Testing of tube amps:
This is not a casual undertaking because:
• There are lethal voltages in most tube equipment.
• Often there are tight clearances such that probing about could get spectacularly interesting in
very short order.
• What information gotten by probing about may be misleading - or may not be easy to
• Failure modes are many, often quite simple, but still many.
You mention measuring voltages within the tubes – there is a simple expedient for that, they are called “test sockets” and plug into the amp, the tube into them, and then all the pin voltages may be measured. Like this: http://onlyjamesj.info/wp-content/uploads/2018/12/socket-savers-tube-socket-test-adaptors-are-becoming-very-hard-to-find-and-i-consider-them-a-valuable-part-of-my-work-bench-test-gear-these-are-used-to-check-voltage-pvc-socket-saver-video-9-pin-noval.jpg
Then, there is the dummy load – as amps of most natures do not like operating without a load, the Dummy Load is a non-reactive (constant resistance at all frequencies, unlike a speaker) load that allows running the amp safely with, or without a signal. These are not complicated beasts, and may be elegant, or not, as the need arises. Like this: http://diyaudioprojects.com/Testeq/Dummy-Speaker-Load/Wiring-Inside-Speaker-Dummy-Load.jpg Or this: https://music-electronics-forum.com/attachment.php?attachmentid=27422&d=1391997926
The key is that the load is able to accept the full output of the amp for a sustained period without damage or overheating. Watts-is-watts, and a 35-watt-per-channel amp will make just as much heat as two 35-watt light-bulbs on those resistors. Nor is a precise 4, 8 or 16-ohm load required. Pretty much “more-than-4, less-than-16 will do, with some calculations. I keep some 6-ohm, 100-watt resistors similar to the green wire-would devices in the second link that do just fine. Note, however, that if one is getting as sophisticated as using a dummy load, that also suggests that one is using a scope, which is for the most part more than the casual hobbyist may wish to undertake.
Critical Items for Proper Basic Diagnosis:
a) Metered Iso-Variac. Metered for voltage, reading accurately within one (1) volt (AC).
b) Metered for AC Current within one (1) watt on a 0-100 watt scale, or two (2) watts on a 0-300-
c) True RMS DVM/VOM capable of measuring AC components on a DC line and not be ‘fooled’ by
d) Excellent light
e) Dental picks
f) Wooden sticks (1/4” dowels, or similar.
g) Decent hand-tools as needed – not many required, but they should be good.
h) Magnifying option for those not extremely near-sighted.
Stuff beyond the basics:
a) Decent soldering equipment. Does not have to be fancy, just hot enough and clean enough.
b) Eutectic solder. Nothing else.
c) Tube Tester – one that does “Shorts” and “Gas” as a minimum – which will be enough for 95%
of all needs. That other 5% gets into “matching” and other niceties which emissions-testers
With the above array, and some decent bench space, one can do very nearly all necessary diagnostics for most tube amps. And once past the basics, a headphone box on the amp ILO speakers will allow for qualitative tests. Consider that high-impedance headphones are very nearly the most merciless load that may be put on an amp, and will reveal all sorts of hum, noise, hash and other defects concealed by most speakers.
A bit on Isolation: An isolation transformer will NOT make the user immune to electric shocks. If one inserts one’s self into a circuit, one will get shock. But, it will protect against certain defects, such as a shorted transformer winding, or a shorted ‘death-cap’ and similar. It is a very necessary precaution for anyone seriously into the electronics hobby and with any ambition to make repairs at any level other than tube replacement.
Why meter the Variac?: The load expected from about any piece of equipment may be calculated, and fairly closely. A meter will allow one to observe how much the actual load might be, and, especially, the onset of B+ when the rectifier starts to pass current – and what that does. Note that 100% of the current going into any given amp is expressed as heat somewhere – either in the amp itself, the speakers (as sound), the transformers, somewhere. Five or six ‘extra’ watts that should not be there is yet more heat. Which, if, for instance, in an output transformer – eventually means *POOF*.
One of the members here dropped off a heavily modified ST70 (Original pattern) on Sunday. There are two items in the queue ahead of it, but here is what will happen:
1. I will do a basic cleaning (stiff paint-brush) and check all the tubes for proper function.
2. I will then test every connection within the system (dental picks).
3. Given that the unit operated properly (per the owner) for some period of time, I will look for any
unusual or suspicious conditions that may account for the change-in-behavior.
4. I will check the time-delay relay (plug-in device, looks like a miniature tube in size), and the
wiring to it.
5. I will check all transformers and chokes for continuity, and the proper internal resistance – this
will be calculated as I will not do any disconnections at this time – just tube-removal.
6. This has been converted to solid-state rectification – verify that chain.
7. Apply current *SLOWLY* keeping an eye on B+ I will use a headphone box and a pair of high-
impedance headphones to listen through this process as well.
8. Create a revised schematic based on what I find ‘under the hood’ (hand-drawn).
9. Make necessary repairs as-needed, assuming I get to the bottom of the failure, noting all
changes on the revised schematic.
This is pretty much how anything gets ‘done’ on my bench. Nothing special, nothing exotic. Nor anything particularly difficult – after 40 years of being in the hobby.