Relay Contact Arc Suppression?

[w8khk]

As I am rebuilding my Valiant, I put in brand new relays for antenna T/R, receiver muting, and HV transformer switching.  I have seen many designs that have a capacitor and resistor in series across the HV transformer contactor, I believe to reduce arcing.  I wonder if there is a formula, or a rule of thumb, to determine resistor and capacitor values?

[flintstone mop]

Just stick a .01 disc across the contacts. That should reduce the arcing. Make sure the cap is rated for the voltage.

Fred

[WQ9E]

Rick,

Typically .01 or .015 for the capacitor will be close enough, the resistor should be around 1.5 ohms per volt of open contact voltage.  The purpose of the resistor in series is to limit the discharge current from the capacitor when the contacts close.  If significant arcing is still a problem then you will need to experiment with the capacitor value.  You can find formulas in books (and the internet I am sure) which will provide a closer approximation of the capacitor value but you probably will have to do a lot of circuit measurement and calculation to use these formulas so an empirical approach is more practical.

I believe Mouser (and probably Digikey also) sell an integrated resistor capacitor for this purpose; I can't recall the tradename however.  But it is just as easy (and probably cheaper) to do it yourself. 

Rodger WQ9E

[w8khk]

Thanks Fred and Rodger for the information.  I will put in the suppressor with components on hand.  I have done most of the mods to the Valiant with junkbox parts, the relays were NIB from my shelf.  I did buy new caps and ceramic tube sockets though.  Still looking for the right transistor for the drive pot mod......

The modulator and speech amp are finished, and I am now working on the power supplies, meter multipliers, and mode switching.  I have eliminated the SSB mode, thus getting more of the high voltage stuff off the mode switch.

Will have pics available soon, and on-the-air tests as soon as the dummy load and scope say it is OK.  If all works as expected, I will do a write-up to share with everyone.

73,
Rick

[KE6DF]

I don't think you would want to put a .01 uF capacitor across the contacts on the antenna T/R relay. It would look like a short circuit to RF. Could lead to disaster if you feed xmitter RF into the receiver thru the .01 uF capacitor. OK for power relay, though.

[flintstone mop]

Good point, Dave.
Rick, I hope you are talking about contacts for switching high voltage and not RF

fred

[N3DRB The Derb]

post away when done -  I love teh transmittr pr0n. Plenty of pix too.

[w8khk]

Yup, only putting the cap on the contacts switching the primary of the HV tranny.  Not any of the LV (mod screen supply), keyer, receiver muting, or antenna. 

I am putting a diode and cap across the coil of each relay, to suppress reverse emf when releasing each relay, so they don't zorch the transistors.  Yes, there is some non-hollow state in the rig; I designed a sequencer to make sure the antenna relay kicks in before the HV, and the HV kicks off before anything else switches back to receive.

Also used a three pole, double throw relay for antenna switching.  Separate contacts short the receiver on transmit, and short the transmitter on receive, etc.  Making sure the receiver is protected!

[WA1GFZ]

Don't bother with a Cap across relay coils. Diode is all you  need.
A cap across relay contacts isn't enough you need to limit current into the cap. I hag a .001 3KV cap explode across the big rig breaker when I turned it off once.

[w8khk]

Thanks for the tip, Frank.  Real estate is getting short under the chassis, the fewer unnecessary parts, the better.

I was going to wait til it passed scope tests before posting any pics, but just for Derb, I will share a few previews.

[w8khk]

OK, here are some preview shots.  The rig was damaged and torched underneath when it followed me home from Huntsville fest, so to repair it was best to strip it and straighten the chassis.  After removing all the heavy metal, I decided a bit of re-arranging would make space for a quad of 1625s in push-pull parallel.  I am considering the capacitor-input HV mod for the modulator, but with the lower impedance of paralleled toobs, it may not be necessary.  Here are some photos showing the blue felt pen artwork of the previous owner, and some sheet metal removal.....

[w8khk]

Here are views of the modulator section of the chassis.  Four 1625 modders, driven by a 6SN7 push-pull cathode follower and 6SN7 push pull gain stage.  The audio starts at 600 ohm line input (XLR on the back) to a UTC A-11 input transformer.  This feeds the gain pot, 12AX7 gain stage, and 12AX7 phase splitter.   I will start out testing with no feedback, just a dummy load on the UTC S-21 tranny secondary.

