Ampeg SVT CL preventative maintenance question - to fil transformer fuse? or not to?

[tubeswell]

So I've got this SVT CL in for repair , and someone's had a go at it in the past I see (tell tale leftover scorch marks on the Fil transformer 6.3VAC lead going to the power tube board (see picture). Whoever looked at it previously obviously cleaned up the board and put 220R 1W Rg2s in (I could detect faint traces of alcohol cleaning marks), but they didn't replace the scorched heater insulation wire.

So today - in between jobs, I thinking, hmmm... could these carbon traces on the wire present a potential short circuit problem:

a) to the other components on boards or
b) in terms of a bad experience with the filament transformer itself

I don't know the full history of this particular amp, but before he brought it in to me, the owner tired to replace a full set of output tubes and ran into biasing problems and it died. I managed to resurrect the power (diodes in the +/-15V power supply for the bias sensor circuit had shorted, so I replaced all of those and the filter caps there and its powering up again, and it can withstand all the pre-amp tubes being loaded in it (1.5A) (but I haven't started to load up the 6550's yet and they are 1.6A each).

The colour of the filament transformer 6VAC lead worries me slightly.

Was it just collateral superficial coloration from a (likely) previous Rg2 fire (it seems to be on the wrong side of the board for an Rg2 fire don't it?)?

Or was it the filament transformer itself?

Why would only one lead be dirty?

I can try putting in an inline 15A fuse on the fil transformer secondary - which would at least possibly help to isolate the problem a bit. Trouble is that to order a new fil tranny here is about $500+ just for the part, and the owner seems to think the amp was working fine before he tried to put the new 6550s in.

Or should I just chuck some heat shrink over that scorched wire insulation?

Opine away...

[52 Bill]

It could be that the physical connector was becoming oxidized and created a high enough resistance to cause heating when under load. That could explain the single wire being toasted.

[Enzo]

Ah. One of those push-on spades burnt up. Just like a burnt up molex pin on a smaller connection.

In my view, the connector female got loose, and resistive. Arcing and resistive heating caused it to get REAL hot, they do that. I'd cut off the last centimeter or two of that wire and mount a new female push-on. I guess they call them quick-connects officially. Clean up the male spade. use a dremel wire wheel, or a fine file or emery cloth or what have you. Probably a good idea to refresh the solder too, the heat may have damaged the original solder.

Shouldn't need more than that.

Disconnect the two wires and powr up the heater transformer. Do the loose wires have 6v across them? I really doubt the transformer was damaged, and even if so it wouldn't be responsible for the burnt connector.

Take an alcohol wipe to the wire and see if the soot cleans off.

[tubeswell]

Thanks guys - yep there is a good 6.6VAC across the wires with all the pre-amp tubes loaded in. I might clean up that old connector

[Enzo]

Clean up the male, but definitely get rid of the female. Once it gets that hot the metal loses its temper and it will never be a reliable connector.

[Mick Bailey]

When the resistance goes up on a high-current connector you get lots of heat. P=I squared*R. Those connectors need to be good quality and a tight fit. I always close them up a little just to be sure - even new ones can be a little slack.

[tubeswell]

How would you describe this part in a catalogue?

"6.3mm Right Angle L Type Crimp Terminal Spade Connector w Cover"?

[J M Fahey]

Why don't you just solder it and get rid of connector problem?

I guess the general idea behind connectors is to allow quick solderless wiring modifications when going to different Countries (why didn't they just use a f*ck*ng "230V-115V" switch?) and/or quick solderless original transformer replacement and/or quick solderless multiconnector board replacement.

Problem is people complicate things until the "solution" becomes a problem.
One variation of: https://en.wikipedia.org/wiki/Peter_principle

Ok, so instead of pulling this wire from this spade and plugging it into that one (repeat a dozen times) you have to actually warm up old trusty and actually solder? ... what's the big deal?

