Welcome to Solid State Guitar Amp Forum | DIY Guitar Amplifiers. Please login or sign up.

March 28, 2024, 01:32:54 PM

Login with username, password and session length

Recent Posts

 

New Electrolytic Caps for 80's Peavey Backstage Plus?

Started by Sirenus, February 02, 2015, 01:30:38 PM

Previous topic - Next topic

Sirenus

Hi,

I recently bought an '84 Peavey Backstage Plus.  I think it sounds great, especially after I replaced the original, 30 year old, stock OEM Eminence speaker with an Eminence Legend 1058.  It has killer clean jazz tones.  My questions is: would there be any benefit to me replacing all of the electrolytic caps in the amp?  Would it improve the sound at all or reduce noise?  The amp isn't super noisy but there is a slight constant hiss when it's on with low gain.

I took out the chassis when I bought it to clean the pots; it looks to be 100% stock.  I have experience modding my tube VHT Special 6 Ultra and a few Boss effects in the past (I probably know just enough to be dangerous!) but this is the first time I've thought about changing components in a solid-state amp.  Thanks in advance for any input.


Roly

Quote from: Sirenuswould there be any benefit to me replacing all of the electrolytic caps in the amp?  Would it improve the sound at all or reduce noise?  The amp isn't super noisy but there is a slight constant hiss when it's on with low gain.

No, no, and no.

If it ain't busted, don't fix it.  As it is not showing any symptoms of failing electros you are more likely to do harm than good by replacing them.

Noise, specifically hiss, is very unlikely to be caused by electrolytic caps.


(thanks to elektrotanya)

If you have a problem with hiss then it is much more likely to originate in the 4558 op-amps, and if you really want to do something about it then changing them for more modern lower noise types like LM833's would be the way to go, however I again caution you that "the game has to be worth the candle"; if the hiss is tolerable, and the op-amps are soldered in rather than socketed, I'd leave it well enough alone.


(ed: typo)
If you say theory and practice don't agree you haven't applied enough theory.

Enzo

Agree, the only noise a weak filter cap will make is hum, not hiss.  Hiss will usually involve a gain stage, and I agree, most likely an op amp IC.

Sirenus

Thanks so much for the help!  I'll leave well enough alone.  What a great little amp!


Anomaly

I've kinda wondered about this as well. Someone else told me that these old SS amps generally don't need to have their caps replaced as often as tube amps because the caps have much lower voltages, so they dont dry out as soon as the caps in a tube amp would. My mid 80's Randall is still ticking and sounding great 30 years later. I've got another rackmount model that has had it's caps replaced (and looks like it may have been modded) at some time but it doesn't really sound much different.

Enzo

And there are ancient caps that still work too.   One thing about solid state amps is that for the most part, they are 1980s or newer, wwhile there are plenty of 50-60 year old tube amps in daily use.  Once there are 50-60 year old SS amps, I bet there will be more dried caps in them.

Roly

Absolute working voltage doesn't have a lot to do with it.  Of rather more importance would be the ESR, and average lifetime ripple current, but it's a bit moot.

If it ain't busted, don't fix it!

If you have symptoms (e.g. buzz, hiss, hum, distortion) then you start thinking about causes, but if the hum level, particularly of the power amp stage alone, is acceptable in a stage or studio situation as you need, then what's to fix?  Leave it alone.  (but there is plenty to bust in the process)

If you have a studio sound engineer say that your amp needs looking at for, say, excessive idle hum, then you go looking for the cause of that.  It's possible that it's due to an earthing problem rather than caps, so you renew all the cap$ and still have the problem.




Generally:

Another factor apart from absolute age, is use.  Seemingly paradoxically elecros in an amp (of any sort) will last a very long time if the amp is regularly used.  What can kill electrolytic caps is disuse.

When an electro cap is manufactured the insulating layer is electro-formed on the foil surface.  Every time the cap is energised this electro-formed layer is reinforced, but if left inactive for a long period the layer will decay and thin and the cap start to get electrically leaky.





It is possible to re-form a slightly decayed electo, but when they are far gone they will just draw DC current and heat up.

One of the reasons that there seems to be an awful lot of bulldust talked in some (guitar) forums about cap, and electro caps in particular, is because with electros "failure" is a bit of a judgement thing in many cases.  You've got the "47uF" bypass cap that measures 0.012uF, or 38 ohms, or the one with the bulging end leaking goo, and they are very obviously duff; but what about 12% low on value and showing falling leakage on reforming voltage?  Is that good or duff?

It's kinda like Chinese Whispers.  A tech explains a little of this detail to somebody, possibly concerning a repair, and by stages this gets morphed into simplistic forum posts that respond to any and every amp problem with "change all the caps!".   

It's not just that a huge background of electronics, maths, and experience in faultfinding all manner of equipment, not least guitar amps, is simply thrown out the window with a simplistic one-size-fits-all four word answer - it's mainly that it doesn't work!  If anything it tends to make matters worse.  It's a version of what we generally call blunderbuss or scattergun servicing - and it always ends in tears (and a backtrack to start over again properly).


So this is unlike most other forums, there accumulated, washed up on this digital beach, is a widely scattered collection of a couple of hundred years of experience fixing stuff.
If you say theory and practice don't agree you haven't applied enough theory.