• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

tube rectifiers and electrolytic reforming w/ variac

Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.
I've been doing some searches looking for info about electrolytic reforming. I've got a variac and two pieces of old tube rectified gear (one a PA amp and the other an old Conn strobotune musical instrument tuner). I've always heard you can just "bring it up slowly" on variac, but many folks claim that you can hurt your tube rectifier. I've read several different methods, and some of them are moderately complicated. What are the odds of hurting the rectifier tube by just pulling the tubes and plugging the units into a variac? At the least, it sounds as if it would be best to start off around 50% voltage to get the rectifier going.

pj
 
By "I've been doing some searches," I meant that I've read several good articles on capacitor reforming. All of them dictate better methods than hooking the amp up to a variac, however, that's what many folks talk about doing. My question isn't so much, "what's the best way", but rather, is there an easy way with a decent chance of working or, if I crank it up to 60v over the course of a few seconds and leave it there for 12 hours or so, what are the chances it will destroy the rectifier tube?

I'd like to get the units running as cheap as possible. If they have some potential, I'll certainly replace the 'lytics to ensure long lives.

pj
 
what are the chances it will destroy the rectifier tube?

Very good if your cap is shorted. Most seem to dry out and become ineffective but I've seen one that was a dead short and made the 5U4 glow red. What will new caps cost you, $10 or so? Caps that old have really gone past their usable lifespan and trying to reform them is just prolonging the inevitable.

If you still want to reform the caps then you must somehow limit the current through a resistor and keep a constant watch on the leakage current.
 
Perhaps the 'ol dim lightbulb to limit current? Is the low voltage on the rectifier heater an issue?

I'm behind schedule today anyway. In the midst of other stuff, I've been working on hooking up the old variac. It's a surplus 240V 10A Staco that a kindly gent gave me via this board. It's mounted on a U channel base (open back, I really need to build a box for it), and initially I was reading about 50V between the chassis and earth with my DMM (regardless of output voltage). I can't for the life of me figure out why. I then connected the chassis to my earth terminal (a good precaution anyway, I figure) and it didn't blow the newish GFI on the outlet. As I can't find any short (chassis to earth read 0 ohms on the DMM prior to installation of the safety ground), I'm wondering what's going on.

pj
 
I just picked up a Hammond M3, and the junk dealer selling it was
gonna fire it up for me (or whoever). I told him Please NO!!!! I'm
the one buying it, don't want it turned on till I do some tests first.

M3 now safe in my neighbor's garage, you suggest Variac? (I don't
have one). Or worth the effort to form and test those old caps out
of circuit, one at a time? ( I can probably manage that trick, but time
consuming...)

I got a Hammond M3 schematic in my archives, just have to find it...
What are the common failures to watch for?

This thing looks in good shape except the "expression pedal" is
missing the pivot bolt through the center... I probably used the
wrong word to describe which pedal, the one on the right.

Where (if any) do I look to see the connector for an external Leslie?
Not look to buy a connector, but look where on the organ???

All the tubes are here, I have an emission tester to check them...
 
You can't use a variac very effectively with a tube rectifier to bring the voltage up slowly. The reason is that the rectifier doesn't function until the AC voltage is pretty high. And by the time the rectifier sees enough voltage to start to conduct, it will be passing a lot more voltage than you'd want to have applied to old suspect caps, etc. downstream.

You won't hurt your tube rectifier by trying to bring the voltage up slowly on a variac, but the components downstream will not get the benefit of the slow increase in voltage.
 
Sounds like I should just suck it up and replace the caps. At least I won't have to worry about the variac killing me. Though I may read up and see if I can use SS diodes up to a point and then toss the rectifier back in.

I had an M3, but it was in pretty rough shape. I found it at an auction when I was an undergrad. After I left the state, my dad wanted it out of his garage. I pulled the amp from the tone cabinet and sold it here. Pretty much everything else was burned in a ditch. Sad.

pj
 
Sounds like I should just suck it up and replace the caps. At least I won't have to worry about the variac killing me. Though I may read up and see if I can use SS diodes up to a point and then toss the rectifier back in.

Like I mentioned its best to replace the caps and be done with it.
As far as the variac killing you....the variac isn't going to be the problem. Proper safety is important when working with high voltage. It is the lack of safety that will send you to the happy sound room in the sky.

To reform a cap or try to a resistor would have to be placed in series with the cap. The voltage is slowly brought up and the current is monitored. You are looking for excessive current draw and if its present the reforming isn't going to work in most cases. Its not a case of bringing the voltage up to 60 volts on the variac and letting her rip. As mentioned above the usage of diode rectification or the usage of a DC power supply is the only proper way to try to reform.
 
I purchased two pair of 1n4005 diodes from radio shack and wired them up in place of the 5AS4. I also wired a light bulb in series with the AC. I hooked it up to my variac and over the course of a couple of hours brought it up to 60ish VAC (which, with the tubes out and the transformer thus unloaded, brought the B+ to 300V). Then I pulled out the diodes and brought it up a little higher with the 5AS4 in place, tossed the rest of the tubes in and again, over some time, brought it up to 117V. I pulled out the bulb and variac and tried it out.

It works! It doesn't sound great, I hear some hum and distortion, but I know believe the the iron is in fine shape. I'll replace the big PS cap and the other under chassis electrolytic. It probably needs new tubes. With any luck, I'll find a schematic and then I'll really have something. I initially grabbed this thing to try the mic input trannies as MC SUT's, but I think my brother-in-law might get a neat bass amp out of the deal.

pj
 
Status
This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.