On the back I put a standard power connector from an old HP disk drive, replaced the SO-239s with N connectors (ant, and receiver input), and added fuses for the LV and HV.  The 9-pin accessory socket was replaced with 8 pin, but none of the HV will go on that socket. Strictly for TR switching, receiver muting, and audio sampling for the scope.

[KF8XO]

When putting the HV Relay mod in the DX-60 I was given some info regarding "Snubbers" across the HV lines to supress arcing, here is the formula as posted on the Yahoo Heathkit reflectore, I will try to find and post the link as well.

"Due to prodding by Garey Barrell :-)

Here is what I have from my RSGB handbook concerning
snubbers:

There are various circuit methods of reducing
the effects of transients arising from switching
surges and mains over-voltages.
One of the most satisfactory methods of
restricting these is to connect suitable series-
connected capacitors and resistors across either
primary or secondary of the power transformer.
Suitable values for these components can be
calculated from the following expressions:
For use across the primary:
C = (100 I mag)/V in microfarads (uF)
R = 150/C uF, in ohms
For use across the secondary:
C = (225 I mag T )/V in microfarads
R = 220/C uF, in ohms

Where :
Imag = magnetizing R.M.S. current. For this
purpose a value of 10% of the primary or
secondary current (at full load - Ed.) as
appropriate may be used.
V = Transformer primary R.M.S. voltage.
T = Transformer primary/secondary
R.M.S. voltage.
Controlled avalanche rectifiers are likely to be
increasingly used (in future).

I hope you all find this useful.

Ken Gordon W7EKB"

[WA1GFZ]

You might consider a stronger switch wafer for the pi network. Also better loading caps.

[w8khk]

Here is the porn, Derb!

I moved the LV transformer where the 866As used to live.  I only use 866s where you can see em.  No window in the front of the Valiant.  I have destroyed stock status of the inside, but from the front the Valiant will look STOCK!  Since I no longer need a clipping control, the final bias will be adjustable from the front panel, right next to the drive pot mod.

The UTC S-21 is mounted where the LV transformer used to be. The 6SN7s fit where the old mod tranny lived.  I also added an electronic screen voltage regulator for the 1625s, so it will be easy to play with various voltages while scoping it on the bench.  The adjustments for screen voltage and mod bias are where the final bias and mod bias pots used to live, on the side of the chassis.  Screen regulator uses an 0B2 reference, 6AU6 error amp, and 6L6 series tube, all mounted near the other audio tubes.

Under chassis, I added a filament transformer for the 1625s, 26.2 volts at 2amps.  The 1625s only take .9 amps total at 26.2 volts.  I also use the transformer to supply 28v for the relay circuit (transistors) and a voltage doubler for the keep alive supply for the three-diode peak limiter.

In the back of the chassis you can see a 12 amp line filter, and the antenna relay, three pole, double throw.

The original wiring harness was badly burned by a previous owner.  Today I finished stripping it out completely.  The entire rewire is now completed using teflon insulated wire.  I rewired the mode switch, eliminating the SSB linear amp function.  This gets the HV off the switch.  No more VR string, just the electronic regulator for the screens.  This runs off the 300 volt supply, eliminating the big heat generating resistor when screens ran from 650 supply.  In order to switch the modulator on and off, I toggle the screen of the 6L6 series regulator, killing the modder screens.  This eliminates the hassle of dealing with screen voltage on while the HV transformer is off.  Screens left on would zorch the 1625s in short order.

Behind the band switch, you can see the transmitting micas that replaced the pissweak postage stamp loading caps.

I am open for any other mods anyone would like to suggest.   One I am considering is using the original mod tranny, all windings in series, for modified heising modulation.  This would get the DC secondary current off the UTC S-21.  Will it help?  Don't know, but open for opinions.  There is still room to mount it where the VR-105s used to live.

73,
Rick

[w8khk]

You might consider a stronger switch wafer for the pi network. Also better loading caps.

Got the loading caps covered in the last post.  Will replace the other stock ones if necessary.

