8khk
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« on: September 12, 2022, 12:34:50 AM »

Hello All,

I am rebuilding a totally-butchered Knight T-60, just for the sheer fun of it.  I am interested in creating a simple, modest screen-modulated transmitter or exciter, around 8 or 10 watts carrier power, with very clean and linear modulation.  This may be used to drive a small amplifier, but initially it will just be used for testing audio processing, and examining the resulting modulated envelope with a spectrum analyzer, distortion analyzer, and of course an SDR.

As manufactured, the Knight T-60 appears to be a watered-down DX-60; a very similar circuit, but wimpy almost to the point of being PW.  Plate voltage is only 440 volts, (about 280 volts less than the DX-60B) and the final is a 6DQ6 B&W sweep tube.  In my view, the low power is not an issue.  But what I am looking for is an educated opinion, based upon actual experience, as to whether the 6DQ6 would screen modulate linearly, all other things being equal.  I could easily switch to a 6146 at this point, if it is truly warranted.  I will retain the existing power transformer, and build within its limitations.  Some of my intended additions will be a challenge, squeezing 15 pounds of components in a 10-pound box!

The T-60, as I received it from an estate sale, was totally gutted, bare chassis and cabinet with a box of parts.  I plan to reuse all the variable capacitors, power transformer, enclosure, chassis, front panel, and meter, but nothing else.  

I will integrate a custom DDS VFO, with an 8-digit, 7-segment frequency display, eliminating the need for rocks or an external VFO.  Band-switching and T/R sequencing will be performed automatically by the DDS microcontroller and relays, as the frequency is altered.  Initially I am only interested in 75 and 40 meters.  The T-60 was not a stellar performer on 10 or 6 meters.  

The Modulator will NOT be doing any controlled-carrier bologna.  It will provide a linear transition from 0 to 125%, using a time-proven design, with negative-peak limiting.

I have completed all required drilling and blasting of the chassis and panel, which I have also refinished, as required.  (As received, the chassis and panel had several nasty extra holes, so I have no guilty feelings about further torturing the sheet metal to make something worth using.  If it was pristine, I would of course have exercised due restraint.)

The chassis opening for the 6146 would need to be enlarged slightly, as the final tube is sub-mounted within the 5-1/2" high cabinet.  This may be accomplished easily if it is truly warranted.  (I have only one 6DQ6, and many 6146s, but having spares is not enough stimulus to blast and drill even more.)  I will post photos of the rebuild as it progresses.

I welcome any input as to whether the 6DQ6 should be retained, or scrapped and replaced with a 6146, to the goal of the best modulation linearity possible with this class of PW rig.

If this exercise results in total failure, I will attempt a decent PW Series-Modulated rig as a follow-on project.

Thanks in advance and 73!
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« Reply #1 on: September 12, 2022, 06:35:03 PM »

The Sweep tubes actually provide more linear screen grid modulation than does the 6146.

Here are my suggestions for modifying the various circuits as you rebuild it.

The power supply is enhanced for a higher voltage and more stability.

The modulator provides variable power output. The puny 6DR7 is replaced with the more capable 6DE7.

* Knight T-60 AM Improvements for AMPHONE.pdf (2365.29 KB - downloaded 58 times.)
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« Reply #2 on: September 12, 2022, 09:19:42 PM »

Greetings, Pheel,
Thank you so much, not only for answering my specific question, but also for providing a wealth of information on your custom mods to the T-60.  Your rig was in much better condition than mine, so I do not feel any remorse for hacking it further. 

I had most of the changes spelled out already, but the technical details you provided will save me several more hours of measurement and planning.  For example, your meter circuit and meter specifications eliminate the need to do measurements on my meter.  Rather than a relative output indicator, and cathode current as you have implemented, I will switch between grid and plate current, using other external means to monitor the RF output level.  I already planned to have an RF output control pot on the panel, adjusting the modulator grid bias.

I ordered a couple 6DE7 tubes on Friday, and one of the two I ordered from Tube World Express arrived today, the other was canceled due to no additional stock.  Also canceled was my order for a couple 6HF8 tubes.  Funny thing, Saturday, only one day after ordering, I found a 6DR7 in my stash, and it tested OK-Fine on my Hickok mutual conductance tester.  Also, while looking at specifications for alternatives, I discovered that the 12B4 (plenty of these in the stash) appears to be a potential replacement for the high-current, low plate resistance second half of the 6DE7.  Most any audio triode would sub for the first half. 

