The AM Forum

I'm having a problem with biasing this apache.  It runs in ab1 mode in sideband and class c in am.  the sideband bias is 55ma and amplitude modulation is supposed to be 0.  I'm only getting able to adjust for a few milliamps in sideband and if I adjust the clamp for 0 ma on am and go on with the tuneup I have low plate current, it's supposed to be 250ma and I'm gettin less than 100 ma even with drive all the way up.  Sometimes I here a squealing? sound and the power jumps up and I immediately put the transmitter to standby.  Also I checked and the ob2 tubes are lighting up on sideband also.

actually I made a mistake there is no bias current at all on sideband

With the clamp adjustment, you want to adjust it where the plate current falls to almost zero without excitation; around 5 mils is a good point.  The purpose of the clamp circuit is to protect the final and power supply if grid drive is lost and it does so by pulling the final screen voltage down to a low level when there is no grid drive.  If you adjust the clamp control too far then it will greatly limit the plate current even with normal drive.

Be very careful where you set the drive control since 6146 tubes do not like excessive grid drive and will be damaged by it.  You don't need more than 6 mils for a pair of these and 8 mils is approaching the danger point.  Johnson added a special warning to the Valiant manual after they and RCA had a lot of warranty issues early on.  I imagine amateurs were used to driving the 807 which seems pretty robust with respect to high grid drive.

I don't have a TX-1 schematic on this PC so I am not sure about the SSB bias issue.  Practicing proper HV safety, measure the control grid bias and screen voltage on the finals while in the SSB position and this should point you in the right direction for the SSB bias problem.

Rodger WQ9E

Don't forget to use a jumper between two pins on the rear Accessory socket for setting the Clamp and/or Bias (I don't remember which one without the manual in front of me). Follow the procedure in the manual.

I keep it at 6ma like the manual says on the drive.  I don't see anything in the manual about jumpering anything on the acc socket.

woops I missed that, I can't believe I missed it.  I jumpered it and am doing everything else correct and the same result.  I'm going to look more at it tomorrow.  thanks

ok I was to tired last night.  checked it out again this morning and that was my problem with the ssb bias was the jumper.  I inserted the jumper and adjusted it to 55ma as it says.  then with no crystal in socket as it says in the manual I tried to adjust the clamp circuit for no plate current as the manual says and it rings a little louder as I decrease it but it will not go to 0

You don't need to get the plate current all the way to zero with the clamp control.  If it will go down to 10 mils or so you are fine. 

The only purpose of the clamp circuit is to protect the final and power supply circuitry if grid drive is lost while the rig is keyed on AM or CW and it will hurt nothing if even 20 mils is being drawn since the finals would only be running at 12 to 15 watts total dissipation.  Without a clamp circuit, if drive was lost the finals would be running at a few hundred watts input with all of that being dissipated in the plate circuit.  Of course it wouldn't run very long like that before smoke came out :)

If you adjust the clamp circuit too far then you cannot get normal input.

Ok.  Do you have any idea what the ringing would be from?

also when it starts to squeal if I try to increase loading and dip final like it says in the manual power shoots way up and ofcourse I flip it to standby!  Are the tubes self oscillating?

Are you in the CW or AM mode when this ringing occurs?  If in AM it could be in the audio section (RF getting into the audio for example).

If you haven't done so already, tighten all of the grounding screws in the vicinity of the final and driver.   A loose ground if often the cause of flaky behavior.

It could certainly be the final taking off; does it do this on 80 meters?  Try adjusting the neutralizing on 80 first because if it is way off it will definitely take off on the higher bands.  Once roughly adjusted on 80 then you can follow the manual instructions to complete the neutralization.

It may also be the driver.  Once again I am away from the manual and I can't recall how the driver is set up on the Apache.  I will post back later from home.

Rodger WQ9E

The front panel gain control (controls the amount of clipping) should be set to near zero during tune up. Also, the actual audio gain control is located behind the key jack. Make sure that's not turned up too far or you'll hear some squealing.

Well I found the problems I think.  There was a couple grounds that were loose, some bolts on the chassis.  And when I made sure the anode screw was tight the cap that is supposed to be on the anode slipped right off of the thin wire that comes out of the tube.  I don't know if this can be fixed, I think there is still vacuum but I don't no if I can get that back on properly or maybe solder direct?

For the anode connection; carefully clean and then tin the wire.  Don't go crazy with the heat in order to avoid damaging the seal and avoid physical stress to the wire-I use a very fine grit sandpaper for this cleaning followed by wiping with a clean cloth.  Make sure the cap is also clean and then slide it on and apply heat and then solder to both parts.  Although silver solder has a higher melting point I don't think you need it for a 6146.

