The AM Forum

I'm looking to replace my 6L6GCs in my Ranger.

Who would like to comment on substituting  Philips/ECG  6BG6GA for 6L6? 

Michael at SMD has these botels fer a good price.   I cant seem to find anyone who wants to get rid of the '6GCs.

klc

Hi Kevin you ol' rascal,

Very rarely do I order tubes , but I came across this additional (new for me) source recently:

http://www.dougstubes.com/

as I recall the plate voltage in the ranger is a little high for 6l6gc .... if you want a plug in replacement use a 1614 or 7027 or 6550 ... the 6bg6 are fb with the plate connection above the chassis ... even a el34 is good .... bias adjustment will be necessary with some ... I would suggest adding a 10 ohm resistor in each cathode to check idle current/balance

Right now, i'm running the plates at 440v; the Design Max is 450V ... yes, its still a bit too high.... the Ranger came with the GCs, so thats whatt I've been running.  The 6550 or the 7027 s are a bit to expen$ive for me. But i'd still like to find a few of them.

The Philips tube is purported to be  "  ABSOLUTELY IDENTICAL to the Sylvania 7027A/6L6GC because they were made  with exactly the same components.  They have the larger 35 watt plates found in only the 7581A/6L6GC or 7027A which means they will work in ANY 6L6 or 7027A application.

http://www.vacuumtubes.com/6l6.html "

On another note, my audio out of the Ranger 2 is 'muddy' and needs more audio.... so i'm plugging in new tubes to see if that will help.

klc

The 1614 is just a metal 6L6 that was specially selected from QC tests.  I have seen tubes labelled "1614" on the envelope, but "6L6" engraved on the bakelite base.

There are other modern tubes that would probably work.

Sovtek offers 5881WXTs and 6L6WXTs which are Russian military tubes.

The audio types claim both work well at 500 volts with long life.

Of those the 6L6WXTs have higher PD ratings.

They are available new from ebay or amazon.com

Dave

6550s pull more filament current of a 6L6, 1.6 amps each.   Metal 6L6s are rated for about 360 plate volts.

I run 6550s for my single 6146 rig.  650 plate volts.  The 6550s need about 30-35 volts bias.  At that voltage limit the idle current to about 80 ma.

Fred

Thanks fer the input(s).

I'd like to run 6550s like the Tron, but these things sound real expensive...... 


I'd forgotten about the el34 .....  Ive got two 6CA7s in the 500 Watt modulator (that has not been married to the 813 rig). But I'd rather keep them where thay are. They have nice pictures of some kid blowing a horn on 'em.

The price on the 6BG6GA is very, very reasonable......

After replacing the 12ax7 and 12au7, the Ranger still sounds muddy, so the next (easy) thing to do is replace the modders.

klc

I have swapped in everything that will fit into my ranger about 5 years back. 

Dont over look the 8417.

C

If the rig sounds muddy changing tubes will make little difference. You have some sort of restriction of the mid and/or high frequencies. Fix that before worrying about the modulator tubes.

" If the rig sounds muddy changing tubes will make little difference. You have some sort of restriction of the mid and/or high frequencies. Fix that before worrying about the modulator tubes."

S, Yer right... I shuda left out the "muddy" part and added the "needs more audio".

klc

Ah, mo betta. Can't go wrong with the 6550s.

The 1614 was a production 6L6 that was tested for RF at 6-8mc, stamped a 1614 and charged more money.

Its just like Eimac and some of their Y* tubes.

If a pair of full emission 6L6's of any flavor cant modulate a Ranger something else is wrong.

Carl

One thing to be careful about - and I buy several hundred of these tubes a year - they are not NOS and they do not necessarily meet the old tube specs.

They are guitar amp tubes  - not Russian military tubes as often advertised - they are made in three facilities (two in Russia and one in the the Czech Republic). Some are also from China.

Some  are packaged in a pretty Mullard or Tung Sol box and are essentially the same tube that sells for 50% less not in the pretty box. The box is the only thing they have in common with Mullard or Tung Sol.

The whole guitar amp tube business is a bit scammy and often reject tubes hit the market at low prices. Recently, I could not get any Sovtek 5Y3s because a giant shipment of likely rejects came in and every one had glass chips rattling in them.

I built a  40 meter transmitter with 6L6s last year and was amazed at how the different tubes performed. Some guitar amp tubes were OK and some were awful - the best by far was a 6550 10 years old made by Svetlana before they went out of business. It was better than several NOS 6L6s that I tried.


Pat
N4LTA

The 6l6 is rated at 375 volts max. The 1614 is rated at 550.

