Ranger modulator tubes

[wa3dsp]

I am completely overhauling/restoring a Ranger. It has metal 6L6 modulator tubes. The high B+ runs about 650+ and could go as high as close to 700 with the modulator quiescent. I have run 6L6's to over 600 volts before without any problems but this is pushing them. I plan on using a regulated 280-300 volts on the screen with fixed bias.

I have a large stock of NOS tubes. Besides 6L6's, 6L6GC's, I have 7027A's, and KT88's.  The KT88' are a little overkill but they do fit. The ones I have are NOS from many years ago made in England. Some are in boxes labeled Red Lion. One or two of these tubes is probably worth more in the audiophile world then the Ranger itself!

KT88's have an 800 volt plate and 600 volt screen max.

Unfortunately I don't have any 6550's

Any thoughts on using the KT88's ?

73 Doug 

[Pete, WA2CWA]

Discussed here:
http://amfone.net/Amforum/index.php?topic=25744.0
More info here:
http://amfone.net/Amforum/index.php?topic=30107.0
and here:
http://amfone.net/Amforum/index.php?topic=26722.0

[WD5JKO]

  Doug,

   The metal 6L6 is rated at 400V maximum. Remember that the plate might swing twice the B+, and under fault conditions more. The 6L6 like many of its beam power descendants has the plate at pin 3 with both pins 2, and 4 used for something else. Many sockets will fail to provide enough path distance to hold off the voltage, as in air we need 1/10" minimum per kilo-volt, and that is with no sharp points. It would be better to find a suitable tube with a plate cap on it, like maybe the short stubby 5933 which is a rugged 807, and the 807 is a 6L6 adapted for RF by adding a few shields. If it would fit, the switch would be just a socket change.

   With the kind of voltage you are going to be using on that rig, be prepared for numerous flame outs, melt downs, etc. You should pull all the B+ wiring out of the harness, and replace it with higher voltage rated wire.

  I wonder what you have planned for that little Ranger?

Jim
WD5JKO

[wa3dsp]

Actually I am making a lot of changes to the Ranger including some new Teflon insulated wiring but the end result will be a 60 watt input AM transmitter, the same as it was designed.  The voltages I mentioned are not that uncommon when removing the vacuum tube rectifiers and going solid-state in combination with today's high line voltages. Even using the unused 5 volt winding as a buck on the primary the high voltage is well over 650 volts unloaded. Loaded to 200 ma with a test load I get about 650. So this would not be an uncommon high voltage in a modern Ranger.

[wa3dsp]

Here is some data on research I have done -

The 7027A looks like an OK tube to use in the Ranger. It has a maximum plate voltage of 600 which is 100 more than the 6L6GC and more than 200 more than the 6L6 and a maximum plate dissipation of 35 watts. The 1614 which was the original tube used in the Ranger has a max plate voltage of 550 and 25 watt dissipation.

The 6550A and the KT88 are very similar tubes. The KT88 has more max plate voltage - 800 compared to 660 for the 6550A. Both have 42 watts plate dissipation and similar AB1 characteristics.

Pinouts are the same - KT88 uses pin 1 as shell (base) ground as does the original Ranger 1614 metal tube. 6550 has NC on pin 1. Both NC pin 6. So no wiring changes are necessary.

The 7027 ties pins 1 and 4 together as g2 and 5 and 6 together as g1. Pin 1 is grounded and pin 6 is used as a tie point. Rewiring would be necessary.

Other  characteristics are very similar. Either of these tubes would loaf along driving a 6146 at 60-70 watts input.

Any replacement is based on availability and should consider future availability. So in my book even if I have a pair, strange tubes are out. They just won't be available in the future when existing supplies are exhausted. These audiophile tubes will be around for awhile. Maybe not as good as they were made in the past but at least around.

So I am going to strongly consider the 7027A or KT88 both tubes that I have a supply of. I kind of favor the KT88 as it requires no socket changes.

Downsides of the KT88 / 6550 is that two require 10 more watts of filament than the 6L6, 7027, 1614.

Note that my object here is not to turbo my Ranger but rather make it a reliable AM transmitter for years to come. I just need to work around the voltages that are available. With higher line voltages and solid-stare rectifiers the high voltage is pushed well above 6L6 ratings and really pushing the original 1614 if you still have them.

I welcome your comments.

73 Doug

[WD5JKO]

  Doug,

   You have done your homework on those tubes. I wonder though, seems to me, the combination of high line voltage + solid state rectifiers can lead to more then just high B+ and the issues that come with that. What about the power transformer saturating, even unloaded...every transformer has their limit where we run out of iron as the core saturates. Some of the vintage transformers meant for ~ 110 vac will saturate on 125 vac, and will not survive. Then there is the issue of filaments where you will likely be running your tube filaments on 7 volts instead of 6.3 vac..

