Capacitor Replacement in a Guitar Amp

[betelgeuse]

I've decided to refurbish a Guitar Amp that I built in high school in the 70s. It worked for a while and then something went wrong - I'm not sure, I'll figure it out as I go (I suspect a power supply or power amp problem), but the first step is I am going to replace every component except the power transformer, maybe some of the transistors (all of the power amp transistors will be replaced).

I've got a handle on the electrolytic capacitors, although I couldn't find radials for some so I went to a close value - 4.7 uf instead of 5 uf, 220 uf instead of 200 uf, etc.

I'm unsure of the other capacitors on the amp and preamp boards:
- what are better alternative for ceramics,
- there are some rectangular capacitors on the preamp boards, I'm not sure what those are,
- there is a tubular capacitor in the power supply from the switch to ground, I'm not sure what type to get, I assume at least 120 volt rating ...

I've included links to the original articles in Popular Electronics as well as a link to a picture of one of the preamp boards:

http://www.swtpc.com/mholley/Popular...PE_Apr1968.htm
http://www.swtpc.com/mholley/Popular...PE_May1968.htm
http://www.swtpc.com/mholley/Popular..._1968_pg99.jpg

Any advice would be appreciated. Since I'm going to the trouble of doing this, I don't mind putting in better parts where practical.

[mjf]

you can use close values for the electrolytic caps,but i would recommend 63v minimum,because the powersupply is round about 60v(and these types sound better).
the rectifier is a little bit weak,i wood take 4amps or more.the cap behind the rectifier can go up to 3300 or 4700uf/min.63v,better100v -take care that there is not more than 60v at the cap -or it will be destroyed.
the cap from ps to ground has to be rated(certificated) for use in powerlines!take care of this.
the rectangular parts are foilcaps(mkt,mkp....)i think.
substituting ceramic caps is a question of sound.some like it,other's prefer foilcaps(e.g. polypropylene caps=mkp).
regards............

[betelgeuse]

Thanks very, very much for the response. Due to rarity of the large capacitors, I've decided to go with a 2500 uf 100 volt and a 4000 uf 100 volt capacitor (cornell dubilier 338-1263-nd and 338-1264-nd) in the power supply and power amp. I've not spec'd a lot of the other caps at 63 volts, I will do so ...

Originally when the amp "blew" I had a "putt, putt, putting" going through the speakers - I suspected either something in the power supply such as the rectifier or the large capacitor, or the power amp transistors ... I would like to beef everything up ...

What are some examples of capacitors rated for power lines? I'm assuming at least something at least at 250 volts - not sure about a power/energy rating ...

[mjf]

indeed there can be heard some bangs when an electrolytic gives up slowly.
there are some ways to damage this device:higher supply voltage than the specifications allow.or built wrong in on the board(inverted,+pole to ground and minus pole to supply v+).or overheating (nearby a heatsink or a tube ).or physical damage(perhaps very loud speaker in the box;rare failure).
the line filter capacitor (ac line rated) you can get from your local part dealer.
will continue.........
regards....

[mjf]

the linefiltercap is 0,01uf = 10nf. for usa i think it has to be UL-certificated.there are some manufacturers like evox-rifa,siemens,.........
it should not be greater than 10nf -for the reason of electrical safety(national laws .....).
this cap should filter radio frequency or so comming from the powerlines.if yuo don't like it let it away.
the fuse in the powersupply is a little bit weak for 60w output-you can change it later on to 3amps when the amp works.
examples for wellknown electrolytics are panasonic,nichicon,vishay,..............
greetings...........

[betelgeuse]

Here is a link of hopefully a better picture of the power supply schematic:

http://i23.photobucket.com/albums/b3...hen/mmm-ps.jpg

I understand that c1, on the load side of the rectifier needs to be "beefed up" as much as possible. The amplifier is advertised as 60 volts out from the power supply and "continuous 60 watts, corresponding to a peak music power rating of about 140 watts" (I'm thinking that I should be concerned about meeting the 60 watts rating with speaker load).

I'm curious as to the rating of c2, which comes off the center contact of the spdt switch - to ground - on the line side of the power supply transformer. It is listed as a .01 uf - it looks to be on the ground sode of the circuit so that a "power line" capacitor is not needed? I'm curious as to the type of capacitor which would best work?

-----

ok - I didn't give you enough time to respond, thanks, that's enough information to keep moving forward ...

[betelgeuse]

It looks like this line filter capacitor will work:

http://search.digikey.com/scripts/Dk...me=495-1655-ND

[mjf]

yes,that's one (epcos=siemens).but it isn't very cheap;
sorry my english is not the best one.these caps can be used between the powerlines or line to ground,or earthground.
regards...........

[mjf]

60vdc from the supply will give you round about 50watt in an 8ohm speaker(continuous) ,and perhaps 100w in a 4ohm(peak);140w i can't believe.
a bigger powersupply will help -perhaps you change it:4000uf100v in the supply and 2500 at the output.and a bigger rectifier is essential,because a big electrolytic is a heavy load.minimum 4amps/100v (kbu4d,........kbu8d or so).and this one got a lower inner resistance(that could mean a small amount more punch).
regards..........

[betelgeuse]

The designer states that the amplifier is designed to supply any 4 ohm speaker, and suggests connecting two 8 ohm speakers - at least 35 watts - in parallel.

Everything that I've read on matching amps to speakers suggest a close match or sizing the amp above the speaker ratings. I'm looking at 2 of either the Jensen c12Q or c10Q speakers - all rated at 35 watts - in parallel to get the 4 ohms.

I'm not one to crank the amp to 10 so I believe this will work. Am I correct in this thinking?

I'm going with all of your power supply suggestions.