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 Post subject: Homebrew audio amplifier
PostPosted: Nov Thu 05, 2009 6:48 pm 
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Joined: Sep Fri 04, 2009 6:25 pm
Posts: 5
Hi!
I have some 17cu5 and 12au7 tubes. I would like to build a audio amplifier using this tubes. I seached schematics on the web without succes. Does anyone have a schematic using this tubes?
Thanks in advance!


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 Post subject:
PostPosted: Nov Thu 05, 2009 7:07 pm 
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Joined: Jan Thu 01, 1970 1:00 am
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Location: Livermore, CA
Hi

Use a circuit showing a pentode output with dual triode driving the tube. Don't worry about actual tube numbers shown.

What will you be using to light filaments on these tubes? A 12AU7 can be operated off of 6.3 or 12.6 volts. These are common voltages.

A 17CU5 was made for use on series string in TV. It requires 16.8 volts to operate the filament. Not as common of voltage.

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 Post subject:
PostPosted: Nov Thu 05, 2009 7:29 pm 
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I have an old power transformer here and the secondary voltages are 13 Volts, 5 Volts and 34 Volts. I think to use two 17cu5 with serie filaments and 12au7 with 13 volts secundary voltage. I have a doubt about the gain necessary to exciting teh 17cu5. The 12au7 provide the necessary gain?


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 Post subject:
PostPosted: Nov Thu 05, 2009 7:36 pm 
Silent Key

Joined: Jan Thu 01, 1970 1:00 am
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Location: Sandpoint, IDAHO 83864
I just want to mention one thing. I have come up with many ideas over the years that used rather unique tubes. If you have a source of the tubes, or a good stock of them fine. But I all too often find that the unique tubes I want to use are only one or two of them. And it makes me wonder what would the project amount to if the tubes were bad, or went bad. I would make sure a supply of them would be around later just in case.
Curt

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 Post subject:
PostPosted: Nov Thu 05, 2009 7:41 pm 
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Location: Livermore, CA
Hi

Using 2 stages of a 12AU7 will give you enough gain for most amplifiers. Since you are planning to use push-pull output one section of 12AU7 may be used as an inverter. An inverter doesn't give you gain. In this case you may want to add another amplifier tube.

You should be able to find a circuit using 6V6's, 6L6's or similar tube for the output. A 12AU7 is similar to 6SN7. Also look for circuits using other dual triodes, 12AX7, 12AT7, 6BQ7, 6SL7.

Check the internet for "tube amplifier circuits".

12AU7 is a good choice but careful of 17CU5. It's wan't very popular. Like Curt mentioned, you may someday want replacements.

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 Post subject:
PostPosted: Nov Thu 05, 2009 8:32 pm 
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The 17CU5 is nothing more than a 50C5 with a 16.8v heater, as already mentioned it was used in series string TVs, almost always as audio output... While there is virtually no demand for those and I'd guess they'll be around for years to come, it'd be far easier to build a series string amp with the 12AU7 and a pair of 50C5 in series... Was done millions of times in the '50s/'60s for small phonograph amplifier...

Now that said, if you have a suitable transformer and the tubes on hand why not??? Why not is, unless you have a 120v secondary winding on that transformer to isolate the B+ voltage from the AC line, the amp(or the 12AU7/50C5 version) will easily knock you on your butt if you don't know what you are doing...

A far better choice would be to use something like a 6AQ5 output and a power transformer for B+ and heater voltage... You could use a 6CU5 but those have a 1.2A heater so would need a healthy heater winding(considering it's a small amp) on the transformer...

Tom


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 Post subject:
PostPosted: Nov Sat 07, 2009 8:29 pm 
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Thanks for all the replies. I'll search on internet about schematics. Thanks for advises about +B isolating. Regarding about de 17cu5 tubes, recently I bought a lot with 20 of them and I would like to use them in any project.


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 Post subject:
PostPosted: Nov Sat 07, 2009 9:48 pm 
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If you'd like, I can come up with a schematic easy enough for the 12AU7/50C5 version, you'd just need a transformer to power it...

BTW those small amps almost always used a 12AX7, but no doubt the 12AU7 will work...

It would be possible to use a couple 24v center tapped transformers back to back for the tube heaters(use a 1N4007 in series to cut heater voltage to the 17CU5) and have isolation from the AC line as well... Depending on exactly what your transformer specs are, may be possible to use it and one 12 or 24v transformer... Radio Shack did have some fairly decent 12 & 24v xformers, haven't checked lately...