I have looked at the 6DE7 v. 6DR7 specs, and to me it appears the vertical output half is identical, while the high mu, low dissipation half is quite different in amplification factor and plate resistance.  The DR has a mu of 68 and plate resistance of 40K, where the DE is 17.5 and 8.75K respectively.  Perhaps it would be a viable sub with revisions to the resistance-coupled amplifier component values.

In any case, I am going with the new RCA 6DE7, and a 6CL6 for the driver, with a buffer between the DDS VFO and the driver stage.

I plan to return the cathode of the modulator to a negative bias supply, and add a negative peak limiter.  I also will use negative grid bias for T/R switching instead of keying the cathodes, as this will be AM only, no plans for CW.  The bias, limited by a 5-volt zener, will provide the switching signal needed for the DDS, thus no additional relay contacts!  My T-60 could also sub as an RF exciter to drive one of the larger plate-modulated rigs.

The mode switch will provide spotting capability, (old standby position) with the DDS switched to the displayed frequency, and enable the buffer and driver when in the spot position, allowing the grid drive level to be set.  The original tune position will  function unchanged, allowing RF output coarse tuning.  The CW position will provide high-level carrier loading adjustments, and the AM position will remain unchanged.

I certainly will beef-up the power supply, and I am adding a separate supply for the DDS and relay circuits.  I blasted another socket hole for an additional RF tube, because the 6CL6 is a single-function tube.  I got rid of the original output tank inductor, substituting one removed from a defunct SB-400.  I moved the loading cap to the right about 3/4", and the big knob covers the old shaft hole in the panel.  Now the plate tank coil sits nicely between the tune and load capacitors.  This mod left space for the additions needed to support the DDS, power supply upgrades, relay switching (including T/R switch, and sequencer.

I thought about upgrading the power transformer, but I am not really interested in increasing the power level, only the modulation quality.  And the DDS PC board fits nicely between the transformer and the panel, leaving room for shielding of the DDS to preclude RFI issues.  The red seven-segment, 8-digit LED display fits nicely above the meter.  The old crystal and VFO input holes now have buttons to manage the DDS software menus.

Now that almost all the planning is completed, I can focus on the wiring, and squeezing all the additional components in the sparse voids both above and under the chassis.  It will not bear much similarity to the Knight product when I am finished.

I will share more details and photos after the initial smoke-letting ceremony.

Thanks again Pheel, you saved me lots of time, and confirmed my original beliefs.  I really did not want to hack that big hole for 6146 clearance!
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« Reply #3 on: September 12, 2022, 11:43:17 PM »

Just a few additional comments.

The 6CX8 is a very viable alternative to the 6HF8 and functions just as well.

The Unit 1 of a 6DR7 has a different set of characteristic curves wrt 6DE7 so you really can't interchange those tubes without a change in the plate and cathode resistors.

I had originally planned for a bias supply for the modulator circuit but couldn't really find enough space. A bias supply for the modulator Unit 2 has the advantage of preventing flat-bottoming of the modulator waveform and allows for a higher percentage of modulation. In other SG transmitters, I put the bias voltage at the bottom of the 250k power control resistor and use a 100k resistor from pin 9 of Unit 2 to the bias voltage.

The Knight T-60 is essentially a clone of the Hallicrafters HT-40 except for the modulator and the Final tube. The 6DQ6 was selected because of its lower profile.

In my view, the 6DQ5 is the superior sweep tube.

I run 6DQ5's in my Turbo HT-40's with 750Volts on the plate (at 0.5mA to 1mA of control grid current) and can easily get 55 Watts out of it; but I usually run about 25Watts output into my Amps. The 6DQ5 lasts forever.

The original T-60 power transformers have a lot of secondary voltage variations so some minor adjustments of the Voltage dropping and divider resistors may have to be trimmed in value.

