How to power up an old unknown quantity tube amp?

[Ian444]

Hi guys just wondering exactly that. I have a couple of old amps I would like to work on. What procedure do you guys use to power up an old unknown quantity tube amp? Something like this?

1. Remove and test all tubes.

2. Inspect wiring and electro's.

3. If solid state rectification, check diodes.

4. Check the mains earth is good (and the power cord).

5. Apply power with a variac without power tubes and check all voltages, making sure not to exceed filter cap voltage limits.

6. Apply a sinewave and check preamp and phase splitter signals, and check there is a bias voltage for output tubes (assuming fixed bias.)

7. Then fit power tubes and power up again.

I just got a variac but have not used it before, how do you know if the amp has a fault and is pulling too much current, does the variac start humming, or do you use a current meter in series, or do you watch for flashes, smoke Do you use the light bulb in series and the variac?

Thanks.

[Gibsonman63]

I don't have a variac, but I would highly recommend using the light bulb limiter on the amp side of the variac.

I would check all of the electrolitic caps closely to see if any are getting blisters on the ends or are physically leaking.

If you have a schematic, I would check voltages against the schematics before installing tubes.

Have a really good look at the tube side of the power tube sockets and look for carbon traces. Those will be conductive.

Clean and retension all of the tube sockets.

I would power up with known good tubes, but not anything that would upset me greatly if they fried.

I usually check for DC on all of the pots. It is a down and dirty check for leaky coupling capacitors.

Check the main fuse and make sure it is the right type and rating. It is amazing what some people will stick in there.

[Structo]

What kind of construction are the amps?

Are they point to point or circuit board?

I agree with Gibsonman63, give each a very hard visual inspection.

It's pretty much guaranteed that the electrolytic capacitors need replacing so if it looks like an amp you would like or would sell ok, then that would be the first step along with unstalling a three prong cord.

[rdjones]

Every pro shop I've ever worked in had a Sencore PR57 Powerite.
I eventually got my own.

Once you get used to using a professional tool like this you never want to go back.
Several other brands have (or had) similar units, such as B&K, Tenma.

RedDog

[selloutrr]

I use a variac with an Amperage meter and watch the amperage as I bring it up from zero volts up to it's operating voltage (old amps ~ 115 / 117volts.)

If it's an old amp just assume it needs work. all new electrolytic capacitors and anything that looks waxy or blistered is probably leaky and bad.

Again watching the amperage if it goes up it means you have a problem. POWER down at once and then find out why.
Bring it up slowly on a variac helps no shock the caps and helps them reform.

I'm personally not a fan of the lightbulb limiter. It works but it has a grey area of dim and lit. I'd rather now for sure what is going on. In a pinch they work as a basic good bad test.

[rdjones]

For someone with a Variac (especially an unmetered one) there are the power monitor meters like the Sencore PM157.
Nice big voltage, current and derived wattage readings that you can hang on the output of the Variac.

Dad had the light bulb limiter on the workbench when I was just a tyke.
This was in the '60s when we would bring '40s era amps back to life.
On one hand the bulb limiter is primitive and imprecise, on the other something is better than nothing.

rd

[Ian444]

Some very good points put forward, thanks very much for the input. I am hoping to be seeing a few amps like this in the future, so my thinking is to make up a checklist to follow to minimize possible damage from just going in balls and all and flipping the switch. I don't want to destroy anything, especially transformers.

From the posts, and the Sencore units, it looks like an output voltage meter and current meter need to be mounted into the same box as the variac. I prefer analogue meters for this purpose. I have an AC voltmeter on hand but will have to find an AC current meter, maybe 0 to 2A would be a good choice for up to 100W amps? It sounds like a lightbulb on the output of the variac would be some extra insurance too, especially if something suddenly let go. I'll do that at the same time. So the ammended checklist:

1. Remove and test all tubes. Clean and retension tube sockets and check them for carbon traces.

2. Inspect wiring, components, and inspect electro caps for blistering or leakage. Check diodes for shorts/open.

3. Check the mains earth is good (and the power cord). Check the size and type of mains fuse fitted.

4. Apply power with a variac without any tubes at all, slowly, so as to let the high voltage caps reform, making sure not to exceed filter cap voltage limits, and check voltages. Check for DC on the pots (leaky caps), the output tube bias (if fixed bias), the B+ voltages, and the heater voltage/s.

5. Fit tubes and carefully apply power via the variac, keeping an eye on the current draw. Best to use known good cheap tubes for this.

OK I'll give this a go. Need to build the variac test box up first. Thanks for taking the time to post guys, much appreciated.

[selloutrr]

read up on variac use and safety some require an additional isolation transformer to operate safely.

[Ian444]

Will do selloutrr. The one I have is an autotransformer, so no isolation. Here's a pic of an amp that I haven't run up yet, an old Australian guitar amp from late 1960's. You can see the output tubes do not want to seat properly. Yep the case is missing too. There's another 4 empty holes for output tubes, must have been for a 200W model. It looks like few repairs have been done on this one. Nice big Ferguson trannies. Up until maybe mid 1970's there was a big tax on imported guitar amps in Australia (e.g. Fender, Marshall), which made local amp manufacture viable. This was one of the local amps.

[rdjones]

4. Apply power with a variac without any tubes at all, slowly, so as to let the high voltage caps reform, making sure not to exceed filter cap voltage limits, and check voltages.

When you do this make sure you are checking the voltage at the _end_ of the dropping string (usually near the first preamp) since with no tubes in and no current being drawn there will be little if any voltage drop, and these caps may be rated at lower voltage.

reddog

[selloutrr]

http://www.ozvalveamps.org/lenard.htm

[Ian444]

Have a really good look at the tube side of the power tube sockets and look for carbon traces. Those will be conductive.

Like this? This will need to be replaced? I have found it is near impossible to remove all the carbon trace. Arcing from anode to a heater pin.

[thereminator]

Necessarily replace the socket. This carbon trace can kill output tubes and transformer (if they are not dead yet).

[Ian444]

Thanks, will do. I am glad that the OPT is OK.

[Cygnus X1]

https://tubeamparchive.com/viewtopic.php?t=14542