50's 5E5A - Repairs for Reliability

[Mick Bailey]

An amp can't be made any more reliable than its tubes - that's how I ended up using SS amps for live work. You have to be an optimist if you run tubes.

[drewl]

I've been doing a bunch of 40's and 50's Fender, Gibson and Ampeg amps for a guy lately.
He wants them to scream and be reliable.
Aside from adding a 3 prong cord they all almost always need filter caps, cathode caps and coupling caps.
Most resistors are still within 20% so they can stay.
Replace the caps with good quality new ones and these amps now sound fantastic and are reliable.
Parts are returned if they want to put the amp back to museum condition.

[Robert Technology]

Selenium rectifier replacement orientation?

Finally getting a chance to dig into this amp and I just noticed that the selenium rectifier for the bias supply is shown as being backwards (positive to negative) compared to modern equivalents (such as the Mojotone layout for the 5E5-A clone), as well as backwards to what I would expect since I want negative alternations to get through and be blocked from returning back to the supply.

Anyone have insight on this just to confirm my suspicions? I'm referring to the diode between the 10K and 56K resistors in the bias supply for the power tubes on the original schematic (positive to 10K, negative to 56K) vs the Mojotone schematic and layout that lists the opposite direction (negative to 6.8K and positive to 56K). My gut is telling me that Mojotone schematic is correct, but that would mean that selenium and silicon rectifiers are labeled backwards from each other, unless I'm overlooking something incredibly simple.

MK-PRO-SCH.pdf
MK-PRO-WD.pdf

[Randall]

Concerning those Astrons, I had a tweed Twin build recently that the builder used Astrons pulled from some other amp. 5 out of 6 of them were leaky, a couple very much so. So out they came and replaced with Mallory 150s. Since then I have a suspicious eye for those Astrons.

Also, consider Warehouse Guitar Speakers Vintage American if saving the original is on the agenda. Great sounding and not too pricey.

[Leo_Gnardo]

Concerning those Astrons, I had a tweed Twin build recently that the builder used Astrons pulled from some other amp. 5 out of 6 of them were leaky, a couple very much so. So out they came and replaced with Mallory 150s. Since then I have a suspicious eye for those Astrons.

As you've also asked about the treble cap, similar warnings about "vintage" mica caps. Last year I had a veddy pricey boo-tiki copy of a tweed Pro, customer complaint was it randomly starts spitting & crackling like a miniature thunderstorm. The builder used mostly new parts, but made a point of installing an authentic 1930's vintage mica cap to "get that vintage tone." Care to take a guess? That's right, the "vintage" mica cap was the source of his problems. So... if you must go there, be warned.

An ordinary disc cap will work just fine. I use ones rated 1000V, never a failure. And 470 pF is close enough to 500, don't drive yourself krazy trying to track down perfectly rated parts.

[Robert Technology]

Thanks for the advice on the silver mica cap! I'll go ahead and replace it with a ceramic disc cap. Any thoughts on the direction of the silicon diode for the bias supply?

[Enzo]

The Fender 5E5A schematic is obviously drawn wrong. negative voltage supplies are negative voltage supplies, they don't wire diodes backwards on specific models.

These days we use diodes, which have anodes and cathodes. In the days of selenium rectifiers, they had plus and minus signs printed on them most times, and that can confuse people.

I doubt you'd need two 1N400x, this is a low voltage circuit. Of the entire 4000x series I only stock 1N4007, like most shops, it works anywhere the 100v one does so why stock both?

I would replace the selenium with a 1N400x, and see what results. All the voltages in these amps are usually way off the old Fender drawing anyway, so decide if your bias voltage is too far out of whack, and deal with it by resistors only if necessary. In other words make no decisions until one is required.

[Robert Technology]

Thank you so much for the response Enzo!

I couldn't imagine how the Fender schematic could be correct, but my confidence was shaken by the diagram. I was just about to throw an 1N4007 in there and put it on the light-bulb limiter but just having someone else tell me I'm not crazy for thinking contrary to the diagram is all I need to move forward at this point. Thanks again!

[Robert Technology]

Just to confirm... The diode arrangement on the Mojotone schematic has done the trick for the bias supply rectification.

Just checking and replacing any coupling caps that have excessive DC leakage now, and replacing the silver-mica cap.

Is it safe to assume I should go ahead and replace the polyester cap on the standby switch? Does it even need to be there at all? It's not even shown on the original Fender schematic but is present on the Mojotone layout (not the schematic though). It isn't the "death cap" that was on the ground switch, but is instead just connected to the side of the standby switch that is directly coupled to the tube rectifier. I'm gonna replace it with a Mallory 150 unless anyone advises differently.

[Robert Technology]

Okay, now I'm really feeling like it's been too long since I've done some of this type of work.

I seem to recall doing leakage tests by pulling the lower voltage leg of a coupling cap then measuring the DC voltage between that "downstream" leg and ground, but I just did some quick googling to confirm the validity of this test and saw some arguments for doing a 50uA current test in the same situation. Anyone have any thoughts on the more accurate way to do this test?

I'm using a Fluke 87 V meter so input impedance is 10M ohm.

[Robert Technology]

Just to close the loop on this... Every cap I've tested with the DC voltage method has had 20+ volts of DC leakage, so I'm replacing them all with Mallory 150s.

Thanks again to everyone for the advice!

[Chuck H]

Hey Robert, is that Corey Feldman in "The Burbs" there in your avatar? One of my favorite movies

[Robert Technology]

Hey Robert, is that Corey Feldman in "The Burbs" there in your avatar? One of my favorite movies

Haha! You nailed it!

It is one of my favorite movies as well!