Phil - AC0OB
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« Reply #4 on: September 13, 2022, 08:22:28 AM »



There is always the Rothman Modulation version of screen modulation....uses a pair of 6DQ6's...translation from Carlos, LU8JB:

"The construction project was born a few days ago, in February 2018, talking as usual one night with my friend Daniel (LU5 JAU), the consideration of the modulation system devised in the U.S.A by Ing. Max Rothman, the one we had once seen, there far and long ago. Word comes, comment goes and recalling old times past where only a minimal pretext was enough, no matter how crazy it was, for both of us to start working like crazy to achieve the goal, which always meant drawing the most interesting conclusions and having a lot of fun, Let's say what concerns experimentation. I saw this system for the first time, many years ago, something modernized in the well-known and popular book Learn transmission in 15 days. As the on-screen modulation system was always less considered than the traditional on-board modulation, then bypassed and without much consideration it quickly went into oblivion at the station. But by restless and after several decades passed, always looking for something to experiment, it was the turn of the forgotten and unknown (at least for us) Rothman system. This is how, once again (and there are infinite numbers), the LU5JAU and LU8JB “laboratories” were activated once more. Daniel, with a traditional two 6DQ6 pants already assembled, made the modifications of the case and the Rothman, immediately it was a reality. In my case, I decided to start from scratch (I didn't really have anything armed to hasten the attempt). In a few days, with an old aluminum chassis and integral construction through, a prototype with the new on-screen modulation system was born in the shack. I must say that the expectations were widely met, what is more, I was surprised by the very high performance of the system; that it is absolutely nothing like the traditionally known and that I did not really expect as such. After experimenting with a pair of 807s, going through 6DQ6 and having obtained an excellent result, I ended up using a parallel of two 6146s at the output. Any stage of radio frequency is useful; This is not critical at all, quite the contrary and you should only be careful to find the precise derivation of the output tank coil to obtain the necessary rf so that when it is rectified through a pair of triodes, they behave As a wrench, they can polarize the screens of the end valves. Said electrodes, since the system is correctly adjusted, self-polarize according to the characteristics of the lamps used, achieving the necessary voltage excursion on the screens to achieve the modulation peaks that an emission with a controlled carrier implies. It all depends on the rf sample obtained from the final tank. In my case, the two 6146s take a quiescent current of about 40 ma, reflecting a carrier (no audio) of around 15 watts of output. In the presence of an audio signal, the current increases immediately, the maximum peaks depending on the inflection produced in the vocal modulation. The maximum plate current value in my case is around 250 ma, which means a real peak power over 50 ohms of load of about 150w; staying permanently with a normal modulation in about 200 ma approximately with a not inconsiderable average power of about 100 real watts. The voltage variation on the 6146 screens ranges from 30 v to 200 v, approximately. What is striking about the system is the linearity that maintains the audio in a very wide frequency range (absolutely the entire useful range) that goes from 30 cycles to around 8 khz. Another interesting characteristic is that the final stage, being polarized with a voltage generated by the audio excursion, means that in the absence of excitation, the screens acquire such a low polarization voltage (around a few volts) that they leave the tubes ends practically without the possibility of amplifying and almost in absolute rest, thus being protected. On the other hand, since the modulation is practically direct from the modulator to the rf screens and there is no resistive or inductive element in between (transformer, reactor, resistors, etc.), the audio quality is absolute, with zero distortion and only it will depend on how elaborate it may be from the microphone itself and the respective chain with which it is processed. Another important fact is that to obtain a certain output power, you must work with a source voltage that is slightly higher than normal. In my case, the final lamps have on their plates around 750 volts at full load, while the rest of the circuit, driver, key and modulator included are powered with 350 volts. The working voltage will depend on the valves to be used due to the tension they can withstand in their working regime. In rf rectification I started using a rectifier diode, no more and no less than a regular 3 amp silicone. and about 1500 v. In any case, the type of rectifier to use is important, since not all respond in the same way in terms of the power to be obtained. The performance in general was very good, but something did not quite convince me and referring to the original circuit I opted to try a gaseous grinding machine, as he expressed it, a 6X5. In my case I used a 6X4; the only difference is in the socket to use. (miniature by octal) Another fact to take into account, the need to place an rf shock before the grinding machine, in order to provide mass reference to the continuous one. I placed it at the immediate outlet of the Pi tank or, failing that, close to the diode plates in case of using a blocking capacitor. In my case, said capacitor was not necessary. (It worked better) From the replacement, added to the neutralization of the final stage and a little work in the tuning of the exciter (EF 183), the results were really surprising. So much so that I also ended up converting a small valve qrp with about 10 watts of output that I abandoned with cathode modulation (nothing interesting) to the Rothman system. That simple The simplicity of the circuit and its very high performance is an invitation impossible to refuse given the results obtained. ... the magic is still intact ... and the legend continues ... Carlos-LU8JB-"