And both will hi-pot the same. RCA had to do something to get the big bucks for the 1614 and that was write new specs.

All we have to go on is the published data from the manufacturer.  Anything past that is just speculation. 

C

In all this, the only real complaint is that the sound is muddy. What has been done to find out if the modulator (or mudulator in this case) is actually able to fully modulate the transmitter? Output tubes are seldom responsible for poor sound unless they are bad. Muddy sound is usually a frequency response problem or lack of enough modulation.

Considering the modulator B+ supply is fixed, there can only be so much voltage swing transformed to the RF stage.

Changing tubes won't fix that unless you change to some that can deliver more plate current at lower screen voltages than the stock ones. Then you might get a little more swing towards zero on each plate at the point where the tube is saturated. The so-called "sweep tube" is a good candidate.

The plate dissipation is not as important as cathode current capacity. This is because it is intermittent voice service.

Welll..... I did ask if anyone had comments 'bout the Philips/ECG  6BG6GA.  The price is very gud; I'd like to grab a few if they really are any gud.

I've been told that the % of mudulation was around 50%. When pressed for a critical report, the sound was reported to be muddie.

I swapped the D10-4 leads around so the peaks point the right way, this made a few happier.

I swapped out the 12ax7 and 12au7 and it didn't seem to do too much. I don't know if the mod tubs are gud. I didn't have spares, so I didn't change the mod tubes. I do have a pair of ruskies on the way. (where is Tolly?)

The interesting values for the 6l6gc are, plate 480, screen 290 and the grid -25.   (the book wants 28V, but this keeps the mod curent down around 60)


klc


 

The Ranger should modulate at or near 100 percent, even with 6L6s. Might be time to take a critical look with a scope.

Rangers quite often sound good right out of the box, and dont have any problems making 100% (or at least real close to it).

Mod resting current and grid biass should reflect AB1 (or AB2) operation with a lot of current swing. (Munky Swing)

Early Rangerz used cathode biass on the modders and the later ones had a biass supply. If ur audio is low and / or distorted, check the cathode resistor for the modders, it may have gone high on you. Or ur schreen supply may be sagging.

As far as mods to the audio section go, there are more recipes than grandpa's BBQ sauce. So a little reverse engineering may be in order.

Sounds like you're due for some voltage and resistance tests........

I tend to believe those that worked with them when new and TCA members.

"Sounds like you're due for some voltage and resistance tests........ "

C     Problem is, they are mostly within 'specs'.  Ergo, the head pounding.  I've got a new pair of 6l6GCs to plug in sat afternoon.  Oh well, its probably something easy and I'm just a bone head who can't see it.


klc


... but I have value

About 20 years ago, I was working in a recording studio and we were ordering a lot of Russian tubes. Again, this is before Svetlana went out of business. We found that the Sovtek brand tubes had a very "hard" sound to them, and the Svetlanas tended to have a much warmer, 'rounder' sound. So, we tubed up a guitar amp with Sovteks for rock tracks and the Svetlanas were a natural for a country lead.

Im sure that a careful use of a distortion meter while rebiasing can make any tube sound hard or soft.

In the ham world hard means splatter.

Just to clarify, Carl, all we did was a straight swap of tube brands of the same type with no rebiasing or other circuit changes of any kind.

Weekend Update.

What were thought to be 6l6gc was actually 6l6GA. Oh well. Whatts an extra 120 volts on the plate gonna hurtz?

Pluging in the Ruskie tubes. Ive got much more swing on the modulator current. After asking for a few 'critical' audio reports ......  The audio sounds 'nice', the depth of modulation is still crappy. Time to switch out the D10-4 and see if a different mic helps things out....


klc

Kevin

A lot of AMers use substantial amounts of compression (audio chain) to increase the average loudness of their signals. I use a moderate amount of compression; but even with that, I sometimes receive reports (from AMers using spectral displays or meters that provide peak and average audio power measurements) that my modulation depth is low. But, looking at my transmitter's modulated rf, on my digital oscilloscope... triggered by the input audio signal, I can clearly see that my modulation levels (positive and negative) are fine. Also, I can see this using my WA1QIX modulation monitor.

Unless you monitor your modulation with an rf sniffer and an oscilloscope, you won't get a good "picture" of what your transmitter is really putting out.

Also, using substantial amounts of compression significantly increases the distortion products ("splatter") in one's signal. People who use a lot of compression sound louder... but they have very wide signals.

Stu

Stu,

Yes, the scope is the berries....  My bench set up is lacking. I've got hummm all over the patterns and its tough to read things with my Tek 453. Anyway, with a 100mV, 3khz signal feeding the Ranger, I'm getting about 60% mod with the modulator biased for 60mA static, it goes up to ~71% at a 70mA current. This is with the pot maxed out.