   Sure lots of folks do this, I'm sure some get away with it, and maybe others don't. It just seems to me that running high line + SS rectifier + 7 volt filaments will give you 1-2 db more power along with 20 db more headaches. I don't have specific Ranger experience, so maybe this works out OK, dunno.

   I once ran 8417's (600v B+), and I had the pin 3 problem (flash over to ground), even with ceramic big  Johnson sockets. I had to recess the sockets below chassis about 1/8" as I recall to finally stop it.

    There are many Octal sockets available, how about a solid Teflon block socket with gold plated CNC milled pins? These are interesting:

http://www.jacmusic.com/sockets/Socket-Yamamoto.htm

   If you have room for KT-88's, then surely you can consider 5933's which can be had NOS for $12:

http://www.abcvacuumtubes.com/vacuumtubes_tubelist_tubes_FM1000-9006.html

Picture of the 5933:

http://www.comaat.com/product/5/5933wa.html

ABC also sells 1614's...

Good Luck,
Jim
WD5JKO

[wa3dsp]

Jim,

  I am not sure where people are getting the idea that 600 volts is too much for an octal tube socket. I guess one guy had a bad experience with an octal socket and then it was the rule.  These connectors are routinely used at up to 900+ volts in ham equipment. Examples are most of the 60/70's era SSB equipment. Collins, Heath, etc. They ran 800+ volts on the finals and it came in from the power supply on an octal socket along with other voltages including a minus 80 volt bias. My 32S1 has close to 900 volts unloaded on the 6146's and on the PS socket. Oh and BTW the socket is an 11 pin - closer spacing. I have never had an octal socket fail and I have a lot of rigs that pass much higher than 600 volts through them. Non of these sockets are ceramic. A ceramic socket would certainly be even better. Really limited by the contact spacing.

Granted that the voltages at the plates of the PP modulators are complex as opposed to just pure DC but you would need a whole lot more voltage to jump across the pin spacing of an octal socket than 600 volts.

[WD5JKO]

  Doug,

    I think we both know that the Octal socket can take a lot, it is the wild things that happen to an audio amp that bridge the gap with a carbonized path. One example would be cranking up the audio gain, with big coupling caps for bass, and then dropping the microphone..The load is inductive, and when wrapped up, will release like the end of a bull whip..It happens, and that is why guitar amp repair folks replace tube sockets fairly often. I don't think many guitar amps use more then 500 volts either.

   So just for kicks since you are trying to narrow the options to a nice Octal tube that is readily available, consider the lowly 6AV5. Here the TV makers put the plate at pin 5, and had pins 4,6 not connected. The sockets used had a little more beef to them as I remember, and the result was the capability to hold off 5KV or so peak in a horizontal deflection circuit. Not too bad for a tube without a plate cap!

   Now look at how the 6AV5 can work in audio, driving G2 only (the Gonset way) from a low impedance audio source. The schematic at the following link gets over 80 watts RMS out of a pair of 6AV5's without sweat or blush; schematic is at the bottom for the push pull amp:

http://www.tubelab.com/6AV5.htm

Lots of fun!
Jim
WD5JKO

[KM1H]

While earning my keep as a USN ET I replaced a few ceramic sockets using 5R4's at 900V AC; granted it probably took 10-15 years to happen. I'd certainly not trust Chinese versions.

If you have room for KT-88's, then surely you can consider 5933's which can be had NOS for $12:

Whats so special about a 807 besides having to change the socket and that they dont sound good in AB2 without a lot of TLC? My V-I and V-II required a lot of rework to sound good. In a Ranger the B+ would likely have to be in the 450-500V area as running an inefficient AB1 would be bad for the stock mod transformer.

Carl

[wa3dsp]

Somehow a discussion of tubes has turned into sockets. I have no problem with sockets!

[Steve - K4HX]

In general, I agree with your thoughts on replacement/availability. Going with more common tubes is usually a safer bet. However, I don't know your age. What is the service life you expect for the Ranger? How many hours a year do you expect to put on the tubes? Replacement may not even be a concern. And in the case of the Ranger, I thinks it's less of a concern than with some other rigs that afford you less options.

Bottom line, I don't think it's that much of a concern. You named several different tubes that will work (and have been used) in the Ranger. Seems to me that picking any one of those now, does not limit your from using something different in the future. Locking yourself into just one tube type is more likely to limit your ability to get replacements in the future.

Any replacement is based on availability and should consider future availability. So in my book even if I have a pair, strange tubes are out. They just won't be available in the future when existing supplies are exhausted. These audiophile tubes will be around for awhile. Maybe not as good as they were made in the past but at least around.

I welcome your comments.