Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Erik Aa on February 25, 2023, 08:44:59 am

Title: PT no secondary voltage. Grounding issue or blown?
Post by: Erik Aa on February 25, 2023, 08:44:59 am

Hi. I am rebuilding an old tube radio into an AC4 type amp. Finished all the wiring today, but when I bring it up on my variac with current limiter (and no tubes), there are no secondary voltages on the pt. The current limiter starts to glow at about 40 volts. I have voltage on the primary of the pt.

Do I have a grounding problem with the pt? Or is the pt blown?

Any and all suggestions would be most welcome. Thanks!

Best, Erik

Title: Re: PT no secondary voltage. Grounding issue or blown?
Post by: sluckey on February 25, 2023, 08:50:55 am

Hard to say without more info. Disconnect all the secondary leads and try again. Surely you checked the PT BEFORE converting to AC4???

Title: Re: PT no secondary voltage. Grounding issue or blown?
Post by: Erik Aa on February 25, 2023, 09:47:39 am

Thanks Sluckey.

When preparing to unsolder the secondaries, I also went over other soldering points on the pt. It turned out two of the old solderings close to the coils were starting to fall off. The pt works now.

How high plate current would you say is ideal for this AC4 type build? I use a 12ax7 and an el84. I suspect my plate currents are too high. I did not turn the variac all the way up to full voltage (230 volts here in Norway). 

Thanks!

Title: Re: PT no secondary voltage. Grounding issue or blown?
Post by: sluckey on February 25, 2023, 10:16:59 am

You'll need to take it off the variac to get realistic measurements. Compare to schematic readings.