First Time Power-up Advice

[GregS]

I'm building the Portaflex project in Kevin O'Conner's book (The Ultimate Tone 3) and am near ready to try it out. The amp uses solid state diodes for the rectifier and does not have a Standby.

I have read the information about powering up for the first time on Paul Ruby's site numerous times. I am currently in the process of learning tube amplifier electronics and am not confident I understand all of it fully. As such I have some questions concerning this.

1. Is there ever any risk of damage from turning on an amp with no tubes installed? Although I would think no as this is part of the testing procedure documented, I wanted to be absolutely sure that there aren't any circuit designs where it might be a problem.
He mentions putting on the Standby for the pilot lamp/heater test which makes sense. I assume that for the following tests that check B+ at the filter caps and OT the amp is NOT in Standby (my amp has no Standby)

2. I assume it is safe to turn on the amp without any load on the OT secondaries when there are no tubes installed because there will be no current flow in the primary - it is effectively disconnected from the circuit. Correct?

3. What about the PT? Can it be damaged by having not enough or no load on it's secondaries? One of the first tests in Paul Ruby's test procedure involves checking the voltage across the HV secondary not connected into the circuit. He states to do it just long enough to get a reading. Could there be damage if you took too long? My PT has a center tap on the HV secondary that is grounded.
The rectifier secondaries are taped up and not used as I'm using diodes. I assume an open circuit here isn't a problem?

4. There are further tests in the procedure with tubes installed that test voltages on grids, screens etc. However, I don't have any specs on what these should be so I'm not sure of the value for me to do this right away. Rather, check these if I find the amp isn't working properly. Thoughts?

All advice, comments welcome. Sorry for the long post.

Greg

[PRNDL]

1. Is there ever any risk of damage from turning on an amp with no tubes installed?

There's no risk if we assume your build is absolutely perfect, which is hardly ever the case.

The OT is effectively disconnected from the circuit. Correct?

Not really ... it has B+ to the primary center tap, which is high voltage.
I always plug in a speaker or load, just in case.

A light bulb current limiter is really nice and not that hard to make.

[Bruce / Mission Amps]

...Not really ... it has B+ to the primary center tap, which is high voltage.
I always plug in a speaker or load, just in case.

A light bulb current limiter is really nice and not that hard to make.

The light bulb limiter is a good idea for all novice builders.
But, a speaker, "just in case", isn't going to do anything if there are no power tubes, well, except blow up if the secondary is accidentally wired to the OT primary or there is an internal short in the OT.

[GregS]

Thanks for the replies.

A light bulb current limiter is really nice and not that hard to make.

Is this done by wiring a light bulb socket to a plug which you plug into the speaker jack?

Also, are there any issues with not having enough load on the PT secondaries?

Thanks,

Greg

[Bruce / Mission Amps]

Thanks for the replies.


Is this done by wiring a light bulb socket to a plug which you plug into the speaker jack?

Also, are there any issues with not having enough load on the PT secondaries?

Thanks,

Greg

No, you basically use the 100 watt light bulb in series with your main 120v-240v power cord to act like a current limiter.
It's like three resistances in series.... the power cord, the 100w bulb and the actual amp.
If there was a dead short somewhere in the amp, there is a huge current surge and the light bulb will glow white hot and bright, just like it was stuck in a desk lamp or overhead light fixture... so you'd know right away to shut it off or at least ASAP.
It will glow bright for only a moment and then quickly dim if there is no serious short in the amp.
So, if the amp is built and wired correctly, (with respect to dangerous shorts in the chassis or power cord) the power cord has nearly no resistance, the incandescent bulb's resistance, when cold, is very very low so it's portion of the voltage drop in series is also very very low.
Since there hardly any loss of voltage/current through the bulb and power cord, the amp (if wired right) works correctly because it's total resistance is now limiting the current flow... and at the right current flow based on being way below your other fuse link, the 1a-3a fuse in the chassis from the power cord line.
Hope that kinda made sense.... pretty wordy, huh?
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Somebody must know right where a link to a drawing of this simple circuit is... yes?

[tubeman]

Here ya go...

[GregS]

Great. Thanks for the information. I'll rig that up.

Greg