The AM Forum

It seems everything that can go wrong with this thing is.  I finally thought I had it fixed up.  I got the amplifier tuned up and putting out full power.  I went ahead and did the w3scc mod(I didn't mess with the audio before)  My uncle was sending me a d104 so I waited to just wire it up when I got it).  Got it tuned up and then adjusted the bias on the modulators.  everything as stated in the manual. I also moved the bias adjustment to behind the key jack.  Makes much more sense.  Got the bias adjusted and plugged in the mic.  heres where the fun began.  I began slowly increasing the mic gain control and herd a loud spark sound.  it appeared the modulation current soared but happened fast.  Now my plate voltage is at 400volts and theres a loud hum coming from I THINK either the rf choke or the modulation transformer.  I don't think I did anything wrong when I did the w3scc mod.  I sure hope that nothing is shorted. 

I guess I will need the resistance of the transformer and choke.  By the way I did turn back down everything to presets. 

I would disconnect the load on the HV supply and see what it reads.  Normally if its being loaded and the voltage drops that low I would expect the primary fuse to pop.  You need to re-establish the base line.  If one half of your full wave rectifier opened, you will be essentially running as a half wave rectifier.  In the mode, you will have much higher ripple voltage and hence much lower plate voltage when loaded.  Your problem as you describe it does not point to a bad mod xmfr.  Also if your HV choke shorted your voltage would tend to go higher but have more ripple voltage.  Some DC resistance measurements on those components should help in locating the problem.  Again, unload the HV supply and see how things look.  One step at a time! 
Good luck,
Joe, W3GMS     

I had to walk away from it for a little while before I got into messing with it  >:(    disconnecting the hv after the choke but before the filter caps it does not hum.  if I disconnect it after the caps it hums and I'm just getting the 400v.  I checked the caps and they aren't shorted atleast with an ohm meter.  capacitance atleast on my dvm is 150uf 25uf more than they are supposed to be each.  I'm going to get some other work done I have to do and go back and look some more.  I suppose it's probably those capacitors. 

I think I might have a problem now...I wanted to double check to see how the capacitors responded with no load.  Everything was fine.  I then disconnected the wires going to the transformer (including the jumper that leaves the transformer and returns back into the center tap of the modulator side).  I only hooked up the wire that goes to the top of the rf side so I had high voltage goingall the way to the plates finals but nothing on the plates....fine....then I hooked back up the modulation transformer wires and problems again!!!!  I then took an ohm meter and measured the modulator side of the transformer...everything was hooked up so theres error. This is what I measured....one side to ground was 183ohms, the other side to ground was 535 ohms.  across the whole transformer was 365 ohms.  I need to unhook the center tap and do this again but looking at the schematic I don't think this is right at all. 

I'm done for the night. 

By switching to CW you eliminate the modulation transformer.  So if you have full plate voltage and it operates properly on CW then look at the mod transformer.

If you have any resistance to ground with the modulation transformer disconnected, then it has a short.  Remove both leads from the secondary and measure to ground.

It bypasses it for cw but the same hv source that the finals use  still goes to the modulation side of the modulation transformer.  thats why I had to disconnect it.  I'm going to isolate the transformer some time next week and make sure if that is what is goin on.  I hope that that part of the transformer is on the outside.  I've never rewound a transformer I bet it's a real pain.

Went to go to sleep just now and it popped in my head to pull the el34s...maybe it's a shorted tube.  I did and thats what it was.  Shorted tube.  I guess the only thing that would cause that is a defective old part.  That makes me very happy.  Not cheap but better than the mod transformer

and what other mods besides the w3scc would you all recommend?  I'm planning on solid stating the rectifiers.  Also I've heard about the timtron turbo mod.  I guess this is the same mod for the dx100 http://www.amwindow.org/tech/htm/dx100.htm.  taking the 500ohm tap and tying it to the plate of a modulator tube. 