I have re-soldered a number of plate caps on 6146 tubes along with replacing plate and grid caps on some 810's and have generally been successful.  It probably is a good idea to wear protective glasses while doing this sort of work just-in-case.

Rodger WQ9E

the wire goes in the cap so do you tin the wire then heat the hole cap to cause it to melt inside the cap?

You want the wire to be tinned so it will be easy to solder.  There should be a little clearance since the cap hole is a little bigger than the wire diameter.  Apply the iron to where the wire exits the cap so that you are heating both the cap and wire simultaneously and apply solder to the wire/cap junction.  The heat should cause the solder to flow down the wire into the cap but you don't need to try and fill the cap with solder; just a little solder is needed for a good joint.

There was a thread on here about different types of cement or sealant to "glue" caps back on after soldering.  You might want to search for and read that thread to help you decide about what to use.  Since none of the gear I have repaired tubes in puts any real stress on the caps and I don't use them mobile I did not "glue" the caps back in place.

Rodger WQ9E

ok thanks I'll take a look.  I was looking at some extra 6146s I have and noticed they are soldered at the top.  These are not it doesn't look like it's solid.  maybe I can drill a small hole

tightened up the grounds and fixed the plate connection.  i noticed that someone wrote 42% weak on a tube so someone tested them.  I've got a new set of tubes but I want to get the problem fixed first.  It wouldn't cause a spuealing right?  And also I am talking about a transmitter spueal I have no receive hooked up. 

Did you check both audio pots? Pull the EL-34's and see if you still get squeal.

Be sure the mic connector is tight on the front panel and the spring shield on that audio input wire is grounded good all the way through the system.

The audio inpoot section SUCK rat b@llz on the apache.  I used this mod: http://www.amwindow.org/tech/htm/w3sccmods.htm

and it works great.  It does away with the hidden gain control behind the key jack and uses the gain control as gain (makes sense).  It also does away with the lousy clipper(just pull out the 6al5).

As for the 6146s, I replaced mine with 8298 tubes which can handle a little more scrote.  I do still have the Raytheon 6146s that I got with the rig, but they don't fit in the sockes too well.  I also have and RCA 6146 and a Westinghouse 6146.  I am going to the Gloucester County hamfest in South NJ on Sunday and will probably pick up more.

Make sure the 6146s you have fit in the sockets well.  It sounds like you may only be driving one tube.  I had this problem with the Raytheons.

Tom - AB3FL

they fit well.  I don't even have a mic hooked up.  I'm just trying to tune the thing up.  If I go any farther and dip the final tune to start tuning the transmitter it will start arcing over on the capacitor.  I'm not putting any audio into the radio I'm just doing as the manual says my mic is completely unplugged right now

Are both audio gain controls set to zero?

What are you using for a dummy load?

If the connector is not tight against the front panel you can get RF into the first audio stage and away it goes.  Doesn't matter if the mic is connected or not.

Try doing as Pete says too, but if RF is floating around in that audio section it could be causing your squeal.

A good step to do is always Keep Separate the Audio chain from the equipments I use the iBox on a few of my things an The Audio comes through quite cleanly it offers good separation takes up minimal room it has a Pot in it for level adjustment and you can built it yourself. You can copy the skizmatic from the web sight an use a radio schmak case.

http://w2ihy.com/ibox.asp

Have all the Bad Condensers been changed out of this thing and the usual circuit cleanup been done First.

73
Jack.

I turned down both the audio pots.  I put the good set of tubes in.  I readjusted the bias but still when I start the final step of tuning up and start increasing loading while dipping the final(it says to do this up to 250ma).  but as soon as I start to increase drive it fries eggs and arcs the capacitor.  I need to go through and test the capacitors I guess like other people said.

What are you using for a dummy load? A 100 or 150 light bulb can also work as a temporary load. No load, or short in the coax to the load, can arc the heck out of the loading capacitor.

oh sorry I'm using a 50ohm mfj dummy load.  I found out that the loading capacitor was shorted out when fully meshed.  I starting with the capacitor a little less than meshed and followed the manual.  I got it to tune up to 50watts carrier out no ringing and no arcing.  However I did not get close to 250 ma like the manual says to keep increasing load until you get to 250 ma when tuning up.  But when I tap my meter i found out it sometimes goes to a different reading or goes to 0.  I'm going to check the meter connections.  Man one problem after another but it sure will be worth it when i get this thing on the air!