I realize the wafers are PW.  I keep thinking about Derb's Gonset, and wonder if these wafers can handle the snot.  If they fail, I will go ahead and rip out the old ones.  Kinda complicated to deal with the main bandswitch and the JS VFO cam, with off-the-shelf parts.  SO if the tank zorches, I will follow up with K1DEUs suggestions, and replace the plate tuning cap with a heavy duty hammarlund that fits, and replace the final band switch with something heavier.  At this point I hope to get it online with almost no mods to the RF section, saving those mods for later.  Thanks for the tips, Frank!

[WA1GFZ]

Screw VR tubes ,use power zeners. RF parts sells them cheap. Just heat sink them to the chassis.

[w8khk]

Frank, I agree the zeners are superior to the vr tubes.  But, I had several reasons for using the 0B2, just like I and several others, like the glow of 866As.

1) Had them in my junkbox
2) Hole and socket already in the chassis
3) Keeps the rig a bit more vintage (for what shows above chassis)
4) Limited space below chassis, lots of room above

I still need to squeeze in the three diode peak limiter, and the rectifiers and filters for HV and LV.  At least I am no longer using any vacuum rectifiers......

If I was building new, from scratch, I would order from mouser. 

[w8khk]

When putting the HV Relay mod in the DX-60 I was given some info regarding "Snubbers" across the HV lines to supress arcing, here is the formula as posted on the Yahoo Heathkit reflectore, I will try to find and post the link as well.

Jeffrey, thanks for the suggestion.  I will research that for future projects, especially the KW rigs....    I forgot to mention that I already ordered several parts per the suggestion of K1DEU, as follows:

"To protect our power, filament and plate transformers I locate a slow MOV and fast transient absorber (both in parallel) near and across (in parallel) transformer primaries.
For 120 VAC protection I Use the following peak voltage devices;
A Panasonic ( Matshuta ) 220 Volt Bi Directional ( AC ) Transient Voltage Suppressor rated at 1500 Watts
Digi-key PN # 1.5KE220CALFCT-ND
And A Panasonic ( Matshuta ) 220 Volt Surge Absorber ( MOV i.e. Metal Oxide Varistor ) rated at 6,500 Watts
Digi-key PN # P7230-ND"

I installed these parts today, since the porn pics were taken.  I also added small fuse resistors to the secondary of the HV supply. 

I also added soft start to both the HV and LV supplies, and adjusted them to get the fils to the rated voltage.  Also, since the 6L6 needs the filament circuit referenced to the DC output on the cathode, I used the available 2.5v 866 fil winding, and the 5.0v 5U4 winding in series for 7.5 volts, dropped to 6.3v for the .9 amp 6L6 filament with a pair of 2.6 resistors in parallel.  This is one modded Valiant!

[KF8XO]

FBOM!

[w4bfs]

excellent job Rick ...sounds well thought out .. sorry I missed eyeball qso at Huntsville ...73 ...John

[K1DEU]

Excellent work and pictures Rick.  I am now reminded and will bring one of my Valiants down from the second floor and take a few pictures of the better plate tuning condenser and the work around at the pi net switching output wafer which allows the output tank to run cool at 1400 PeP out with asymmetrical B+ internal 811A zero bias modulators. The original wafer is more than adequate, but not the way Johnson wired it!  Regards John

[w8khk]

Excellent work and pictures Rick.  I am now reminded and will bring one of my Valiants down from the second floor and take a few pictures of the better plate tuning condenser and the work around at the pi net switching output wafer which allows the output tank to run cool at 1400 PeP out with asymmetrical B+ internal 811A zero bias modulators. The original wafer is more than adequate, but not the way Johnson wired it!  Regards John

Thanks, John. 

I look forward to see pics of your mods, especially the switch wafer mods.  Feel free to post to this thread.

By the way, after discussing all the other mods with you on the landline, I was a bit disappointed that I had gone with the 1625s.  I never thought of using triodes where the 866s lived.  I gave my stash of 811s to my brother, but a couple Taylor TZ-40s could do wonders for the positive mod peaks!  Anyway, I have wanted to build a scroteful power amp using four tetrodes, so this was a good opportunity. 

The real challenge was fitting everything on the chassis.  I also wanted to eliminate the driver transformer.  I added the input transformer to eliminate ground loops on the audio input.  I first tested that tranny to make sure it could handle the input level, as I had only used that device in the past for mic input.  Tomorrow I expect to have it on the bench to test the entire audio section for response and distortion.

