How To Safely Power Up Tube Amp After 10 Years?

[Paul Marossy]

How do you start up an old tube amp that hasn't been used for probably 10 years and you don't have a Variac? Can you safely do it with a light bulb current limiter? I have an all original 1965 Fender Showman that a relative gave me and after physical inspection and a lot of cleaning up I'm ready to fire it up but I'm concerned that the filter caps might possibly fail on power up.

Any suggestions?

[thermionix]

A light bulb current limiter is FAR better than nothing, and would allow you to turn it on without the risk of major damage or blowing fuses.  BUT...1965 filter caps, that have sat idle for 10 years...there's probably no chance of reforming those to usefulness.  Replace them.

Use good caps, F&T are good quality and a fraction the cost of Sprague Atoms, for example.  It won't diminish the value of the amp, it's not like swapping parts on a vintage guitar.  A better analogy is dry-rotted "factory original" tires and hoses on a vintage car.  Keep them for a non-functioning museum piece maybe, but not if you plan on driving the thing.

[thermionix]

Also I don't know when they added the word "DUAL" on the faceplate, but I think there is a period where the Showman and Dual Showman looked the same, but the Dual has a 4-ohm output, and the Showman has an 8-ohm output.  If unsure, you can check the number on the OT.

[ElectricDruid]

+1 agree. I'd probably replace the electrolytics as a matter of course on something that old. They'll be dry as old bones by now and not doing their job. Plus a decent modern replacement will probably be better than the originals anyway. I don't regard mains hum as part of vintage tone!

Tom

[bool]

FT caps are excellent. Not only technically, but they look good visually as well.

[amptramp]

This is a problem we run into all the time and by we I mean those of us who restore antique radios as a hobby.  New electrolytic capacitors should be part of the refurbishment because as capacitors age, they lose capacitance.  There are several versions of the Showman amp so I don't know which schematic yours follows but you can guarantee the capacitors are bad.  Be especially careful with the cap for the bias supply (one version has two caps) because if it shorts, you have no bias on the output tubes and the only question is whether you blow the output transformer or the power transformer first.

If you are concerned about the collectability and originality, this article:

https://antiqueradio.org/recap.htm

shows how to dissect old electrolytic caps and stuff new electrolytics in them.  I have done this several times with old Philco radios and the results have been reduction of hum and buzz down to the noise level.

One thing to check and it may be worth unsoldering them to get a good measurement but check for leakage in the coupling capacitors going to the grids of the output stages.  I had a VTVM with a jack on the side that put out 70 volts from the power supply just for this type of testing - put 70 volts on one side of the capacitor and measure the voltage on the other side with the 11 megohms of the meter input as the DC load.  Anything more than millivolts is cause for replacement with modern plastic film 0.1/630 capacitors even if the originals were 0.05/400.  I did one restoration of a radio where one coupling cap was 7 megohms and the other was 5 megohms.  You don't need that fighting against your bias supply.

With the amp on standby, you can check your bias voltage at each output tube grid.  Some Showman versions have a balance control but no overall bias setting, so the voltages will be different.  Be sure you have enough bias.  The 6L6 likes values around -23 volts or greater.  If your bias filter cap goes bad, this voltage may go more positive making the output tubes draw more current than they should.

[R.G.]

All very good advice. Electro caps don't last forever, even with careful use. Careful use implies frequent power-on time, which has the side effect of mildly re-forming the insulation. This will eventually fail, but it makes them last a lot longer than leaving the caps un-powered for years, then turning on the power switch. Good on you for realizing this and asking.

New caps is the best strategy. Next best is re-forming.

Re-forming is pretty simple, and does not require a variac, or even a light bulb limiter. It requires only a resistor or two. Bare aluminum can be oxidized to have an insulating layer with only a an electrical current. Getting a layer that's 400V thick is trickier. The proper polarity and limited current in a conductive water bath and the right chemical content grows a layer of aluminium oxide.Too much current makes this layer grow with holes of less resistance in it, and there are lower resistance holes in it. Very low currents grows a more consistent layer, which is what you want.

One trick that works is to put a large resistor in series with the rectifiers, before the first filter cap. 100K to 200K is a good starting point. This is large enough that the current is limited even if the cap would otherwise short. For instance, if the power supply in the equipment would be 400V if the cap was good, that means that the rectifiers put out a half-wave rectified sine with 400V peaks all by themselves. If the caps will not withstand this voltage, they conduct heavily at lower voltages. A big resistor limits the current down into the "safe for building" region. So if the caps would only hold 100V, a 100K resistor limits currents at the peaks to (400V-100V)/100K = 3ma. This is a safe voltage for oxidizing the weak spots.

