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Testing tubes without a tube tester???

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Hi Folks!
Dumb Question #1: How can I test vacuum tubes without a tube tester? Namely, 6AK6, 6BE6, 6HR6, and 12AX7.

Dumb Question #2: Is there a solid-state equivalent to these tubes? They are from an old Knight Star Roamer that keeps dying.

Dumb Question #3: Is the above worth doing?

Thanks for your help.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
captainkirksdog said:
Hi Folks!
Dumb Question #1: How can I test vacuum tubes without a tube tester? Namely, 6AK6, 6BE6, 6HR6, and 12AX7.

Dumb Question #2: Is there a solid-state equivalent to these tubes? They are from an old Knight Star Roamer that keeps dying.

Dumb Question #3: Is the above worth doing?

#1 - You can test them in circuit, by measuring the voltages around them, and the signal flow through them - but you need to know what's happening and why.

#2 - You can't really replace valves (tubes) with solid-state devices, although there have been a very few devices made over the years for that purpose - but they were only ever for a few types, and I've not heard of them for years. What's a 'Knight Star Roamer' anyway?.

#3 - Repairing it may be, it all depends on what you think it's worth, and if you could replace it cheaply - like anything else, you need to fault find in the unit, many faults are common over the years - for instance anode loads of triodes going high value, an extremely common fault.
 
Hi Nigel,
A "Knight Star Roamer" was a kit-built shortwave receiver sold back in the Fab Fifties in the USA.
As to in-circuit testing: Since I don't have any data on the radio, I don't have any way of knowing what to look for. When it comes to tubes, I know about this (.) much! Any thoughts?
 

suptjud

New Member
Advice from a member of the antique radio restoration hobby crowd... Replace ALL of the paper capacitors. Replace ALL of the electrolytic capacitors. Check ALL resistors and replace all that are more than 20% out of tolerance. Odds are the radio will work after performing these actions...
 

Trini

New Member
I always say the best tube tester is the socket/circuit for which it was intended.

I think I still have somewhere, a printout of a couple of schematics for making your own solid state replacement for a few valves/tubes. I don't remember all but I do remember there was one for the double triode type like the 12AX, 12AT, 12AU7. They were wired on to 9BA plugs to be plugged in directly into the original socket. I seem to remember they used FETs that were fed from the valve/tube anode voltage via a dropping resistor and regulated to about 27 volts using a zener diode.

If you are really interested I can do a search and see what I find. There was no circuit explanation but parts were labelled. These came from an electronics circuit handbook I think.

If you are going to refurbish the receiver and have not powered it up yet , there is one thing you ought to pay attention to. Fire it up gradually. Put a 60 to 100 watt bulb in series with the mains input to reform the electrolytics and dryout some of that moisture accumulated during the time that the set had been out of service. Failure to do this can cost you the power transformer, the rectifier and filter capacitor(s). If you haven't seen a capacitor explode before you, won't like to.

Trini.
 

Nigel Goodwin

Super Moderator
Most Helpful Member
captainkirksdog said:
Hi Nigel,
A "Knight Star Roamer" was a kit-built shortwave receiver sold back in the Fab Fifties in the USA.
As to in-circuit testing: Since I don't have any data on the radio, I don't have any way of knowing what to look for. When it comes to tubes, I know about this (.) much! Any thoughts?

They are generally pretty straight forward, if you don't have the circuit (and presumably as a kit it originally came with one) you may be able to find one to download on the net. Failing that, as a minimum you need to know the pin connections to the valves.

The 12AX7 is a standard double triode, I can still remember the connections for those (at least for the European equivalent!). Counting from pin 1:

Anode
Grid
Cathode
Heater
Heater
Anode
Grid
Cathode
Heater - centre tap

The advice given about replacing all the capacitors, and checking all the resistors is pretty sound - components from that era are very prone to failure by now. Mind you, modern parts are unlikely to last anywhere near as long!.
 

Dean Huster

Well-Known Member
You can tell the relative ages of the posters when the question pops up, "What's a Star Roamer"! Knight-Kit, a division of Allied Radio back in the 1950s and 1960s, made a wealth of test equipment and other kits and back then, was probably Heathkit's closest competitor. They also made the R-55A, R-100A, Spanmaster and Ocean Hopper shortwave receivers.

I've put together lots of Knight-Kits over the years .... well, over my Wonder Bread years, anyway, (I know ... What are "Wonder Bread years"?) and I still have every one of them and regularly use my R-100A shortwave receiver, KG-670 RC Tester and KG-620 VTVM.

Anyway, there are a lot of Star Roamers out there working just fine. I'd suggest getting some GOOD tubes to put in there. Check out Antique Radio Supply at http://www.tubesandmore.com for a good assortment of used, new and NOS vacuum tubes.

There's a forum on the Internet designed solely for Knight-Kits at http://www.senac.com/boards/1270/ where you can find help getting parts and other stuff.

For those of you that don't know of the resource, the BAMA site is one of the largest available for downloadable information on antique gear. It has the complete manual for the Knight-Kit Star Roamer available as a free download. It's at **broken link removed**

And, you CAN use a good Tektronix semiconducor curve tracer with an outboard heater supply to test vaccum tubes! But if you don't have a tube tester, the curve tracer isn't likely either, is it?

Good luck with your Star Roamer. It'll be worth restoring. Oh yeah. Marc Ellis spent several articles in Electronics Now restoring a Star Roamer. If you need the specifics on that, let me know. In better times when my stuff wasn't in storage, I'd have looked those articles up for you and sent you a copy.

Dean
 

Nigel Goodwin

Super Moderator
Most Helpful Member
Dean Huster said:
You can tell the relative ages of the posters when the question pops up, "What's a Star Roamer"! Knight-Kit, a division of Allied Radio back in the 1950s and 1960s, made a wealth of test equipment and other kits and back then, was probably Heathkit's closest competitor.

You're making the mistake of assuming everyone is an American!, presumably Knight-Kit were only based in the USA?. Heathkit was also available in the UK, in fact we've still got a couple of items of Heathkit test equipment in the service department at work.

It's a pity you can't still get Heathkit stuff, it was great fun to build - but the costs would be just too high - it was pretty expensive back then, but the quality of the kits was superb!.
 
Hi guys!
Thanks for the input. Hey Dean, if you ever do find a reprint or a link to Marc Ellis' article, I'd love to have a copy. Or maybe just what issues it ran in. I subscribed to both GB pubs for a long time. Maybe I have it too. Thanks again.
 
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