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Might still need to tweak the pre-amp voltages, we'll see.
V1 and V2 both low but I had a 5751 on v2 so I expected it to be different from my original build voltages. v3 wasOK but the output valves were down around 350v. I was expecting nearer 415-420 from a 320-0-320 PT. I previously had 454v from a 345-0-345 PT. Investigating further one side of the HT was much lower than the other at the rectifier.
So I got a little more time today and I pulled both HT's and the bias supply from the rectifier and checked the raw voltages. 256-0-315. Bias 23v. I suspected I had made a schoolboy error and mixed up the CT and Bias wires, but no, it seems not, and the resistances all tally. Now I am wondering if they could have connected the wires incorrectly internally, the schematic is quite clear, Red and red are HT, Red/Yellow is CT, Red/Blue is Bias (should be 60v)
Not quite sure what to make of it, wondering if my level of knowledge is just inadequate and I'm making a stupid mistake!!
Red-red 21.7 ohms Spec sheet says 20.15 +- 20% so Ok there
Red 10.2 red/yellow 11.2 red
Red/yellow 2.2 red/blue
Red 12.5 red/blue 9.2 red
Doesn't make any sense to me. Definitely wired as per schematic so could they have mixed up the wire colours? But if Red/Blue was CT thats more out of balance than Red/Yellow as CT
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
@DJH83004. I did originally measure the secondaries whilst connected from the rectifier 285-one side 334 the other. All valves installed and biased at 38mA. I was getting 351v on the anode of the 6L6's, which is what alerted me to the problem.
If you get much more than about 70-80mA mains draw with the secondaries unloaded I would guess you've got a shorted turn in the HT winding. In theory that should produce a near-short on the primary too, but I've come across some where it doesn't seem to - I don't know why.
Although I had a really odd one a few months ago, an old Fender PT which drew about 300mA but otherwise seemed completely normal, including all the secondary voltages. I would never have noticed if I didn't routinely monitor the current on power-up. I still don't know what it was... the amp ran fine, even on an all-day full power test.
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Swapped back to Red/Yellow CT and get 256-0-314, 22v
@ICBM, current draw with secondaries disconnected (heater wires connected but no valves installed) 0.17mA.
Stumped....
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
I desoldered the white primary which was connected to Neutral on the IEC and connected my MM in series at this point (thus removing the current draw , if any, of the relay transformer from the measurement). MM was definitely reading 0.17mA, I changed the range a few times and the decimal point kept moving as expected and the 'm' next to the 'A' was always present. Must admit its a cheap MM and I really do need to get a better one if I'm going to be doing much more troubleshooting!
Transformer is coming out.....
Backtracking and checking everything that I hadn't already checked, in order to prevent me looking a fool and sending a perfectly good PT back to the supplier. Fuse on the CT had blown!! When the old PT was blowing HT fuses I checked the other fuse as well, so it was good, now it isn't. Swapped it and I get
337-0-337, bias 61.7 volts Bang on spec.
How do you delete a thread to save ones blushes? ">
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
"Take these three items, some WD-40, a vise grip, and a roll of duct tape. Any man worth his salt can fix almost any problem with this stuff alone." - Walt Kowalski
"Only two things are infinite - the universe, and human stupidity. And I'm not sure about the universe." - Albert Einstein
Anyway, far from being sorted, now the amp is blowing CT fuses regularly, within a couple of seconds of turning off standby, even with no valves installed, and the rectifier board hanging free (I thought I might have made a botch of resoldering the PT wires, but no). WHich is different from when I powered up first time. Then I was able to set the bias to 38mV and between then and checking the internal voltages the CT fuse blew and gave me the weird unbalanced HT supply voltages.
Now its getting it bit beyond my knowledge/patience level so its booked in for an experienced tech to look at.
The PT is capable of nearly twice the current required, being for Fender Twin. When goingoff standby the inrush current is jus too much for a 500mA fast blow fuse. a slow blow/timedelay of the same value is fine.
Amp sounds fine though the preamp voltages are alittle off, though I do have a 5751 in v2 which isnt stock.