Nathan,
I am glad you found the problem with your Apache.  Before you do any mods, I recommend that you get the basic rig working and on the air.  There are many mod's that you can do to the Apache that will improve the audio quality.  I did my own using the stock mod xmfr but did change the interstage transformer from one purchased from Antique Electronic Supply.  The Mod's were fairly basic and did not require any drilling and blasting.  The results on the air sounded excellent.  Basically, I redesigned all the class A voltage amplifier along with a installing a new driver transformer.  I also utilized a small amount of negative feedback and the results were very pleasing.  The clipper was also bypassed.  Fran's mods should work out well also. 
Good luck with your project and hopefully your having fun! 
Joe, W3GMS     

Am I mistaken or can't you just pull the 6AL5 clipper tube and disable the clipper?  The audio is rectified to ground.

The Apache circuit is similar to what Johnson used in the Valiant and Viking 500.  If you adjust it for minimum clipping then it doesn't clip the audio anyway.

In the case of Johnson, the "secret" is that minimum clipping is with the clipper control turned fully clockwise.  In the case of the Apache, you have to properly set the control hidden behind the key jack or otherwise you end up with a lot of clipping.

Finally, great audio is only great if the receiving stations can actually hear you.  It is fine for local and good conditions but there are many times where the ability to dial in some clipping is very helpful to the receiving station.  There is a reason why these circuits were installed in the first place.  It is no different than the speech processing in modern SSB gear-it sounds better without it but it has its place in contesting, DXing, and most importantly weak signal work.  One of the funniest things I heard on an AM net was net control telling a station I am sure your audio would sound great if only I could hear you...

Rodger WQ9E

Yep, that was a fortunate fix.

I always try to learn from any repairs done by others. In this case, when a rig has been known to work in the past and the problem is in the high power area, the tube is a very likely suspect. Tubes, unlike most other components, have limited lives and are especially prone to becoming gassy from sitting unused. I've tested many many power tubes over the years and by far, the biggest problem is usually a short when HV is applied due to gas. Once the seals get grosssly overheated or the cement around the pin-to-glass fails, it's all over.  


These days, whenever I get a power tube that hasn't been in use for 2 months or more, I'll let it sit with the filaments on for at least 10 hours, or more. This gives it a channce to getter out the gas, if any.  Even a potentially good power tube can arc if put to the HV too soon if dormant too long.


I just made up a test fixture that puts air and fil power to my spare linear amp tubes. I plan to circulate the spares every few months thru the tester. It does pay to warm them up on a continual schedule.

T

On a stock/unmodified Apache, the clipping control is on the front panel. The first few degrees up from zero will provide only a few db's of clipping. The audio control is located behind the key jack.

   I once tried using the EL-34 as a RF linear amplifier in the Central Electronics 20a. I found that several of the cheaper imported brands were fragile in case of an overload, quickly became gassy, and would self destruct. The one exception was this one:

http://www.pentalabs.com/pdfs/EL34S.pdf

   I highly recommend the Teslovak E34LS for any application that originially used a 6CA7, or EL-34.

Here is the ordering page for a matched pair:

http://www.pentalabs.com/Product.aspx?dept_id=1516&pf_id=e34ls%20mp

Good Luck with that Apache!
Jim
WD5JKO

http://www.dilbert.com/strips/comic/2009-12-15/

haha I liked the comic strip.  I atleast always learn something new when I have problems.  thanks for the info on the tube.  I will look into it theres alota el34s out there.

When the tube shorted out was it by any chace in the horizontal position in other words was the Apache propted up on its side. Sometimes this is a convenient position to temporarily run the rig so that you can acess the bottom of the chassis while making measurements but unfortunately tubes with high voltage on them, especially the EL34s are prone to shorting out when you run them in the horizontal position, ask me how I know.

QHQ makes a great point and I would add to that be careful with rigs that use MV rectifiers and be sure to run them through a proper start up procedure as detailed in the manual.

Due to a 6AG7 short I had a near meltdown with a Central Electronics 20A while doing alignment.  I had it on its side and noticed the power output dropping and I then saw the rectifier plates starting to blush.  A 6AG7 had shorted and I had forgotten to check the fuse in this particular hamfest special.  It had a 20 amp fuse (which I promptly replaced); no lasting damage other than 1 bad 6AG7 but only because I got lucky that time.  I simply forgot to check the fuse that time; a mistake I have not made since.

