Solid State Guitar Amp Forum | DIY Guitar Amplifiers

Solid State Amplifiers => Amplifier Discussion => Topic started by: joecool85 on February 27, 2014, 12:17:45 PM

Title: My K-20X
Post by: joecool85 on February 27, 2014, 12:17:45 PM
I've been playing guitar for 12 years now and the whole time I've owned this amp - Dean Markley K-20X.  It's been through a lot.  Over the years I have added a line out (just like the K-20, not sure why they got rid of it on the K-20X), added a speaker out jack and removed the rear wooden "bar" on the back side of the amp.  I now have replaced the permanently attached power cord with an IEC jack w/fuse.  Not only does this allow for quick cord replacement, but also easier storage, the use of a 90 degree cord so you can put the amp closer to other devices, and it puts the fuse in an easily accessible area (back panel).  The old fuse location being on the PCB was not only inconvient, but arguably unsafe (putting mains current and voltage to the PCB).  The mains wiring now bypasses the PCB completely making the amp safer and easier to service.

As a side effect, I needed to relocate the power transformer to have room for the back side of the IEC jack.  It is now directly under where it use to be, same mounting holes and all.  This puts the transformer outside of the metal enclosure.  This potentially removes some noise (though so far I haven't noticed a difference).

Now the pics!

**EDIT**
Attached schematic as it currently stands, minus speaker cutout switch which is spst inline with speaker + lead (2/24/2020)
Title: Re: My K-20X
Post by: joecool85 on February 27, 2014, 12:20:03 PM
Of note, I added those little wooden "brackets" at the bottom of the amp.  They allow me to put a cord in there and not have it fall out.  This is what it looked like without them. 
Title: Re: My K-20X
Post by: mexicanyella on February 27, 2014, 06:19:45 PM
Nice work, and nicely documented! I like your Squier Tele too...that's a neat orangey vintage finish.

This is an inspiring thread, because I've got a pre-X K-20 that's working but pretty ratty, with paint spills on the tolex and a cracked MDF baffle board. Not sure if I just want to keep it around as an extra flavor of small amp, or if I want to get creative with it.

I've considered removing the chassis and making it into a pedalboard-mounted amp, so I could just carry my pedalboard and a small cabinet for low-volume stuff.

I've also thought about swapping in a 4-ohm 8" midbass/PA-type driver and enclosing the back (below the chassis) with some cleats and panels screwed/glued in place, and using it as a bass practice amp. Not sure I want to get that non-reversible with it, but at very low volumes, it sounds excellent with a P-bass (drive channel set with gain very low--1-1/2 or 2--and the output vol halfway or more). Even with the stock speaker and open-back cab, I bet it'd be a great bass recording amp, miked up for some low-volume grunt and twang and combined with a clean, maybe compressed, DI for lows... 
Title: Re: My K-20X
Post by: joecool85 on February 27, 2014, 07:54:47 PM
I've thought about cutting down the existing cab and turning it into a small head. It drives a 10" wonderfully BTW.

Sent from my XT1055 using Tapatalk

Title: Re: My K-20X
Post by: mexicanyella on February 27, 2014, 10:33:27 PM
Mine did a pretty good job of driving a 2 x 10 cab I sold recently, loaded with a pair of 50-watt SLM speakers. Wish I still had a 4 x 12 sitting around to try it with!

The X models might be different, but to my ear the pre-X K-20 has a pretty lazy transient response, and I'm not sure if that's a power supply limitation, or a specific attempt to emulate a saggy tube rectifier, or what. My early 80s Peavey Audition 20 seems to have a lot more clean headroom and feels a lot "faster" in terms of transient response.

On the other hand, I recently discovered that if I put a compressor pedal in front of the K-20 and set the amp's input gain about halfway, the bass about 1/4 of the way up, and dime everything else, the K-20 goes nuts with sparkly upper mids and loose, wild-ass lows...kind of like Joe Walsh's James Gang-era tones. It makes me think of cranked tweed Fender, sort of. I'm happy to discover something that amp can do that my other amps cannot.
Title: Re: My K-20X
Post by: teemuk on February 28, 2014, 07:30:36 AM
I see the PC board is used for other, more feature packed, amps of that series as well. Have you thought about building those unpopulated areas in the printed circuit board? What's omitted by the way? I reckon at least spring reverb driver and recovery stages...?
Title: Re: My K-20X
Post by: joecool85 on February 28, 2014, 08:08:59 AM
Teemu, it is a reverb circuit. The K-30RX shares a PCB. It has reverb, a slightly larger cab and a 10" speaker. Same output power though despite being a "30" vs a "20".