Tom


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 Post subject:
PostPosted: Nov Sat 07, 2009 10:24 pm 
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Joined: Sep Fri 04, 2009 6:25 pm
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Hi !!
I would like to receive your schematic.
It is will help me a lot !!
Did you send this via e-mail?

[]s

Marc


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 Post subject:
PostPosted: Nov Sun 08, 2009 12:55 am 
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Yes I can email it...

Will need your address, you can PM that to me...

Tom


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 Post subject:
PostPosted: Nov Sun 08, 2009 3:51 am 
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Location: aston, pa, usa
http://webpages.charter.net/dawill/tmor ... C5SET.html
--------------------------------------------------------
One 50C5 and one half of a 12AX7.

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Last edited by bill hamre on Nov Sun 08, 2009 5:12 am, edited 2 times in total.

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 Post subject:
PostPosted: Nov Sun 08, 2009 4:51 am 
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Location: aston, pa, usa
http://www.dogstar.dantimax.dk/tubestuf/4x4intro.htm
-------------------------------------------------
scroll down to schematic for lafayette kt-92 circa 1959. Pair of 35C5/50C5 and a 12AX7.

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 Post subject:
PostPosted: Nov Sun 08, 2009 5:19 am 
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Location: aston, pa, usa
http://www.dogstar.dantimax.dk/tubestuf/4x4const.htm
------------------------------------------------------------
"li'l" 4 X 4 has 12AX7, 12AU7 and a pair of 35C5/50C5

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 Post subject:
PostPosted: Nov Sun 08, 2009 7:31 am 
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Joined: Jan Tue 16, 2007 7:02 am
Posts: 925
Location: Lexington, KY USA
Mang,

The other replies offer you good advice. I think it’s a good thing to find something to do with oddball tubes. Save the popular types for replacement use.

Do use a transformer, or transformers, to isolate the AC power to your amp. The old LaFayette amp’s design, with no power transformer, was a very bad idea!

Please post a schematic, once you decide what you are going to be building. We will be glad to pick it to pieces for you! You will get a bunch of interesting, and possibly useful opinions from forum members.

The 17CU5 should be a fairly safe choice; there seem to be some around, and nobody wants them. If you do get your amp working well enough to want to assure being able to keep it running long term, you might pick up some spares to keep on hand.

While I was writing the above, you let us know that for you, spares are not a problem! In case anyone else wants to try this, ESRC is selling both 16CU5s and 6CU5s ( NOS ) for $1.00 each:

http://www.esrcvacuumtubes.com/dollar_days.html

The tube manual says that this 7-pin miniature tube is intended for audio output stage use. The plate voltage is rated for only 150V, and the plate dissipation is only 7W, max, so even a pair of them won’t make the world’s most powerful amp. It should be similar to a 50C5, rather than a 6L6GC.

( Bill, I have to mention that my LaFayette amp has a Telefunken 12AX7! Darn tube is now worth more used than the amp cost new! Thanks for the link. Lots of good information there.)

I think another 12AU7 will cost much more than the 17CU5, but these are still being made, so should be available for quite some time.

Has anyone tried any of the TV / FM RF duo-triode tubes for audio? These can still be had for next to nothing, especially the ones with funny heater voltages. There are also a bunch of RF triode-pentode tubes that might go into homebrew amps or radios very cheaply. See the ESRC $1 list.

You might guess thatI had a good time looking at that $1 list, trying to see what these oddball tubes might be good-for. There are some series-string sweep tubes that might replace pricey 6L6s, if you can figure out how to light ‘em up. The 3C6 might be considered a sort of loktal 19 for your twinplex regen set. There are some beam tubes, dual control pentodes, and other unusual functions, if you are interested in trying some different sorts of circuits.

Ted


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 Post subject:
PostPosted: Nov Sat 28, 2009 4:07 am 
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Posts: 385
Location: Lehigh Acres, FL, USA
I agree on the oddballs here. My Stereo homebrew amp uses four tubes. 25CK3 as a rectifier, 25C5s for output (Ok maybe not as odd, but what uses these?) And a 12AT7 as the amp (Not as sought after as a 12AU/AX7 but it does the job, so probably not an oddball.) Isolated from the AC line of course using a radioshack tranny and one from an old satellite receiver (one heavy sunnovagun, but gives good filament voltages)


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