Swapping op amps in a Peavey KB-100

Help Support GroupDIY:

Tubetec

Well-known member
Joined
Nov 18, 2015
Messages
3,439
I recently took in an old Peavey KB-100 keyboard amp that a buddy was throwing away , needed a bit of work circuit wise , but nothing major . In the end I scrapped the cabinet and fitted the amp into a huge Yamaha B100-115II  cab  I had lying around . 

There was a couple of small issues needed addressing , a diode in channel one had a disintegrated lead wire, I replaced it with a similar small glass signal diode and the channel was working again , ch2's grounding in the jack wasnt making contact so it was noisy , just simply bending the contact fixed that issue .
There was a mix of op amps in various positions , the later schem stated 4558's throughout
but what was in there was something else , one input channel even had a TL072 . I had a bunch of 4560 op amps from a more modern Peavey , I decided to try them as input stage , gain remained more or less the same but noise was very much reduced maybe 10db less , I did find despite the higher noise the fet input TL072 sounded better with hi-z sources like guitar pickups ,but for ordinary line level inputs the BA4560 gave much quieter performance . Repositioning the reverb tank further away in the big Yamaha cab , I also managed to reduce the induced noise from the mains transformer in the reverb return by a good margin. I probably spent more time than it was worth but at least now I have something I can offer as a working amp and saved a few bits from the ever growing mountain of electronic detritus . I might re-fit the Motorola Piezo tweeter and make it switchable  for  a little more tizz along with the boom of the big scooped Yamaha cab . The sound from this cab reminds me of the bass sound on 'Madness' records from the late 70's early 80's or Aston Barrett from the Wailers who used an Acoustic rig . Extra THD in return for more low end oooomph is always a good deal in ska/reggae.
 

JohnRoberts

Well-known member
Staff member
Moderator
Joined
Nov 30, 2006
Messages
21,896
Location
Hickory, MS
The 4560 was a good soldier... relatively quiet and had good turn on/off transient control (I used truckloads).

The keyboard amps were pretty popular and good sellers.

JR
 

Tubetec

Well-known member
Joined
Nov 18, 2015
Messages
3,439
Thanks John ,
Im glad I saved it from the crusher , its probably 35 years old already and electronically its still perfect ,
the broken diode seems to have been corroded due to spillage of probably beer near the input , the jack contact not making ground is most likely due to someone yanking the cable out violently , so you could attribute the failures to user error . Theres another lifetimes worth of usage left in it yet .

I might revert one of the input channels to the TL072 for passive instrument input as it seemed like it was loading the pickup  better than the BA4560 .
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,750
Location
Marcelland
Tubetec said:
I might revert one of the input channels to the TL072 for passive instrument input as it seemed like it was loading the pickup  better than the BA4560 .
OPA2134 will give you both high-Z compatibility and low-noise (not as good as 4560, but close enough, and anyway much better than TL072).
 

CJ

Well-known member
Joined
Jun 3, 2004
Messages
14,332
Location
California
maybe tweak the bias current resistors for the 72, they need mush less current at the input stage, right?
 
Top