Refurbishing a Peavey KB60

[chaosStrings]

Hi all! 1st post in this forum ... a great one, truly a goldmine for information

I bought a used Peavey KB60, schematic is attached below.

It works fairly well, but there are some issues I would like to solve:
- channel 1 is fairly noisy, there is lots of hiss (channel 2 works OK though);
- there is a loud pop when I turn off the amp, which I fear could damage the speakers.

Therefore I decided to refurbish the amp, since i think it is fairly old. A quick look at the PCB shows that it has not been modified, there is still the pencil signature of the tester
I would like to change:
- the electrolitic capacitors, since I heard that it is better to do so after 10-15 years of use;
- the opamps, for some lower noise, better ones;
- solve the issues mentioned above.

So here are my questions:
- would changing the caps be beneficial?
- what kind of opamps would you suggest in place of the 4558s?
- would these changings be enough to solve the aforementioned issues?
- finally, some caps are held by glue. Since I think this can increase reliability by damping vibrations and reducing stresses on capacitor leads, it may be better to use glue on the new caps, too. Which one should I use?

Thanks,

Daniele

[Robert M. Martinelli]

Hi,
I've been replacing 4558s with NE5532As many times in the past with good results, they're pin-to-pin compatible, furthermore their noise figure is way lower, so the hiss you complain about should be considerably reduced. Be aware that they draw more current than 4558 though....

If the old ICs are not socket-mounted, I'd also advise you to add DIP sockets before replacing the ICs, for future ease of testing/troubleshooting/replacement.

As to the loud "pop" you hear when turning the amp off, given the amp's topology, I' d divert the output signal away from the speaker and into a suitable "dummy load" power resistor ( 4 or 8 Ohms depending on the speaker's impedance, which is not shown on the schematic ) by means of a relay, operating the relay coil in parallel with the amp's power supply ( power switch ). this way, when turning off the amp, the capacitor is left free to discharge through the "dummy load" power resistor, eliminating the chances for the speaker to get damaged by that loud "pop". As to the resistor's rated power, 5W should be enough IMHO, because the transient current through it will be of a very short duration, however, being resistors rather cheap, you can use a 10W one for additional peace of mind.

Now, about the filter capacitors, we're not talking about a high value vintage amp, so, if you're not concerned about keeping it original, and since you plan to work on the amp anyway, changing them after 20 or so years of service is not a bad idea.

Hope this helps

Best regards

Bob

[Enzo]

I have a different view of this.

First, the turn-off pop. The loudedst the pop can possibly be is to the rail voltage. This then is no louder than the maximal signal output. In other words one good lick drum slap. If your speakers can handle the full power of the amp, then the pop is no threat. In practice the caps are bleeding when it occurs, so it can't even get that loud. In short, it may be annoying, but it won't hurt anything.

And really, though I consider it a needless exercise, if you want to open the speaker line to keep it away from the speaker, there is no particular need to send it to a dummy load. Just open the line, The solid state amp doesn't need to be loaded.

Popping at turn off is VERY common in solid state amps.

If one preamp channel is noisier than another, then it most likely is a noisy op amp. A new one will likely fix it.

As to the noise level overall, how do the other channels sound? Clean enough? If so, then any appropriate op amps will be sufficient. 4558s are still readily available and cheap, but so are newer ones like 4560, 4580. And of course good old TL072s. 5532s are old school, but sound OK, but as Bob points out, they do draw about twice the current as 4558s, so one or two in place of the 4558 is OK, but don;t go refitting a large mixer with a pile of them, you might run out of power supply.

And then there is the gilding of the lily department. We can put some sort of cosmic NASA grade op amps in there, but remember they are in the context of a very basic consumer electronics circuit, so the expected beneifit may never materialize. If the other channel is clean enough, remember it was made that way with plain old 4558s.