As the weak spots "heal" the voltage the cap will withstand rises, and less voltage appears across the resistor. When the voltage across the resistor drops to less than about 10% of the voltage across the cap, the cap will almost always withstand the full voltage without the resistor. It's formed now.

There are two issues with this. First, it's a little better to feed the cap under form-ation a clean DC signal instead of a half-wave rectified voltage. Second, some of the electro caps down the voltage string in a normal amp are lower voltage. It may be that the first filter cap is rated at 450v, the screen cap at 450, but 350 or even 250 down at the preamp tube plate supply. Those caps don't need forming up to high voltages. So you get to exercise your soldering skills. The process is like this:

1. Unplug, wait for draining, make sure the voltage on all high voltage caps is under a 20-30V. Sure, they ought to be, because this thing hasn't been turned on for years, but what if?
2. Open one end of each resistor in the dropping chain of plate supplies. Also open one end of the inductor to the screens.
3. Remove all tubes excepting the rectifier if it's a rectifier tube type.
4. Optional: temporarily install a new 10uF to 22uF 450V cap after the rectifiers. This will even out the DC for the first cap forming. It's not absolutely necessary, but will smooth things out.
5. Insert a 100K to 220K resistor from the rectifiers (and new temporary cap, if used) to the first filter cap.
6. Check your work. Make sure what you think you did is what you actually did.
7. Clip your voltmeter, set to high DC volts, across the rectifier output and ground.
8. Turn on power. Check the voltmeter for high DC volts. The half wave rectified waveform will do funny things to most DMM, so this may look screwy, but there ought to be volts. Now move the + lead to the cap. This ought to have a nice, smooth DC voltage. Because of the high resistance, it may take some time for this voltage to ramp up to full. The time constant of a 47uF cap and a 100K resistor is 4.7 seconds. It takes about five time constants for a cap to be fully formed, so the voltage may ramp up for as much as 30 seconds to a minute. You can tell when it's stable because the changes in reading will get smaller and slower.
9. Think: what does that nice, smooth DC voltage mean? If it's lower than 90% of what you think the power supply ought to be, the cap needs forming, and you can proceed. If it's over 90% of what the power supply ought to be, The cap will likely be fine with full voltage without the resistor.
10. This fixes the first cap. When the voltage across that current limiting resistor gets below 10% of the peak expected output voltage on the cap, the cap is done. So you can move to the next cap in the forming line. If the amp has an inductor to the second filter cap, it's probably OK to leave the inductor connected and form up the first two caps at the same time. If you're having good luck with this, you can probably put the tubes back in and reconnect the dropping resistors. If not, you may have to do it one section at a time, moving the limiting resistor down to that section and forming a cap at a time.  Since each of these steps may take a day to finish, while you're waiting, think about whether you should have ordered and soldered in new caps in the first place.

[Paul Marossy]

This is a problem we run into all the time and by we I mean those of us who restore antique radios as a hobby.  New electrolytic capacitors should be part of the refurbishment because as capacitors age, they lose capacitance.  There are several versions of the Showman amp so I don't know which schematic yours follows but you can guarantee the capacitors are bad.  Be especially careful with the cap for the bias supply (one version has two caps) because if it shorts, you have no bias on the output tubes and the only question is whether you blow the output transformer or the power transformer first.

The tube chart says it's an AB763 circuit.

[Paul Marossy]

Also I don't know when they added the word "DUAL" on the faceplate, but I think there is a period where the Showman and Dual Showman looked the same, but the Dual has a 4-ohm output, and the Showman has an 8-ohm output.  If unsure, you can check the number on the OT.

I checked the OT, it's 8 ohm. It only says "Showman" on the faceplate and was definitely made in 1965.

[aron]

It's really exciting to fire it up after doing the necessary fixes!!! Good Luck!

[thermionix]

Also I don't know when they added the word "DUAL" on the faceplate, but I think there is a period where the Showman and Dual Showman looked the same, but the Dual has a 4-ohm output, and the Showman has an 8-ohm output.  If unsure, you can check the number on the OT.

