Revoicing a Dean Markley K-20B preamp for guitar use

[Tone Meister]

Yeah I know, but I really want to do it anyway.

Dean Markley K-20B bass practice amp. Works perfectly fine, but sounds thin in the bass and.low-midrange with electric guitar. I've already changed the stock 8" speaker to a Celestion Eight 15. I want to retune the preamp and use it for very small shows where low volumes are paramount.

I have no desire for it to distort. I'd like to retune the preamp to perform similar to a blackface Fender. Nothing more.

In looking at the schematic, the input resistor is 10K (R1) and in series is C1 0.047 capacitor. Also, there is a treble boost circuit that might be usable if retuned to perform like a Fender Bright switch.

Schematics attached.

[Enzo]

Um...

perform similar to a blackface Fender. Nothing more.

How can I make my steak taste like lobster, nothing more.

This is a basic op amp circuit, and I fear getting it to sound like a Fender tube amp will only happen in the most casual of senses.

First try this: Disconnect the 8" speaker and connect the amp to a 2x12 or 4x12 cab for a listen. Or even a 1x12. You want more bottom, listen through a speaker that can make bottom. You may want to keep the 8" form for ease of transport etc, but do the experiment anyway so you will know just how much the speaker matters.

It is already a bass amp, so one hopes it is optimized for bass. If it lacks enough bass for guitar, just imagine how short it must fall for bass. And keep in mind they designed this for the 8" speaker, so they likely needed to LIMIT the bass so as not to flab out the poor little speaker.

Your tone stack is familiar Fender turf, even if it is drawn a little different. As it is cut only, all the way up is all the bass it will pass.

You wanna mess with C1? Go ahead, tack a 0.1uf in parallel and see if it gets you anywhere you want to be. Someone else can calculate the freq response at C13, C5, and elsewhere.

[Tone Meister]

Alrighty then. I was hoping someone could run the preamp circuit through Spice and make some reasonable predictions.
So yeah, I'd like this ghetto steak to sound somewhat like a Missouri lobster, .

[J M Fahey]

Agree and add:
1) as Enzo said, you have an 8" guitar speaker inside a shoebox.
Exact same Amp, as is, into a real cabinet , will surprise you.

2) C1 is already way too large, I would expect a .0047 uF cap there.
In any case adding "electronic" bass will only fart because speaker can neither stand nor reproduce it.

3) only possible improvement would be to halve C11 or R10 which will rise turnover frequency and give you more midbass, which sounds warmer.

4) that said, little DM amps sound very good, EQ is spot on ... they may suffer small cabinets and speakers and not too much power, but basic sound is good.

5) in any case, use it as a personal monitor, people in their seats can hear a far better sound than you.

The secret is **close** miking, as in having microphone push grill cloth towards dustcap.

In that case, cabinet size and room acoustics practically lose all meaning, and inside that tight space, even an 8" speaker is King ..... you will be surprised by what the audience gets..

2 practical examples:

a) a couple Months ago, I went to a Tourist favoured square near home.
In general, they have Tango players, singers and dancers, and frown on Rock , etc. , but sometimes accept a *low volume* band, playing old Classics, forget Heavy Metal or worse.

This band was playing with a very basic small drum kit, unamplified Sax, "everything plugged straight in the mixer", a 150W or so 6 channel one driving tiny PA speakers .
I saw Guitar player plug a Strat into a simple Distortion pedal, think a Rat or one of the simplest Boss.
I expected Guitar sound to be ratty buzzy beehive because thatīs what you get when you go from pedal straight to mixer ... yet sound was VERY good (besides, the guy was a killer player).
I sat besides them, and looked around ... until I found his secret weapon: his pedal went into a tiny 10W Valvestate, practically hidden in plain view a few meters behind the drummer, among a random pile of drum and instrument cases, leather jackets, etc. , with a microphone stuck against the speaker.
Not sure it was a 6" or 8" speaker, very light in any case ... but reamplified acoustic sound was very good, specially very smooth which is not usual.

b) an SS Guitar member told us that in his heyday he got killer sound from a 15W 8" speaker SS Japan made "Fender" Squier, held backstage inside a drum case and with an SM57 again snug against front grill.
Judge for yourself (I have already posted this YT video many times, illustrating this point):

Tha Marshall wall backline you see is mostly OFF, the rhythm player is using *one* and heīs using what I described above, only reamplified through stage monitors to "stupid high voume" (his words) so as to have amp feedback and sustain:

[Tone Meister]

I understand that concept more than you know Juan. I earned my nickname 40 years ago by playing appropriate parts at the appropriate time at the appropriate volume. I also have one of those Squire amps that you speak of and I actually use it regularly. I just thought it would be fun to tweak this DM amp just a little bit and try using it for certain music types.

