Kustom quad 65DFX

[9Lives]

Does anyone have any experience with this amp at all? I have one that I've never used much.  Bc I just don't like the way it sounds. It's to bassy and extreme mid hump. I love my mids and all.. But damn. My real question is about the OD channel. It sucks royally the best sound I can get out of it is with the eq all the way down. The tonestack on this amp is fairly powerful it's just not there.. Musically I guess. The amps just to mushy or muddy, can't explain it. Does anyone know where I can find a schematic for this thing or any existing mods? I figured it's a long shot but never hurts to ask. Also a foot switch would be nice. I'm wondering if it's the same as a typical spst switch would work?

[oldhousescott]

Best bet is to call Kustom service dept and request a schematic. Might charge you a bit for it, or might not make it available at all (except to authorized service centers).

Once in hand, we can venture some suggestions.

[9Lives]

actually I was expecting that exact answer.. Actually. Never hurts to ask. It's not a very popular amp. Never hurts to ask. The amps  got some potential. It's got a powerful crystal clean channel. I'm gonna take your advise. Maybe I can work the "its broke angle"

[oldhousescott]

OK, I found some schematics online, and it looks like both the lead and rhythm channels use a combination of bridged T for the mids and standard Fender two-knob blackface for the tone stacks. The lead channel has a bit more going on in the bridged T circuit, but you could certainly model it in SPICE and get some frequency plots. That would give you some idea where to start tweaking.

Here's the lead channel tonestack:


I'll need to find a clearer overall schematic to see what's going on for distortion in the drive channel.

[oldhousescott]

The mid circuit is centered at about 1kHz. As the control is turned up, it simultaneously reduces the amount of mid scoop while cutting more highs. Conversely, as the control is turned down, it scoops out more mids while allowing more highs to pass.

At first, the bass/treble tonestack looked like it might be wonky with a mix of normal and scaled values, but it works out to be fairly normal looking, except for a slightly higher mid notch frequency (in USA mode).

You can move the mid circuit frequency by playing around with R26 and R27 (keep them equal to each other). Making them both 10k will move the frequency notch down to about 450Hz. That might give you more control of your mids. If that's too low of a mid frequency to adjust, try 6k8 for R26 and R27. That will put the center frequency between 600 and 700Hz.

Likewise in your bass/treble tonestack, changing R57 to 6k8, C24 to 1uF, and C40 to 220nF will move the dip down to around 650Hz. Increasing C39 to 47nF will give you a similar effect when switching into UK mode. You can play with these values in Duncan's Tonestack Calculator to simulate the response.

The overdrive looks like back-to-back zeners in the feedback of the fourth stage. You could use lower reverse breakdown voltage zeners to get a smoother OD, albeit yielding less volume. Or you could remove the zeners and temp in a clipping selector board (I can't recall who makes it) and experiment to find the best sounding clipping diodes.

Note: all the above pertains to the drive channel. The clean channel can similarly be modded, except it's impossible to make out the part designations and values from the overall schematic floating around in cyberspace.

[9Lives]

dude, I really appreciate that. I'm gonna stare at this post and read it about 20 times until I get an idea of what I want to do. The only scem I could find was for the 100quad dfx and I figured it was close. One cool thing about this amp is it scoops the hell out of the mids very effectivly taking me back to 1990 when my friends and I didn't believe in mids lol. Now I feek it's a crime to scoop to hard but this amp is just strange. I wonder if the zener clipping is what I'm not so fond of.. Is it clipping freq to the inside cathodes or does it goto ground. Making sense? Let me study b4 I post more. Thanks again.

[9Lives]

also I've this is my first amp so I'm a lil intimidated by it. The mod you suggested.. Is that a freq patter you like? That's where my lack of experience bonks me on my head bc looking at freq waves confuses the hell out of me. Hopfully this well teach me somthing. I'm going to pull up the tonesatck page tonight and see what I get. I'm not really sure what I'm looking for, what's good and what's bad.. At first glance is there anything you noticed about it that you wouldn't like? I saw your word "wonky" lol. Sorry you have to break this down so far. But I really wanna fix this thing.

[oldhousescott]

OK, found the schematic for the 100DFX online. Looks similar to the 65DFX, we can probably use it for a reference.

The mods for the dirty channel tonestack I suggested move it closer to a standard Fender blackface response, although the response with the stock values isn't too different, except for the location of the mid dip as previously noted. It's kind of between a Fender and Marshall response as is. As you discovered, you can really scoop the mids with the mid control. My suggestion to mod the tonestack and mid control is to give you more control of the lower mids.

The clean tonestack and mid control are similar to the dirty channel. The mid control is centered at about 850Hz, which is a bit high. You might increase C37 to 3n3 or 4n7 to lower the frequency. The tonestack has a prominent rise in the treble response.

Both channels pass through a Sallen-Key bandpass filter before the output amp, probably to trim off the high-highs and low-lows.

