No worries, you’ve probably made a wee mistake.
It happens, the important thing, as you mentioned, is to logically go through the circuits and figure out what’s wrong.
So, if no sound, other than hiss, on both channels, then we know we have a wiring and or component error.
It will probably only take a wee bit of troubleshooting to figure it out, then glorious sound shall commence!
CAUTION! HIGH VOLTAGE PRESENT, ALWAYS CLIP ON WITH THE AMP OFF AND THE POWER CORD DISCONNECTED, EXCERCISE EXTREME CAUTION! NOTE: YOU SHOULD DISCHARGE THE CAPACITORS BEFORE YOU WORK INSIDE YOUR UNPLUGGED AMP.
1. Get out the schematics, flip the amp over (on a soft cotton dish towel), clip your voltmeter ground to the middle star ground.
2. Start at the beginning and check all voltages, compare them to the schematics as you go, mark the actual voltage reading beside my readings.
a) J2 B+ Tubes loaded s/b (should be) 230-250vdc take all measurement with no Signal IN – No RCA’s connected (so basically the pre is in idle)
b) P6 s/b ~134vdc
c) P8 s/b ~2.7vdc
d) P3 s/b ~136vdc
e) P7 s/b 0vdc
f) P4 across P5 ~12vdc
3. Now if any of the voltages are out significantly, then go backward through that circuit and look for your error.
4. Resistors can be checked in circuit, just disconnect the power cord, discharge the capacitors and set the volt/ohmmeter into the ohm function, probe each end of the resistor and check the value against the schematic – check it off as you go.
5. Capacitors can’t be checked in circuit, so just check your polarity and value.
6. If you’ve gotten this far and all the voltages are within spec, then it is time to have a good look at the signal path. Unplug, discharge capacitors. Plug a quality RCA cord into one IN jack, turn your volume to max, set your volt ohmmeter into the ohm function, clip one lead onto the center post at the end of the RCA cord (make sure it isn’t touching the outer ground connection). Now check the path from the RCA IN center post (s/b very low resistance), next volume IN/OUT, next P7 (v.low), next any star ground s/b ~470K, you are reading through R1. Do the same thing from the RCA OUT using a RCA cord go back to P3. Last check the ground of the RCA cord to any start ground (s/b v.low). Basically you are looking for either a dead short from the signal to ground = 0 signal in or out, or a signal or ground connection that is poor or compromised. This is harder to explain than to do in reality – just take your time and look for a section of the signal circuit that’s compromised in some way – then figure out why.
7. Note with very high efficiency speakers +95db you will normally here a very low level hiss at your tweeters – this is normal for Pure Class A amps. I test the noise floor of my Kits both electrically and with my ears. With ~93db efficient speakers, I set my preamp for normal listening volume (2 o’clock), then without touching anything else bring my ear close to the tweeter – at about 3-6″ I can start to hear hiss at a low volume – this is acceptable. With the R8 because you are essentially adding a preamp gain to a preamp gain, you will automatically double the noise floor, so at your listening position with very high efficiency speakers you will hear a very low level of hiss, this is normal.
That’s a good first pass, if you have any questions let me know, but first dive in, within a short while you’ll start to understand what you are up to.
And NO Worries – the Pre will start to sing soon and if all attempts to locate the problem fail, i can always take care of it on my bench.
Focus & Luck!
Jim