* Screen Modulated Amplifier.jpg (99.31 KB, 1127x809 - viewed 101 times.)
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« Reply #5 on: September 13, 2022, 01:20:46 PM »

Phil, thanks for the additional details.  I appreciate the 6CX8 suggestion, but I have already wired the buffer and driver, using a 6CL6 for the latter, and I will test in stages using my variable bench supply.  The 1/2 to 1 mil grid current lead was helpful as well.

I agree space is at a premium with that enclosure, and I do not know if I will fit everything in successfully until I am finished.  It is important to arrange the dissipating components where they can enjoy some convection currents.

I understand the 6DR7 is not a direct sub, as I mentioned it would need changes to plate and cathode resistance values.  But I will stick with the new 5DE7 as you suggested.

I measured 180 volts on the power transformer secondary, unloaded.  I will test under load, and calculate the various resistors, using your map as a starting point.  My audio and buffer stages will be quite different.  I have a separate power transformer for the DDS and relay switching circuits.  I had originally planned to use a voltage-doubler from the 6.3 volt filament winding, but I feel it might be best to avoid overloading that transformer, as it would be difficult to replace.  It is really not likely to fail, with the low duty-cycle on the HV winding, but why take chances if it is unnecessary....
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« Reply #6 on: September 13, 2022, 01:30:01 PM »

Jim, the Rothman modulator circuit looks interesting.  It reminds me of the G2DAF circuit I used for my dual 4-400A linear back in 1967.  I uses a pair of 6AX4 damper tubes in a voltage-doubler, using the rectified RF input power to tickle the screens.  It worked very well for many years.

If I tackled the Rothman, I could reduce the size by subbing a 6CG7 for the 6SN7, and a 6AQ5 for the 6V6, but still I think it is a lot to pack into that little T-60 cabinet.  It could probably be done if I used an HP-23A external power supply, but at this point I would rather avoid the controlled carrier approach, and just produce a clean ten or so watts of AM conventionally.  It was very interesting reading; I followed several other related google links, including the patent, which expired in 1973.  I saw that Bacon, WA3WDR (SK) even considered using that circuit.  I worked him many times back in the early 60s when he was running the up-side-down tube.  Maybe inspiration for a future project, who knows.....
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« Reply #7 on: September 13, 2022, 06:57:33 PM »

I have already wired the buffer and driver, using a 6CL6 for the latter, and I will test in stages using my variable bench supply.  The 1/2 to 1 mil grid current lead was helpful as well.

I measured 180 volts on the power transformer secondary, unloaded.  I will test under load, and calculate the various resistors, using your map as a starting point.  

If that transformer is that low a voltage, then here is another version of the power supply since you're going to need an extra 12-15ma for the 6CL6 stage over the 6HF8 pentode buffer/multiplier stage, which would have lowered the overall voltage on keydown.

So you're keeping the 6FH8 triode Osc. stage and that will feed the 6CL6?

Phil - AC0OB

* Knight T- 60 HV Power Supply for AMPHONE V2.pdf (48.26 KB - downloaded 28 times.)
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« Reply #8 on: September 13, 2022, 07:06:59 PM »

Thanks, Phil...  Your simulations will save a bit of time as I wrap up the power supply changes.  The only challenge is fitting everything in and maintaining clearance for air circulation.  Should be a fun rig once it is completed.
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« Reply #9 on: September 13, 2022, 09:14:36 PM »

Thanks, Phil...  Your simulations will save a bit of time as I wrap up the power supply changes.  The only challenge is fitting everything in and maintaining clearance for air circulation.  Should be a fun rig once it is completed.