And filter caps? I hate to just throw parts at things. Does the amp sound funny, hum, or sag? If not, then your caps are probably good for a long time yet. Unlike in a tube amp, these caps are not baked to within an inch of their lives, and they are not operating at high voltages, so leaky is a lot less of a problem.

[chaosStrings]

Thanks for the suggestions!

I was already planning on putting some commonly available opamps, though I didn't know what kind of characteristics I should look for on the datasheet. I'll try the 5532s and TL072s, as soon as the electronic shop reopens from holidays

Just a question: for the reverb opamp, would one with a higher source current help? (I read that 5532s are better at driving loads ...)

About the caps, the amp sounds fine. There is some noise going on, but I do not really know whethter that comes from the opamps, the other gear I use the KB60 with, dying caps or something normal ... I guess I'll try changing the opamps first and see if there is an improvement.

[Enzo]

Just a question: for the reverb opamp, would one with a higher source current help? (I read that 5532s are better at driving loads ...)

"Better" in what way? What is the reverb not doing now that you expect it to do with different ICs?

Failing - as in dried out - filter caps result in increased hum mostly. Hiss is a product of gain. And it usually comes from a semiconductor.

As to the signal sources, just turn the amp on and don;t plug ANY inputs into it. Turn up the volume controls and you can hear the "self" noise of the amp. Then any higher noise levels when signal are applied must be coming from the signal sources.

[Robert M. Martinelli]

I have a different view of this.

First, the turn-off pop. The loudedst the pop can possibly be is to the rail voltage. This then is no louder than the maximal signal output. In other words one good lick drum slap. If your speakers can handle the full power of the amp, then the pop is no threat. In practice the caps are bleeding when it occurs, so it can't even get that loud. In short, it may be annoying, but it won't hurt anything.

And really, though I consider it a needless exercise, if you want to open the speaker line to keep it away from the speaker, there is no particular need to send it to a dummy load. Just open the line, The solid state amp doesn't need to be loaded.

Popping at turn off is VERY common in solid state amps.

Enzo,
I think you're right about the "impossibility" for the speaker to get damaged, OTOH that "pop" can be really loud; I have a mid '70 Vox AC30 SS that emits huge "pops" whenever the power switch is operated, and I'm so annoyed by this I'm considering adding a diverting relay to it myself...

Though I know SS output devices, if properly sized, allow the load to be disconnected, I advised to add a dummy load resistor to provide a discharge path for the output capacitor. A 5 W resistor is cheap and adds "cleanliness" to the design IMHO, but, as you said, it's not "mandatory". BTW that's the way I'm going to do it on my AC30, due to its "quasi-complementary symmetry w/single supply design topology ( which calls for a huge output capacitor ).

Another point I'd like to mention is that, by using that diverting relay the "back EMF" from the speaker doesn't make it back to the circuit, so it's not allowed to add itself to the rail voltage, which can be still there due to the filter supply caps still not discharged, so IMHO, the relay can also help in protecting the output devices from a momentary "over-voltage".

Different views are what makes us all richer IMHO....all the more so when they're expressed peacefully and politely like you do....that's why I enjoy spending some of my time here so much

Cheers

Bob

[chaosStrings]

"Better" in what way? What is the reverb not doing now that you expect it to do with different ICs?

The reverb effect remains quite subtle (no cathedral-like as in Fender Amps), even when the control is maxed out. I am wondering if it is a question of how the opamp gain is set, or it is due to the capability of the 4558 to drive the reverb tank.

Enzo,
Different views are what makes us all richer IMHO....all the more so when they're expressed peacefully and politely like you do....that's why I enjoy spending some of my time here so much

... and it gives the possibility to people with little knowledge in electronics and none in power amp design (that would be me ) to learn a lot

[Enzo]

The reverb effect remains quite subtle (no cathedral-like as in Fender Amps), even when the control is maxed out. I am wondering if it is a question of how the opamp gain is set, or it is due to the capability of the 4558 to drive the reverb tank.

