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Hello,

I am a first time amp builder working on my Tweed kit.

I have wired up the heaters, power transformer, fuse holder, stand-by switch and the pilot light socket. The instructions say it is now a good time to check that the power transformer is working properly. It says to install the fuse and carefully apply power to the circuit (i have built a current limiting light bulb) and check the AC voltages are within range of spec.

I have not had a lot of experience using my multi meter, mostly I have checked my resistors. I have watched some videos on setting up my multi meter to measure AC but I am not sure how to proceed measuring on my amp (where specifically should I be placing my leads when doing the measuring). Also the instructions say the voltages will be higher with no tubes installed, how much higher is acceptable?

I have attached some pictures of what I have wired up so far.

Any advice/directions would be appreciated.

Cheers

Stephen

Set your meter for AC volts. Be very careful measuring the high voltage. With one probe connected to the chassis (it doesn't matter which one) and the other probe on pin 4 of the rectifier tube socket you should read 350 volts or more. The readings will be higher with no tubes in place. Maybe 375 volts or so, the reading isn't that critical for this test.

With the probe still connected to the chassis, move the other probe to pin 6 of the rectifier tube socket and you should get the pretty much the same reading you got on pin 4.

Now check the heater voltage running to the rectifier tube. Put one probe on pin 2 and the other on pin 8. You should read about 5 or 6 volts.

Now check the heater connections to the other tubes. With one probe connected to the chassis you should read at least 3.15 volts at each heater pin. If you measure across the heater pins, for example with one probe on pins 4 and 5 of a 12AX7 socket and the other probe on pin 9, you should read 6.3 volts. It will probably be more like 7 volts with no tubes in place. You should also note that the pilot light is lit.

Thanks Mitch,

I really appreciate you taking the time to reply, this is just what I needed.

I will take the measurements tomorrow.

Cheers

Stephen

Hey Lew;

Please double check your wiring from the mains to the switches with the layout again... (I am using Rev 10)
Something does not seem right...
Remember the black wire from the mains should go to the fuse before going to the power switch which then feeds the transformer...

Andrew

You can't check things too many times where line voltage or high voltage is present.

It looks like the wiring is OK, though. In the photos it looks like a yellow wire runs from the live terminal on the IEC connector to the tip terminal on the fuse holder. But it looks like the photo was taken before the wire that runs from the sleeve terminal on the fuse holder to the power switch was installed.

The transformer is wired properly. Black and Black/Blue to the neutral terminal on the IEC connector and Black/Red and Black/White to the power switch.

Hi Mitch/Andrew,

Thanks for checking my wiring. Yes I did use yellow wire to run from the live terminal on the ICE connector to the tip terminal of the fuse holder. And yes, in the photo I did not have the wire from the sleeve terminal to the power switch installed, it is now.

I measured from chassis to pin 4 and then to pin 6 of the rectifier tube socket and got 360 volts on each one.

All the heater pins are correct, 3.15 volts individually and 6.3 volts pin to pin.

However when I measured from chassis to pin 2 of the rectifier socket I get 123 volts not 5 or 6. From chassis to pin 8 I get 127 volts. When I measure across pin 2 to pin 8 I get 5.6 volts (which I believe is correct based on another thread on the forum).

Any ideas why the pin 2 voltage is so high?

Cheers

Stephen

Sorry, I shouldn't have told you to measure the heater voltage at the rectifier tube with respect to ground. It is not referenced to ground so right now it is free to float up to whatever voltage it wants. You are reading 5.6 volts across the heater pins which is correct and all that matters right now.

Don't worry about the 127 volt reading to ground. Once the rectifier is in place its heater voltage is referenced to B+.

No problem Mitch, thanks so much for taking the time to help me out. I really appreciate it.