Old peavey century 120

[OB311]

Working on a Peavey century 120 bass series amp head.

While clean the pots on this amp, I noticed the boost input jack (have no idea what it's for) and the foot switch input jacks were not wired to anything.

Found a picture of a complete circuit board online and can see where they should be wired.

Now to my point.

When I fired it up for the first time, ever (it was in a buddy's basement for 8 years collecting dust), it is always in distortion mode. Could the disconnecting of the switch wires left it in distortion mode?

Is this as easy as wiring it back up!

 

[OB311]

Thanks all

 

[bassrich]

You use the footswitch to turn on /off the distortion but yes if no fs is used it is on

 

[OB311]

Thank you

 

[bassrich]

do you have the original footswitch? if not I just turn the distortion knob to Zero bt it is nice ti be able to switch

 

[OB311]

No foot switch. It wouldn't of mattered anyway, the Jack was disconnected from the board. Putting it in constant distortion mode. Even with the distortion dial all the way down, it still was distorting.

But last night I soldered a wire from the Jack to the pin on the circuit board (where it was originally attached) and voila! No more distortion. Works great now, just need a foot switch.

 

[groovaholic]

Let me know if you need anything else; I've owned a couple of these amps and they are surprisingly fantastic-sounding.

I got into a discussion of it with Bobby Baldwin (Peavey amp engineer) and he made some great suggestions about the output coupling capacitor that were easy to implement:

"Basically, capacitors pass higher frequencies with ease, but lower frequencies have a harder time passing through. This means that your power amp puts out a little less in the low frequency range than it does at 1K. The other thing that happens is that when you start to clip the power amp with low frequencies, instead of a pleasant growl, you get a nasty buzz sound. The way to fix that is to use a larger capacitor, but...

The other thing is that capacitors can be inductive, especially electrotytic caps, because the plates are rolled into a coil. This means that your power amp also puts out a little less in the upper audio range because the inductance starts to factor in a little. Inductors pass lower frequencies freely, but tend to block higher frequencies. Just using a larger capacitor can increase the inductance because the plates can be longer, thus more inductance.

Ultimately, the output response of your power amp is sort of a bell curve.

This capacitor is also subject to high ripple current (rapid charging and discharging) because there is pure, high AC current passing through it all the time. This can dramatically shorten the life of a capacitor.

The way to get around all of this is to use multiple capacitors in parallel, one of them being a film capacitor so there is no high frequency degradation. The inductance will go down because inductors in parallel divide. The ripple current will also divide so you won't be stressing a single cap."​

Implementing this idea cost me less than $5 in parts and made a HUGE improvement in the sound of the amp -- much bigger down low and more open on top.

See the attached pic; the coupling cap is located in the lower right hand corner of the power amp board. For reference, in the attached pic, the speaker jacks are in the upper right and the mounting tab for the filter cap is in the lower right.

 

[groovaholic]

Dunno why it keeps rotating my picture -- the yellow (film) capacitor should be on the left hand side of the pic

 

[groovaholic]

Like this:

 

[bassrich]

Let me know if you need anything else; I've owned a couple of these amps and they are surprisingly fantastic-sounding.

I got into a discussion of it with Bobby Baldwin (Peavey amp engineer) and he made some great suggestions about the output coupling capacitor that were easy to implement:

"Basically, capacitors pass higher frequencies with ease, but lower frequencies have a harder time passing through. This means that your power amp puts out a little less in the low frequency range than it does at 1K. The other thing that happens is that when you start to clip the power amp with low frequencies, instead of a pleasant growl, you get a nasty buzz sound. The way to fix that is to use a larger capacitor, but...

The other thing is that capacitors can be inductive, especially electrotytic caps, because the plates are rolled into a coil. This means that your power amp also puts out a little less in the upper audio range because the inductance starts to factor in a little. Inductors pass lower frequencies freely, but tend to block higher frequencies. Just using a larger capacitor can increase the inductance because the plates can be longer, thus more inductance.

Ultimately, the output response of your power amp is sort of a bell curve.

This capacitor is also subject to high ripple current (rapid charging and discharging) because there is pure, high AC current passing through it all the time. This can dramatically shorten the life of a capacitor.

The way to get around all of this is to use multiple capacitors in parallel, one of them being a film capacitor so there is no high frequency degradation. The inductance will go down because inductors in parallel divide. The ripple current will also divide so you won't be stressing a single cap."​

Implementing this idea cost me less than $5 in parts and made a HUGE improvement in the sound of the amp -- much bigger down low and more open on top.