Compression....  Yes, its a good way for a lousy signal to sound even lousier. At least that is what Les Ismore says. Maybe I'll find a cheep CBS Audimax to play with, but not for now.

klc

Kevin,

   The Hum on the scope needs to go!

If you are using a random wire as a scope pickup, then just shunt the wire to ground with a RF choke (like a 2.5 MH) at the scope input. Hum gone!

Or at the Ranger power levels, use a 10X scope probe right at the Ranger RF output. It will be safe so long as the Ranger sees a 50 ohm RF load. Use the scope ground clip too. At higher power, I just insert a 10 pf cap (3KV) in series with the scope probe tip.

Since you see an increase in modulation power as you increase the static modulator plate current, that suggests your modulator is limiting after class A operation and something is caving in as you transition to class Ab1 operation. More than likely the modulator screen voltage is falling, or the bias is shifting around (class Ab2).

I find it amazing how good some rigs can sound with nothing more than a D-104. Don't go crazy buying boxes outside the rig when the problem is inside the rig.  :P



Jim
WD5JKO

Kevin

One way to increase the modulation depth is to lower the B+ on the 6146 (but not the B+ on the modulator tubes).

This will, of course, lower the average output power; but from the perspective of how you sound on the air, you will actually sound louder. This is because of the AGC/AVC action in a typical receiver. In a roundtable... after the listener adjusts the volume level... signals with a higher ratio of carrier to modulation will sound too soft.

There are a number of ways to lower the B+ on the 6146. I use a separate, adustable, external B+ supply in conjunction with the 9 pin socket on the back of the Ranger. By adjusting the B+ on the 6146, I can adjust the average power level. Since we are only talking about a maximum of around 600V at 125mA, this is not an expensive proposition. I use a small 120VAC=> 480VAC "control" transformer and a bridge rectifier. I use a small variac at the input to the transformer.

Separately... in my experience, by far the weakest link in the Ranger audio chain is the interstage audio coupling transformer. It produces a great deal of distortion, due to saturation. There are lot's of ways to replace it. One way... that I used... is shown here:

http://mysite.verizon.net/sdp2/id13.html

Stu

Thanks fer the feedback.

I'm using a few turns of wire off of the 10X probe as a pick up. The choke thing is a better way to go. I'll recheck the  bias as I modulate the rig. The disturbing thing is having to max the audio pot to get apreciable output.  Something is wrong with the 12ax7 sections. This is a simple circuit and should be easy to fix. But not fer me.
 
I'm not going to do the add on thing untill this unit starts to act like it should.

Dropping the  B+ on the '46 is an option....  and its kind easy using the socket on the back.


klc

Going along with what Jim said:

When using an analog oscilloscope to look the the modulated RF output... one will generally be forced to use a sine wave audio input signal.

The sine wave audio input signal, when increased to a peak amplitude that is higher than the grid bias voltage, will cause grid current to flow in each of the modulator tubes every audio sine wave cycle. But, the associated average grid current will interact with the grid bias supply to make the grid bias more negative... until the average grid current is reduced to (essentially) zero. I.e. the modulator will automatically re-bias to class AB1 if a sine wave audio signal is applied.

Therefore, if one is trying to operate the modulator tubes in Class AB2... which is typical for a Ranger operating with the same B+ on the modulator tubes and the 6146... the effect described above will make this impossible with sine wave audio applied to the Ranger.

With ordinary speech, peaks (both positive and negative) occur very infrequently (a small percentage of the total time)... the charge associated with the infrequent, short bursts of grid current will be bled off of the grid bias supply bypass capacitor by the various parallel resistors.... and, therefore, the capacitor across the modulator grid bias supply (15uF) will hold the bias voltage at the value it is set at with no audio applied. This will allow one to achieve larger audio peaks with voice than with a sine wave.

Stu

Kevin

When applying a 3kHz sine wave... make sure that there is a capacitor (e.g. 0.1uF) in series between the output of the audio generator and the microphone input to the Ranger. If the audio generator looks like a modest impedance to ground (e.g. 600 ohms), and if you don't have the series capacitor... then you will remove the (grid leak) biasing of the 1st audio preamplifier stage of the Ranger... causing the plate voltage in the associated half of the 12AX7 to drop to a very low value (because of the 470k ohm and 220k ohm series resistors between the plate and the 300V supply), and signifcantly reducing the audio gain of the stage.

