Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: kwm488 on March 30, 2016, 10:49:36 am

Title: what problem of my ac30
Post by: kwm488 on March 30, 2016, 10:49:36 am

hi


my built ac30 have this problem, do anyone know what problem?

Title: Re: what problem of my ac30
Post by: FranciscoPerez on March 30, 2016, 11:54:05 am

Hi, I'd say bad preamp plate resistor, or cold solder somewhere...
is the amp old?

Title: Re: what problem of my ac30
Post by: pompeiisneaks on March 30, 2016, 12:53:22 pm

Also sounds similar to my Vox AC100 when the tubes got into runaway, maybe the bias has gone out of whack in some way?  Is it a sort of pop/chirp sound?  Kind of hard to say with the sound in the video.

~Phil

Title: Re: what problem of my ac30
Post by: kwm488 on March 31, 2016, 10:27:38 am

hi
i built this amp last year. the transformer is Mercury. but the resistor, caps are old.
last year, the amp no problem, i just find out this problem recently.


so i should check all resistor value first?


and i think the power tube is super hot. but some people say it is normal.

Title: Re: what problem of my ac30
Post by: pompeiisneaks on March 31, 2016, 10:52:40 am

Define super hot...

If it is red plating it is definitely too hot, otherwise the only way to know how over the recommended rating the amp bias setting may be, is to measure it. 

Phil

Title: Re: what problem of my ac30
Post by: sluckey on March 31, 2016, 11:43:41 am

AC-30 is cathode biased so the bias is not likely to 'run away'. However, it's well known that the AC-30 is biased hot. Hot enough to steal your finger prints!

Title: Re: what problem of my ac30
Post by: kwm488 on April 18, 2016, 10:42:19 am

hi


i tried to replace the el84 tube today. i guess the problem may comes from the el84. because the bias make the tube too hot.


honestly, should i install the bias resistor to make the tube more cool?


thanks

Title: Re: what problem of my ac30
Post by: pompeiisneaks on April 18, 2016, 12:57:04 pm

No you don't need to install a bias resistor, you should adjust the cathode bias resistor to match a cooler performance.  The default from the schematic was 50 Ohm, I think I raised mine to 100 or 125 ohm to lower the tube current and it sounded a lot less harsh and performs a lot better.  You can try differing values of that resistor and see what gets the right balance of the crunch you like without the tubes running so hot.  Just measure the current after each change to ensure you're also not going too far the opposite direction.

~Phil

Title: Re: what problem of my ac30
Post by: kwm488 on April 18, 2016, 09:30:24 pm

hi
thank you.
sorry , i have question.


i used gold lion El84 before. it runs about 20 hrs. so my amp have problem now. i changed it to china el84 yesterday. i play it few minutes, it is no problem, but i smell something, so i stop. i believe 50ohm cathode resistor will melt the el84. really don't know what current el84 can handle it. is it old el84 can handle it? someone tell me el84 max can handle 300v, but vox ac30 give 400v. i forget

Title: Re: what problem of my ac30
Post by: EKDENTON on April 18, 2016, 10:58:00 pm

the AC30 1990 schematic is showing 320v but you did not say which build you did. If you go to the bottom of this page here there is a link to all the schematics and they show the typical voltages of pretty much all the common amps. A 50 ohm cathode resistor is used  on several of the schematics of the AC30's. One has two 100 ohm 5 watt in parallel which would be 50 ohms.

Title: Re: what problem of my ac30
Post by: kwm488 on April 19, 2016, 11:43:56 am

hi
i follow the 1963 jmi vox ac30 to build it


it is same with weber 6v30


http://www.tedweber.com/media/kits/6v30_layout.jpg (http://www.tedweber.com/media/kits/6v30_layout.jpg)

Title: Re: what problem of my ac30
Post by: sluckey on April 19, 2016, 12:03:22 pm

Quote from: kwm488 on April 18, 2016, 09:30:24 pm

i used gold lion El84 before. it runs about 20 hrs. so my amp have problem now. i changed it to china el84 yesterday. i play it few minutes, it is no problem, but i smell something, so i stop. i believe 50ohm cathode resistor will melt the el84. really don't know what current el84 can handle it. is it old el84 can handle it? someone tell me el84 max can handle 300v, but vox ac30 give 400v. i forget

EL84s will be much happier with 322v. Use a PT that will provide this voltage. Tubes will run cooler and last longer.