Johnson ranger 2 el34 idle current

[kc2etm]

Hi I purchased a used Johnson ranger 2 off the internet it has a lot of the timtron mods in it and some unknown mods  the modulators seem to have been replaced with jj el34 tubes it was working but I think I goofed up I followed the manual to set things up which was probably not to smart I set the idle current for modulator according to the book which from reading I now realize was probably wrong for the el34 tubes well anyway I was modulating it into a dummy load I was getting a ringing from the tubes or transformer and some high frequency feedback then a nice arc and smoke seems pin 2 and 3 on one of the el34 sockets arced over pretty good so I have ordered some ceramic sockets to replace them do you think this happened because the idle current was to low any ideas where I should set idle modulator current for these two el34s who ever modded it did include a nice simple adjustment for that


Thanks
fred

[W3GMS]

Hi Fred,

If the driver has been changed to a hot cathode phase inverter, the modulator needs to be biased for AB1 operation which requires more resting current.  I am "guessing" something around 90 MA for the two tubes.  If it still has a driver transformer that will support some grid current so you could easily run them in AB2 operation.  Best pull some spec sheets off the internet and look at the recommended static resting plate current for the different classes of operation.  If you don't have a schematic of the mods, try to get one or trace out the circuit.  Its nice to know what your dealing with.

Joe, GMS   

[kc2etm]

Hi Joe it is a phase inverter setup everything looks very close to the timtron mods with a few differences I had the current set much lower then that do you think it could have caused my arc or just a bad plastic socket when it was working it would break into distortion before I could turn it up enough to hit 100 percent on the scope

[kc2etm]

well I just tested again with the higher current it did clean up the distortion but as soon as I turned up the mod control zorch on the el34 that arced  im reading short between 2 and 3 heater and plate on the other tube the non arcing tube im not reading a short funny thing is it works until I crank up the mod control then zorch

[WD5JKO]

    Fred,

   You have the pin 3 problem. Folks who repair guitar amps know a lot about this. Many popular tubes, 6L6, 6550, EL-34, 6V6, etc. have this problem. What is it? Well pin 2 is the filament, and pin 4 is the screen grid. That plate at pin 3 will swing twice the plate voltage, and under overload, a lot more. When under loaded (Hi-Z load), a whole lot more. We like to have at least .1" gap / 1 KV, and with no sharp points, like wires protruding.

   If you look at several socket types, this gap varies a fair amount. The ceramic Johnson sockets, although unlikely to carbon track, have a pretty narrow gap between the pins. Some sockets have insulating material around the pins in a way that the gap is increased 50% or more.

   I know the EL-34 is rated at up to 100 watts RMS audio with 800V on the plates, but with what sockets? I would look for not only new sockets, but look for some with some insulating meat around the wire terminals.

   I prefer using screen grid drive, and class B to sweep tubes. Look up the 6AU5 or 6AV5. These don't have the pin 3 problem. They use pin 5 for the plate when pins 4, and 6 are not connected. Also look at that Gonset G77 as it uses two 6BQ6's (these have plate caps) to modulate a single 6146.

  Many folks use the Hi-Fi output tubes in a Ranger like the 6550 or EL-34. You will find that an idle current that approaches the plate dissipation of the tubes is necessary to get low IMD. So for EL-34's, the Pd is 25 watts. That would be about 40-50ma per tube idle current when the B+ is around 500v. With the Gonset arrangement and sweep tubes G2 driven (G1 grounded), the idle current can be about half what I mentioned. There are many Valiants in the NE heard on 10M that use this arrangement. They sound sweat too.

