Converting my Peavey TKO 80 Bass Amplifier over to a preamp.

[DrNomis_44]

Hey Everyone,

Here's yet another one of my amp projects that I've got scheduled for this year, after some debating in my head, I've decided to see if I can convert my old 80's Peavey TKO 80 non-EQ Bass guitar amp over to a pre-amp so that I can use it with my Behringer K3000FX Keyboard/P.A. amp.

Here's what I think I'll need to do in order to complete the conversion:

1, Remove the PCB from the amp chassis.

2, Remove, or de-activate all the power amp circuitry on the PCB, leaving just the preamp circuitry fully working.

3, Design a new +/- Regulated 15V DC supply to power the preamp circuity.

4, Install the PCB and new power supply in a 2U 19 inch rackmount case, re-using the original black plastic front panel.

5, Test it out with my Behringer K3000FX Keyboard/P.A. amp.


Doing that will effectively give me a 300 Watt solid-state Bass guitar amp.

[Simon Barden]

Remember that 'Behringer watts' are normally far less powerful than the same value in RMS or AES watts. At best it will be around 150W RMS, and you may find that the Peavey actually has a very similar power output compared to the Behringer. It may be worth bringing out the speaker connection (upstream of the crossover) from the Behringer and powering it from the Peavey and see how the amp outputs compare using the same speaker configuration. Or just try with the preamp out from the Peavey going into a line-in of the Behringer.

[DrNomis_44]

Remember that 'Behringer watts' are normally far less powerful than the same value in RMS or AES watts. At best it will be around 150W RMS, and you may find that the Peavey actually has a very similar power output compared to the Behringer. It may be worth bringing out the speaker connection (upstream of the crossover) from the Behringer and powering it from the Peavey and see how the amp outputs compare using the same speaker configuration. Or just try with the preamp out from the Peavey going into a line-in of the Behringer.

The TKO 80 I have is actually only a 30 Watt amp, I've got the smaller practice amp version without the EQ, here's a couple of pics of the amp chassis:





Note that the knobs aren't the original ones, but I reckon they look fine.

Pics of the inputs and controlls on the K3000FX amp:






The top pic shows the controls of the K3000FX Keyboard/PA amp and the settings on the EQ to get a sort of useable Bass guitar sound, if I wanted to plug a keyboard into the K3000FX at the same time the EQ would have to be set flat.

[Simon Barden]

Ah. So the TKO is not non-EQ as you described. I think you meant 'non-graphic EQ'.

Even so, I've been caught out by Begringer's 'watts' before. Bought a powered mixing desk that was supposed to be 800W per channel, but it was so quiet that my old Peavey 120W per channel power blew it away in terms of output (and my previous Spirit 300WPC powered mixer was even louder). I returned the desk and got a plain mixing desk and new power amp instead. Needed more channels otherwise I'd have stuck with the Spirit desk (though it weighed a ton).

[DrNomis_44]

Yep, I meant "Non-Graphic EQ", Peavey did make a TKO 80 Bass amp with a Graphic EQ, but it is not only more powerful, it is also bigger than the TKO 80 I have, I had been having a good debate in my head about what to do about it, whether I was going to buy a new speaker for it, or convert it to a preamp, I realized that converting it to a preamp is going to cost me less, both in money and time, these little TKO 80 Bass amps aren't that great sounding due to the configuration of the power transistors, which I believe are in what's called a "Quasi-Complementary" output stage configuration ( I think), but my main rationale for doing the conversion is so that I can use my Squire P-Bass with my Behringer amp, I have tried plugging the P-Bass straight into one of the line inputs and it does work, the tone is a bit lacking though, because the inputs aren't really designed for a Bass guitar, it's more of a full-range P.A./Keyboard system, I've tried tweaking the EQ on the Behringer K3000FX and that somewhat works with the bass, but, if I also want to connect a keyboard up to the K3000FX, then I'd have to set the EQ flat to work with the keyboards.


So, converting my TKO 80 Bass amp into a preamp for use with the K3000FX amp, is going to benefit a Bass guitar in at least two ways as far as I can see, firstly the input impedance is going to be a better match for a Bass guitar, secondly it will provide a separate set of tone controls for a Bass guitar and I will still be able to set the graphic EQ on the K3000FX flat for keyboards.

[DrNomis_44]

Update:


I decided to do an experiment a few minutes ago, this involved connecting the pre-amp out socket on the front panel of the TKO 80 up to one of the line-inputs of the 300 Watt Behringer K3000FX Keyboard/PA amp, after plugging in my Squire P Bass, I first powered up the TKO 80 and then the K3000FX, no loud bangs or smoke occurred, so that was a good sign, with the bass, mid, and treble controls set to the 12 o clock position and turned up the pre and post-gain controls on the TKO 80, I was greeted by a nice loud and clean bass tone coming from the Behringer K3000FX when I plucked a string on the Squire P-Bass, so the idea of converting the TKO 80 into a bass preamp for use with the K3000FX is going to work perfectly.


The Squire P-Bass sounded nice and punchy through the setup, if I turned the volume up I found that I could easily get the glass panes of my loungeroom window to rattle, I had both the line input level, and master volume of the K3000FX set to the 12 o clock position and it sounded plenty loud enough to my ears with volume to spare.

Interestingly, the sounds I got were brighter than plugging the P Bass straight into the line input so I was correct in my thinking, the TKO 80's Bass, Mid, and Treble controls seem to give quite a useful range of tone from a deep Reggae style Bass, to a bright Rock Bass.

[Marcel]

Ah. So the TKO is not non-EQ as you described. I think you meant 'non-graphic EQ'.

Even so, I've been caught out by Begringer's 'watts' before. Bought a powered mixing desk that was supposed to be 800W per channel, but it was so quiet that my old Peavey 120W per channel power blew it away in terms of output (and my previous Spirit 300WPC powered mixer was even louder). I returned the desk and got a plain mixing desk and new power amp instead. Needed more channels otherwise I'd have stuck with the Spirit desk (though it weighed a ton).

Not the dreaded PMPO Wattage raising its ugly head again.... So sad that the marketing departments of amp makers keep dragging that meaningless term out under the guise of 'bigger is better' banner...

ONLY figure I ever consider as real is RMS Watts.... and Peak Watts when with a huge dash of sceptical salt.... Also probably why I'm easily satisfied with 15W of EL84 tube goodness...

[wazkelly]

Hey Doc, sounds like you just hook the rig up as described rather than go to a whole lot of trouble. That way you can keep the 30 watter in case you need it for a quieter practice amp or smaller venue and still be able to use the line out/line in on keyboard amp when you need more grunt.

[DrNomis_44]

Hey Doc, sounds like you just hook the rig up as described rather than go to a whole lot of trouble. That way you can keep the 30 watter in case you need it for a quieter practice amp or smaller venue and still be able to use the line out/line in on keyboard amp when you need more grunt.

I was considering that, but the problem is that I can't really get enough volume out of the TKO 80's existing power amp because it starts to distort when I turn it up anywhere past half way, I found that the best solution was to convert the TKO 80 into a bass preamp and use it with my Behringer K3000FX amp, and yes, I did replace the original power supply smoothing caps beforehand.


To use it at smaller venues, or bedroom, I just simply turn the post gain down and the tone is much better sounding.

The TKO 80's existing power amp design isn't the greatest, being a Quasi-complementary design, it inherently has some residual distortion in the output signal, it's a pretty cheap design.