Info on Fender SK-20 amp wanted

[Mark Hammer]

I picked one of these up yesterday. http://profile.ultimate-guitar.com/p...49_image_0.JPG Nothing terribly special, but cute and functional and it was a good deal. I'd like to learn a little more about it to help bring out it's finer points, whether by mod or by use.

This is essentially a Sidekick amp that is sort of a "mini-JC-120", with built in chorus and a stereo output. It has two 8" speakers and I'm assuming 10W a side. There is a send-return loop that permits a stereo return (something I'm looking forward to using with my various stereo Line 6 pedals and other stereo DIY pedals), although I don't know if the send is mono or tip/ring stereo. The overdrive is nothing to write home about, but it is footswitchable.

What I would like to know more about is:
a) the particulars of the send/return patch point
b) the topology of the 3-control gain structure
c) the functioning of the "presence" control

I haven't been able to locate a schematic on-line, though I have found many of the various Bullet, Squier, Frontman, and similar low wattage fender SS practice amps. None of these have a presence control. Any information or link to a schematic would be appreciated. If you have any e-documents you could send me, send them to: mhammer<at>295.ca

Thanks a batch.
Mark

[Enzo]

Watch your email.

The preamp is straight mono, the effects send is mono.

The preamp has a couple op amp stages, and each one just has its own gain control, thus OD and GAIN. Then a simple transistor circuit is the preamp out with a conventional volume control there. Three controls.

At the loop point, the thing branches. The two returns are into each branch. Left return is dry, going straight into the left power amp. Right return runs through the chorus stuff and on to the right channel.

The presence control is in series with the bypass cap on the emitter of the preamp out stage, allowing variable bypass.

[Enzo]

I sent you a 2MB file, and just got a not-delivered notice. But often as not, when I get those, the thing actually got delivered.

Did you receive the file?

[Mark Hammer]

Thanks, Enzo. Nope, didn't show up. Possible to reduce it in size or break it up into 2 files or something?

BTW, opened it up last night, briefly. It was pretty much as I expected from looking at other low wattage fenders. Pair of TDA2030s for power, and a pair of red LEDs for clipping. Though I did not check underneath to see what they were connected to, it seemed like they use a 470uf cap on the output of each TDA2030. Given that the unit lacks a bit in bottom, I though it might prove profitable to swap those out for 1000uf for just a bit more oomph. Obviously the low end is constrained by the cab and speakers, but 470uf just seems kinda "lean".

I guess the other thing I need to know is how to rig up a footswitch for this unit. Many amps just have a stereo jack and two SPSTs connecting to ground for remote switching, but I know Fender sometimes has something a little different.

[Mark Hammer]

I'm not impatient, but if you had tried again, nuthin' yet. The second part of the address is simply 295.ca (a dialup service that costs $2.95 per month). Maybe my feeble attempt to prevent spamming has misled you.

[Enzo]

I sent the file from a disk, with Adobe reader, I can't break out a page. But I have a paper copy, I will scan just the schemo and try that. The failure message said your something or other was full.

I'll send the original again just to see. No I hadn't retried, I have not been in the shop since last night. My new day is just under way.

[Mark Hammer]

I just deleted about 20M of stuff that a friend had sent me. There should be room now.

[Enzo]

OK, one more time

[Mark Hammer]

Nope. nothing, nada, garnicht, rien, bapkes. Turns out I deleted the e-mail but never purged the trash, so it's possible that's the holdup. I just finished purging it, so...

[Enzo]

OK, sent again. I might as well just scan it and quit messing around.

[Enzo]

Well, so far it hasn't come back undeliverable. That's a good sign.

[Mark Hammer]

Indeed, it WAS received. Just checked now. Thank you so much. I owe you big time. I'd offer you my first-born, but the food bills would bankrupt you.

[Enzo]

I was starting to think it was a new Passover plague.

[Mark Hammer]

Now, I realize it is not a fabulopus amp, but even sows ears can be turned into something a little more useful now and then. With the aid of the schematic Enzo sent, I identified and carried out a number of improvements to the amp:

1) made a foot pedal for selecting chorus and boost (amp didn't come with one)

2) made the overdrive tone a bit less harsh-sounding by increasing some of the lowpass filtering on it (may still swap out the LEDs for silicon diodes to get more crunch at lower drive settings, and less volume boost when overdriving)

3) gave myself a tad more bass by increasing the 470uf power-amp output caps to 1000uf

4) made the treble control more useful and tolerable by reducing an 1800pf cap down to 1200pf, the tone controls are now useful again

5) gave a little more speed to the chorus by dropping a .082uf cap in the LFO down to .068

6) added juuuuuust a bit more delay time to the chorus range to thicken it up a bit by increasing a 68pf cap in the HF clock circuit to 82pf; sounds richer now

7) removed a pair of clipping diodes at the return point on the effects loop so that it is possible to have higher volume AND clean sound

All those changes made it a nicer amp, although I still find a bit too much hum for my tastes. The master plan is to co-opt the Presence control (which I have yet to find any use for) as a reverb control, and build in a reverb unit on the bottom of the amp chassis. I've been tinkering with using springs from Home Depot to make a totally DIY spring reverb, and it is actually capable of sounding pretty decent. I have a small 3" speaker as the spring driver. The spring is epoxied onto the coil cap on the speaker, and the other end of the spring is soldered to two other springs, which branch out to their respectve sensors. The sensors are some nice crystal phone cartridges I picked up cheap. The springs can be soldered directly to the piezo disc. Having one primary spring and two secondary springs still permits a bit of diffusion. The piezo discs put out a nice hefty signal that does not require quite so much recovery gain, so I won't have to worry quite so much about hum and noise from the reverb. The speaker used to drive the primary spring is driven by either a pair of NE5534 chips or a 386 power-amp chip. Haven't made up my mind yet. I'm hoping to incorporate a Dwell type control to adjust spring drive, plus some reverb tone-shaping.

What remains to be decided, though, is where to tap the signal for sending to the reverb circuit and where to return the reverb signal to. It is probably wisest, I think, to tap the signal for reverb-send as early as possible in the signal path, so that it is untouched by the EQ settings, amount of overdrive, and such. That way, when it gets blended back in, it will be a clean reverb wash to situate in the back of a sound that has otherwise been modified. Now, where to return it is what has me perplexed. I can think of reasons to return it to both channels, buit I can think of reasons to want to return it to only one channel.

I'll let you know when I get there.

Of course, given the stereo-return aspect, it might also be interesting to consider implementing RG Keen's solid-state Pro Reverb Vibrato in stereo ( http://www.geofex.com/FX_images/FakeFenderVib3.pdf ). Need ot pop some holes in the back of the chassis, though. The very idea of a stereo 2 x 8" amp with overdrive, chorus, reverb and stereo vibrato is kinda nifty if you ask me.

So, thanks again Enzo. Your help made one heckuva difference.

[Enzo]

DOn't overlook one reverb tap possibility - right off the speaker. A little voltage division and the speaker out can drive the reverb pan. Then make the recovery stage return wherever you liike.