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Title: Re: My K-20X
Post by: joecool85 on April 06, 2014, 08:08:29 PM
Added a "kick stand" to the little guy. I used a brass rod, a cable clamp and two screws. All stuff I had laying around. The bar is from our bed's headboard, our Son dismantled it when he was about a year old lol.
Title: Re: My K-20X
Post by: joecool85 on April 06, 2014, 08:09:31 PM
More pics
Title: Re: My K-20X
Post by: Bill Moore on April 07, 2014, 11:54:58 AM
Thanks for the info! I lost my K-20 to my wife's sister, but if it ever comes back home, I have a few more ideas, (reverb would be cool!)
Title: Re: My K-20X
Post by: galaxiex on June 10, 2017, 01:56:20 PM
Hi Joe, Hope you don't mind me putting this here.

Thought we could compare our K-20 amps.
I opened it up to install a speaker jack and was mildly surprised at the differences to yours.

Mine was made 1993, which probably accounts for the differences.
Any idea what year yours is?

The layout and circuit board are obviously different.
Mine has the line out on the front panel and a fuse holder on back.

I was hoping to find the same circuit board as yours with the un-populated areas for the reverb.

Oh well, I'll just use a reverb pedal.

A minor feature I like is the slanted speaker baffle.

Cheers!

Edit; looks like it would be easy to make the drive switch, foot switchable.  :)
Title: Re: My K-20X
Post by: joecool85 on June 10, 2017, 02:20:50 PM
Quote from: galaxiex on June 10, 2017, 01:56:20 PM
Hi Joe, Hope you don't mind me putting this here.

Thought we could compare our K-20 amps.
I opened it up to install a speaker jack and was mildly surprised at the differences to yours.

Mine was made 1993, which probably accounts for the differences.
Any idea what year yours is?

The layout and circuit board are obviously different.
Mine has the line out on the front panel and a fuse holder on back.

I was hoping to find the same circuit board as yours with the un-populated areas for the reverb.

Oh well, I'll just use a reverb pedal.

A minor feature I like is the slanted speaker baffle.

Cheers!

Edit; looks like it would be easy to make the drive switch, foot switchable.  :)
Yours is a K-20, mine is a K-20X. Almost identical circuit, but there are differences especially in the PCB. I'm not sure of the year on mine because it isn't stamped. But I bought it around 2000.

Sent from my XT1055 using Tapatalk

Title: Re: My K-20X
Post by: galaxiex on June 18, 2017, 03:25:16 PM
Well I'm done tinkering with this amp.

I installed a speaker jack (not much to show there, so no pics) on the bottom of the chassis.

I noticed a huge difference between the volume of the drive and clean sounds.
eg; if I get the clean setup to where I like it, and then switch in the drive, there is a huge volume jump that doesn't work for me.
I prefer just a modest vol increase + the distortion.

The culprit seems to be the 2 resistors, R5 and R6 in the attached schematic.

BTW this schem part numbering does not match my board, but no matter... the circuit seems to be the same.

Anyways.... after some experimenting I left R6 alone and changed R5 to a 22K.
Now the clean sound dials in at much lower settings, but that's ok, cuz switching in the drive gives me the slight vol boost + dist that I want.

Down side is... if I crank the clean setting too much, it starts sounding just like the drive is switched in, and very little difference when the drive is actually switched in.