73,
Rick

[w8khk]

Well, last night I FINALLY finished the mods to my Valiant, purchased at the Huntsville fest last August.  I needed to do that before going to the Lawrenceville fest today - finish one project before acquiring another - guess what followed me home today (but that's another story!)

A sincere THANK YOU to everyone who provided suggestions and input, especially Rodger, WQ9E who spent untold hours sharing his experience with this boat anchor!  Without all the help, I would have missed many fixes and improvements....  then I got carried away and created a few of my own.

It is now ready for testing, with scope and dummy load.  I will post photos in the near future.  (Next week I will be in FL, visiting dad, W2DU.)

My primary goal was to make a "stand-alone" transmitter with good voice quality that would work with an external audio chain, receiver, tuner, and scope, but with no other external components, such as TR switching, heising circuits, etc.  Here is a list of the "Features and Modifications" applied to my new "Valiant":

Completely stripped chassis (except for VFO and some RF stage components
Straightened bent chassis, cleaned and polished away all crud and felt pen markings
Punched holes for new tube and transformer layout
Entire zorched wire harness removed, re-wired with teflon wiring
Standard power connector (IEC like in a PC) replaces power cord
Cornell Dubilier 12A 120V line filter
Solid State Bias, LV, HV, Relay, and Diode Peak Limiter power supplies
Soft Start circuits added to LV and HV transformer primarys
MOVs and Transorbs added for LV and HV transformer primaries
Rear-panel mounted fuses for LV and HV transformer primaries
25.2 Volt CT 2 amp transformer added for 1625s, relays, and peak limiter KeepAlive
Regulated supply for Modulator screens using OB2 reference, 6AU6 error amp, and 6L6 series regulator
Transistor Drive Pot Mod
Front panel final bias adjustment pot replaces "clipper" pot
Modulator Screen voltage adjust pot installed where final bias pot lived previously
Entire modulator driver circuit redesigned as follows:
UTC A-10 input transformer for 600 ohm balanced line (XLR) input
(No mic input, uses external audio chain only)
12AX7 gain stage and phase inverter follows audio level control
6SN7 push pull gain stage with inverse feedback from modulator plates
Driver transformer eliminated
6SN7 push pull cathode follower stage drives 1625's
Four 1625's in push pull parallel modulator
Parasitic suppression on all 1625 control grids, screen grids, and plates
UTC S-21 115 watt modulation transformer,with no DC current through secondary
Installed original modulation transformer with primary and secondary in series as Heising reactor
1UF 5KV oil cap for Heising coupling from mod xfmr secondary to gnd
Provision for local and global feedback in audio circuits
"2-diode" Peak Limiter ala WA1QIX and AB2EZ included under chassis
SSB linear amplifier function removed (Who needs SlopBucket, anyway?)
Multiplier 6CL6 to driver 5763 directly coupled, no longer through mode switch
HV contactor uses two poles of three pole relay, includes arc suppression components
Third pole of relay enables modulator by enabling 6L6 screen (screen voltage regulator tube)
Main TR control relay provides "keyer" switching and external NO and NC contacts
Antenna TR has Three pole double throw relay, shorting Rec on Xmt, shorting Xmt on Rec
All relays driven by transistors with appropriate delay sequencing
Original SO239 UHF connectors replaced  with "N" type coax connectors. 
Orig "RF in" connector for SSB now functions as Receiver input via relay
9 pin accessory connector replaced with octal connector for relay control
External TR control via low current, low voltage transistor input circuit
VFO phenolic coupler replaced with flexible "epoxy" coupler
All loading caps replaced with transmitting mica caps
Replaced all out-of-spec resistors and capacitors
Caps at bottom of plate choke replaced with smaller value for improved highs

Still open for modifications suggestions for band switch from K1DEU.....

The most significant challenge was "space management" but EVERYTHING fit under the chassis!

Power supplies and variable regulator passed smoke tests.
VFO stable, and the finals heat up the Cantenna.
Will do response and distortion tests on modulator before the final dummy load tests.

Alignment and scope testing still needs to be performed, then it will be connected to the ANTENNA!