Some blindingly obvious things to do in an amp build

[R.G.]

Here are some things that at least to me are incredibly obvious things that need included in an amp build that hardly ever are.
1. One fuse per winding on the power transformer. The AC power inlet fuse is there to prevent fires. It's a happy accident if it protects the power transformer. Put a fuse in the heater winding(s) and one in each side of the centertapped high voltage if so provided.
2. Install silicon safety diodes in series with a tube rectifer, one in series with each anode (not cathode like I first wrote 8-0 ). Then if the tube shorts, the silicon picks up the slack and does not let your power transformer and filter caps die.
3. Put bleeder resistors across the first filter cap. This ensures that it will bleed down in some small time to save YOU from shocks when you service it. Pick how much current you can allocate to them, or how much power you can waste, then figure the resistors. For instance, if you have a 400V supply that goes to 500V if there are no tubes in, and you don't want to waste more than half a watt, calculate P = Vsquared/R = 0.5 and get R = 500K, and rated at 1W. A 470K 1W would do, as would two 220K half-watts in series, or two 1M half watts in parallel.
4. Build it with a 3-wire AC cord and safety grounded chassis. There is no two-wire magic.

There are other things, but these scream to be done on any new build. Build it so it is harder to kill, and easier to service.

[jmaf]

Nice, RG. Now I feel I must head back to the shop and acommodate more fuses. On item 2 I think you meant the diodes in series with the plates?

[R.G.]

On item 2 I think you meant the diodes in series with the plates?

DOH!! Yes, anodes, not cathodes! The diodes go in series with the incoming HV in a manner which would make HV correctly on the first filter cap if the tube were shorted on all active pins except the heaters. I edited the post. Thank you.

Nice, RG. Now I feel I must head back to the shop and acommodate more fuses.

It's a bit of a PITA to site the fuses. I like side-by-side strips with several fuses in them. They screw or pop-rivet right to the chassis. Probably better to use a 2X block for the HV separate from any heater fuses. Note that the heater current is large, so get fuse holders with plenty of current rating for it. Maybe 10A or more.

[jmaf]

I'll go back and review some circuits I have lying around.... The fuse holders I have here are rated 3A, not good enough for the heaters I guess, but probably ok for the HT. I don't have double/triple holders, just individual ones that get the screw on the center. Shouldn't be too much work to get them in near the standby switch, for example. Thanks again for raising the topic and for the tips.

[jmaf]

Related question for the wizard of RG: a technician friend closed his shop and gave me a bag of axial-wired fuses shrinkwrapped in blue. All I have is a bag of them, no specs, nothing. I guess you know where this is going....

How do I find out their rating? Series them with a known resistor and push the variac until they pop? Then record the voltage and current?

They're great for amp builds, because I can wire them like a resistor, in fact they look like resistors... Any tips extremely appreciated.

[R.G.]

Related question for the wizard of RG: a technician friend closed his shop and gave me a bag of axial-wired fuses shrinkwrapped in blue. All I have is a bag of them, no specs, nothing. I guess you know where this is going....

Unfortunately yes, I do. There's really only two ways. One is to detonate them, and find out where they died. This isn't very accurate, because how much overcurrent and how fast it changes determine the actual current when a fuse opens. All you can say is "it's less than that" once it opens.

Worse than that, if they're all shrinkwrapped, you can't tell that they're all the same rating for sure. They may have all the same color/type of shrink tubing, but different values. Probably not, but you can't really tell.

The second way is to take a sharp knife and slit the shrinkwrap, then read the fuse. On each one as you use it. You can re-shrink-wrap them after that if you want.

I wish I knew of a sure-fire electrical test to tell the carry current of a fuse, but I don't, and worse yet, this is one of those things where I can't even come up with a good theory of how to. Fuses are just too variable.

You might tell by watching the fuse element for sagging or signs of softening as you increase current, but you'd have to remove the shrink wrap for that, too. Might as well read the cap. Resistance is no good indicator without knowing the alloy of the fuse, and knowing where the last one popped isn't any indication of the next one unless you have other knowledge that they are in fact identical inside, which the shrink tubing prevents.

Ugh. I hate PITA.

[guitarmike2107]

These are good point’s I have just started inserting two inline fuse holders for the HT secondary, but haven’t gone as far as doing the heaters yet.
Another thing I would say is a must is that the IEC pin terminals should be shrink wrapped, this is the only place I have ever been shocked. Amp turned off forgetting that its plugged into the wall.

[Gibsonman63]

One more thing to add. The light-bulb limiter. Build one and always do your initial power up with it. It will save you a lot of agony later.

[R.G.]

These are good point’s I have just started inserting two inline fuse holders for the HT secondary, but haven’t gone as far as doing the heaters yet.
Another thing I would say is a must is that the IEC pin terminals should be shrink wrapped, this is the only place I have ever been shocked. Amp turned off forgetting that its plugged into the wall.

Good one.

As an embellishment, how about soldering on one of the neon power indicators to the back of the IEC so it's always lit when there's power coming in? I put LEDs in series with the bleeder resistors on the Workhorse line as an indicator that there was voltage on the filter caps and as a debugging aid. No light, voltage stopped before here.

One more thing to add. The light-bulb limiter. Build one and always do your initial power up with it. It will save you a lot of agony later.

Another good one. Why build an amp and not build the tool that will let you bring it up carefully and well?