Zenith HF-16 Cobramatic tubes

[samanthaharlow]

I'm trying to figure out what kind of tubes to put in my HF-16 record player. One of them is a 12AU7, but I can't seem to find a number for the other one. Does anyone know where I can find this info?

[David Spanovich]

I'll admit that this is way out of my league, since this type of phonograph doesn't have a crank sticking out of the side, but according to the information posted here -- http://www.youtube.com/watch?v=x_UeQZcnJd0 -- "The tube lay out is as follows: 6X5 Rectifier, 12AU7 pre amp, and 6V6 power amp."

Perhaps the person who posted this video has a copy of the schematic.

[samanthaharlow]

Thanks for the info, this is just what I was looking for.

[samanthaharlow]

Along the same lines, is there a certain type of tube tester I should get, just for working on my own turntables? There seem to be so many online to choose from and I am very new to this. Thanks!

[gregbogantz]

There are about as many theories regarding tube testers as there are people who use them. Long story short - none of them are faultless and none of them tell you everything there is to know about how a tube will work in a circuit.

There are two basic types and a few intermediate ones. The most basic is an "emission" tester and it will tell you if the tube is totally dead or basically functioning. Most have additional features that will indicate if the tube has internal shorts that could cause it to damage the circuit. There are a lot of these available on eBay and they are pretty cheap. One of these will be suitable if you are just a casual user.

The other type is the "transconductance" tester, the most popular and common being the Hickok brand. These are much more elaborate and still bring big prices on the used market. They are good for matching tubes where you want matched pairs such as in a power amplifier appliction. But even these can be misleading to even the most skilled user. I wouldn't recommend paying the price for one of these unless you want to devote considerable time learning its quirks and how to use it. But it often boils down to this - the best way to decide if a tube is good for your circuit is to put one in and try it. I have seen tubes that test quite good on a Hickok and still cause distortion in the piece of stereo equipment that I want to use it in. So, like so many other things, caveat emptor.

[samanthaharlow]

Thanks Greg, this is great info for me to have!

[anchorman]

if you are into this kind of thing, perusing the antique radio forum would do you a world of good. lots of helpful people hang out there who really know their stuff regarding radios and old tube amps. browse around there and see if you find what you need, then start asking questions if you don't.

-jon

[anchorman]

like greg said, you can spend a lot of money buying complicated tube testing gear, but unless it is a hobby you enjoy, it is unecessary. most important in my book is being able to test for internal shorts since these can cause catastrophic failure of your equipment, especially if it is the rectifier tube(s). next most important is to see that the filament lights up, after that you need to know if the cathode is still capable of emitting electrons at the level needed for the circuit to function. you can find this out, and if the filament is good by just plugging the tube into the circuit. with some tubes that have gone bad, you will only get a short between elements after it heats up. I wouldn't worry about buying the most expensive mutual conductance tester you can find, as the amps you will find when working on old record players are probably not so sensitive in their design that they will sound any better from having matched tubes or any special brand of tubes.

be forewarned, though, that it's not just the tubes that go bad in these old amps, resistors and especially capacitors can go bad and cause anything from hum to a serious electric shock depending on the circuit design, and how the part fails. the basic rules of radio repair that you can learn from the antique radio forum members or any good book on the subject will apply to your electric phonographs too.

www.antiqueradios.com

have fun!