Oh well... its still better than the huge jump in volume from before.  8)
Title: Re: My K-20X
Post by: Bill Moore on June 30, 2017, 04:32:43 PM
I have posted before on my little K-20, and had intended to experiment some with it.
Our (only) grandson was down a month ago, and likes an amp with OD, but most I have at the house need to really be loud for that. I showed him the Dean Markley, and he played through it a little, and said it was OK. For several days he would go into the music room, and play, always choosing the little K-20. When we were getting ready to take him back to the airport, I found a bag that would fit the little amp in, and told him to take it home. (I had to pay to check his other bag, but I was happy to do so). It now resides in Alaska, and his mom said he is really proud of it!
Title: Re: My K-20X
Post by: J M Fahey on July 01, 2017, 08:15:20 AM
 :dbtu: :dbtu: :dbtu:
Title: Re: My K-20X
Post by: joecool85 on February 24, 2020, 02:28:12 PM
I've finally done it.  I added reverb to the K-20X.  I chose to do a digital reverb based on the Accutronics/Belton "brick."  The PCB I chose to use is from 1776 Effects, the Rub-a-dub reverb (https://1776effects.com/products/rub-a-dub-reverb).  All other parts were sourced from SmallBear (http://smallbear-electronics.mybigcommerce.com/) and I used the short delay Belton brick.  I think next time I might use the mid or long though...or the model 3 so you can adjust length.  The decay is a little faster than I would like, but still sounds pretty good.

While in there I also added a speaker kill switch so that I can turn the amp on and off without pops or noises.  No gut shots, but the power for the brick comes from an LM7809 drawing power from the positive leg of the stock power section.  There aren't any holes for standoffs on the PCB, so it is attached to the enclosure with a zip tie and foam between the board and the enclosure for electrical insulation.

Also not shown (and happened after these two pictures), the line out and speaker out jack have been changed to insulated plastic type jacks as the reverb brought out an issue I had made back in 2005 when I added those two jacks - a ground loop!  All fixed now and working better than ever.  I honestly never noticed it before since it was so subtle, but the reverb made it pretty obnoxious.

I like that I put the knob on the back - hidden reverb   8|
Title: Re: My K-20X
Post by: galaxiex on February 24, 2020, 06:46:10 PM
That's pretty cool, Joe...  :cheesy:

Where in the amp circuit did you put the reverb?

I assume at the front end since it's a stompbox type circuit?

Gut shots would be great.  8)
Title: Re: My K-20X
Post by: joecool85 on February 25, 2020, 08:49:31 AM
Here it is.

Please bear in mind that this looks different now as this was a test fitment.  The wires in this picture were connected to either end of R11 (one end lifted).  I have since removed R11 and now connect on the bottom side of the PCB to help with noise.  I have also added star grounding and replaced the metal speaker and line out jacks with insulated plastic types.

The schematic attached to the first post in this thread shows were I connected in.
Title: Re: My K-20X
Post by: galaxiex on February 25, 2020, 11:14:12 AM
Quote from: joecool85 on February 25, 2020, 08:49:31 AM
Here it is.

Please bear in mind that this looks different now as this was a test fitment.  The wires in this picture were connected to either end of R11 (one end lifted).  I have since removed R11 and now connect on the bottom side of the PCB to help with noise.  I have also added star grounding and replaced the metal speaker and line out jacks with insulated plastic types.

The schematic attached to the first post in this thread shows were I connected in.

Thanks for the gut shot, but I don't see the schematic attachment?
Title: Re: My K-20X
Post by: joecool85 on February 25, 2020, 11:37:54 AM
The schematic is on page 1, first post, last attachment: https://www.ssguitar.com/index.php?topic=3403.msg25231#msg25231
Title: Re: My K-20X
Post by: galaxiex on February 25, 2020, 11:51:53 AM
Ohhh, DUH!

I didn't see we are on the second page!  ::)
Title: Re: My K-20X
Post by: Bill Moore on March 02, 2020, 05:46:19 PM
Now I'll have to try that!
I gave my first little amp away, but last year a fellow wanted to trade a "practice amp" for a pedal I had. I was surprised when he showed up with another Dean Markley. One of the pots had the shaft pulled loose, and after searching for a replacement. I ended up repairing the original. Speaker is a little rough, but at low volume it isn't noticeable.
Now reverb?
Title: Re: My K-20X
Post by: joecool85 on March 02, 2020, 08:08:13 PM
Quote from: Bill Moore on March 02, 2020, 05:46:19 PM
Now I'll have to try that!
I gave my first little amp away, but last year a fellow wanted to trade a "practice amp" for a pedal I had. I was surprised when he showed up with another Dean Markley. One of the pots had the shaft pulled loose, and after searching for a replacement. I ended up repairing the original. Speaker is a little rough, but at low volume it isn't noticeable.
Now reverb?

