Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Dimitree on February 22, 2020, 09:27:58 am

Title: probably short PT
Post by: Dimitree on February 22, 2020, 09:27:58 am

so I'm building a Vox AC30 amplifier,
I completed everything (except installing the tubes), but before powering it up, I decided to obtain some infos about what to do on the first power up, to avoid problems.
So I learned that I should verify that the connection points for the PT secondaries are open circuits.
I desoldered the PT and the OT, and started checking the PT secondaries.
Unfortunately I noticed that there is little resistance between each pair of lead:

- 1ohm between the two 5V leads
- 1ohm between the two 6.3V leads
- 50ohm between the HV leads

the PT is this one:
http://www.classictone.net/40-18050.html (http://www.classictone.net/40-18050.html)
that I bought new from ClassicTone suggested reseller some months ago.

Am I missing something, or the PT is damaged somehow? I'm really frustrated  :w2:
thank you for the help

Title: Re: probably short PT
Post by: sluckey on February 22, 2020, 09:41:13 am

It's OK

Title: Re: probably short PT
Post by: PRR on February 22, 2020, 12:23:31 pm

> verify that the connection points for the PT secondaries are open circuits.

Open to ground!!

Each winding must have pretty good conductance end to end (otherwise the electrons could not go through).

This is not your PT, numbers will differ. And most bench meters don't read sub-Ohms correctly. But you see that your readings are in this ballpark. (A very big ballpark, like Sardar Patel Stadium, but not "wrong".)

Title: Re: probably short PT
Post by: Dimitree on February 22, 2020, 01:54:41 pm

Thank you, and sorry for my ignorance

So how should I check the open circuit to ground? I mean, what do you mean for ground in this situation (where the PT is unconnected from the circuit)? The chassis?

Title: Re: probably short PT
Post by: sluckey on February 22, 2020, 02:31:46 pm

Quote from: Dimitree on February 22, 2020, 09:27:58 am

So I learned that I should verify that the connection points for the PT secondaries are open circuits.

I think PRR was just commenting about this statement you made. When taken out of context this statement doesn't have much meaning. Where did you learn this? Post a link please.

Title: Re: probably short PT
Post by: Dimitree on February 22, 2020, 03:52:43 pm

I read that here:
http://www.paulrubyamps.com/info.html (http://www.paulrubyamps.com/info.html)

Quote

Start with no tubes installed, MAINS UNPLUGGED, trannies NOT soldered in and get out your ohmmeter. Also remove the pilot lamp if you have one. Use your ohmmeter to verify that the connection points for the PT secondaries are open circuits. With no tubes and no pilot lamp, the 6.3VAC and 5VAC secondaries should be open circuits, no continuity. If you used a tube recto, then the HV secondary of the PT should also read as an open circuit.

But probably also on another link

Title: Re: probably short PT
Post by: sluckey on February 22, 2020, 04:29:20 pm

Now that makes perfect sense. Ruby wants you to check for an open circuit (infinity resistance) between chassis and the point where the PT wires would be connected in the circuit. He doesn't mean to check resistance of the PT wires.

I can think of several things that would make this procedure useless. What if you have 100Ω resistors on your filament circuit. You will NOT read an open circuit in this case. There will e 100Ω resistance to ground. DUH!

Or, suppose you have solid state rectifiers feeding a big ole filter cap with a 220K bleeder resistor to ground. Depending on how your meter is connected you may read 220K to ground.

Or maybe you have a bias tap on your PT that would connect to a simple bias circuit. Ain't gonna be an open circuit here either.

I understand where Ruby was coming from with this suggestion, but it's too riddled with exceptions. I bet Ruby doesn't do this step!
  :icon_biggrin:   All in all, I think this check is pretty useless.