So did the idle anode or cathode current ever get checked?I am still measuring bias current
I did and got a bit under 400VDid you do the tests that were suggested? It's like you're not even reading our comments, because you certainly aren't responding directly to ANY of them.
If you disconnect the bias tap from the transformer to the bias supply, pull the tubes, turn the amp on, and measure plate voltage, you will then have a really solid indicator of the condition of the transformer. It should deliver more than 480 volts with no bias tap load. Probably over 500.
You returned the amp too soon. You made the problem go away for now but my suspicion is that you neither found the root cause problem nor did you fix it. And that will come back to bite you in the ass when the amp melts down another set of expensive 6550s.
According to the owner this problem began years ago and hasn't ever been "solved" by others before me as well. I told him to keep an eye on it and if the problem returns we will delve deeper. I suspect the previous techs also were baffled why the voltage is so off and bumped up the bias resistance to keep it working. Apparently the problem began in the 80s.I did and got a bit under 400V
I see 393V (V4-5) & 394V (V6-7) on plates.
I did and got a bit under 400V
I’m somewhat sceptical the info is good. My guess is that to redplate 4x6550 at nearly 400V will take well over 600mA continuous HT current, yet the HT voltage barely drops a volt compared to a only few mA loading from the preamp valves?Under 400 volts B+ with good supply capacitors, no tubes installed, and the bias circuit disconnected?
If there are no power tubes in it, and no caps are leaking to ground, how else can you explain the low B+ voltage? With no load it should be sky high. I'd expect 500+.
It's very doubtful that the line voltage jumper got moved to the wrong tap.
I didn’t mean I’d rewind it for them. They can buy a replacement for half what I charge for winding it. I want the lamination steel and I also want to properly condemn it as shorted windings before any dissection would occur. He appears to lack the interest testing it properly and maybe doesn’t have the equipment but I do and would love the opportunity for the transformer or the parts to make it good again.As Pete says, the voltages stated don't really make sense. I'd wager it's also at least worth checking the primary tap is correct before condemning the PT.
I'm a total amateur, but there are a few red flags here in what the OP is saying:
1. Used negative voltage to set bias (rather than idle current).
2. Asked for help identifying secondary wires on PT (while in circuit, which should be obvious)
3. The discrepancy in voltages reported with no/redplating load.
Now there's nothing wrong with helping a friend - in fact it's commendable! - but it would be a real shame if this seemingly original PT is written off on this basis.
@william vogel has made an offer to test the PT, and even rewind it if necessary. As @Steve Fundy is in the US, that sounds like a sensible option to consider.
I didn’t mean I’d rewind it for them. They can buy a replacement for half what I charge for winding it. I want the lamination steel and I also want to properly condemn it as shorted windings before any dissection would occur. He appears to lack the interest testing it properly and maybe doesn’t have the equipment but I do and would love the opportunity for the transformer or the parts to make it good again.
It sounds like he’s through with it and returned it. I also think that was a mistake without properly biasing the amp with a current measurement but it was apparent that wasn’t happening.Sorry, my mistake. Perhaps if we knew where the OP was, someone might be able to suggest a local tech who could offer a second opinion?
Yes of course. I know that, except Bassmans had a 5U4 instead of solid state rectification. It's exactly what you said: Layout. Unfamiliar territory. The amps work the essentially same way.Just be aware, the basic Marshall schematic (JTM45 to Superlead) and the basic Fender schematic (Bassman) are very nearly identical. The first Marshalls were the Bassman circuit, with minor tweaks only. So if you can work on Fenders, you know how Marshalls work.
Of course there are differences in construction and layout, but basically they're the same amplifier concept.
Due to valve and mains supply variance, it’s insufficient to just set the negative bias voltage to some nominal level. Rather its effect on anode or cathode current should be checked, and further adjustment made as required.I have used the both negative voltage and idle current measurement method to bias many Fenders.