R390A in Trouble Again

[flintstone mop]

Hello
I was tuning around and noticed my 390A was almost deaf below 8mhz. The I.C.E. RF limiter was arced out inside , so there must have been a close by hit from a storm.
By pass the limiter and still not as deaf as before, and testing with my IFR told me that I needed 10 microvolts to hear anything. 50 microvolts did not produce an almost 50 on the carrier meter, more like 20.
Going through the tuning, I discover that 8mhz and above the radio was rock on. Slightly more than .5 microvolt to hear the faintest signal and 50 microvolt for carrier meter to read 50.
So, I replaced the 1st mixer tube 6C4 and all back to normal. This must be a weak link in this radio cuz most of the sensitivity problems have been the plug-in coils in the RF deck or the first mixer stage.

[WD8KDG]

Haven't read the R-390 e-mail reflector for a while:

But if memory serves me, the 17MHz crystal in the 75 degree C oven on the RF deck can cause the same thing, deaf below 8 MHz.

Craig,

[w3jn]

6C4s were invented by the devil.  I see more problems caused by those than by any other toob.  Hammarlund and others loved to use them as oscillators, and they're prone to going dead at higher bands, spurious oscillation, etc.  Some NOS 6C4s won't work in particular circuits at all.  I went thru a half dozen when trying to fix a divider circuit in a FRR-59 before settling on a used one - it was the only one that would work.

[flintstone mop]

Hey JN
Would a 6C4WA JAN tube be better or still the same shortfalls?

[kb3ouk]

wonder if there is a way to just completely get rid of the 6C4 and use another tube that has the same characteristics. i saw somewhere that a 6C4 is just one half of a 12AU7, wonder if you could puit one of those in there and just use one side of it? or are they just as bad as the 6C4 is?

[KM1H]

Ive been stockpiling NOS JAN Sylvania and GE as they seem to do the job in Hammies, Nationals and the occassional 390A.

The 12AU7 is 9pin

The 6J6 and triode connected 6AH6 may be possibilities, havent done any yet but they are readily available.

[w3jn]

Hey JN
Would a 6C4WA JAN tube be better or still the same shortfalls?

Dunno, Fred.  All I can tell you is NOS Raytheons apparently will NOT work in a FRR-59, nor in a Hammarlund LO.

[kb3ouk]

ok, i knew the 12AU7 was a 9 pin, couldn't you just change the socket and rewire it for the tube?

[w3jn]

Easier just to find a 6C4 that works in the circuit

[KC4VWU]

Make adapters to test possible substitutes?

[The Slab Bacon]

I have had the same problem here as well. The 6C4 LO in Hammarlund receivers and 6C4s hate each other. Like Johnny said, just because they are new doesn't mean they will work in that circuit. You just have to keep trying them till you find one that works properly. You can have brand new ones that won't work and used ones that test piss-weak work fine. There seems to be no method to the madness. In audio applications, though, I have never seen a problem with any of them.

Another tube that I friggin hate with a passion are 12BY7s. They are also hateful.
A 'BY7 is the only tube that I have ever seen still light up but have absolutely no emission at all. I have been bit by them a few times as well.

[kb3ouk]

i know how bad 12BY7's are, i had one in my FT-101 that one time got so hot it's a wonder it wasn't melted in a puddle on the chassis. i touched it with my thumb and had an instant blister. that thing was HOT.

[The Slab Bacon]

Johnson Invaders seem to eat those things for breakfast. Both of mine (the 200 and the 2000) had totally dead ones that lit up but had no emission whatsoever. Every other Invader I have worked on had the same problem, I have also seen this in several other rigs as well. That is a piss-poor excuse for a driver tube! ! ! I have often thought about rewiring the sockets and changing the associated circuitry to use a 6CL6 or a 5763 instead. Both are tough tubes and hard to kill.

[w3jn]

They're also almost always dead in the SX-101 LO.

[Todd, KA1KAQ]

Still have a 101 stored in VT at the folks place. Works great so long as you get in there and tweak the slug associated with the crystal after you turn it on. The old guy who gave it to me had it opened up so everytime he got on the air, he could give it a crank.

Never had a problem with the 6C4, but not a lot of gear here uses them. There's a 12BY7 in the T-599A that gets nice 'n hot, though I think the metal 6F6s in the old Super Pros win the 'peel off fingerprints instantly' award. Or the 5Z4 in the 32V-2.

Tubes I've seen the most failure with seem to be 6U8s in the KWM-2A and 7360s in Clegg gear. They seem to go wonky far too fast.

OTOH, I picked up a nice RCA SC-88 from a guy in Colorado about 10 years ago which 'worked', or so it seemed. He sent me a video of it picking up one of those 50 megawatt bible thumper stations, just booming in. When I got the set it was very quiet, even the local BC station barely came in. Turned out every single octal tube in it was flat as a turd, only a couple gave any amount of wiggle on the B&K tester. Apparently this guy lived next door to the station, which is why it boomed in so well on the video. He probably received it on his dental fillings. Changed out the tubes and it worked like new. But even flat tubes can still work, to some degree and in some cases.

[kb3ouk]

i have a crosley console set that i've ran a couple sets of 6K6GT's through it. i had one i stuck in there, i think it was a sylvania, that looked alright until i tapped the side of the tube and the whole thing suddenly starts glowing blue and purple! jiggle it around some more and it stops glowing.

[WU2D]

It might be asking for trouble, but the 6AF4 (not the 6AF4A which is the little guy) might be an interesting triode to try in the LO. It has half the plate dissipation of a 6C4 and you would be close to its plate voltage limits, but that tube has all kinds of gain at high frequencies since it was designed specifically as a V/UHF LO in TVs.

Mike WU2D

[flintstone mop]

It might be asking for trouble, but the 6AF4 (not the 6AF4A which is the little guy) might be an interesting triode to try in the LO. It has half the plate dissipation of a 6C4 and you would be close to its plate voltage limits, but that tube has all kinds of gain at high frequencies since it was designed specifically as a V/UHF LO in TVs.

Mike WU2D

That's interesting.........It might be more reliable as a replacement osc.........my problem seems to always be associated with the 1st mixer........I guess same function.