by Peter W. Mon Jul 02, 2018 8:48 am
Some interpolations to this excellent diagnostic process: j beede wrote:No need to damage anything during troubleshooting.
1) Power down the amp and remove all tubes and "Weber devices"
2) Insert a 60W (75W or 90W will do) incandescent bulb in
series with your mains supply
This lamp will light in proportion to the current running through it - and why it MUST be a conventional incandescent lamp. By the way, a 90 or 100-watt lamp would be preferred as it should be near full brightness ONLY when the Amp is operating at peak load.3) Power up the amp and watch the bulb. It may glow for a moment at initial turn-on then quickly dim down to a steady state.
The only current on this lamp with all tubes removed should be the power-transformer quiescent current. The bulb should be dim, indeed. And there should be no turn-on surge as there are no cold filaments involved.4) If the bulb dimmed or went dark (favorable) move on to step 5. If the bulb did not dim or lit up brightly you have a short circuit to locate.
This short will be in any of four common locations:
a) The line-cord shorting to the chassis - easy to locate and fix.
b) One of the transformer input leads shorting to the chassis - easy to locate and fix.
c) One of the transformer output leads shorting to the chassis - not as easy, but not hard.
d) An internal winding short on the main power-transformer. - very nearly always fatal.
ARE THERE GROMMETS on the chassis penetrations for the transformers? If none, start looking for shorts there. 5) Power down, then insert all tubes
except the 5AR4 or Weber device
6) Power up and observe the bulb. It should glow brightly for a few seconds then begin to dim noticeably as the filaments heat up. It should not dim down as much as it did in step 3.
Cold filaments pass a lot of current (bulb is bright). As they heat, their internal resistance goes up - bulb dims. Without the rectifier in place, there will be no B+, so you are seeing only filament and quiescent transformer loads - about 25 watts or so.7) If the bulb dimmed move on to step 8. If the bulb did not dim or lit up brightly you likely have a short circuit to locate or failed tube(s) to isolate.
With the amp in this condition, it is acceptable to remove one tube at at a time. When/if the lamp dims suddenly, and re-brightens, that tube is the (a) culprit. Always keep going with any diagnostic test even if a cause is identified. It may not be the only issue. 
Power down and insert a relatively expendable rectifier (not the Mullard GZ-34 that your great uncle hand carried home from the war).
9) Power up and observe the bulb. It should glow brightly for a few seconds then dim noticeably as the filaments heat up. It should not dim down as much as it did in step 6.
Yes. You are now seeing B+, and the Amp will be pulling about 90% of full load (which will still be below the nameplate rating). 10) If the bulb dimmed your amp may actually play music at this point and you can move on to step eleven. If the bulb did not dim or lit up brightly, or blew a supply fuse you may have a bad segment(s) in your high voltage filtering. All tube audio hobbyists should keep a supply of 450V, 22µF electrolytic caps to swap in for just these sorts of circumstances. Start swapping 22µF in for the segments in your quad cap. DANGER HIGH VOLTAGE even when the amp is unplugged.
Or, some sort of mechanical short in the rectifier path, or a bad section in the rectifier itself. BUT - when swapping, each section, the OEM rectifier section must be OUT-OF-CIRCUIT as if it is shorting, it will remain short. One issue NOT spoken to here if the amp shorts out on this last step is a failed output transformer - only when the rectifier is in place will it see B+, and a short to the chassis will pop the fuse. 11) Power down and remove the dim bulb setup. Put on your safety glasses and power up the amp. Voila! Or not... in which case come back and give an update.
...j
Now, a few things I would do first, and I do assume you have a decent VOM and perhaps alligator clips for one lead.
a) Get pin-out diagrams for all tubes.
b) Connect the black lead from the VOM (on OHMS) to the first filament pin. This is a good place for that alligator lead. Touch each other pin with the read lead - playing attention to the pin-out diagram. You should be getting a reading only on filament or filament center-tap connections. Nowhere else. And you should be able to see why on the diagram. If you do, the tube is toast.
c) Do a complete visual exam of the amp, top, bottom and sides. Use a dental pick to check every connection and look for cold-solders, loose wires and so forth. Then:
d) Use the VOM (on OHMS) to look for shorts between any of the transformer leads to the chassis - use the schematic to determine which ones are relevant - and any of the filter cap elements to the chassis - as some of th ese leads are connected via a resistor - keep that in mind. The schematic is your best friend.
Another "by the way" - sounds like eggs frying from an AMP and not at the speakers, is usually because something is cooking. As the main filter cap is surrounded by semi-liquid salts, that could be one source. And as the main power-transformer is a bunch of enameled wire tightly wound together, that could be another. And lastly, same for the output transformers when carrying B+.
Lastly, there is a Kill-Or-Cure for OPT or Power Transformer winding shorts: Obtain a medium-voltage (100V or higher) high-uF capacitor (1,000 uF or better). Charge it fully - requires a DC power-supply or similar meants - and discharge it across the shorted winding. Works about 1/3 of the time.