Since you didn't present a schematic I give you this as my assumption of what you have or have planned.

Phil

* Knight T-60 AM Improvements for AMPHONE.pdf (2371.75 KB - downloaded 40 times.)
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« Reply #10 on: September 14, 2022, 12:04:17 PM »

Thanks for the suggested schematic, covering my stated changes to the components.

I do not yet have a complete schematic to share.  I plan to build this up in stages, testing the drive from the DDS, then adding the modulator sections.  As space is at a premium, I am not totally sure that I can squeeze everything in without compromising the thermal stability.  If I run out of space, I may just power the transmitter from my Heath HP-23 supply.  This option would leave space for a second 6DQ6 final tube, although not actually desired at this time.

I plan to perform initial testing without any internal power supply; instead I will use the regulated bench supply visible in the background of the attached photo of the T-60 atop the audio processor.  This supply provides 6 or 12 volt filament power at 10 or 5 amperes respectively, the following DC voltages: regulated + and - 5, + and - 12, fixed + and - 10, + and - 20, as well as vacuum-tube regulated 0 to - 150, 0 to + 250, 0 to +350, and unregulated + 400.  All voltages are metered, and the higher voltage DC outputs are current-metered.  Once the RF and audio designs are nailed down, the potential internal power supply component values may be determined.

The low-impedance audio input will be fed from the audio processor in the photo.  There will be no microphone input. Other photos show the front and back view, partially assembled.

I am sharing some preview photos as the box comes together.  

On the front panel, to the left of the frequency display is a center-off toggle switch, to enable tuning.  In the down position, spotting and drive may be accomplished, with no plate or screen voltage on the 6DQ6.  In the up position, all voltages will be present, and the output will be connected to a dummy load.  With the function switch at the bottom right, either full carrier (CW) for loading, or AM mode may be selected.

Under the meter is another toggle switch to select either grid or plate current measurement.

To the left of the frequency display is the encoder for frequency adjustment.  Pressing this knob causes the DDS code to enter menu mode.  Below the meter, where the crystal and VFO jacks were present in the original T-60 design, are four buttons.  The top two change the encoder frequency steps down or up one level, and the bottom two change the encoder speed to either minimum (1 Hz) or maximum (1 MHz) step size.

Bottom left two knobs are audio modulation level and output power level (final screen voltage).  Because the panel was hacked up before I got it, I had to move the grid tuning control to the left one position, and I added a small vernier dial.  To the right of the grid tune control is the grid drive pot, adjusting the 6CL6 screen voltage through a pass transistor.  

The band switch position will probably not be used, as the DDS software will select the proper drive and final inductance via relays.  T/R switching and a simple sequencer will be included.

The rear view shows the loading capacitor relocated to allow space for the replacement output tank coil, this one recovered from a defunct Heath SB-400.

When in receive mode, the DDS frequency will be shifted up 456 KHz, and an output is provided as a local oscillator signal to either the HRO or the RME-69, allowing so-called "transceive" operation, thus making zero-beating ultra simple (somewhat of a lost art these days).   When in receive mode, the DDS menu allows the frequency encoder to function as a RIT, allowing proper receive tuning of stations not on the net frequency.

Does anyone care to speculate on the audio processor manufacturer?


* T60WithProcessor.jpg (2741.23 KB, 3264x2448 - viewed 75 times.)

* T60FrontView.jpg (2755.76 KB, 3264x2448 - viewed 74 times.)

* T60_RearView.jpg (2423.32 KB, 3264x2448 - viewed 64 times.)
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« Reply #11 on: September 14, 2022, 12:07:28 PM »

Here are a few additional photos, showing the DDS board nestled between the power transformer and the front panel, and the DDS in test mode on the bench.

The DDS code will support either LED digit displays, or LCD alphanumeric displays, either separately or in unison.  I would prefer the LCD display, but there is not enough room on the panel to include it in the T-60 rebuild.  The LCD shows the code I modified for my brother's DDS  (Bill, W2WM).


* T60WithDDSBoard.jpg (2581.69 KB, 3264x2448 - viewed 68 times.)

* DDS_DualDisplay.jpg (2054.59 KB, 3264x2448 - viewed 63 times.)