I love it, well worth the time and money.  I think it was $45 or so in parts.  At first I wasn't sure I was sold on the "short" Belton brick, but I really jive with it now.  That said, I've heard clips of the medium and long bricks and they sound great too.  Others have said that the difference between the three is pretty small anyway.  Or, of course, you could go with the "3H" version using the Rub-a-dub Deluxe PCB.  It's an extra $2 and would require 2 more pots.  Other than that it's the same basically and would allow you to control the decay length, and tone of the reverb as well as the normal mix amount.
Title: Re: My K-20X
Post by: linty on July 20, 2021, 11:21:42 AM
Hoping JoeCool85 is still around for me to necro this thread.  Would you be able to elaborate on the difference (if any) between the line out and the headphone out?  I'm having weird issues with using a K30RX to add reverb and pass the signal along to a mixer and am wondering if maybe this could be a better option than using the ph out.

Thanks!
Steve
Title: Re: My K-20X
Post by: Enzo on July 20, 2021, 10:07:50 PM
LIne out is a signal that is suitable for a mixer or another amp or some line level effect unit.  Headphones out is...tada.... for headphones.
Title: Re: My K-20X
Post by: willpirkle on July 21, 2021, 08:08:17 AM
In addition, a line out is designed to drive a high impedance load (>10k, such as the unbalanced input to a mixer, FX return, etc...) while the HP out is designed for very low impedances (8-32 ohms, speakers). A more important difference is that line out on an RCA or 1/4" jack, is designed (by spec) to deliver a specific nominal value of -10dBV, or ~316 mVRMS into that load. There is no formal spec for headphone outputs, so you never know what you're going to get, which is why some headphone outs are really loud and others are quiet, when driving the same load. Headphone outs are typically noisier as they usually use headphone (low power) IC amps, rather than op-amps but in many cases you can't really tell the difference (high gain guitar preamps, etc...).
Title: Re: My K-20X
Post by: linty on July 21, 2021, 10:46:11 AM
Thanks will, that makes sense.  From what I see in the modded circuit diagram at the start of this thread, it seems like line out volume would follow speaker volume.  If I wanted to silence the speaker when using line out would I just need to add an spdt to switch between the speaker and a 4 ohm power resistor?
Title: Re: My K-20X
Post by: Enzo on July 21, 2021, 01:37:52 PM
That solid state amp does not require a load, you can simply turn off the speaker with a switch.

What I would do is connect the line out 10k resistor to the phones jack tip connection instead of the speaker.  Now the line out works when you are on speaker or when on phones.  Now the phones still work, but you can use the phones jack to kill the speaker.   Just plug an unwired plug into the phones jack, and that opens the speaker connection.But a switch works fine too.
Title: Re: My K-20X
Post by: joecool85 on July 21, 2021, 03:13:16 PM
Quote from: Enzo on July 21, 2021, 01:37:52 PM
That solid state amp does not require a load, you can simply turn off the speaker with a switch.

What I would do is connect the line out 10k resistor to the phones jack tip connection instead of the speaker.  Now the line out works when you are on speaker or when on phones.  Now the phones still work, but you can use the phones jack to kill the speaker.   Just plug an unwired plug into the phones jack, and that opens the speaker connection.But a switch works fine too.

Enzo, that's a pretty slick idea! I'm not going to re-wire mine since I rarely use the line out now anyway, but if I ever did another I would probably do it this way.
Title: Re: My K-20X
Post by: flester on July 23, 2021, 03:44:52 AM
Great to read stories of people modifying gear like this, and also handing it on to a younger generation. My kids showing no interest in guitar gear yet, sadly. I'm holding out for their  kids.
Title: Re: My K-20X
Post by: Trino on May 28, 2022, 05:38:30 PM
Hello joecool85 Hello, I need a thing
, can you give me a photo of the pcb of your k20x please?
Title: Re: My K-20X
Post by: joecool85 on May 31, 2022, 11:40:00 AM
Quote from: Trino on May 28, 2022, 05:38:30 PMHello joecool85 Hello, I need a thing
, can you give me a photo of the pcb of your k20x please?

Anything in particular you are looking for?  Earlier on in the thread I have pictures posted of the topside of the board.  There are also attachments showing the schematic as well.