Time for a recap?

revolver · Post by **revolver** » Thu Oct 27, 2005 11:01 am

1972 Hiwatt Custom 50. I have owned it for ten years and replaced the power tubes twice. Keep hearing about getting your amp recapped, understand the job can be quite pricey.

How important is this? if a cap goes will it take the whole amp with it?

soundmasterg · Post by **soundmasterg** » Thu Oct 27, 2005 2:29 pm

The caps that are talked about when people say to replace them are any electrolytic caps or paper caps in the amp. Sometimes the coupling caps can leak also, and should be replaced, but that doesn't happen nearly as often. Usually, and in the case of your Hiwatt, the electrolytic caps are mostly in the power supply and their function is to take AC ripple that is riding on the DC B+ that supplies the power tubes, and short the AC ripple to ground. If your caps are tired, they can fail in a couple ways.

One of the ways is that they will stop filtering the ripple to ground, so your amp starts to hum more and more until it becomes unusable. My uncle's 1958 Gibson amp was like this, and all the filter caps had leaked black goo.

Another way that the caps can fail is that they can explode, but that doesn't happen often. Still another way is that the caps gets so leaky (not physically leaking stuff but electrically leaky) that it almost causes a dead short and your power transformer is being stressed WAY more than it should be, which causes the power transformer to overheat and sometimes blow. Your power tubes can also object to the conditions and blow. Power transformers are expensive, so if one goes, especially with a Hiwatt, (Partridge transformers are expensive) then it costs almost as much as a new amp to fix it.

Replacing the electrolytic caps isn't all that hard or expensive compared to replacing the power transformer. If your amp used a multi-cap can capacitor, then it can get tricky sometimes finding the correct values, and usually using individual caps is cheaper and provides better performance than using a multi-cap can. Any good tech should be able to too at the amp and advice you as to costs and best approach to replace the caps. If they have't been done, I would suggest to have them replaced. The amp will sound better and you can rest assured that it will perform correctly for the next 10-15 years. Just make sure that any other caps are not replaced unless they are bad. Some techs will replace all the caps in the amp, regardless of if the coupling caps in the amp work fine or not, and this can change the sound.

revolver · Post by **revolver** » Fri Oct 28, 2005 5:51 am

Thanks for your thorough response Greg. Perhaps my wife can have my amp serviced as my Christmas present!

bassman · Post by **bassman** » Fri Oct 28, 2005 12:41 pm

Do you recommend changing the caps in solid state amps made during the 60's?

I still have my Vox Essex Bass Amp (Thomas Organ) that I bought new in 1967. Physically its still in perfect condition. But sound wise it really sucks, and it's pretty noisy (hum). It still has it's original 2-prong electrical cord for power.

What do you recommend for this type of amp?

soundmasterg · Post by **soundmasterg** » Fri Oct 28, 2005 3:14 pm

Electrolytic caps have a finite lifespan, meaning that they will wear out eventually. They actually last longer if the amp is used constantly rather than if it is used rarely, because the material in the cap doesn't dry out as quickly since it keeps getting reformed every time you use it. Most people say 10-15 years for the lifespan of an electrolytic cap, but we have all seen some 50 year old amps that still work fine with original caps. I would guess with those old amps if you tested the performance of the caps or looked closely at them, you would fine their performance has degraded, and that they are either leaking or bubbling. Any amp that uses electrolytic caps should have them replaced after 10-15 years for optimum performance, and the lifespan of the caps will vary depending on the circuit they are in and how often the amp is used. I would guess that most of that hum can be fixed if the electroytic caps are replaced Jon. I'd also recommend to rewire the 2 wire power cord to a modern 3 wire setup.

bassman · Post by **bassman** » Fri Oct 28, 2005 3:32 pm

Thanks Greg for this. I've thought about changing the power cord over to a 3-prong many times in the past. But i've never got around to having it done. The amp sits "on display" so to speak in my rec room and rarely gets plugged in and turned on.

I'll see if I can find a local amp tech who can replace the caps in it and change the power cord at the same time.

Is there a specific brand of electrolytic caps that you recommend, or more importantly don't recommend for use in these old Thomas Organ Vox amps?

soundmasterg · Post by **soundmasterg** » Fri Oct 28, 2005 11:41 pm

Jon, I have no idea what type or values of caps that they used in your Thomas Organ Vox amp. If they used can caps, then regular axial or radial discrete (one cap to a can) caps would be cheaper and give you a higher voltage rating easier if you need it. Since its solid state, you probably don't have to worry about high voltage like most tube amps, so the radial caps that you can get from Mouser.com would probably work just fine. Any of the cap brands ending in ***con like Xicon or Rubicon for instance would be fine. Other good brands are F & T, and Sprague Atom. I would probably look at the schematic and see if you can find the cap values you need, or have your tech do it. You may also email Don Butler, aka Toneman, (www.toneman.com)as he has had lots of experience with those particular amps. Where are you located Jon? There are some good techs around depending on where you are.

When changing the power cord to a 3 prong, make sure it is rewired correctly so the fuse and switch are inline with the black wire and not the white as it is currently. Lots of techs don't remember to do that and it can be potentially unsafe.

bassman · Post by **bassman** » Sat Oct 29, 2005 4:20 am

I'm in the (GTA) Greg. Greater Toronto Area.

triode · Post by **triode** » Sat Oct 29, 2005 10:28 am

Electrolytic caps have a service life of 10-20 years max. After that point, they develop more series resistance and start to get warmer when in use. They have less capacitance and start to become ineffective as filters. The aluminum oxide paste inside just dries out. When I look at amps, I check the electrolytics first.

Coupling caps and signal caps from early 1950s to early 1960s amps can be made from wax, paper and foil. The paper inside deteriorates at the rate of 2% per year according to Capacitor Manufacturing Technical data. A 50 year old wax/paper cap is dead meat and should be replaced. Several books could be written about caps. I happen to have a lot of experience with most caps, as I have sold and replaced tens of thousands of them.

They all sound different. Those of you who are not circuit jockies like me, probably don't believe that, but it is true, everything matters in the music signal path, even the emotions that are put into the assembly of the amp, the guitar and the attitude of the amp tech who is doing the repair.

soundmasterg · Post by **soundmasterg** » Sat Oct 29, 2005 12:23 pm

Jon, Kevin O' Connor (he writes amp books and runs London Power)is up there in Canada, though I can't recall if he is in Toronto area or not. You may give him a call and see if he knows any good local techs, or if he can do the work himself. I'm not sure if he does tech work liek that still. Check his site out at www.londonpower.com.

Charlie, every older amp I've worked on that uses paper caps leaks DC, so I agree that they should be replaced. The paper caps sound a bit different than modern metallized stuff, but unless you want a leaky cap, you have to go with the new stuff IMHO.