* DDS_Testing.jpg (2578.12 KB, 3264x2448 - viewed 57 times.)
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« Reply #12 on: September 14, 2022, 03:17:20 PM »

Max audio processor; nice front panel work!


Does anyone care to speculate on the audio processor manufacturer?

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« Reply #13 on: September 14, 2022, 04:14:11 PM »

Going to call it it "TED," for T-60 Electronic and Digital?   Cheesy

Phil - AC0OB
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« Reply #14 on: September 16, 2022, 12:37:25 AM »

I'm glad the 6DQ6 was kept. The curves for the screen where G1=0V show it looks sort of like a volume expander when G2 is driven to the higher voltages. (GE page 3). I wish a similar curve was ob the 6146 data sheet. The Rothman circuit should really make it sit up and howl, if that was to be used.

Most impressive is the great appearance of the transmitter panel.
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« Reply #15 on: September 16, 2022, 01:26:19 PM »

I was wondering where you went lately, Rick -

That will be the coolest T-60 on the planet.  A worthwhile project.

I don't know if you guys know it, but Rick has a relatively inexpensive CNC machine that is set up to do panel engraving among other things. (The Max panel)  I can imagine having all the rigs in my shack engraved like that...  wow.

Just like building up a race car.  There is something to be said about taking an old car and making it perform like a superstar.

Keep posting the progress pics and test results, Rick!

T
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« Reply #16 on: September 16, 2022, 03:55:34 PM »

That is a real “Pimped out” T-60. 😎😁
I built and used a Knight T60 as a Novice in 1965.
I picked up one last year out of a feeling of reconnecting with the place where I got into the hobby.

The transmitter has a lot of flaws including the low voltage on the plates of the 6DQ6.
It is actually a nice sweep tube, though not as great as the 6DQ5.
 I have run a pair of those tubes with 750volts on the plates in a later rig.
The meter is also a weak design point.
Still- it was the rig on which I made my first contacts and my first DX (France on 15meters CW), so it has my affection.
You have also gotten rid of the pile of FT243 crystals I had to use…
Great work!
73, Mike
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« Reply #17 on: September 18, 2022, 05:07:00 AM »

Tom,

I also have a cnc router.  It was a 300 dollar and some change kit I had to put together.  Took a little time, but I was also working 8 to 10 hour days and raising two boys alone.....  Honestly it could be built in a day or two with your building skills.  It connects to the computer via USB and there are free software packages for use.

I also purchased a laser for engraving stainless.

Mine doesn't have a huge table, maybe 10x10 inches.  But I can do longer stuff by doing sections at a time.  Width I'm stuck with, however.

I bought it to make pc boards.  Works good at that, too!

--Shane
WP2ASS / ex KD6VXI
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« Reply #18 on: September 19, 2022, 10:49:26 AM »

That is a real “Pimped out” T-60. 😎😁

The transmitter has a lot of flaws including the low voltage on the plates of the 6DQ6.
It is actually a nice sweep tube, though not as great as the 6DQ5.

Mike, thanks for the positive comments on the T-60 build.  Actually, I originally planned to use a 6146, as I knew it was a viable choice for screen modulation.  But I found a good 6DQ6 in my stash, so I thought I would inquire of the AMfone brain trust as to whether it might be a good choice.  I did not know that sweep tubes were superior in regards to this form of modulation.

I was surprised to see the 6DQ6 is so stout regarding voltage and dissipation, really better than the 6146.  And now you mention the 6DQ5, which is even better.  Yes, the voltage in the T-60 is too low to obtain the maximum performance, but the box is small, as is the power transformer.  I am mainly looking at having ultra-clean modulation, as a test bed for analyzing modulated audio from the processor, so a great deal of power is not necessary for this build.  I would not be able to replace the transformer if I let the smoke out, so I am very careful about overloading it, including the filament winding.  The 6DQ5 is double the filament current, so it is out of the question.  I will be running the DDS VFO, relays, and sequencer from the 6.3 volt winding, so I decided to add a 120 volt isolation transformer to provide the positive 300 and negative bias for the RF drivers and audio stages, eliminating the dropping resistors from the 450 volt supply.  Now the only load on the main transformer will be on the filament winding during standby periods, so it should be much safer.

