Powering up a new amp build

[funkgang49]

I just completed a Champ/Princeton type build and am about ready to power this baby up. I've checked all wiring and everything seems to be correct. Upon powering up do I need to have all tubes installed and a speaker load connected?

[martin manning]

I just wrote some instructions out for a 5F6-A build here: https://tubeamparchive.com/viewtopic.ph ... 321#226321 See pg. 5. More than you need for a simple single-ended amp, but gennerally applicable.

Yes, you need a load connected if you power up with output tubes installed. Which circuit did you build?

[funkgang49]

Well... It's based loosely on a 5F1. I am using upgraded Merc. Mag. transformers that allow larger ouput tubes; 6L6, KT66, EL34. OT has taps for 4/8/16 impedances and is rated at 3K. If I want to run a 6V6 I can turn imp. selector to 4 ohm when driving 8 ohm cab and get 6K. 5Y3 recto. I veer from the original circuit further in the preamp. I am running 2 different preamp tubes from a single input each with their own Volume controls with a shared Tone control (like a 5C5) which then feeds into a driver stage and on to the Power tube. Can I powerup without the tubes installed? Then if nothing blowsup or catches fire I can check voltages then install tubes and recheck, yes?

[martin manning]

The most cautious approach would be to power up with no tubes installed, which would test the power supply primary circuit, and you could also check filament voltages and HT AC voltage at the rectifier (pins 4 and 6 to ground). Then put the rectifier in and see if you get the DC voltage at rectifier pin 8. If that all checks out put the rest of the tubes in and check voltages. Do you have any way to adjust the idle current for the different power tube types?

[Firestorm]

Well... It's based loosely on a 5F1. I am using upgraded Merc. Mag. transformers that allow larger ouput tubes; 6L6, KT66, EL34. OT has taps for 4/8/16 impedances and is rated at 3K. If I want to run a 6V6 I can turn imp. selector to 4 ohm when driving 8 ohm cab and get 6K. 5Y3 recto. I veer from the original circuit further in the preamp. I am running 2 different preamp tubes from a single input each with their own Volume controls with a shared Tone control (like a 5C5) which then feeds into a driver stage and on to the Power tube. Can I powerup without the tubes installed? Then if nothing blowsup or catches fire I can check voltages then install tubes and recheck, yes?

Do the light bulb limiter thing at first to make sure you didn't short something. As Martin said, when you power up with output tubes installed, you should have a load connected to the speaker jack.

Your impedance choice is interesting. Doubled to 6K should make 6V6s happy. At 3K, EL34s might sound nice (depending on the B+), but Fender typically ran their 6L6/5881s at 4K, so you're a tad low. Won't be a problem, but even those might sound better at 6K.

[thejaf]

I haven't seen it mentioned here before, but I've generally powered up in steps during the build process. For instance, once the heater wiring is done but HT not hooked up, I power up with tubes installed to ensure all that works fine, then I proceeded with the build.

Second power up is without tubes installed using a light bulb limiter and check voltages to make sure nothing out of the ordinary is happening (like B+ on the chassis or something stupid like that).

Third power up is with everything hooked up.

[funkgang49]

The most cautious approach would be to power up with no tubes installed, which would test the power supply primary circuit, and you could also check filament voltages and HT AC voltage at the rectifier (pins 4 and 6 to ground). Then put the rectifier in and see if you get the DC voltage at rectifier pin 8. If that all checks out put the rest of the tubes in and check voltages. Do you have any way to adjust the idle current for the different power tube types?

The Power tube is Cathode biased. I will be using a 680 ohm 10watt resistor and 25uf @50v bypas cap at first. The 5F1 circuit uses a 470 ohm w/ 25@50. I was hoping to use a single value to find a 'happy medium' for all the output tubes.
I do not have a light bulb type limiter rigged up, but I do have a Variac and a DMM to measure the voltages.

[funkgang49]

Ok, so I'm ready to plug in the amp to the Variac and slowly raise the voltage but I would appreciate other more experienced eyes to check my wiring. I took some photos and hopefully I can manage to post them correctly.
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[Structo]

If your variac has an ammeter on it, watch the current draw as you raise the voltage.

[funkgang49]

If your variac has an ammeter on it, watch the current draw as you raise the voltage.
_________________
Tom

Thanks Tom. My Variac does have an ammeter and I'll definately be watching!
I've added a spec sheet for the OT transformer in the attachments. I would appreciate it if someone could check my wiring before I powerup. Thanks.

[funkgang49]

Anyone?

[ChrisM]

From what I can see it looks right.

[JazzGuitarGimp]

A technique I use with a tube-rectified amp and variac, is this: This only applies if your build has a standby switch. Place the standby switch in the standby position and power up the amp at full line voltage (or bring it up slowly on the variac if you're uncomfortable) until the tubes warm up - 30 seconds should be sufficient. You could also use this step to meter your filament voltages. When the tubes are warm, quickly turn the variac all the way down and flip the standby switch to the operate position, then slowly bring the variac up. That way, the rectifier tube is already warmed up, and can really bring the B+ up slowly.

[funkgang49]

OK, hooked-up power from Variac to amp turned on power switch while in standby raised voltage to 120v... nothing. No measurable voltage on heaters, nothing lights up, no power to tubes. I'm guessing there's a problem with the PT?

[ChrisM]

More likely you forgot a fuse in the fuse holder or there is a wiring error.