crystal heating

[KK4RF]

Gentlemen,
     I experience significant heating of my FT-243 crystals when used in my Globe Scout 680A. They don't burn my fingers but they get pretty warm to the touch as I test my rig in the AM mode. Key-down time of less than a minute clearly does this. I'm afraid I'll damage my crystals. Is this (heating)  to be expected? I'm tempted to replace all the components in the oscillator circuit. Any ideas on this?
               ---Marty, KK4RF---

[W9GT]

Are you using real FT-243 xtals, or retro-fitted FT-243 xtal holders that have miniature xtals mounted inside?  Those miniature xtals can be a problem when used with tube-type equipment and the associated higher xtal currents that go with some tube-type xtal oscillators.  Some vendors are providing the retro-fitted xtals in FT-243 holders.  They are not as rugged as the original FT-243 xtals.

I had a 6146 xtal oscillator xmtr as a novice that I used FT-243 xtals with, but it had a small incandecent pilot bulb in series with the xtal to limit the xtal current.  I have even seen 813s used as xtal oscillators, but xtal current was also limited with a series bulb.

73,  Jack, W9GT

[KK4RF]

Jack,
     Thanks for the ideas. I checked out your 6146 oscillator/transmitter in my 1956 ARRL Handbook, and they do indeed have a 2volt 60 mA lamp in series with the crystal. My Globe Scout 680A uses a 6V6 as oscillator tube, in what appears to be a colpitts-type circuit, and uses a 47K resistor in parallel with the crystal, both of which are connected on one end to ground. I'm going to drink a cup of coffee and go out into the shack and check the 47K resistor and generally check out some voltages in operation.
     The crystals which heat up for me are old C-W crystals in FT-243 holders. I think they're the real deal. I did buy recently a 3880 and 3885 xtal in FT-243 holders from Bry, AF4K. These, I suspect, are the retro-fitted, lighter weight crystals that you mention. I'm not sure, however. I'm just afraid I might fracture these new crystals, hence I'm trying to figure it out before I try them out in this circuit.
     Thanks again.
                     ---Marty, KK4RF---

[WA5VGO]

I changed the 6V6 in my Scout to a 6AG7. Now the oscillator has more output and the crystal runs noticably cooler. Also, crystals that were sluggish with the 6V6 work perfectly with the 6AG7.

73,
Darrell, WA5VGO

[KK4RF]

Darrell.
     Did you have to change the wiring on the oscillator tube socket? Any change in the tuned circuits? I do have a few 6AG7 tubes lying around the shack. Thanks for the idea.
                            ---Marty, KK4RF---

[WU2D]

The 6AG7 idea sounds like the ticket.

But you could also simply try reducing the value of the grid bias resistor from 47K (I assume that it is somewhere around there) lower to say 10 - 27K.

You could also reduce the screen voltage by inserting a screen resistor (if you do not have one now). Running lower power input on the oscillator will directly lower crystal current.

You can reduce feedback by lowering the value of the bottom Colpitts capacitor  (if it is a colpitts type crystal oscillator)

Finally clean your crystal with Vodka and then reduce the contents of the bottle.

WU2D Mike

[KK4RF]

Mike,
     Thanks for the ideas. Darrell mentioned changing the 6V6 to a 6AG7. I checked the tube diagrams and I'd have to move parts around to different pins. Not a big deal. I'm off work this week and will be tinkering on it more later this week.
     73s and thanks to all for the ideas.
                      ---Marty, KK4RF---

[WA5VGO]

All you need to do is move a few wires around on the tube socket. If you don't like the results, just put it back the way it was. The 6AG7 is a much better oscillator tube than the 6V6.

73,
Darrell, WA5VGO

[KK4RF]

Darrell,
     I tried the 6AG7. It was actually easier than I thought it would be. While I was at it, I put in a few newer resistors and capacitors. It fired right up. The crystals run cool now with no noticeable heating to the touch. A 40 meter crystal that wouldn't oscillate with the 6V6, oscillates fine with the 6AG7. The rig now keys better on CW.
     It doesn't seem to like my new crystals for 3880 and 3885, however. They key OK on CW but sound awful on AM. My older FT-243s sound good on AM. Weird. Tomorrow, I'm going to check the screen and plate voltages. I did put in a larger screen resistor, using a new 27K resistor instead of the old 22K that was in the rig.
     Anyway, thanks for the neat idea. I'm going to keep tinkering with it this week. Next week, it's back to work.
                        73s,  Marty, KK4RF

[The Slab Bacon]

I have a Scrote 680 on the shelf downstairs, i havent looked at it or fired it up for some years now, but here's a thought. The original osc tube, was it a 6V6, or a  6V6-GT ? ? If it was originally a metal tube the circuit may be designed to need the shielding and/or additional capacitance from the metal envelope. Or is it possibly missing a slip on shield if it is a glass tube? ?

Mine is all original, and I dont remember anything too wierd to tune it up. I dont remember having to do any mods to it just to make it work. IIRC the osc and the final are fairly close to each other, so this could possibly be some kind of odd shielding (or lack thereof) problem.

                                                        the Slab Bacon                                 

[KK4RF]

Slab Bacon,
     That is an interesting thought. The manual on this rig shows a 6V6. The tube I pulled was in a glass envelope. It's an old Mazda 6V6-GT. I do have a few old, old 6V6 tubes in metal around here somewhere. It would have been interesting to have tried different ones in the circuit before I changed the circuit.
     In any event, I've already made the change to the 6AG7. Any other opinions on the difference between the metal tubes or the glass-enveloped ones in this rig would be appreciated.
     I do have 2 old Globe Chief transmitters here also. Different models but both used a 6AG7 into a pair of 807 tubes.
     Thanks again.
                      ---Marty, KK4RF---

[W9GT]

Hi Marty,

Interesting experience with the 6AG7 vs the 6V6.  I had a similar experience with an old 1947 vintage John Meck transmitter.  It originally used a 6L6 oscillator/multiplier driving a pair of 6L6s in the final.  The 6L6 just didn't seem to work reliably with xtals and with a VFO.  I ended up changing it to a 6AG7, as well.  The 6AG7 seems to be much more reliable, however, I did, in fact change the oscillator circuit a bit.  I can't remember the specifics, but I do remember that I used the same oscillator cirucuit as that used in the Knight T-50, which uses a 6AG7.  I also have a couple of Globe Scouts and they have always worked fine with the stock oscillator circuits.

There are tons of old rigs using 6V6s and 6L6s and they work very well.  Depends upon the xtals you are using and the specific type of oscillator circuit.  Anyway,  sounds as though you have resolved your problem.

73,  Jack, W9GT

[The Slab Bacon]

Marty,
         Just for the hell of it, look at the bottom of the toob socket and see what is connected to pin 1 If it was originally a metal tube, pin 1 will usually be grounded as it usually is tied directly to the metal shell for shielding purposes.

I usually try to keep a few metal 6V6s and 6L6s around just dor RF applications in old buzzard gear.

I cant remember ever seeing any 6AG7s with glass envelopes.    

As far as FT-243 rocks are concerned, I just dont trust the ones with the little biddy modern crapstals in them. I just look for ones that are cut for the lower parts of the bands and regrind them up to the AM frequencies. Its not really that hard to do.              
                                                               the Slab Bacon

[W0GSQ]

Marty,
         Just for the hell of it, look at the bottom of the toob socket and see what is connected to pin 1. If it was originally a metal tube, pin 1 will usually be grounded as it usually is tied directly to the metal shell for shielding purposes.

I usually try to keep a few metal 6V6s and 6L6s around just dor RF applications in old buzzard gear.

I cant remember ever seeing any 6AG7s with glass envelopes.   

As far as FT-243 rocks are concerned, I just dont trust the ones with the little biddy modern crapstals in them. I just look for ones that are cut for the lower parts of the bands and regrind them up to the AM frequencies. Its not really that hard to do.               
                                                               the Slab Bacon

Good stuff here.  http://faculty.frostburg.edu/phys/latta/ee/6cl6xmtr/6cl6why6cl6.html

Steve, W0GSQ

[KK4RF]

Jack, Slab Bacon, and Steve,
     Thanks for the thoughts. I did check the oscillator tube socket and pin 1 was indeed grounded. Curiously, however, the original schematic showed the 6V6 without a pin 1 on the diagram. Interesting. Feels like we're doing an archaeological dig on this old 50's style transmitter.
     Steve, the article on 6AG7 and the smaller 6CL6 tubes was neat. I'm going to try to get the referenced March 1950 QST article comparing the 6AG7 and 6V6 tubes in oscillator circuits. It sounds like the debate was settled 59 years ago. Thanks for the reference.
     All in all, I've had a lot of fun making the switch to the 6AG7 and learned quite a bit as well. Many thanks to all.
              ---Marty, KK4RF---

[WU2D]

I have used the 6V6 metal in my Paraset spy rig for 6 months and it does fine at about 6 Watts out. It keys nicely. That is not bad for a crystal oscillator. That oscillator uses a standard tuned plate approach but adds a gimmick for more feedback from plate to grid.

My first rig was an ARC-5 on 80M but I used an ordinary 6V6-GT in my first homebrew rig when I was a novice. That was a 6V6 into a 6DQ6 made almost completely out of old TV parts right down to the transformer and 5U4. It worked great on 40M and even tripled to 15M.

Back to the paraset: For fun I tried a bunch of old metal 6V6's in the Paraset to get a distribution. It was tight with the lowest output around 5.5W and the highest around 7W.

I also tried a 12A6 with higher fill voltage and even tried glass 6V6's a 6Y6 and a 6W6. All oscillated but the 12A6 was only 4W out and the 6W6 was almost 10W out and it cracked a 40M crystal!   

Mike WU2D

[KM1H]

6V6's seem to be touchy and I suspect its a combination of the actual manufacturer and condition. It is also a tube known for heater/cathode leakage doing bad things to the grid. Ive no experience with imports. OTOH the 6AG7 is always reliable.

I wouldnt trust any restuffed holder with the little ones. It takes very careful design to keep the current down.

Carl
KM1H

[KK4RF]

Gentlemen,
     I made a 2 transistor xtal oscillator/power amp and now I can use my 3880 and 3885 retro-fit crystals in FT-243 holders, and the thing has enough output to drive my Globe Scout 680-A! I'm hoping maybe someone will hear my Globe Scout tonight on 3885. I'll be running it barefoot. It's rated at 50 watts carrier input power so hopefully I would have 25 or 30 watts out.
     I'll be on around 8 PM eastern daylight savings time. Hopefully the thing won't sound too bad on AM.
     Thanks again to everyone for the help getting this old rig back on the air.
73s,  Marty, KK4RF