It will be lots more convenient to push buttons for frequency changes, than to dig through a basket of quartz!
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.”   Ronald Reagan
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« Reply #19 on: September 19, 2022, 10:55:04 AM »

I'm glad the 6DQ6 was kept. The curves for the screen where G1=0V show it looks sort of like a volume expander when G2 is driven to the higher voltages. (GE page 3). I wish a similar curve was ob the 6146 data sheet. The Rothman circuit should really make it sit up and howl, if that was to be used.

Most impressive is the great appearance of the transmitter panel.

Patrick, thanks for the kind words on the panel.  It was a bit of a challenge to fit everything I desired, in 3 dimensions (back of panel clearance) while utilizing extra holes left by the previous owner.  After moving the loading capacitor and knob, I could no longer retain the original T-60 artwork, so out came the spray paint can.  It is a black metallic finish, and it almost appears similar to the dark gray of the Collins cabinets.  Since this is a "work-it-out as-I-go" project, I will not attempt to engrave the legends, but defer instead to the p-touch labels for this one.  I have actually spent much more time on the planning than the building, because it is very difficult to remove misplaced holes!
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.”   Ronald Reagan
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« Reply #20 on: September 19, 2022, 11:02:48 AM »

I was wondering where you went lately, Rick -

That will be the coolest T-60 on the planet.  A worthwhile project.

I don't know if you guys know it, but Rick has a relatively inexpensive CNC machine that is set up to do panel engraving among other things. (The Max panel)  I can imagine having all the rigs in my shack engraved like that...  wow.

Just like building up a race car.  There is something to be said about taking an old car and making it perform like a superstar.

Keep posting the progress pics and test results, Rick!

T
Hello Tom, I am still here, spending a good bit of time in the background on new PCB schematic and artwork for the next version of The MAX.  In between I did an independence day project, reworking the remains of an HP 608C RF generator into a utility bench supply, and I spent a weekend assembling the new K7DYY modulation monitor, a very fine product in my opinion.

As to engraving, I use a rather small kit-built CNC, which can also perform blasting and drilling on the smaller, light-weight panels.  The heavy work is relegated to my behemoth floor-standing Harbor Freight drill press, to which I added a heavy-duty cross-slide table with lead screws driven by stepper motors and timing belts.  The X, Y, and Z axes are now driven by signals from a $4.00 Arduino Nano and G-code from either Fusion-360 or Easel, and now, instead of using socket punches or fly cutters and hole saws, I can sit back and watch the 3-inch meter holes appear like magic in a 19 inch panel, up to 1/4" thick.  Meter holes were never a fun part of building for this ham!
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.”   Ronald Reagan
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« Reply #21 on: September 19, 2022, 12:49:29 PM »

That is a real “Pimped out” T-60. 😎😁

The transmitter has a lot of flaws including the low voltage on the plates of the 6DQ6.
It is actually a nice sweep tube, though not as great as the 6DQ5.

Mike, thanks for the positive comments on the T-60 build.  Actually, I originally planned to use a 6146, as I knew it was a viable choice for screen modulation.  But I found a good 6DQ6 in my stash, so I thought I would inquire of the AMfone brain trust as to whether it might be a good choice.  I did not know that sweep tubes were superior in regards to this form of modulation.

I was surprised to see the 6DQ6 is so stout regarding voltage and dissipation, really better than the 6146.  And now you mention the 6DQ5, which is even better.  Yes, the voltage in the T-60 is too low to obtain the maximum performance, but the box is small, as is the power transformer.  I am mainly looking at having ultra-clean modulation, as a test bed for analyzing modulated audio from the processor, so a great deal of power is not necessary for this build.  I would not be able to replace the transformer if I let the smoke out, so I am very careful about overloading it, including the filament winding.  The 6DQ5 is double the filament current, so it is out of the question.  I will be running the DDS VFO, relays, and sequencer from the 6.3 volt winding, so I decided to add a 120 volt isolation transformer to provide the positive 300 and negative bias for the RF drivers and audio stages, eliminating the dropping resistors from the 450 volt supply.  Now the only load on the main transformer will be on the filament winding during standby periods, so it should be much safer.

