'74 Dual Showman Reverb problem...one for the ages!

[Steve Fundy]

Aloha from Hawaii all..
My beloved DSR was down & I finally got around to working on it. There was no audio, but not blowing fuse/all tubes glowing. Long & short of it turned out to be no B+4 (~400VDC). It was getting knocked down to 9+VDC. I began to trace out the circuit & lift B+4 off at each point it connected to preamp tubes and then voltage returned at each point until I got to V4. No matter what I tried, removing and testing components,as soon as I reconnected B+4 to V4 (2 100K 1W resistors) it got knocked back down. So, I was just about to post here and ask for advice, when I remembered other times when strange things happened as a result of the fiber & turret boards' being so close together. So I decided to unscrew & slightly lift up the turret board & prop it up & VIOLA! B+4 was back (300 some volts) and amp fixed..I used it last night! Yep one for the ages!

[mhuss]

Those fiber boards get leaky after 40 years particularly in more humid environments. See this a lot in older Fenders...

[Randall]

You say it is a "turret board", do you really mean it is a standard Fender black eyelet board? I'm confused about "fiber and turret boards being too close together". I would think losing almost of of the B+ to a short would cause the fuse to blow if it is indeed the correct fuse installed. And what do you mean by B+4?

[Dave H]

I would think losing almost of of the B+ to a short would cause the fuse to blow if it is indeed the correct fuse installed. And what do you mean by B+4?

B+4 is the fourth node down on the power supply chain from the first filter cap. It's marked 'D' on the schematic. Shorting it to ground doesn't blow a fuse because current is limited by the series resistors in the chain.

[Steve Fundy]

You say it is a "turret board", do you really mean it is a standard Fender black eyelet board? I'm confused about "fiber and turret boards being too close together". I would think losing almost of of the B+ to a short would cause the fuse to blow if it is indeed the correct fuse installed. And what do you mean by B+4?

Turret board, eyelet board...whatever, but yes eyelet. B+4 is the voltage derived from the last 20uF cap under the can. In the schematic it's "Letter D". Go review some DSR schematics. I would have thought that an almost short would have taken the fuse out, but it did not, making this one even more baffling. Correct fuse installed. No way I would ever do otherwise

[Steve Fundy]

for sure. I have had fiberboard trouble with most of my older fenders

[Randall]

"Turret board, eyelet board...whatever"

Well actually, no.

Turret board

Turret

Eyelet board

Eyelet

[Leo_Gnardo]

I would think losing almost of of the B+ to a short would cause the fuse to blow if it is indeed the correct fuse installed. And what do you mean by B+4?

With the short at the lower-voltage end of the power supply string, I'm not surprised the fuse didn't pop. Resistors in the power supply string limited the amount of current expressed into the short.

What I'm wondering about: did the resistors in hi voltage supply recover from this trauma? I'd have a close look at those resistors, including verifying their values haven't gone out of spec.

[Steve Fundy]

Yeah I know. I just built a deluxe. It uses a turret board. I used the term incorrectly. I KNOW the old fenders use an eyelet board. You just took a certain percentage of the joy out of my amp repair with your being a stickler for use of absolute correct terminology.

[Steve Fundy]

I did check each one and all within spec. Yeah I an still surprised it didn't pop the fuse, but it did not. Yes, it was of correct value.

[glebert]

You just took a certain percentage of the joy out of my amp repair

Wait, there is joy in amp repair?

[Steve Fundy]

Wait, there is joy in amp repair?

No confusion. Joy for sure.

[Enzo]

I can't read my schematic, are the B+ series resistors 2.2k and 10k? I will assume so. SO about 12k total. Now if we have 400v B+, and short the D capacitor, than means we have 12k resistance in series with the shorted cap. SO the 400v faces 12k, not a dead short to ground. That resistance draws only about 25ma. And that means about 15 watts is consumed there.

Looks like the mains fuse is 2.5A slow. Those hold at 2.5A all day, so let's work with 3A to blow it. 3A in the 120v mains circuit is 360 watts. The amp needs to draw 360 watts from the wall to blow the mains fuse. SO our 15 extra watts from a downstream B+ cap short falls WAY short of that.

And THAT is why the fuse never blew.


Now if the FIRST filter cap shorted, then you have a short right across the rectifier node. The voltage will be dragged down, but even assuming 400v, all we need is ONE amp for long enough and we are drawing 400 watts, so POOF, fuse blows.

This of course is a very crude technical analysis, but should give the basic idea.

[Steve Fundy]

I can't read my schematic, are the B+ series resistors 2.2k and 10k? I will assume so. SO about 12k total. Now if we have 400v B+, and short the D capacitor, than means we have 12k resistance in series with the shorted cap. SO the 400v faces 12k, not a dead short to ground. That resistance draws only about 25ma. And that means about 15 watts is consumed there.

Looks like the mains fuse is 2.5A slow. Those hold at 2.5A all day, so let's work with 3A to blow it. 3A in the 120v mains circuit is 360 watts. The amp needs to draw 360 watts from the wall to blow the mains fuse. SO our 15 extra watts from a downstream B+ cap short falls WAY short of that.

And THAT is why the fuse never blew.


Now if the FIRST filter cap shorted, then you have a short right across the rectifier node. The voltage will be dragged down, but even assuming 400v, all we need is ONE amp for long enough and we are drawing 400 watts, so POOF, fuse blows.

This of course is a very crude technical analysis, but should give the basic idea.

makes sense

[Randall]

"You just took a certain percentage of the joy out of my amp repair with your being a stickler for use of absolute correct terminology. "

Well, you'd better get used to that right quick if you are going to hang around here. Ask me how I know.