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Peavey KB 300 - Worth changing the 4558 op-amps?

Started by WimWalther, November 03, 2023, 11:30:08 PM

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WimWalther

Hi folks,

Is there any point in changing out the 4558s in a (first model, mid-1983) Peavey KB 300?

Is there much of anything to be gained by this? The amp works fine, though it's maybe a bit noisier than I'd wish.

Thanks!

WimWalther

#1
While we're here.. I'd like to improve the low end - add another octave or two down low.. to at least one channel. But I'm not really sure which parts are determining the present cutoff.

Obviously I'll probably need to at least double the PSU storage, but that's fairly straightforward.

PDF schematic attached. Note that this is only correct to the early version without an XLR input and only a single row of jacks on the left

Tassieviking

If it already has sockets mounted for the IC's I would say go for it, as long as the new op-amps are compatible.

If you are after less noise then it might be better to check those 40 year old capacitors as they might have downgraded a bit over 40 years.
I would definitely test the main filtering caps for the power supplies, those big electrolytic caps can cause noise if they go bad.

There are better quality capacitors around now, a lot better then 40 years ago.
The biggest question is do you need it to sound clearer or do you just want to tinker, I would leave it alone if everything is working ok and it's not too noisy.
The amps were always a bit noisy back then, some more then others.
There are no stupid questions.
There are only stupid mistakes.

WimWalther

Hi Tassieviking,

Maybe I just want to tinker.. a guy needs something to do!

No, the noise is not so bad, just more than I'm used to, coming at this from the hi-end / hifi world. Yes, the OAs are all socketed.. so very easy** to change.

The big psu filters are Cap-Tech parts, which were premium quality in that era. Yes, newer stuff is "better", but yet those parts are so much better than 60s-70s stuff already that I don't suspect them. They also have a weird mount pattern.. I don't think a 1-3/8" snap-cap will just pop in.

(** if we don't count r&r the chassis,which is a pain.)

phatt

I'm assuming you are using this for el guitar? If so then the lack of bottom end is likely due to the presence of the horns producing so much treble it drowns out the Bass.

If you are chasing the sweeter tones that good guitar rigs produce then the last thing you want is hi Freq horns.

Also if the amp has a Scorpion speaker they are already very bright when compared to other famous guitar speakers.

Should be easy enough to disconnect the horn and see if that sounds sweeter and improves the low end.
Phil.

joecool85

Quote from: WimWalther on November 03, 2023, 11:30:08 PMHi folks,

Is there any point in changing out the 4558s in a (first model, mid-1983) Peavey KB 300?

Is there much of anything to be gained by this? The amp works fine, though it's maybe a bit noisier than I'd wish.

Thanks!

What are you going to swap in place of the old 4558s?  TL072?  For what it's worth, those old 4558's are considered magic by many.  They aren't though, they just have an awful slew rate and are kind of noisy.  The DO happen to work well for electric guitar though.
Life is what you make it.
Still rockin' the Dean Markley K-20X
thatraymond.com

WimWalther

#6
Quote from: phatt on November 06, 2023, 04:43:19 AMI'm assuming you are using this for el guitar? If so then the lack of bottom end is likely due to the presence of the horns producing so much treble it drowns out the Bass.

If you are chasing the sweeter tones that good guitar rigs produce then the last thing you want is hi Freq horns.

Analog synth, actually. It's primarily a keyboard amp.

QuoteAlso if the amp has a Scorpion speaker they are already very bright when compared to other famous guitar speakers.

This is the "type 1" KB 300 ca. mid-1983. It doesn't have a speaker badge on it, so I'm assuming it's a fairly decent average driver of that era. Definitely not a scorpion or widow.

The preamp has 3-band eq, so it's easy to dial the highs way down. I don't have the synth stuff setup right now, so I'm using some commercial reference recordings to evaluate the sound. The fact that I'm using a cheap CD player & BT link might be some of the issue.

Do you know if BT audio has a lossy compression step? Does it send raw PCM / WAV or something like mp3 on the fly?

WimWalther

Quote from: joecool85 link=msg=414954558's are considered magic by many.  They aren't though, they just have an awful slew rate and are kind of noisy.  The DO happen to work well for electric guitar though.

Man, you sure do hear a range of opinions on old 4558s! But I wouldn't know, I come out of the vacuum tube world.

So how are they for synth stuff?

Jazz P Bass

Not too sure about synths.
 But I did run in to circuits that where designed around the slow slew rate of older 4558 ics.
one in particular was a Tube Works amp that had a 12AX7 blow out that toasted the 4558.
I installed a modern version of the ic & the customer wanted to know what was wrong with the amp.
The distortion was NOT the same.
So, I grabbed the ic from the reverb circuit & swapped them around & wallah. Customer happy!
 Another instance was the original Tube Screamer pedal.
Same thing.
Modern ic is too fast. slew rate wise.
By this time I had accumilated a 'stash' of older 4558s.
Installed one of them. Problem solved.

WimWalther

Quote from: Jazz P Bass on November 08, 2023, 01:57:15 AMI installed a modern version of the ic & the customer wanted to know what was wrong with the amp.
The distortion was NOT the same.
So, I grabbed the ic from the reverb circuit & swapped them around & wallah.

Interesting. So it's not simply a matter of using newer *types*, like 4332 or TL072, but 4558 of more recent manufacture..

So did the manufacturer(s) update the datasheets to reflect the improved specifications, or did they leave them alone, figuring that as long as the new parts "meet or exceed" published values, they were ok?

Either way, kind of an annoying situation. Do you know, roughly, when the new-spec 4558 devices began appearing?