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Author Topic: Circuit for these tubes  (Read 2969 times)


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Circuit for these tubes
« on: May 26, 2016, 10:35:58 AM »
Im looking for a circuit based on one of these tubes. Also if any of them are 12ax7 equivalent please tell me.

12B4A (i have two of these but only one of the rest on the list)

Im looking for a preamp but if any of these can be used as a power tube i need one of those too. Im just a beginner and these are all i have access too at the momemt


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Re: Circuit for these tubes
« Reply #1 on: May 26, 2016, 03:55:18 PM »
Data I have from the book Essential Characteristics:

12B4A   Low gain preamp triode can work with 6.3V heater
12CA5   Small power tube good for 1.5W
12AL5    Dual low current diode
35C5     Small power tube good for 1.5W  Needs 35V heater supply
12BD6   RF Pentode might work as a preamp
12X4     Rectifier tube
12FX5   ???? Did you mean 12FX8?
12BDK5  Did you mean 12BD5?
12BA7   Radio tube not useful for guitar amp


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Re: Circuit for these tubes
« Reply #2 on: May 26, 2016, 04:00:06 PM »
Thanks that helps me alot. Heres what i have


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Re: Circuit for these tubes
« Reply #3 on: May 27, 2016, 12:48:23 AM »
And also be aware that all those tubes starting with 12 need 12.6v for heaters, rather than the common 6.3v.  12AX7s can be wired for either, making them an exception.

Those low power output tubes were used in things like table radios, TVs, and home intercoms.


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Re: Circuit for these tubes
« Reply #4 on: June 10, 2016, 09:32:49 PM »
You're probably going to have to either design a circuit yourself or adapt a non-guitar circuit to your needs if you want to use those tubes. Even back in the day when tubes were current technology and there was a cornucopia of tubes for designers to play with the major amp manufacturers generally stuck to a few models of tube which are the same ones still being produced and used in guitar amps to this day.

That's not to say there aren't unusual designs out there but there were so many types of tube made back then that even if you happen upon a whole box of old obscure tubes chances are none have been used in that application. Some won't even be suitable for audio use at all. Some weren't made for audio but can be adapted for audio and almost all of them are power tubes. There are a few interesting obscure preamp tubes but they tend to be rare and just as expensive as the common tubes and I've found it hard to justify using them.

It all depends what you want to do. If you're dead set on using those tubes because you like the challenge of turning trash into treasure (which is how I got into electronics) you've got a lot of research, planning, simulating and failing to do before you're going to get a working amp out of it (but you get a lot of knowledge too!). If you just want to build an amp right now that has a decent chance of working you might as well just put them away and go buy some more traditional "guitar tubes" and build a little Fender clone or some other well established design. We've all got fantasies about turning a box of junk into something nice but you're not going to save money by using obscure tubes. The transformers are where you'll end up spending the bulk of your money. Not to mention small components like capacitors, potentiometers, sockets, etc... all that stuff adds up and you need it no matter what tubes you use. Once you buy all that, a 6V6 and a couple 12AX7s aren't much of an expense.

To answer your question fully, the only tube on that list I've heard of being used in a guitar amp is the 35C5 (actually it's 50v heater cousin the 50C5). These designs come with a big catch, though. The reason they run at such high heater voltages is that they were designed for the heaters to be run in series right off the 120v mains. They also operate at low plate voltages which (kinda sorta) eliminated the need for a power transformer (which was and still is far and away the most expensive part of a tube amp) and therefore they were commonly used in cheap radios and record players of that era. Some makers of super cheapo guitar amplifiers like Kay and Danelectro followed suit and you can find schematics on the net that will help you but they should not be built as is! They have no isolation from the mains and are commonly called "widowmaker" amps because they are dangerous. Nevertheless the preamp and power amp parts of the schematic can be useful. They are usually simple designs and easy for a beginner to understand. It's the rectified mains power supply that is dangerous and you need an isolation transformer to make it safe. Do not skip the power transformer! You have been warned!
« Last Edit: June 10, 2016, 09:37:24 PM by sa230e »

J M Fahey

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Re: Circuit for these tubes
« Reply #5 on: June 11, 2016, 05:01:40 AM »
Million Dollar Question: do you have sockets for all of them?
Beside novals and octals, any other sockets, once very popular (rimlock / locktal / 7 pin  / "U/A Technology" , etc) are now impossible to find.

No sockets no builds, as simple as that.
Unless you are very handy and improvise them from printer connectors and such plus some clay or epoxy paste.

Then you will have to design your own stuff, they won't straightbfit in , say, an old Fender circuit.

It all depends on commitment and available time.

I bought 1000 Ecc/Pcc189 dual triodes for peanuts (less than $1 each)  some 2 or 3 years ago, I know they are used in Audiophile preamps, with the idea of making either some tube distortion pedal, some hybrid preamp or some hybrid amp.

There they are , still gathering dust, mold and spiderwebs, I haven't had a free time slot needed for that.