Hoffman Amplifiers Tube Amplifier Forum

Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: JeffAlanAmps on November 17, 2020, 02:41:52 pm

Title: Higher than normal AC voltages on PT secondary during startup?
Post by: JeffAlanAmps on November 17, 2020, 02:41:52 pm

Hi All,
I’ve just finished a 5F1 build and am ready to progress through the first-time startup procedure.  I’m following Rob Robinette's startup instructions https://robrobinette.com/Tube_Amp_Startup.htm (https://robrobinette.com/Tube_Amp_Startup.htm), taking things slow, trying to be as safe and diligent as I can. I have my chassis plugged into a light bulb current limiter with a 100W bulb that I’ve independently verified is good, and that runs into a GFC interrupter and then into standard house wall voltage (mine measures 120).  The PT is a Hammond 290AX.  I’m using a CEN-TECH P98674 digital multimeter to make all my measurements. 

With power on and no rectifier tube in, there's no dead short (current limiter bulb stays dark) the amp pilot light comes on, there's no arcing or smoke, and I measure 120VAC at the fuse and power switch (red probe on fuse or power switch and black probe to chassis ground).  So far so good, I think.

But on the PT secondaries: where I should be getting 5VAC on the rectifier tube heater wires (wire to wire), I measure 5.4VAC.  Where I should be getting 6.3VAC on the preamp and power tube heater wires, I measure 6.8VAC.  And where I should be getting 325VAC on the HV AC (wire to ground), I instead am measuring 350VAC.  Are these differences acceptable or within expected ranges?  If not, what – apart from a bad PT could be causing this?  I don’t want to proceed any further with the startup until I get a handle on this.  Thanks in advance for your input and ideas!

Title: Re: Higher than normal AC voltages on PT secondary during startup?
Post by: vampwizzard on November 17, 2020, 03:58:32 pm

all of those seem normal. you get +/- 10% in most cases and im not worrying. things look fine so far.

Title: Re: Higher than normal AC voltages on PT secondary during startup?
Post by: JeffAlanAmps on November 17, 2020, 04:25:07 pm

Thank you - that is reassuring!

Title: Re: Higher than normal AC voltages on PT secondary during startup?
Post by: PRR on November 17, 2020, 06:22:05 pm

EVERYTHING sags. When my well-pump starts, the lights dim a bit. When I throw rocks in my Honda the springs sag. If I have the car headlights on and hit the starter, the headlights dim. When Las Vegas turns-on, Hoover Dam dims.

Tube guitar amp transformers may sag 5%, 10%, or so. Your ~~8% is not wrong. It will get near-enough when everything is sucking.

Title: Re: Higher than normal AC voltages on PT secondary during startup?
Post by: JeffAlanAmps on November 17, 2020, 09:57:27 pm

You guys are awesome.  Thank you!! :icon_biggrin: