Too high supply voltage in Deluxe Reverb

[BDM]

In this vintage silverface Deluxe Reverb there are two bypass capacitors on the rectifier tube. In standby mode, these two caps are the only load on the rectifier tube. The schematic shows them as 450 volts and they are marked 450 volts. But they measure at 498 volts. I have just replaced them with new 450 volt caps. Why would the actual voltage exceed the rating right out of the unloaded rectifier?

I'm using the "AMPAB763 schematic which shows a GZ34 rectifier, though my tube label in the cabinet shows (and I have) a 5U4.

When I flip the standy switch to normal operation, the voltage drops to 430.

[Rob Mercure]

You've described your own problem - the power supply is unloaded and the result is a peak voltage 1.414 times the RMS AC input. You might also have locally high line voltages but this is more a blackface and ealier problem and not as common with silverfaces. But the schematic won't be the AB763 as this designates the "b" version of the July 1963 design.

I've literally got "fish frying" as we speak so I don't have time to track down the specific schemo but if you check the Fender Amp Field Guide online you can find the silverface schemo.

Hope this helps

Rob

[BDM]

Thanks for the reply. But why would they design it to have a standby voltage greater than the rating of the first two bypass caps? How can 498 volts be correct when the two caps it feeds are only rated for 450 volts?

The back of the chassis says 117 volts, so it's not a 110 to 120 volt problem. My current line voltage measures 118.8 volts.

I got another schematic from the site you reference, and it's no different in the areas I'm currently looking at.

[stokes]

Looking at the schem for the AB763,which is the Blackface, you must have one of the "crossover" SF's,an early SF which still had parts and circuits which were a mix of the older BF and newer SF amps.Like Rob started to explain,your first caps are on the "hot" side of the standby switch,so in standby your caps are hit with "unloaded" volts which will drop when taken out of standby and put under full load.It would be a good idea to use 500v caps here,although the 450 v rating will take a little more volts for a short time,it is safer to use caps that are rated for the actual volts they will see,if you leave it in standby for too long you will stress those caps more.If you look at the SF schematic these caps are on the "cold" side of that switch,so the 450 cap is okay.Check the volts with the amp on and out of standby,the volts will fall as we said earlier.

[Rick Erickson]

But why would they design it to have a standby voltage greater than the rating of the first two bypass caps?

I seem to ask a question like this at least once a day. Ever notice that Fender used a 50V cap in the bias supply of most all the bf amps, yet often had 60V or more across the cap? I see manufacturers exceding the safe limits of components all the time. The Marshall JCM2000 amps for example, have at least a half a dozen resistors that burn up from overheating. As a tech, one must determine if the problem is an amplifier fault or a design error. Personally if I had an amp with 498V on the filter caps I would use a pair of 350V caps in series - like the larger AB763 Fender amps had. A couple 100uF/350V caps would give you 50uF @ 700V capacity. Now that's what I call safety margin. Don't forget to string a couple 220K 2W resistors across them to distribute the voltage evenly.

RE

[Rob Mercure]

A few years ago I recapped a Deluxe that was born the same year as me - 1954 - and the recapping was "prophylactic" as the original caps still seemed to be working well with little or no hum (no bets as to what a 'scope would have shown). But this amp was designed for a maximum line voltage of 110 - brownouts and dropped voltages were common 50 years ago. Still the original caps held on 120 VAC circuits without fail. So, to risk an opinion, "they don't make 'em like they used to." Yeah, I really believe that there was a more "conservative" safety margin built into caps, tubes, and other parts, than you see now. Engineering was more "sloppy' cuz the world was more "sloppy' - voltages were erratic, the commonest carbon comp resistor tolerance was +/-20% and the most common schematic voltage tolerance within a circuit was +/-10% (and a 20K ohm/Volt volt meter was considered state of the art unless you were using a VTVM, many cheaper meters loaded the circuits considerably). Sputter, sputter - really now, why do you think that there is enough variation between manufacturers of the "same" tube that a NOS Sylvania 7025 now sells for around $30 while a pair of Telefunken E83CCs sold for $1,000 on fleabay? This "modern world" just lacks variety (and perhaps sense)!

Buttttttt........the caps were 4 times as large as now! And when they failed they failed miserably spraying electrolyte all over the rest of the circuit!

Using a variac and a limiting resistor I've "restored" electrolytic caps that were 25 years in storage and I still use 'em in my own stuff - and only about 10% have deteriorated sufficiently to be unuseable (filter caps used to have a tolerance that was something like +60/-20% so a "high" tolerance unit that has deteriorated 60% is now "on spec" and the test of a filter cap in circuit is whether it "filters" and whether the ESR causes too much current drain and gets hot {and some other stuff but it's late}) Hell, where can I find 600VDC electros without selecting from NOS?

Rant, rage, rave, sputter and g'nite

Rob

PS: Oh, and the Tilapia turned out great - I'd not fixed that variety of fish before so I didn't want to spend too much time at the computer and overcook it.

[Rick Erickson]

Hell, where can I find 600VDC electros without selecting from NOS?

A NOS 600V electrolytic is usually just a couple 300V electros in series, no splitter resistors, with a cardboard wrap around them to make them appear as a single unit.

RE

[Rob Mercure]

Hey Rick,

Not the Sprague Atom TVA-1966 20uf, 600V units that I've got - as one did meet it's end, but not it's "maker" <grin> while being restored and split the aluminum can I can assure you that it was one bit roll of aluminum, paper, and paste (and mess!). But these were made in August of 1993..

Rob

[BDM]

Yes, when taken out of standby the voltage drops to 418.

As for 600 volt caps, Mouser has them. I'll get a pair of 20 ufd at $16 each.

The amp has two 12AT7 tubes. I swapped places and the motorboating stopped. All passive components measured okay.

The reamining problem is that the 180 volt plate voltage measures 220 volts. Should I just live with this?

Is it possible that the main transformer shorted a turn in the primary?

[stokes]

I dont think the 220v's on the plate is that serious.Even tho your amp has 117v stamped on it instead of 110v that amp was made around '68 and wall voltages were still lower than they are today.Add to that the fact that as Rob pointed out components were +/- 10% and some were 20%,age and component value drift and now you are at +/- 20%,you could bring it down by changing the resistor in the B+ rail feeding that plate,it could have drifted some,but I wouldnt be overly concerned with it.I dont see any reason to suspect you have a short in the PT.

[Rick Erickson]

As for 600 volt caps, Mouser has them. I'll get a pair of 20 ufd at $16 each.

Did you check the dimensions? 3.875" Long! I don't think you will be able to fit them under the doghouse. Take a look at Stokes message again - the obvious solution is right there.-
{quote} "If you look at the SF schematic these caps are on the "cold" side of that switch,so the 450 cap is okay".
Just rewire the standby switch so the caps are on the other side and your problem will be solved. No expensive over-size caps necessary.

RE

[Enzo]

There you go, move ONE wire and you are set.

Keep in mind also that these voltages on caps are "working voltage," and it is perfectly OK to run a 450v cap at 450v all day long. They could have a surge voltage of 525 or something. After all, our shops are not piled to the ceiling with Deluxe REverbs with exploded main filter caps.

[BDM]

Move one wire....

I guess, but I imagined that they may have had a reason to keep two filter caps on the rectifier tube even in standby. Does the 5U4 care?

What about finding the value of resistor that would drop the standby voltage to 450? It might also help pull down the high plate voltages.

[Enzo]

That would also soften the supply - more sag. Why not use Rick's idea and just move the wire to those caps over to the other side of the standby switch? Now they have no voltage on them untill you flip the standby on, and at that time, they won't see that high voltage. Once they are charged up, they are invisible to the recto tube except for their small leakage current.

Having those first stage of filters stay hot might make for a bit less of a turn-on pop, just guessing. ANd as to there being two of them, it was in those days cheaper to use a pair of 16uf caps than one 32uf cap at the voltage in question.

[bob p]

I'm looking at the problem a couple of steps farther back in the power supply. Was your SF amp's power transformer really designed for the 120 V supply mains? Or as stokes mentioned, does your SF amp have a BF PT designed to run with a lower supply voltage?

In the event that you have a transitional SF amp (with a BF PT) you might want to build a bucking transformer to use with your vintage amps. Just in case you're not familair with this, if you have amps designed for 110 VAC supply mains that end up in an overvoltage state when run on today's 123 VAC mains, there's a simple solution. Buy a 120VAC-12VAC transformer, and wire it up as a bucking transformer to throw away 10% of your supply voltage. Doing that, you'll bring the 123VAC voltage down by 10% to 110VAC and all of your 110VAC vintage gear will be happy.

This situation may or may not apply to your amp, so I'm just throwing it out as food for thought.