The capacitor is needed with any audio input source or any microphone (except an un-amplified D-104 or a microphone that has a high impedance to ground at DC)

Stu

"  the audio generator and the microphone input to the Ranger. If the audio generator looks like a modest impedance to ground (e.g. 600 ohms), "

Duh.       Like I said earlier, it aint easy.  Lets see, that was explaind in class back in the 70's. And I still screwed it up.

The HP 200CD is transformer coupled, 600 ohm out .......   Well, I'll isolate the generator and see what things look like. I did hang a 100K pot off the generator to get the signal voltage down to 100mV. But that still doesn't do the bias any good.

 
"Going along with what Jim said: "

Thanks Stu, I wasn't following it too well.


"one will generally be forced to use a sine wave audio input signal"

I played with the 1khz cal output off the scope and fed it into the rig. But the scope really didn't like that waveform.

FWIW, Per Wa1HLR, I solid stated the bias rectifier, increased the cap to 100uF, and added a pot to vary the bias. Both the HV and LV were solid stated; the modulator is on the low voltage supply.   

I'll play some more tomorrow.  Thanks guys,

klc

8) 8) 8) 8) 8) 8) 8) 8) 8) 8) 8) 8) 8)

hum on a scope is almost always filter capacitors.

P,

I shuda been clearer.... The hum is only humming when i'm doing modulation monitoring.


klc

Kevin

If you have a ferrite core, try making an isolation transformer that will let through the modulated RF, but block 60 Hz and harmonics of 60Hz:

Use a few turns on the primary and an equal number (actually it doesn't matter that much if the numbers of turns are equal) on the secondary.

The output of the RF sniffer should connect to one of the windings, and the scope to the other.

Make sure that there is no ground connection directly between the scope and the Ranger. Make sure that this ferrite core transformer is located outside of the Ranger, at least several feet from the Ranger's AC supplies (transformers). Use a twisted pair of wires to connect the pickup to the ferrite transformer,

Note: when you use a few turns of wire... located inside the Ranger or near the Ranger's power supply transformers... as a pick-up for the scope...   you have a problem with the magnetic fields of the Ranger's power supply transformers inducing hum into the pickup . [This is exactly the same as hum induced into guitar microphone pickups or ribbon microphones]

For that reason, it is best to use an RF sniffer that is coupled to the output of the Ranger somewhere along the coaxial cable (external to the Ranger).... but if you add the extra transformer (made from a ferrite core), as above, you will block the low frequency (60 Hz, 120 Hz, ...) hum components that have been induced into the pickup.

The use of an RF choke across the scope input, as suggested by Jim, performs a similar function... but the transformer will also isiolate the scope's ground from the Ranger's ground.

Stu

What I did was take some #12 Awg and make a 2 turn, 1/2 inch coil. The scope gnd on one end, the 'hook' on the other. Placing the pick up coil near the tank, I would adjust the un modulated trace to display 2 grads on the screen.... With the cover off, ther's rf floating all over the place. 

The ferrite isolation coil is a gud idea. I'll try that....  I do have a pick off point on the dummy load, maybee i'll just mount a  toroid off of it....

back to the bench.


klc

Kevin

You wrote:

"FWIW, Per Wa1HLR, I solid stated the bias rectifier, increased the cap to 100uF, and added a pot to vary the bias. Both the HV and LV were solid stated; the modulator is on the low voltage supply."

These are good things to do... but even with the 100uF capacitor, application of a periodic input signal (like a sine wave) will cause the bias voltage to become more negative (i.e. the capacitor will charge up to a more negative value) if you increase the amplitude of the audio signal to the point where grid current flows on audio peaks. This effect is enhanced (made worse) by the use of the resistive voltage divider.

Again, the effect will be negligible with voice input, because the statistics of voice are such that the signal is at peak amplitude only a tiny percentage of the time. E.g., there are long periods of silence between words, etc.

Therefore, you need to be careful about drawing conclusions regarding what your Ranger's peak modulation capabilites are (for voice) when you make measurements using a sine wave.

I had this problem with my KW-1, and with my Ranger. To solve the problem, I made a bias supply (for each of them) using a string of Zener diodes ... whose values were chosen to produce the desired resting current in the modulator. Since there is essentially zero series resistance (for a bias supply that employs Zener diodes), drawing average grid current during modulation does not have any effect on the bias voltage.

Stu

Stu,

Thanks for the input.  This stuff is interesting - the small stuff can sneak up and bite. I've got a 43 volt zener (1N4755) accross a 10k pot. This takes the place of the 'bottom' voltage divider resistor in the bias rect. circuit.

I think i'll try the zener ckt after I get this Ranger checked out and working better. There's a Viking 2 after the Ranger 2.


FWIW, I've got the Viking loaded up on 7290 if anyone is interested.....

klc