As a side note, Musicians buy big heavy expensive SVTs ... not because they actually *need* them but because that's what they see onstage at every major act ... I seriously doubt 99.98% of them need to actually switch voltage regularly because they are on an overseas tour.

My point is: reliably solder what needs to, forget push-in connectors, and if/when necessary , have the Musician bring the amp back to you for voltage conversion ... if going on Tour that is

Just 2 days ago You Tube "suggested" me watching this video.
At first I automatically dismissed it, thinking "what can a kid who barely started shaving teach ME about Bass amplification?" , doubly so one calling himself "Wizard/Guru/whatever"?

I shuddered remembering SGM

But then thought, "I will watch it: I sell Bass amps, 95% of my market are wannabe Rock stars, let's see what they think" ... or who they trust ... certainly not much respect around for greyhaired paunchy old Techs ... I suggest you also watch it:

Surprisingly he makes a lot of sense, specially recognizing his possibilities are playing either in a very small Club or at best a medium sized one.
And *if* playing a larger place, it will have a big PA so Bass amp is just a glorified monitor.
He even explains Impedance properly

Welcome Brave New World.

Edit: and even tell your customer about soldering, casually mentioning that you are improving his amp to good old SVT standards, instead of cheesy modern Vietnamese ones ... which is true by the way.

Worst case, cut and paste a few SGM "truisms" for him
Just omit the "100V" related ones

[tubeswell]

Yeah I might yet resort to soldering those pesky heater terminals Juan - especially if can't find a "6.3mm Right Angle L Type Crimp Terminal Spade Connector w Cover". :-)

[tubeswell]

Just pondering on the failure mode. The bit that I repaired on this amp the day before yesterday was the power supply for the +/- 15VDC (which amongst other bits, runs the bias sensor circuit). This amounts to the 4 x 1N4007 diodes D22, D23, D24 and D25 in the pair of 2-phase rectifiers for that supply circuit. These diodes were all short circuits. I replaced them, and the accompanying filter caps (and the accompanying 15V 1W clamping zeners) just to be on the safe side, and the amp is powering up again. However this power supply draws its AC from the same filament transformer that is supplying the 6VAC for the heaters. Now I wonder, if the build-up of oxidation on the connector terminal that caused the arcing and the charring on the output tube board, also caused transient voltage spikes back through the filament transformer that spiked the diodes? Is that a plausible failure mode? (Only D18, D19, D20 and D21 weren't affected - and they are rectifying off another winding from the same tranny?) Only I'm scratching my head to understand why these 1000V diodes croaked.

[MarkusBass]

If I were to guess what was the root cause of the problem, I'd say that either someone plugged in the amp set to 115V to 230V mains voltage, or he incorrectly rewired the filament transformer to 230V. Either of this caused that Zener diodes were shorted (they are supplied from the filament transformer).

Mark

[tubeswell]

It was the 1N4007s that were shorted

[MarkusBass]

diodes in the +/-15V power supply for the bias sensor circuit had shorted

Sorry, I thought that you mentioned Zener diodes in your first post.

[Leo_Gnardo]

Why don't you just solder it and get rid of connector problem?

+1 When I see roasted filament connectors on SVT, also Fender/Sunn 300, and similar, I just clip off the quik-connects and solder on. It's not the running current that damages them, it's the "startup" jolt that can reach 6x the running current. With half a dozen 6550/KT88 plus a gaggle of pre tubes, 15 amps running current bad enough, imagine how much in that instant when switching-on driving cold filaments. OW, no wonder those connecters fry.

[Jazz P Bass]

Yeah I might yet resort to soldering those pesky heater terminals Juan - especially if can't find a "6.3mm Right Angle L Type Crimp Terminal Spade Connector w Cover". :-)

The right angle terminals are refered to as a 'Flag Terminal'.

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Quick disconnect as Enzo stated will get you there: https://www.google.com/search?hl=en&...33.Dz5KGoLoV08