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Old 1st July 2009, 02:25 AM   #11
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dude reread the quoted text theres a 12AU7 and a 12U7 i got them mixed up. 12U7 is designed for low voltage.

and hes put a 12AX7 into a 12AU7 amp

which can be bad

is this a revised 12AX7?
the reason im asking is the plate voltages
on some tis 250v max on some its 300 and others 330 and ones like 12AX7A it could be anything

i may be wrong on this but if the circuit is depending on the resistance of the heater for part of the circuit, a small value pot for biasing might be a wise investment for that quarter you seen in the BK parkinglot
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Old 1st July 2009, 03:45 AM   #12
anatech is offline anatech  Canada
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Hi bacon665,
Well there are many tubes listed in the quoted section. I was trying to address a common problem seen often enough. I have even seen someone use a 12AX7 to drive a reverb tank. The 12AX7 kept losing emission and it arrived as a repair.

The original 12AX7 was superseded by the 7025 / 12AX7A. Every new manufacture 12AX7 is an "A" device because they improved the heater to cut down on hum. I just don't feel like typing the "A" all the time.

There are three rating systems in use. Absolute maximum, center design value and maximum design value (I think). The RCA tube manuals have an explanation of this, so it's easy to look up. These ratings simply point out guidelines so that a tube will last it's rated operating hours, and that can vary between different manufacturers. It is not wise to exceed these ratings. The absolute maximums are ratings that can not be exceeded at any time as damage to the tube may result.

Quote:
and hes put a 12AX7 into a 12AU7 amp
Not great, but oh well.

The 12U7 and other mobile radio tubes are normally rated for lower plate voltages, like 250 VDC. With vibrator supplies, there are practical limits as to how much current you can run through them. This helps define the maximum supply voltages you may see in a car for instance. These are often "Loctal" tubes, they lock into the socket so they don't rattle free and fall out.

This particular tube is a nine pin, rated for only 30 VDC. Notice that this tube is intended to use the battery as its B+ source, so the normal plate voltage will run around 12 ~ 15 VDC. This device does not use a vibrator to step up the battery voltage to a normal 130 ~ 250 VDC.

Quote:
i may be wrong on this but if the circuit is depending on the resistance of the heater for part of the circuit,
That would be unwise as this parameter is not well controlled. They do make heater - relay "tubes" for timing purposes.

-Chris
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Old 1st July 2009, 03:54 AM   #13
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it may be unwise but its been done before maybe he will post a schematic of his amp?
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Old 1st July 2009, 05:52 AM   #14
Ipanema is offline Ipanema  Malaysia
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will measure the heater current today. Below is the schematic and B+ voltage is 250V.
Attached Images
File Type: gif linestage.gif (5.1 KB, 212 views)
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Old 1st July 2009, 09:55 AM   #15
Ipanema is offline Ipanema  Malaysia
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Just measured heater current of 12AX7. 150mA spot on. When plug in directly to RH preamp, the biases are way off the original value with 12AU7. So I guess, 12AU7, 12AT7 and 12AX7 cannot work by simply swapping the tube. I learn my lesson. Thanks for advise.

Regarding the glow issue. Why the new reissue Tungsol does not design it to produce the magic glow. There are only some small red dots visible when inspect carefully. The main reason for me to build a tube amp is so that I can enjoy the magic glow during listening.
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Old 1st July 2009, 03:09 PM   #16
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Then its time for one of the simplest mods in the world...
Put an LED in the center of your tube socket.

Click the image to open in full size.
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Old 1st July 2009, 03:16 PM   #17
chrish is offline chrish  Australia
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He said the "magic glow". Fake LED glow - not my taste I am afraid...
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Old 1st July 2009, 09:59 PM   #18
anatech is offline anatech  Canada
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Well, you are welcome to choose your tubes for looks rather than audio quality!

Use a voltage regulator tube or two in the power supply. They look really cool! They are available in 8 pin and small glass envelopes. A wire mesh screen is highly recommended for these. Be aware that they will only tolerate a low capacitance in parallel unless you decouple them with a resistor.

Now you will have a light show and probably less noise in the output signal. That depends on the supply you are using now.

-Chris
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Old 1st July 2009, 11:44 PM   #19
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hes not kidding.
Click the image to open in full size.

those two blue ones are mercury half wave rectifiers.
866AX i believe been a while since i opened the pdf schematic and looked.
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Old 1st July 2009, 11:48 PM   #20
anatech is offline anatech  Canada
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Hi bacon665,
Ahhh, shouldn't they be caged or shielded some how?

Mercury rectifiers are pretty, but tend to generate RFI +++. Noisy fellas!

I use #83s for the most part. Past that I go for solid state rectifiers. The HV ones (industrial) are costly as heck though.

-Chris

Edit:
That amp of yours is a thing of beauty to look at. Nicely laid out too.
What's the output? Around 5 W per channel?
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