Jim
Wd5JKO

[kc2etm]

Thanks Jim I have ordered some ceramic sockets for it not only are the ones in it not ceramic they very cheesy looking lol I did have the current set at only 65 when it first happened so I would imagine the combo of cheap sockets and not enough current caused my meltdown so I will put in the new sockets and get a new tube and set the current higher and see what happens I did see lots of pictures of burnt in the audio forums all related to the el34

[W3GMS]

well I just tested again with the higher current it did clean up the distortion but as soon as I turned up the mod control zorch on the el34 that arced  im reading short between 2 and 3 heater and plate on the other tube the non arcing tube im not reading a short funny thing is it works until I crank up the mod control then zorch

Glad that fixed one of the problems!  With a scope, you can observe the audio at the grid of the tube and will instantly show if the tube is drawing any grid current while being driven from a phase inverter.  When biased properly, you should see a clean undistorted waveform providing its a clean speech amp / driver combo.

I modified my Ranger some 25 years ago to use 6550's and its proven to be a very reliable transmitter with zero problems.  Be careful with the negative feedback configuration.  Improper phase shift can cause a problem in addition to small amounts of RF being fed back into the audio amplifier which will make the audio chain right on the edge of stability.  When working on the Ranger, the last thing I hook up is the negative feedback  and in some cases once everything else is right, you will find you need very little of it.       

Now off to your other problems which you have received some good advice about.

Joe, GMS

[WD5JKO]

Interesting place that sells what appears to be a quality Octal socket:

http://www.caryelectronic.com/index.php/amp-parts/tube-sockets/gilded-8pin-ceramic-vacuum-tube-socket-top-mount-valve-fr-vp41-sp4.html

Condition: New

For 8pins tube: VP41 SP42...

Working Voltage:1500V

Contact Resistance >0.01Ω

Insulation Resistance <10000MΩ

Useful Life: 200 times   <-----What does that mean?? 200 insertions??

Notice the little ridges on the back between pins. This increases the path length for higher standoff.

Whatever you do, don't have anything pointy on the lug for pin 3. In fact, a piece of heat shrink over the lug and wire  for each tubes pin 3 should help.

Go a little further and make your own spark gap so a high voltage burst has a place to go such that nothing dies. We do this on modulation transformer secondaries on big rigs...Why not little rigs as well?

What happens when your keyed up, audio gain is way up, and you drop your microphone? 

Jim
Wd5JKO

[KD6VXI]

That's Denny Hadd,  of Dentron and Amp Supply fame,  Jim.

--Shane
KD6VXI

[W3GMS]

Here is another source that offers several different styles of octal sockets.   Scroll through all the pages and you will see the offerings.

Joe, GMS

http://www.tubesandmore.com/search/node/tube%2520sockets

[kc2etm]

I have sockets on the way is there a better tube option then the el34 that will require minimal changes from what I've read it seems these have chronic arcing issues

[kc2etm]

Well I put in the ceramic sockets today two new tubes and the same tube flame out again tried swap the two and same socket flames out so it must be something on that tube

[kc2etm]

just tested some resistors feeding the tubes I have a two 222k feeding pin 5 on each tube one reads 215 the other reads 285 I don't have any on hand right now to swap them out but do you think that could be causing some sort of imbalance in the tubes

[kc2etm]

From looking at the timtron modulator schematic it looks like these are r18 and r19 which it says must be matched
R18,R19 150K - 220K modulator grid resistor - pair must be matched so maybe this is the issue the one that reads 285k is the one that's arcing

[WD5JKO]

Fred,

  I wonder what the plate voltage is in that rig? A slight imbalance in the audio drive, or variation in the DC operating point should not cause the thing to arc. With the guitar amp folks, they lose the socket when they insert that 1/4" plug causing a momentary overload from 60 hz.

  Could the modulator be on when the RF tube is not putting out any RF? If the modulator were unloaded, then the peak plate voltage (pin 3 to pin 2) could be sky high. Is it arcing pins 3 to 2 or 3 to ground? Did you make the pin 3 connection nice and smooth (no pointed sharp points), and heat shrink over the pin connection?