Well, I don;t know just how subtle your unit is, but it would never have all that surf music reverb like an old Fender. Most old Fenders - in my opinion - have way more reverb than you can use. Up to about 4-5 on the dial it is useful, but above that is silly. This is a keyboard amp, and the amount of reverb it produces is by design. It is not designed for surf guitar.

That single 4558 can drive a reverb pan just fine. Peavey has been driving their reverbs with it for decades. Even in the guitar heads.

It is possible your reverb pan is defective, or indeed your 4558, if the reverb is way too subtle.

COmpare the circuit on the KB60 to that is the old Special 130 guitar amp. Note the differences in details. Also note they use different reverb pans. What are the Accutronics numbers on your pan?

[chaosStrings]

I still have got to figure the way to open the part of the amp where the speakers and reverb are placed ... Anyway, the reverb effect is well audible, so the thing works well. I was just wondering if it were possible to get a greater range for the effect by changing the opamp. I'll buy a dedicated effect unit for delay and longer reverb anyway, so it isn't much of a deal.

About channel 1, problem solved! I found a 5532, stuck it in, and now the noise level is acceptable. The amp alone is very silent, apart from some hiss which kicks in in both channels when the gain is set at > 3h00. I'll never use that much gain though.

The power off pop is now also kind of OK ... I turned off the amp when the gain constrols were not set to 0, and there was almost no noise. I guess I'll change my habits then

[Enzo]

The reverb can be a major contributor to popping as circuits go.

The gain of an op amp is set by the resistors in its feedback path, the chip itself doesn;t much matter in that regard. So the amount of reveb wouldn;t much change. A different IC there might change the level of noise.

The grille is held on by an industrial strength velco sort of thing at the corners. Use the metal trim strips up the sides. Use a screwdriver, and get behind one of the upper corner's trim strip. Use the lower front edge of the chassis as a fulcrum and pry the top corner of the grille outwards. It will pop free. Then the other top corner should follow along relatively easy. lean the whole thing out and pop the bottom free. Your are now looking at the speaker. Dismount he spaeker to get at the inside of the cab where the reverb lives.

You now have one channel with a 5532, and one with a 4558. COmpare the hiss/noise level between teh two. ANy difference should be due to the chip type. Your 5532 replaced a defective IC. Now that both channels have good ICs, how much difference is there?

[chaosStrings]

... back from the week-end!

I now have the amp at the rehearsal room, so I cannot work on it for the reverb; I'll check it as soon as I have got time though

About the channels, their noise level is comparable. That means, the amp is pretty silent and usable! The only occasion when it isn't silent is when I turn up the volume ... and I drown the other guitarist's amp

[Musicfreak]

I still have got to figure the way to open the part of the amp where the speakers and reverb are placed ... Anyway, the reverb effect is well audible, so the thing works well. I was just wondering if it were possible to get a greater range for the effect by changing the opamp. I'll buy a dedicated effect unit for delay and longer reverb anyway, so it isn't much of a deal.

About channel 1, problem solved! I found a 5532, stuck it in, and now the noise level is acceptable. The amp alone is very silent, apart from some hiss which kicks in in both channels when the gain is set at > 3h00. I'll never use that much gain though.

The power off pop is now also kind of OK ... I turned off the amp when the gain constrols were not set to 0, and there was almost no noise. I guess I'll change my habits then

How many did you change? 5 or 6?

[smile]

Peavey KB60

Hi
New to forum, just picked up a KB60 for clean jazz guitar, suited my requirement for a good bass sound with no or little clipping and reverb. was going to get a bass amp for the job but saw this with reverb and I think I may have got a good amp for what I need. Any thoughts on using the KB60 for jazz guuitar or mods to make it even better? The amp works fine, only had scratchy pots (now fixed).

Does anyone know what type of speaker is in it e.g sheffield (power handleing watts/ohms) they used?, I have the aqua panel model I guess from early 90's and I cant seem to get the front panel off to get to the speaker.