I checked the OT, it's 8 ohm. It only says "Showman" on the faceplate and was definitely made in 1965.

I might be thinking of the "brown" period, or I might just be mis-remembering altogether.  Either way, sweet amp, and good luck with the cap job.  I do the bypass electros too, but not as critical as the B+ and C- filters.

[blackieNYC]

I am fortunate to have access to a 10a variac.  I have a couple 100w tube heads from 70s/80s that have been off for 20 yrs.
a timely post, Paul.  Been putting this off for a while. 
-can the variac be used to re-insulate, as described by RG?
     If so how is this best done?
-is a load important at this point if there is no signal?  Or is it perhaps important to have the load there in case some component has initiated some unwanted noise?

I'd like to sell these heads.  If they work as-is that would be great.  If they need a re-cap, the price goes up, and then I suppose I have limited my buyers to people who understand the value of a re-cap. 

[R.G.]

After all that typing, I was not clear. So what else is new?   

Re-forming electros the way I described is usually done with all tubes out for the first cap, and only non-output tubes in for the rest of the cap sections The tubes don't do anything except hold the voltages down on their respective decoupling caps so the caps don't go wildly overvoltage, as the caps down the chain from the first filter cap may be made for and rated at lower voltage than the first one or two caps.

And a variac isn't needed in this setup. The rectifiers produce high voltage rectified DC, but the high value resistor in series (the 100K - 220K one) limits current so that the variac isn't needed. Just put in the limiting resistor and turn on the AC. That's one of the screaming advantages.

Not all caps are recoverable this way. Some will fail. But those would probably fail with other methods tool.

[anotherjim]

If it was my amp, I'd definitely do what RG is suggesting. I know that reforming is possible, although I've only done it with post 1970 caps and they may never of had an off period anything like 10 years.
OTOH, if the amp belonged to a paying customer, straight to re-cap makes a lot of sense.

[vigilante397]

I don't regard mains hum as part of vintage tone!

+1 to everything already said here, but especially this. Well put Tom

I had the privilege of working on a '65 Vibrolux a few years back and the owner had me replace the two-prong cord and all the electrolytics for precisely that reason.

[thermionix]

I know that reforming is possible

Personally I consider that more of a NOS cap thing.  In the vast majority of 60s amps I work on, if they still have the original filters they are bulging or leaking electrolyte, and no process is gonna put the toothpaste back in the tube.  And who knows the condition of what you can't see inside.  I understand why some people would want to try reforming in some cases, but with new high-quality caps available for reasonable prices, why bother with the effort and risk.  An entire set of F&Ts is probably less than the cost of two 12AX7s, and you're good to go for a decade at least, as long as you run the amp a few times a year.

[Paul Marossy]

I wouldn't recommend this to anyone really.... but since I knew the history of the amp and how it was taken care of and after doing a careful visual inspection of everything, I went ahead and just used a light bulb current limiter, employing a certain strategy that made sense to me. So far no problems using 100 watt bulb now.

Really does sound great with the all vintage tube compliment.

Now contrast that with a Marshall 9005 rack mount power amp I bought from a friend like 15 years ago. It was stored in a garage here in Las Vegas for I think one summer (which means like 120 deg F min. for like six months), and I was playing it one day for a few minutes and one of the filter caps just literally blew up and spewed nasty smelling smoke throughout my house. That amp was like 30 years younger than this Fender Showman and was fairly regularly used.

[PRR]

> one of the filter caps just literally blew up and spewed nasty smelling smoke throughout my house.

I didn't want to give bad advice.

But my practice is: if it isn't toooooo collectable, put it in the driveway, plug-in, see what happens.

If the caps explode, well, obviously they needed replacing. My "reforming" results are so mixed (and time-consuming) that I don't swing that way anymore. (When I was rehabbing 450W of amps, and was broke, I spent days reforming surplus caps to get 7 of 10 to work for a summer.)

Yes, an "all original 1965 Fender Showman" I would be a bit more careful on. If only so I could salvage the old caps (un-burst and un-mucked) to hollow-out and hide new caps in.

[printer2]

I agree. Just make sure you have the right fuse in to give some protection to the rest of the amp incase of unforeseen problems. I have a 1968 Bassman and it sat for a good 20 years until there was a debate going on about SS to tube watts and I used the Bassman and a Yamaha professional amp to show they are indeed the same. No smoke no fuss, started right up.