Sears Solid State Guitar Amp LA-500

[CJ]

what does that 10 ohm - .1 cap circuit do?

seen it on a few amps, hi freq snubber?

thanks!

 

[JohnRoberts]

It's for stability. Google Zobel network or Bouchert cell.

JR

 

[CJ]

back to this thing again,

installed a chip amp, loud as heck but too much mid range, so we put a cap in series with the amp which caused some weird distortion when cranked, so we build a 2:1 xfmr to isolate signal, based on neve line input, but using UTC 31 UI lams,

 

[CJ]

transformer done twin coil humbucker, nice and flat, 2:1 ratio to drop the level a bit so we are not at max volume when knob set to 3,

 

[CJ]

put in an old UTC can and filled it with poly wax,

sounds alright but amp still acts funny,

 

[CJ]

next we try a build out tone circuit , just a resistor/cap network, a little better but the amp still does not like the cap input, must need a ground or something

works ok, but hard to drop off enough mids with this primitive tone circuit,

could just say screw it and bolt this thing up, but these things can be an educational experience so we trudge on down the path to better tone, hopefully, 

 

[CJ]

now just a 1:1  resistor divider with a tone cap bypassing the first resistor so we still have a direct signal input,

also tried a snubber on the speak terminals,

best sound of all, a bit of weird breakup,  we can still try other things,

like maybe finishing the original discrete circuit,

 

[CJ]

we are now trying the original circuit redone on a DIY board,

 

[CJ]

go ahead and laugh, but it works, 

 

[CJ]

sounds just like the chip amp, little louder, 2N3773 transistors heatsinked to the chassis panel,

no weird distortion clip thing,

still a ton of mids, turns out that the speakers and cabinet have a lot to do with the sour tone, maybe a little foam inside the cab would help,

100 hz to 200 hz is too strong, so we are simulationg a tone network based on the Fender stack that works great but chops a lot of signal, there is no free lunch, the more tone shaping you want, the more signal you have to lose,

but i think this original circuit has enough gain to use a Fender stack dialed in for what we need, then you record the pot values and use fixed resistors and caps,

this thing had a weird latch up problem when tested with the HP dual supply, the speaker cone would get thrown out by DC and the ammeter peaked out, only one of the pwr transistors got warm,

using a cap between the speaker and amp solved the problem, i bet this is why the circuit blew up, Acoustic and other amp makers have that cap in there, this direct output circuit probably latched up and thus the melted traces on the PC board,

now when we installed this with the pwr supply that came with the chip amp, there was no latch up  problem, maybe the supply rails have to be perfectly matched to prevent this,

we could hope that the even rails from the on board supply keeps this DC off the speaker, but an output cap would be cheap insurance,  and maybe cut down on some low mids, depending on the value,

 

[CJ]

ok Fender circuit is a no go, tubes and transistors do not mix well, unless you play a Musicman amp, 

trying to redesign a tube circuit for low z did not look good, hard to get a good dip,

so we are going to use a low z tone circuit,

the Pultec MEQ-5 has a sweet mid dip than we can dial in for our desired frequency of cut,

note that without that first 2.2K resistor, the circuit will not work, you get a flat line response, just like you get after eating a double whopper,  ;D


simulating the Pultec dip section>

 

[CJ]

here is the curve, Q is set with the series resistor that feeds the cap,

 

[CJ]

only problem now is finding an inductor,

so we take some Litton & Hurst Neve lams and spool up 500 turns of #38,

for an inductance of 3 henries at 150 Hz and 1 volt,

this is too high, our curve does not look right,

so we unwind half the turns to get 1/4 the inductance,

use 275T  just in case, now we have a 1 Henry coil which looks good on the simulator,

using a junk bobbin, see the missing flange?  ???  just tape it,

 

[CJ]

done with this project, here is the tone circuit we stuck in between the pre and pwr amp to improve the sound>

 

[CJ]

here is a link to guitar string freqs,

http://forum.cakewalk.com/Guitar-notes-and-frequencies-chart-included-m869331.aspx

we figured out what notes needed to be squelched by playing the guitar thru the amp, then matched the rez freq of the filter to the notes,

speak cab had marginal speakers and a cabinet that needed compensation,

here is the response of the filter,

 

[spoony]

Some of the Sears solid state circuits in the Aspen Pittman book are interesting

Which version of the aspen pittman book?