The zeners are 12v pieces so the clipping occurs at about 13 volts on the plus and minus excursions of the waveform. They are in the feedback loop. The power supply rails are +/- 15 so the zeners keep the signal off the rails and cause hard clipping.

This is all just kind of general info, but maybe it will be a jumping off point for any mods you would like to make.

[9Lives]

I noticed you live in SC, what part? I live in GA statesboro. I think your right the mids are dabbling in the hi range. It's making the end result as kind of muddy. Do you think this the prob? The lows are really low and the highs really high I'm guessing the lower mids being "not so defined" would possibly be causing a gap in what we hear?? Maybe the midrange freq is not blending in audio wise. How smart would it be to solder sockets in place for this tonestack? Bad idea for amps with all the vibration? I have a large bin of parts so what ever you suggest I can make it work. As for the clipping. I don't like it. At all. It's not useful to me in the least bit as it is so a total rehaul is worth the risk to me. I don't use the channel it's very muddy and doesn't playwell at all with the other pedals. It actually brings the dr boogie back to and OD with LOTS of feed back! I don't understand this. The best sound is to scoop the mids completly. I have been into the mosfet type clipping, wondering if somthing like this would suit this channel better. If I could say I had a goal with this is to make it really tight more heavy tones bc I love amp distortion and none of my amps have "that sound" so if we could find a way to do this I would consider myself in debt bc this thing is that bad right now. Thanks for the help. And let me know if this isn't enough info to go on.

[oldhousescott]

Here's a couple of ideas for improving the drive channel clipping. In both cases, the diodes can be LEDs, or silicons, or multiple pairs of diodes, or FETs. Really just trying to show clipping devices in anti-parallel in two orientations. You can leave the zeners in place in the next stage as they'll probably not come into play, or remove them.

As far as socketing the tonestack, you can probably get away with that if the PCB is sturdy enough, at least long enough to determine the values you want to use. Ultimately, I'd solder the components to the PCB.

[9Lives]

ok cool, I see what you did in the 2nd example that would be a typical hard clipping. That would give me a more distorted sound rather than an over driven tone correct? In the first example the diodes are parallel within the feedback loop not shunting GRD. I've seen this b4 but had no chance to ask. Is the bc they are in the fb loop that you don't need to grd them? What effect does this give? I see the 4558 in there. Probably wouldn't hurt to replace that with an 883 would it? I guess I'm just tryin to get good results if I'm goin to break this thing down. I can probably take some pics once I get it open

[oldhousescott]

The first example is typical for an overdrive circuit, like a tube screamer. It tends to produce softer clipping compared to the second example.

I wouldn't bother replacing the 4558. It probably contributes less than 1% to the character of the sound. Most of the tone is determined by the circuit configuration and component value choices. The cabinet and speaker play a big part in the sound, too, but I wouldn't spend a bunch of money on a speaker upgrade. Resistors and capacitors are cheap. Just tweak those until you get it sounding as good as it can for your tastes.

[9Lives]

ok so this maybe a dumb question but what does it sound like if I use both? Clipping within the feedback and after the output? And if I were to mod it to hard clipping style would I have to re configure the pot? Keep in mind I'm looking  for some thick tight hi gain amp dist. Could I get that from this w/o completly over hauling the circuitry? And would there be ne assortments of caps and resistors to compliment each clipping style?

[9Lives]

ok man, I hope your reading this bc your on to somthing. I did the tone stack mod to the lead channel just to so how on track we are.used the 10k pair for the mid knob. It's a little low but I think I'm keeping it there bc the "playability" improved a bunch. Also replaced the caps. It's alot more pedal friendly and just responds better to my setup. I attempted jerry riggind a couple hard clippers in there by tieing to opposing 4148's from 3 lug of the gain pot to the amp chasis. It actually made it very smooth but the volume drop rediculously and it interfered with the clean channel :? So in a panic I cut it out and fixed that prob. The mod tampered with the taper of the mid pot slightly it's like 12:00 is at it's strongest and increasing goes into low freq. Any idea why? Over all it beats the hell out of how it was and I'm not done yet. I had to use a metal film for the 6k8.. All I had. I want to build a channel selector. There's a plug  for it w/ 3 descriptions. Ring/sleeve,  tip,   tip/sleeve.  I'm guessing I can use a spdt to toggle from t/s to r / s  with sleeve tied to the common lug? Thnk again!

[oldhousescott]

Yeah, the 1N4148s will limit the output swing to +/- 0.6 volts. If you use a couple of LEDs instead, that will raise the swing to +/- 2 volts or so. That should get you more volume and more crunch (less smooth). Or you can string together 3 or 4 1N4148s in series in each direction to raise the clipping threshold as follows:

               +---|>|---|>|---|>|---+
    --------|                               |----------
               +---|<|---|<|---|<|---+


Glad the tonestack and mid cut value changes helped. We can try to guess what values will improve the tone by crunching numbers, but as you've discovered, it's often best to use those as starting points and experiment from there.