tube q

[jimmy]

hi everyone.

my firefly works!!! or it did for a little while  :x  i bought a new tube but they only had 12AX7s but the guy in the shop said it would work fine. so i picked one up and plugged it in. nothing. i went to change the volume setting and then it worked. then i discovered it only worked when you push a little on the chassis. i thought dodgy wiring and had a look. nothing was too bad, but i resoldered some stuff for the sake of it, and botled one of the boards back to the chassis because i unscrewed it the other night tto test some stuff. i then swapped the tubes around (theyre both 12AX7s now) just for giggles, and i turned it on, and there was nothing, and then there were pops, and then there were sparks. i turned it off and looked around, and swapped to tubes back and now there is no sound. i followed arons advice an measured voltages on the plates, looking for pops, and it pops all the way up to pin 1 on the first tube, but theres still nothing. can
anyone suggest a problem i might try looking for? and does the second tube HAVE to be an 12AU7? by the way, the pops get steadily louder as i go backwards from tube 2 pin 6, but then when i get to pin 1 tube 1 its alot quieter.

cheers
Jim

[Peter Snowberg]

Glad you had a taste of it it anyway.

That guy at the shop is dead wrong. They may have similar numbers, but those are two different tubes.

A 12AU7 has a gain of about 16 at your voltage and will source a max of 15-20 milliamps depending on make.

A 12AX7 has a gain of about 100 and will only source a milliamp or two.

You can plug an AX into an AU socket for a quick test, but you will be hopelessly overloading the AX and the bias will be wrong. Depending on the circuit, you will see the plates start to glow red or orange after a very short time. You can however plug an AU into an AX socket with safety. It won't be biased correctly, but that may even be a plus.

Start with locating the source of the sparks. Never let sparks be a mystery.

My guess is that you have a blown cathode cap. Measure the cathode voltages for more insight there.

When you have an amp that cuts out when something is moved, it's really important to figure out exactly what the problem is before proceeding. A wooden chopstick makes a great tool for pushing around connection until you figure out what the problem is.

In any case, take that 12AX7 back, yell at the counter twit, and get yourself a 12AU7 before going on too far.

Good luck!,
-Peter

[jimmy]

i hate musos corner. worst shop ever. sure theyve got all the cool stuff, but about 3 of the god-knows how many staff know what they're talking about. ill take it back today, provided it hasnt exploded.

BTW, has anyone ever heard of ruby tubes before???

cheers
Jim

[Peter Snowberg]

As far as I know, Ruby tubes are all just tested and rebranded tubes from good factories. I don't know how involved they've become in the specs as of late, but I've never heard bad things about them other than price. I know some of their tubes are JJs, some are Russian, and some are Chinese.

I have never liked the Chinese 12AX7s I've bought (Shuguang factory) and the internals were very sloppily constructed, but the Shuguang 12AT7 and 12AU7 I like very much indeed. That's what I have in my 12AU7 amp.

If that's the only local choice, consider yourself lucky.

Their distributor is:
http://www.magicparts.com/main.htm

Take care,
-Peter