NPMAD

eSGEe said:
HOLY CRAP MAN --- to get that must feel like a MILLION BUCS!
I am jealous as HELL -- but also VERY VERY happy for you Brother !!!
Congrats!!!!
:2Thumbs:
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It does feel like a million bucks, & thanks, appreciated.
With family gone & no risk of kids coming to see what's going on, & possibly getting electrocuted by doing what nosey kids do, I've got to it conditioning the new filter caps. First, with the amp on standby I checked for proper operation & range of the bias circuit. All good. To condition the filter capacitors I first removed the HT fuse, then, with two alligator clips I jumpered a 100k 1 watt (current limiting) resistor across the two terminals of the HT fuse holder & powered up. It's been on for about 1/2 hour or so now, voltage drop across the 100k resistor is down to 18.2Volts. Anything below 5V & we can consider the filters properly conditioned, but really, the lower it's left to get the better. Here's a pic with new filter caps in & being conditionedIMG_20210130_160221.jpg
The alligator clips, 100k resistor & meter leads attached to the resistor (monitoring voltage & voltage drop across resistor) can be seen attached to the HT fuse. Cheers
 
Mr Grumpy said:
Do you have to warn the local town authorities before you turn it on for the first time?
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Nah, they're pretty used to that sort of thing by now. So long as I don't krank them up late at night it's usually no problem. I usually don't play them after about 4pm, though occasionally the neighbours do ask me to turn it down. Cheers
 
Ok, all unloaded (no valves installed) voltages check out,
Heaters 3.24-0-3.24VAC.
B+ = 443VDC.
Bias supply ranges from 0V to -50VDC.
All coupling caps check out, no DC leakage, though I expected this, the C296 mustard & RS silver mica capacitors are pretty good in that regard.
So, after some lunch I'll re-tension the valve sockets, give it a few shots of contact cleaner, then install valves, bias the output valves, find the best valve for the V3 (PI) position & then,,, plug a guitar through it.
While I think of it, something else that is in line with @67plexi thinking it may be a '67 rather than a '68, the board's inter-turret wiring is all done on the board underside as was done in '67 & earlier amps. In '68 Marshall started doing this wiring on the upper side of the board to make troubleshooting/repairs easier. Cheers
 
Ok, so I got everything done except finding the valve best suited to the phase inverter (V3) position. The bias adjustment slider makes biasing more time consuming than with a trimpot due to the slider making coarser adjustments. Getting the negative bias voltage in the ballpark is relatively easy using a non-conductive implement (nylon screwdriver) to move the sliding contact, but fine adjustment requires shutting the amp off & draining it to make small adjustments by hand. Only VERY small movements of the sliding contact are needed. Ended up plate voltage 416V, bias V4 at 39.9mA (66.5%) & V5 at 40mA (68.2%). Having the cooler biasing valve in V4 is ideal as it gets driven the hardest by the PI.
With it still out of the headcab (so I could keep an eye on the output valves) I plugged an SG into the bright channel, controls set presence 2, bass 4, middle 7, treble 5 1/2, bright volume 3 1/2, normal volume 0. That the noise floor was a little high was expected seeing as the electronics weren't shielded (chassis not in headcab) & the amp was in close proximity to the 4x12 cabinet.
And it sound good, damn good, clear, punchy & ballsy with that aggressive upper mids & high end sizzle that Marshalls do so well. And loud, did I mention that at 3 1/2 on the dial it's getting loud,,, & starting to break up a little. Good sustain too. Chords could be coaxed into feedback by manoeuvring the guitar toward the speaker cab, likewise single notes with some vibrato.
I knew at the time (about 5-20pm) that it was a bit late in the arvo to be playing a 50 Watter at home & after a little more than 5 minutes the young Barby doll next door let me know that it was too loud. I'm very much looking forward to taking it out to the farmhouse (rehearsal space) for a decent workout at a more sensible setting of 6, 7 or 8 on the volume. Cheers
 
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ivan H said:
To condition the filter capacitors I first removed the HT fuse, then, with two alligator clips I jumpered a 100k 1 watt (current limiting) resistor across the two terminals of the HT fuse holder & powered up.
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Nice method. This is much simpler than the method of disconnecting the B+ lead into the first filter cap and then clipping on the current limiting resistor to the cap and to the B+ lead.


ivan H said:
That the noise floor was a little high was expected seeing as the electronics weren't shielded (chassis not in headcab) & the amp was in close proximity to the 4x12 cabinet.
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What works for me if I want to get a good idea of the noise floor of the amp when it's on the bench opened up and unshielded, is to lay a sheet of aluminum big enough to overlap the open chassis and secure that sheet with a couple of magnets. Obviously the magnets won't hold if the chassis is made of aluminum.
 
syscokid said:
What works for me if I want to get a good idea of the noise floor of the amp when it's on the bench opened up and unshielded, is to lay a sheet of aluminum big enough to overlap the open chassis and secure that sheet with a couple of magnets.
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I had a sheet of aluminium alloy that I used for the same thing, for years, but now its shielding my 1 watt JCM800 micro amp (was just the right size). Cheers
 
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