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TEST WITHOUT TUBES
#1
Hi Guys

When we have a tube amp for repair or we have a new build and want to test it, the first thing we should do is REMOVE THE TUBES.

Of course, we never power anything newly built or of unknown status without connecting it through our Power Limiting Safety Socket (PLSS) - this is the mandatory project in TOT (Tonnes of Tone). This test fixture will save you countless $$ on fuses and will keep things from going up in smoke.

At first we limit power as much as possible since there are no tube loads, just caps charging. The surge will cause the lamp to brighten then dim if everything is okay.

We use our DC voltmeter to check voltages within the circuit, to check their polarity and relative sizes. If this has a power section with fixed-bias, check that the bias voltage is present right on the tube socket pins. Sweep the bias pots to assure their function and leave them set to the maximum negative voltage (bias pot '0' end; zero, cold; low bias; CCW)

If the lamp brightens and stays bright, there is a problem in the circuit.

If the voltages seem okay, you can add preamp tubes. This will require changing to a higher wattage lamp in the PLSS. Powering up should be the same as before but the bulb might glow a little bit. You can measure voltages around the tubes and even send a signal through and scope it up to the empty power tube sockets. Check the bias voltage again with the reduced power limiting. Its proportion to the screen or plate voltages should look more typical now.

Add the power tubes and reduce the limiting further using a larger lamp or parallel lamps.

TEST WITHOUT TUBES any time you modify the bias circuitry AND for any new build AND when you add Power Scaling.
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#2
Hi Guys

It is dumbfounding how many "amp builders" believe you cannot test an amp without tubes !

If you mention it to them and they come back with "You can't run the amp without tubes", it puts into question if they really know how amps work - including their own products.

As posted above, TESTING WITHOUT TUBES is the mandatory first step in assessing a build with unknown status(broken, needing a repair), a new build, a modified build and a repaired build.

When we say to test without tubes, we mean without power tubes, except where the unit is only a preamp in which case literally test without ANY tubes.
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#3
Hi;

Got a couple amps that have been sitting broken and unused for some years. Want to bring them back to life. In addition to pulling the tubes and using the PLSS, would a Variac be a good idea as nothing has been powered up in so long, or does the PLSS make the Variac redundant. If the Variac is a good idea, do I use it with the PLSS, and which is the horse, and which is the wagon???
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#4
I assume by a PLSS you mean light bulb limiter? Typically when I do this I would only use it not the variac. Others might weigh in differently on this but I'm not a believer that filter caps should be re-formed using Variac. If you do use a Variac should it be used with an isolation transformer for safety.

Depending on how long the amps have been sitting you might have to plan on re-capping them. Caps don't last long if they are not charged regularly.
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#5
Thanks a lot, appreciate your advice and comments.
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#6
(12-27-2018, 02:42 PM)TiCatFan Wrote: Thanks a lot, appreciate your advice and comments.

Sure thing!
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#7
Hi Guys

Reforming caps should be done using a current limit of some sort. Some cap manufacturers suggest that the charge current should be limited to three times the nominal leakage current for the given capacitor, which means the current will be in the low-milliamp range t most, or even microamps. A high-value resistance will suffice inmost cases but this results in a reducing current a the cap charges up.

PLSS = power Limiting Safety Socket

The PLSS should always be used when testing amp that have unknown operating status, and for new builds and repairs, and for amps that have had a modification to their bis circuit. As our books Tonnes of Tone (TOT) and The Ultimate Tone (TUT) state, a variac does not limit power; rather, it limits voltage. If you have both a variac and the PLSS, place the variac first in line, then the PLSS. It probably makes no real difference to the test result, but this seems like the correct intuitive order.

Have fun
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