pre firing it up for the first time questions #124

[echo44]

Ok closing in on the end of my build here a few firing it up questions:

1) recommendations for setting the Phase inverter trimmer and OD trimmer.

2) Bias info? Never have biased a tube amp using the 4 jacks and a Multimeter method, have always used a bias meter which has two jack that go between the tube and amp and read two tubes simultaneously any info on how to use the multimeter method? Using the Bias king method how do you do it with 4 output tubes?

3) Biasing the FET, Looked at the 18v battery method and confused If i am not planning on hooking up an acoustic guitar do I really need to bias the FET?

4) Is it best to start up the amp the first time without the tubes and use a variac?

5) If I am powering up the amps without tubes I do not need a speaker hooked up correct?

6) is normal to see bright red flames bellowing out of the amp reaching the ceiling?

As always thanks from the rookie

[steve_p]

I would start with the PI Trimmer roughly centered. Then adjust your voltages from there. Small tweaks makes a definite difference.
Try setting the OD trimmer wiper to about 100k above ground.

Using the multimeter to set bias, simply measure the mV drop across the 1 ohm cathode resistors. Using ohms law, this equates 1mV of drop to 1mA of current flow. I have run my 2 amps around 30mA per tube, however this is up to personal taste as well.

Some folks like to use the FET input for electric guitar as well.

Generally not a bad idea to startup without tubes. You are correct, no speakers required without power tubes installed. I have not used a variac, but i guess it couldn't hurt.

Flames hitting the ceiling? Depends.... how high are the ceilings?

Good luck, you are going to love it.

Steve

[echo44]

Ok so I powered up and NO SMOKE )
put a volt meter on B1 and saw some nice voltage!
Next step put the tubes in.

Will be a little tricky to adjust things with tubes in as this was built as a tubes
up amp. Any tricks?

Also I noticed the 50/100 LED didn't Light is this because the tubes are not in
and the circuit for the LED can therefore not complete?

[ic-racer]

I always monitor the output tube cathode current on startup. The output tube plates are the highest electrical potential in the amp. Even with the bias pot set for least bias current (most negative voltage) if there is an error in the bias supply construction or design, the tube acts like a big diode (not reversed) shorted right to ground with 400+volts.

When I first powered up my HRM, I had left off the ground wire for the bias supply (rendering it useless). I was monitoring the EL34 cathode current and it shot over 150ma and kept climbing quickly. I shut it down just before the whiff of smoke from the tube base hit my nose.

[Structo]

Congrats!

When ever power tubes are installed you must have a speaker load connected to the output transformer.

If your chassis has four bias test jacks, just ground the black probe, set the meter to read DC volts and plug the red probe into the jack by the tube you want to check.

It will read in mV (millivolts), try around 28-30mv to start with.

Check your PM box, I sent some info.

[sepulchre]

You might find this an interesting read.

http://www.paulrubyamps.com/info.html#FirstPowerUp

You're already past some of it but there are some good tips.