I'd like to see the circuit run through spice one time and see what emulations we can come up with. And in hindsight, maybe I should have said the amp produces a little bit too much Hi mid and treble rather than saying it was weak in the bass.

[drewl]

I use a DigiTech GSP1101 for effects, but with the amp modelling you can run it into any crappy SS amp and get a decent sound.

I know as I often do that at rehearsals, having to use whatever amps are there.

But a few tweaks could help your amp sound better for guitar.

Maybe increase preamp gain?
Put some diodes in opamp feedback loop like a tube screamer?

[Tone Meister]

I use a DigiTech GSP1101 for effects, but with the amp modelling you can run it into any crappy SS amp and get a decent sound.

I know as I often do that at rehearsals, having to use whatever amps are there.

But a few tweaks could help your amp sound better for guitar.

Maybe increase preamp gain?
Put some diodes in opamp feedback loop like a tube screamer?

Actually that's exactly opposite of what I'd like to do. Since this will be used strictly as a pedal platform, my goal is always to get a solid great sounding clean tone that is full and clear with just guitar as the starting point. I'll shape the tone upstream of the amp.

I have very advanced touring pedalboards in 3 sizes and depend on those for tone shaping in most, but not all, cases. I also have built in direct outputs with Jensen transformers out through an XLR connector in case I need to run direct. The onboard DI is also used when playing acoustic or bass through the pedalboard.

I'm gonna fiddle with it a little and see if I can balance the frequency response some.

[J M Fahey]

Whatever you do with electronic response, it will then be played through a speaker, so any realistic expectation must consider its actual frequency response, since itīs a very real equalizer and filter which affects averything going through it.
You can not avoid it , at most replace it with a better one (better speaker and cabinet).

This is the datasheet Celestion 815 frequency response:



as you see, it reaches flat to some 160Hz and then slopes downwards at none less than 12dB/octave, see that at 80Hz itīs 12dB down.

Now that (and most other published ones) is measured with speaker mounted in a "standard" infinite baffle.

Celestion probably uses the Euro version, earlier called the D.I.N. baffe, here I found the IEC version, almost the same.

Size depends on speaker diameter, but notice for 8" speakers itīs 4.5 feet wide and 5.5 feet tall ... WAY larger than your cabinet , so low frequency loss, not only deep bass but aso low mids, will be catastrophic anyway.

And that considering *any* 8" speaker , if itīs mounted in that tiny enclosure.

My point is that acoustics has a way louder voice than bare electronics in this problem, so if you canīt solve the former, donīt lose sleep on the latter.

I suggested increasing low mids, which has the side effect of apparently lowering high mids and highs by comparison ... not sure you can go much further.

[Tone Meister]

Agreed Juan. It sounds great mic'd up, so I know it can sound great out in the house. Keep in mind that this is nothing too important since I have a large collection of real amps to use. But this amp does sound very clean which makes it a good candidate for a pedal platform. So I might not use it more than 2 or 3 times a year on an actual show, but I take a little bit of pride in being able to take a cheap amplifier like that and make great sounds with it.

I am not going to try to play through an external cabinet because that defeats the point of a lightweight grab and go amp. I'd like to just get as much low mids out of it as possible and call it a day.

[J M Fahey]

Ok, please try the low mids enhancing tone control mod, thatīs about the best (and only) tone improvement you can realistically get.

There is a Plan B, of course, and it means taking the bull by the horns.

It involves:

* you want real low mids and bass but still an 8" speaker?

Fine, get a speaker designed for that, an 8" PA woofer.
In the example Iīm showing below, they originally used an Eminence Beta 8A , an 8" 150 or 200W RMS woofer, large heavy long voice coil, large magnet, heavy cardboard cone.
Miles away from your lightweight 15W 8" 15W Celestion.

In current incarnation, they use about same but made by Celestion, same thing.

* they made a properly tuned cabinet.

* what separates men from kids: above mentioned speaker reaches low, but requires gobs of power: Acoustics says for each octave you go down, you lose 8dB efficiency ... or need 8dB higher power to get same SPL, same thing.
So the example below uses a 200W RMS amplifier.

* to keep full system lightweight, they use a Class D power amp, an SMPS, and I guess a NEO speaker instead of original Ferrite magnet Eminence. Not sure about that