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mjinks 25th February 2013 03:57 PM

Tube amp assembly and testing as I go
I'm about to embark on my 5th and 6th builds of the Triode Electronics clone kit of the Dynaco Mk-III mono blocks. My previous builds have gone okay, but I've needed help to get things right, mostly due to typical newbie goofs like weak solder joints and naive wire routing. I'm hoping that my technique has improved enough that these next efforts will go more smoothly, but I'd like to learn to improve my testing practices.

I understand why it's a good idea to test subcomponents as they're built rather than wiring the whole thing together and hoping for the best, but I know barely enough to be dangerous when it comes to applying power to partial circuits. I've read a few books on tube amp design, which help as far as they go, and the Triode manual is a good guide that includes tips on troubleshooting, but on both fronts there's an unwritten assumption that the whole design is complete before current is applied. I've worked around electronics enough to know how not to injure myself. The gap I'm missing is a general idea of how not to injure my components; the "bench smarts" to know what's safe to try with a partially built or partly disconnected circuit, and how to test meaningfully when the complete system as designed isn't yet present.

I'll hazard a guess that I could assemble the power transformer, filter board, choke and rectifier -- simple enough -- and test that much before moving on to the driver, bias and output portions. But what does a good test of a power supply look like when the rest of the circuit isn't there? And does a power supply like this one require some sort of dummy load to avoid running out of control and cooking itself (or hopefully just eating fuses)?

Analogous questions come up as each stage of the system comes in. For instance, can I test my driver board before the tubes are in, and how? What about the output stage? And at various steps in between? Say I have a working amp that develops a problem; what's a reasonable technique to try to isolate the bad part, vs. making things worse?

I realize this is a wide question, and I don't expect a short course in practical electronics so much as outlines and tips for learning, ideally with a focus on this kind of circuit. Any benefit of experience is welcome, as well as pointers to reading material I may have overlooked. Thanks.

DF96 25th February 2013 05:00 PM

You have to understand the circuit you are building, as it is difficult to give generic advice. It depends on things like whether there is any DC coupling between stages, or components which have marginal ratings so will survive in the complete circuit but not under 'partially built' conditions. For example, I understand that certain guitar amps specified electrolytics which were over voltage under some conditions, but OK when everything was working.

A good kit will guide you. A cheap Chinese clone might not.

mjinks 25th February 2013 06:20 PM

Thanks DF96.

This is a good kit, and the docs as well as the tech support do provide guidance. As it happens, Chad from Triode wrote to me a short while ago to say that for an amp as small as this one, he doesn't bother with modular testing; just recommends observing good technique (solder, routing, etc.) and doing continuity tests after the unit is together but before powering on.

And your point is well taken about understanding the circuit. I knew when I posted that my question was impossible to answer in the general case, and I am indeed trying to give myself an education as I can, thus the design books.

But on the other hand, I've watched experienced repair techs take my amps apart for troubleshooting in a way that made me think there must be some general guidelines for how one goes about exploratory surgery on a circuit like this. Maybe the "pros" I watched weren't skilled so much as lucky, or maybe I'm trying to skip the hard parts.

SY 25th February 2013 06:37 PM

Good instincts- test the power supply first. You'll need a power resistor as a dummy load to simulate the amp's current draw. Once that's sorted, you can use the supply to test the other stages as they're built.

DF96 25th February 2013 08:16 PM

If it has overall negative feedback, test it at first with no feedback. If each stage is AC coupled then each stage ought to work (almost) OK on its own.

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