  73,
Jim
Wd5JKO

[W3GMS]

From looking at the timtron modulator schematic it looks like these are r18 and r19 which it says must be matched
R18,R19 150K - 220K modulator grid resistor - pair must be matched so maybe this is the issue the one that reads 285k is the one that's arcing

Fred,
I don't have Tim's mod's in front of me but I believe the resistors your talking about are in the phase inverter circuitry.  The reason the values need to be matched is to provide equal amount of P to P audio at the grids of the modulator tubes. 

Remember Tim's mod's have been duplicated in rigs many times so I would not focus on design issues since the mods are very well proven.  If someone else got in an messed with things that's another story! 

Another thing is to only put one modulator tube in at a time without applying any audio.  Look at the resting current with one tube and then the other.  That will give you a balance figure for the tubes at least at idle.  If that looks good and the P to P audio at each grid is good then you should be good to go.  There is a chance of an imbalanced winding due to a fault in the mod iron.  If you want to look at that without all the HV, just take a small filament transformer and feed voltage into the secondary and each side of the primary should have the same voltage. 

Just some thoughts...

Joe, GMS
   

[AB2EZ]

R18 and R19 are used to feed both bias and average grid current to the grids of the modulator tubes.

With due respect to Tim's design:

If the values of these resistors are not equal, or if the two audio coupling capacitor values are not equal, or if the values of the plate and cathode resistors of the phase inverter are not equal, or if the modulator tubes are not well matched (in terms of grid current v. grid voltage when the grid is positive with respect to the cathode)... then the tubes will not be biased properly. I.e. the grid bias voltages will not be equal.

Note:

The problem will not be apparent when there is no audio applied to the modulator. The biasing problem will only occur when enough audio is applied to cause the modulator tubes to draw grid current on audio peaks.

As a tweak on Tim's design...

Try using a pair well matched 150k ohm 1W resistors or a pair of well matched 100k ohm 1W resistors. Make sure that  C11 and C12 (the two coupling capacitors) are well matched (e.g. from the same production lot). Make sure that the plate and cathode resistors, R16 and R17, of the phase splitter are also well matched 68k ohm 2W resistors.

Stu

[W3GMS]

Fred,

I guess the resistor values you gave me were not the ones I thought.  I was referring to the phase inverter resistors and not the grid resistors of the modulator tubes.  Again, check and see how the resting current compares with what you have in the circuit now as I described.   

Joe, GMS

[AB2EZ]

Again:

The problem will not be apparent when one looks at the resting current.

The problem only arises when the audio input to the modulator  is large enough to cause grid current to flow into the modulator tubes on audio peaks.

The flow of grid current on each audio peak (particularly if the audio signal is a sine wave) causes the charge stored in C11 and C12 to change (but not equally, unless everything is balanced). These charges are slowly removed via the currents that can flow through the resistors that connect to each side of C11 and C12, respectively. This, in turn causes the grid bias on the two tubes to become unequal (unless everything is balanced)

Stu

[W3GMS]

I certainly understand all of what your saying Stu but,,,,,,I have found merit in checking balance statically on modulator tubes.  I DO realize this does not correlate to being matched as the applied signal drives the tubes into further conduction.

Joe, GMS

[kc2etm]

update well when I pulled the resistors out to test them today and change them yes I tested them in the circuit yesterday don't tell anyway when I pulled them one was reading 215k the other 360k upon closer inspection one was orange orange yellow and a 330kk the other was red red yellow and a 220k it looks like someone wasn't wearing their glasses when they worked on this so I swapped in a new pair of 220k and the problem is gone and yes it was definitely happening on audio spikes for instance when tapping the mic the tube would start melting down so far it seems to be holding on the dummyload though im afraid to crank it open just yet just want to thank everyone for the help on this issue   

[kc2etm]

well as soon as I cranked it up the other socket flamed out but I havent changed that to ceramic yet and I was also using the previously burned tube in that side now so it was probably a carbon short so I ordered two new tubes and I will change that socket to ceramic and see what happens seems these tubes are more trouble then they are worth

[AB2EZ]

If you look at the specification sheet for a Mullard EL34, you will find that, in push-pull AB1 configuration, the recommended plate voltage (DC value) is 375-400V, and the associated fixed grid bias is -33V (at 375V plate voltage) and -36V (at 400V plate voltage).