Rodger WQ9E

Rodger,  your point about the fuse is a very good suggestion.  In the last three pieces of equipment I have worked on, all three had fuses double the recommended size.  None of them had problems so I would bet they blew for some reason and the owner didn't have the proper current value and installed what he had on hand.  Now I check the fuse of each piece of equipment that comes in the door first thing.

My Apache has been resting between my tantrums. I have not touched it for about a year - perhaps it is time to get back on the horse.

Hey QHQ - When are we going to hear you on the air with your Apache?

Mike WU2D

   Mark,

    OK, so tell us more about the EL-34 sensitivity to position. I recall a lot of controversy on this topic with the 811/811A, but never with the 6CA7/KT77/EL-34/E34L family of tubes. So what happens, and what was your experience with these tubes. You got me curious.

Regards,
Jim
WD5JKO

My experience with shorting 6CA7s has fortunately been breif, I tend to learn from expensive mistakes. I am sure that you can run them horizontal under certain conditions, I would imagine proper orientation of the envelope is one thing that might help, elements do sag when they heat up. I also tend to run alot of voltage on my PA and modulators to keep the efficiency up. My Apache has SS rectifiers so that and higher than 1960s line voltage keeps thing on the edge as far as plate voltage is concerned. I noticed in this month's ER a modulator article where 6CA7s are horizontally mounted, hope they survive.

It is good to keep in mind that we are operating most of these tubes many,many years beyond their design life intent.  Element sag or movement with heating is going to be worse for most tubes when operated in the horizontal plane.

I prefer to minimize the risk to large and expensive tubes so if possible I would keep them vertically oriented both during testing and normal operation if possible.

I didn't catch that article in the current ER yet but I assume there was a mechanical reason for wanting the tubes to be horizontally "polarized"?  Except for trying to save space in a mobile unit or perhaps to achieve better exciter to final shielding by mounting a driver tube under chassis it doesn't seem to be good practice to take a chance with expensive tubes today.  I see that 6CA7 tubes have no orientation restrictions imposed by the manufacturer but I still have a feeling that horizontal mounting increases the odds (but doesn't guarantee) a shorter life.

Just my opinion as a member of PETA  (Protect Every Thermionic Amplifier)

Rodger WQ9E

Gee, Mark you are coming up on your QSO of the year. Do you still have a valid license...I'm BYB to get you on the air.

Exactly Frank - when are we going to hear Mark this year? New Years Eve?

I didn't have it on its side THAT time when I was applying plate voltage.  but I had earlier.   It's almost got all new tubes in it now.  I might as well replace the rest while it's down.  Also going to go ahead and replace the rest of the caps to.  I'm going to wait until i get it going first.

ya I thought about changing that to operate on the power switch and just let it warm up a while before operating. 

Put the rig on a power strip. You should not have power to it when you are not home anyways. 

The Japanese Audiophools will give you BIG BIG money for those Mullard EL34s :)

C

  The EL-34s are operating in the Apache on the ragged edge of their design limits. Most power tubes resort to plate cap terminals as they approach these voltages. With the Apache the margin for error is small. Some operators ,in an effort to "take it easy" on the old rig will load the final below the rated current. With the Apache, with just a little too much audio this can result in some higher than intended voltages in the modulator. In damp environments or even in the case of  dusty EL34 sockets arcing is not uncommon. There have been plenty of Apaches with carbon tracked sockets from this condition. I had one that actually welded the tube pin to the socket contact. The next likely spot for an arc is inside the tube itself.
 

Yea, the following web sight sells them for $425 / pair!

http://www.audiotubes.com/el34.htm

** See the two UTUBE video's on the EL-34 at the same website  listed above...

I bought a Gonset G50 off Ebay, and to my surprise instead of a pair of 6L6 modulators, it had a vintage pair of Genalex KT77's! Those are worth a lot if a NOS matched pair. A nice surprise.

Jim
WD5JKO

The El34's will work just fine.
Bill

is there actually a noticable tone difference in the different tubes that would concern us?  I play guitar and I no they say it makes a difference in guitar amps but unless you a/b them I don't know if you could tell.

         Keep the sockets clean and dry. Make sure to back off the audio when the final is loaded lightly .  Recessed sockets help but they can still flash over to adjacent pins.