See the attached pic; the coupling cap is located in the lower right hand corner of the power amp board. For reference, in the attached pic, the speaker jacks are in the upper right and the mounting tab for the filter cap is in the lower right.

Cool might just try this with mine

 

[groovaholic]

As long as it's one of the crappy, no good, horrible (please don't buy one and drive up the prices) ones that look like this:

Then the mod should work.

If it's one of the later ones:

I don't think it'd work the same; the output stage is much different.

 

[OB311]

It's the 120 instead of the 200

But looks exactly the same as the one you have.

 

[groovaholic]

My first Century was a 120; current one is a 200 -- I can't tell any difference (inside or out) although I've seen a schematic for a 120 that only had 2 output transistors, putting out roughly 60 watts.

 

[tbplayer59]

My first amp was like the first pic, but I didn't know it came in two different powers. Thanks for the pic. Thanks for the memory.

 

[bigtone23]

Old thread, but had to comment. I did the coupling cap mod to my Century 200. Wow! What a noticeable difference it made in the amp! The lows are much bigger, louder and extend a bit deeper, I had to turn the bass knob down to get a similar tone that I'm used to with the head. The highs are also different--they are less harsh and peaky in the upper mids and more open, clean and extended. I also had to drop the treble control a little to get my usual tone out of the amp. It's like my treble and bass knobs earned an extra couple numbers of range. I used to run the treble on 2-3. mids on 10 and bass on 6-10 depending on how loud I was playing. Now it's treble on 1-2, mids still on 10 and bass 4-6.
If you have one of these heads, you must try this mod. It really makes a great amp much better!

 

[BassmanPaul]

I’m going to miss Bobby coming in to sort our Peavey problems out. Still I wish him all the best in his new gig.

 

[SOUTH PAW]

Hope you don't mind me asking.

First, a little history and a photo.
I brought mine out of storage after 34 years and while it worked, it was scratchy sounding when adjusting it, as would be expected I know, but could get it to settle in to a pretty clean sound. Unfortunately, it began making more and more noise, like a bad connection, and though I could bang it a little to clear it, the noise would return sooner each time.
I have done a little soldering before, but have never worked on an amp other than a speaker replacement in a combo, so I decided to open it up and have a look. Sure enough, there was a solder joint disconnection, so, I made the repair, had it working without a problem for about 30 minutes, and pop, stopped working completely.

So, my question is, are they worth fixing? Should I go any further doing it myself?

 

[SOUTH PAW]

Anyone?

When I saw this thread, I thought it was cool to see how these amps can be improved, so all the more, with my situation ( two weeks ago ) I had to ask and consider if they are worth repairing as well.

 

[bigtone23]

I think Century 120/200 heads are worth repairing, very much so. I totally get the cult following the 1970s Peaveys have. The circuits are pretty simple and everything inside is discreet, so most anything inside the amp that could go wrong is relatively easy to fix. Chances are that all your amp needs is a 50,000 mile checkup. 34 years is a long time to sit dormant, so I would bet there are some dry caps and possibly out of spec resistors to replace. It could need both output transistors replaced, too. No matter what, it's easy to work on and the parts are cheap.

My 200 head was working fine when I got it several months ago. I cleaned the pots, jacks and connecting wires between the preamp and poweramp boards and replaced the Distortion pot with the proper 10K Linear since someone before me installed a 500K Audio that operated like an on/off switch.
A thorough recapping and the coupling cap mod was done to insure that it will work well into the 2030's. It cost me $140 to do so, but that is what I expect to spend on most any solid state amp every 10-20 years to keep it running properly. The tech I use is not only loves the older solid state stuff, but is very fair and doesn't replace anything that doesn't need it. He also gave my Standel combo a clean bill of health after $120 of work replacing caps, output transistors and giving it a good cleaning. $10 a year for repair and upkeep of an amp is peanuts, in my opinion.

 

[SOUTH PAW]

Thanks for your reply bigtone23. Good to hear that my very first amp is not only worth fixing, it's also relatively easy and affordable to do so.
Because I currently have other amps to use, and more time than money, I'm thinking of doing it myself.

Also, thanks for listing the things that are most likely to need replacement. I will definitely, as a first step, need to gain the ability to visually identify components to know what I'm looking at, and what it does, when I open it up. Can you recommend any beginner manual on amp repair ?