This means that in recommended operation at 400V DC, the peak plate-to-cathode voltage, during modulation (when one tube has 0 volts on its plate and the other tube has maximum voltage on its plate), is 800V, and the peak plate-to-grid voltage is 800V + 72V = 872V


http://www.mif.pg.gda.pl/homepages/frank/sheets/129/e/EL34.pdf

In the Ranger, depending upon the AC line voltage you are using, and depending upon whether the HV B+ has been "solid stated"... the DC plate voltage you are applying to the EL34s may be as high as 700V. Furthermore, the associated fixed grid bias would be around -50V.

Adding to this, the "clamping action" of the RC coupling between the phase inverter and each grid, and the fact that the peak of the applied audio grid voltage + the fixed grid bias voltage may be greater than 0 volts... the peak plate to cathode voltage may be 1400V, and the peak plate to grid voltage may be something like 1400V + 120V* = 1520V.

*If the audio signal applied to the grid of each modulator tube has a peak-to-peak voltage of 120V (i.e. the peak of the audio voltage waveform is 10volts more than the negative DC bias supply voltage), and if the RC coupling to the grid of each modulator tube clamps the positive peak to 0V relative to cathode... then the negative peak will be -120V relative to cathode.

Even though these types of tubes can often be operated at voltages higher than the recommended values... it seems to me that you may be pushing the capabilities of this particular pair of Mullard EL34s.

By balancing up the components in the audio driver/phase splitter... you may have introduced a small reduction in the peak values of modulator tube element-to-element voltages ... but not enough to eliminate the arcing.

You might do better with a different pair of EL34s (sometimes particular tubes will arc over at lower voltages than other tubes of the same type because they are "gassy")... but, maybe not.

Note that the Phillips EL34 specification includes Class B push-pull operating configurations with DC plate voltages that are much higher than those in the Mullard specification sheet. I.e. DC plate voltages above 700V. Perhaps the arcing would not occur with Phillips tubes.

http://www.mif.pg.gda.pl/homepages/frank/sheets/129/e/EL34.pdf

Stu

[AB2EZ]

Thinking about this a little more...

The plate-to-suppressor grid voltage will be the same as the plate-to-cathode voltage if the suppressor grid is grounded. The plate and the suppressor grid are also adjacent... and therefore the field between them (volts per meter) would be the highest.

Therefore, inside the tube, an arc is most likely to form between the plate and the suppressor grid.

The addition of the suppressor grid in the EL34 v. a 6L6 or a 6550 (with the screen grid between the plate and the control grid) may make the EL34 more susceptible to arcing at high plate voltages.

I also agree with Jim (on careful re-reading of this thread from the beginning)... that if the arc is originating at the socket.... between pins 3 and 2... it is because pin 2 is at ground potential (or ground +/- 8.9V peak) and pin 3 is connected to the plate of the tube. So, as Jim says, you need to be sure that these pins are kept as far apart as possible, including the effects of the attached wires and solder.

I wonder if the arc can originate inside the tube... and then migrate to the socket?

Stu

[W3GMS]

well as soon as I cranked it up the other socket flamed out but I havent changed that to ceramic yet and I was also using the previously burned tube in that side now so it was probably a carbon short so I ordered two new tubes and I will change that socket to ceramic and see what happens seems these tubes are more trouble then they are worth

Fred,

Not sure what brand of tubes Heath used, but the Heathkit Apache uses EL-34's with according to the manual, 700VDC on the plates.  I forget what the screen voltage is but could check.  I have never heard or had any problems with those tubes in an Apache and that includes mine as well.  I do realize and have had some sockets issues.  I think I have ceramic sockets in my Apache for the modulator.  So there is a rig that works quite well with whatever version EL-34's they used.   

Joe, GMS