It will be lots more convenient to push buttons for frequency changes, than to dig through a basket of quartz!

Hi Rick,
You make a valid point about the 6DQ5 filament current. My suggestion regarding that tube assumed that you would retrofit a bigger transformer that would have the filament current available plus the additional voltage to give you closer to 500-600vdc for the RF amp.

The DDS VFO is brilliant and this is a hot rod T-60 extraordinaire, but one is limited in what is possible within that small enclosure. The attached photo is an example of “a bridge too far” for all to find amusing but possibly to find instructive on practical limits. 😉😂😁
You have inspired me, however to soup up my own old T-60 with at least the DDS and a few other published mods to improve keying, etc. Thanks for sharing your work. I am also intrigued by your mini-cnc machine. Any info is appreciated.
73, Mike


* 10F129A4-BDD7-4CF3-9C3C-BA9B0BF10DAC.jpeg (378.76 KB, 2115x1309 - viewed 52 times.)
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« Reply #22 on: September 19, 2022, 01:19:38 PM »

Somewhere in the 1952 issues of CQ is a big write up on the Rothman system.  July 1952 I think.

Also:

https://worldradiohistory.com/hd2/IDX-AUSTRALIA/IDX/Amateur-Radio/50s/Amateur-Radio-AU-1952159.pdf

https://patents.google.com/patent/US2765443A/en

Sounds interesting......
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« Reply #23 on: September 19, 2022, 01:50:41 PM »

Hi Rick,
You make a valid point about the 6DQ5 filament current. My suggestion regarding that tube assumed that you would retrofit a bigger transformer that would have the filament current available plus the additional voltage to give you closer to 500-600vdc for the RF amp.

The DDS VFO is brilliant and this is a hot rod T-60 extraordinaire, but one is limited in what is possible within that small enclosure. The attached photo is an example of “a bridge too far” for all to find amusing but possibly to find instructive on practical limits. 😉😂😁
You have inspired me, however to soup up my own old T-60 with at least the DDS and a few other published mods to improve keying, etc. Thanks for sharing your work. I am also intrigued by your mini-cnc machine. Any info is appreciated.
73, Mike

Yes, the T-60 enclosure is very small, 12 inches wide and less than 5 inches high.  I can squeeze in many features, but no more output power.  You have given me an idea....  I am half-finished building an exciter for my P-P 250TH rig modded by P-P 810s.  I am building it in an old Heath SB-400 cabinet with new panel and chassis, using power supply components from the same rig, and a vacuum-tube VFO.  Perhaps I will plant a couple 6DQ5s there instead of the 6146s.  I guess it will depend upon the cost of the DQ5s.  There is room in that build for a screen modulator as well.....

I do not have any current pics of the Harbor Freight drill-press-to-CNC mods, but I attach an old pic showing the cross-slide and stepper motors being tested with G-code and breadboard Arduino controller.  Maybe this is a subject for another thread.  It makes a very inexpensive, but powerful, shop addition!

That bike looks pretty hot, but if I was going to great lengths to pimp it out, it would include a small-block chevy for motivation.


* CrossSlideSteppersArduino.jpg (3217.15 KB, 3264x2448 - viewed 50 times.)
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.”   Ronald Reagan
w8khk
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This ham got his ticket the old fashioned way.


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« Reply #24 on: September 19, 2022, 02:03:04 PM »

Mike, I just noticed you mentioned you were interested in the Mini CNC.

Attached are a couple pictures, one showing the controller based upon Arduino, to which I added limit switches with optically-isolated interface.

This mill is under $400 via Amazon, and is very solid.   If you are interested in one, I suggest you opt for a build that uses machined aluminum for the structure.  Avoid plastics or 3-D printed brackets at all costs, they just do not hold up or provide accurate positioning with the stresses of the high-speed cutting bits.


* MiniCNC.jpg (2704.63 KB, 3264x2448 - viewed 58 times.)

* MiniCNCcontroller.jpg (2579 KB, 3264x2448 - viewed 56 times.)
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Rick / W8KHK  ex WB2HKX, WB4GNR
"Both politicians and diapers need to be changed often and for the same reason.”   Ronald Reagan
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