Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Oddvar on November 24, 2016, 05:55:41 am

Title: AC30 again, too hot?
Post by: Oddvar on November 24, 2016, 05:55:41 am

Hi again.. The Hoffman AC30 proved to be hard to get right, for me at least.  I unconnected  pin 1 on the four powertubes, thanks to Sluckey.  Now the nos Sylvania tubes worked for a while, then the volume just faded and died.  I put in some other tubes in and the worked.  I am beginning to wonder if the voltage is too high, seems like the burn iside is increasing on some tubes.  Is there any way to lower the bias?

Title: Re: AC30 again, too hot?
Post by: sluckey on November 24, 2016, 07:16:45 am

Increase the value of R34. I'd try 68, 75, 82, or 100. Must be rated for 10 watts. Higher values means tubes run cooler.

The AC-30 is well known to be a hot amp, especially with the original chassis/cab design.

Title: Re: AC30 again, too hot?
Post by: pompeiisneaks on November 28, 2016, 11:37:10 am

yes Sluckey helped me with this same issue on mine, it was running a bit hot and I dialed it back down a bit by trying different resistors, i think mine ended up best at 125 Ohm, but I don't recall off the top of my head.  Yours likely will be a touch different because every circuit is a bit different due to the variance in resistors in the path etc, and performance of the overall circuit.  (Or so I understand).

~Phil

Title: Re: AC30 again, too hot?
Post by: Mark on November 29, 2016, 03:09:16 am

What voltage is on the anode/plate of the EL-84's?

What is on the screens?

I assume you are using a 50 ohm resistor and have about 13VDC across the resistor?

These amps run ridiculously hot, they certainly exceed the specifications of the valves.

Regards

Mark