1973 Peavey Century Repair

[Supernaut]

Hoping to get some insight with regards to a Peavey amplifier im repairing just now. I bought the unit as spares or repair last week, cosmetically it was in poor condition but the seller did say it switched on. When the item arrived with me i split it apart to see if there was anything going on inside, I own several late 70's/ early 80's Peavey solid state amps and usually by this stage they are filled with dust, dirt and debris inside, usually they are still holding well up aside from needing a re-cap.

This unit has a 1000uf coupling capacitor that had snapped off on one side but other than that it looks pretty decent for its age, i done some reading online and i came across a forum where a user had modified the circuit by replacing the single 1000uf capacitor with three 1000uf capacitors and a film capacitor running parallel, he done this on the advice of a Peavey engineer who was involved with the construction of this amp back in the day. He explained that the single capacitor setup actually attenuated some frequencies and it would benefit the amp greatly to modify that part of the circuit. I seen two posts on that forum where people had done this modification and both said it sounded great.

The only issue i take is the way that its executed, both users posted pictures of the modification but they had anchored the three extra caps to the single 1000uf capacitor on the board. I noticed my PCB has a hole directly underneath where the 1000uf cap sits, i was wondering if i could use a single nylon bolt and nut assembly to attach some strip board and build out the capacitor mod using radial caps? I feel it would be more mechanically stable and it opens up options for capacitor choice.

I then started to consider if i could do a similar thing for the 6000uf can capacitor and build that out with three 2200uf radial caps to improve filtering? Both caps needs to be replaced anyway since the 1000uf has a snapped leg and the main 6000uf cap is reading 4700uf and has a slight bulge on the top.

Some photos of the modification and some pictures of my own PCB for reference.

[madires]

The output coupling cap is part of a high pass filter, i.e. it attenuates low frequencies. A higher capacitance lowers the cutoff frequency. So it's about the total capacitance in this case, and not about using multiple caps in parallel. 1000µF seems to be on the low side anyway. You could try a 2200µF or 3300µF cap. BTW, placing electrolytic caps next to hot power resistors is a bad idea (reduces the electrolytics' livespan). Regarding the filter cap, it's fine to use multiple caps in parallel. Also, a detached filter cap isn't ideal. Hopefully the amp has a star ground.

[Supernaut]

Thanks for your response, my reasoning for a multi-capacitor setup comes exclusively from the information passed on by an Engineer at Peavey. Ive only just started repairing amps and electronics at a hobby level so I cant comment on this particular capacitor setup that he describes. If I hadn't found this information online then I would most likely have just replaced the components 1:1

Ive copied the exact quote so people can better understand what im trying to achieve and where i got the idea. This capacitor setup only mentions the coupling cap and it was me who thought about doing a similar job with the 6000uf capacitor considering I need to change it out anyway.

Quote:

"I'll also share the rest of my conversation with Bobby Baldwin (@BbbyBld) about this amp:

There were two Centuries. The early one had a discrete preamp that ran off of a single +50V supply.

I built a really simple preamp with discrete transistors and was blown away at how fast and present the response was. It occurred to me that each opamp stage probably has close to 10 transistors inside in the audio path, and a modern preamp can easily use a dozen opamp stages. Each transistor adds a little delay and a little distortion.

The later Century (C1980) used the 200H module, the preamp used opamps and they ran off of +/-15V.

The power module [in the older Century] is one of the original Peavey solid state power amps by the way.

Hartley Peavey contracted a company out of Alabama to design it. That company was Orrtronics, and their claim to fame was manufacturing recording tape.

It is a very early, primitive design.
It was used in various products up to about 1980, even though Peavey had far more advanced designs during that time designed by Jack Sondermyer, the father of modern solid state power amp technology.

The capacitively coupled output is part of the "crudeness" of this design, so I'd recommend modifying that part.
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 electrolytic 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.

So, if you end up beefing up the output coupling cap. it will add some low end.

There may be a little bit of crossover distortion, but that is normal for these modules, and it improves once it heats up, which is also typical of many passively cooled SS power amps.
If you've had to replace the original output transistors, you may want to add a 0.1uF cap. across the speaker output jacks.
It would help to clean up the output signal when using the modern output transistors by dampening out instability.

[After completing these mods, I commented on how loud the Century is for a humble 100-watt amp. Bobby replied as follows:]

Modern power amps are biased class B, but also have a whole lot of gain and lots of negative feedback to get rid of crossover distortion, which in turn gives them a high damping factor. Your amp does not have a lot of gain or feedback, and has a low damping factor. That's where I think the loudness comes from."

[madires]

Yes, caps have an Equivalent Series Inductance (ESL). But it's quite low and the impact on audio frequencies is negligible. Here's some info from a well known manufacturer: https://chemi-con.com/characteristics-of-aluminum-electrolytic-capacitors/. The ESL for a large electrolytic cap can be around 30nH. A laudspeaker cable has more inductance.

[CaptDon]

A true 100 watts of available RMS power without clipping is incredibly loud when coming through a 2 X 12" cabinet with Peavey Scorpion speakers that are rated at around 99dB SPL 1W/1M on axis. The numbers say you could get 120dB SPL and in reality probably more. One thing to watch with the larger speaker coupling capacitors is if the power supply is uni-polar as in chassis ground and +80 volts vs bi-polar chassis ground and +/- 40 volts. I the first example the capacitor may have to charge through the voice coil to 1/2 of the supply voltage and that huge thump is why they sometimes used less than the optimal capacitor value in the output stage. 1000uf is a good coupling value for a guitar with an 8 ohm speaker. It is however not enough capacity for a bass guitar especially a 5 string bass and also not enough for a keyboard amplifier. This represents my opinions only. Ask three audiophools the same questions and you will get 10 answers!! Cheers to you fellow musician!!