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Fender Mini twin amp mods and some help needed

PostPosted: 19 Jan 2017, 03:18
by HoggYBhoY
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I recently modified this little amp to see if i could make it as good as it could be and to learn some more. I have attached a schematic but i dont think some of the values are correct.

I used shellac to lacquer the weed. in hindsight i wish i had used the clear shellac on this type of tweed its not the same as on the
big amps, strangely it has a lot smaller weave and the shellac seemed absorb right in and darken too much but it still looks cool.
also the amp had two 2 inch speakers absolutely pointless and sounded terrible. So i managed to get a 6x4 inch speaker in there !!
i had to move the guts around a bit to make it fit and also remove the pots from the pcb as they were directly connected
i installed a bezel for the LED added a 12v battery holder and IC socket. I finally settled on the Vintage MC1458P opamp I had taken from a 70's
pedal
now im still a bit of a noob at this so i was looking for some advice

this amp comes with a headphone out jack strangely it is a 1/4 inch jack how can i make this into a mono cab out jack? I think
i might have idea how to do this i have attached some pics one of the pics is from a guy modding the amp into a pedal
but i think it should still apply here
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on the pic where he has cut the pcb board he says this "The cut on the left stops the amped signal from reaching the stereo OUTPUT jack.
The cut on the right turns the stereo jack into mono by isolating its tip and ring contacts"
so if i removed the resistor and the capacitor then cut the trace as he sugested would this allow me to connect to a cab?

also i would like to replace the Si clipping diodes with Ge ones and arrange it to be asymmetrical clipping
i want to match the forward voltage of a typical led if possible by soldering a few diodes in series. how would i go about doing this ?

i dont like the tone pot very much its kinda weird its a B50k pot could someone suggest an alternative ?
aslo which caps would be the tone caps here i would like to mellow the tone as its a bit shrill sounding when the drive pot is up full. when drive at 0 and the volume maxed it sounds good.
drive pot is A500k and the volume is A50k

i cant tell you how much better it sounds with a 6x4 speaker in there !!! and the opamp swap made a big difference aslo.

Thanks in advance any info would be greatly appreciated :)

Re: Fender Mini twin amp mods and some help needed

PostPosted: 24 Jan 2017, 22:29
by allesz
Hallo, thank you for the schem, I got the mini deluxe that looks pretty similar. But the board is different: two sides with power amp on top with thru hole components and a smd pre on the bottom... mods will be difficult on mine.

The "deluxe"is made of plastic. This is not so bad, since it was easy to drill a hole for an external cab: just add a switched jack (used a cheap sinthetic one), desolder the wires from the speaker and connect them on the jack; then you just add a couple of wires from the jack (the switched tabs of course) to the speaker. Done.
I also think (not sure) that you can substitue L1 and R10 with jumpers.... but why loose headphones (and perhaps a direct out)?

I agree that this cutie sounds quite good and loud connected to a proper speaker.

The tone is quite strange and not very useful I rekon. Because my pre is smd I let it alone, by now :twisted:
Since you have a mod friendly board, and already removed the pots, I would try the stupidly wonderful tone control on muzique.com (little work required) or a big muff tone control.

Swapping the diodes with germanium ones will probably loose the nice clean and semiclean sounds. Think about it twice.

Re: Fender Mini twin amp mods and some help needed

PostPosted: 25 Jan 2017, 17:52
by HoggYBhoY
Hello
Thanks for the info. its a pity about yours being surface mount :(
I will never use headphones with this. this jack is 1/4 inch so I would rather just modify it
to be a cab out. surely this can be done? If i jumper the cap and the resistor and then cut the board as shown in the pic(to make it mono)
would that not give me a cab out jack ?

in regards to the tone pot what one are you recommending ?
you were saying that changing the diodes to ge would lose the clean tones ? i do not really get that. would they not only start clipping when
the forward voltage was reached? i would plan on soldering few ge diodes in series maybe 4 to increase the forward voltage.

Re: Fender Mini twin amp mods and some help needed

PostPosted: 25 Jan 2017, 18:57
by allesz
Hallo, probably jumpering L1 and R10 will be enough. Mono or stereo jack will not make a difference, granted you use a mono power cable for the external cab.

Both tone circuits are good to me. You can download the duncan tone stack calculator to simulate the sound of a big muff tone stack and have a sort of "preview" of the sound with different components. Big Muff tone stack gives often (but not always, if you select the right components) a mid scooped sound.

The wonderfully stupid tone control is easyer to wire and, in the end, it's just a matter of the selecting the right value of the cap that goes to ground. I like it very much on pedals (better than bmp one actually), but never tried it on an amp.

If you put 4 germanium diodes the forward voltage will be almost like the one of the two plain 1n41418s... put just two GE diodes and the forward voltages will drop a lot and the circuit will start to clip too soon. I like this amp with just a tiny hair of gain (just a little bit over minimum) and I suspect that the sound I like would be lost. With GE diodes the circuit will look almost like a mxd distortion+, wile now we have a dod 250. Never bonded with the dist+ myself.

Re: Fender Mini twin amp mods and some help needed

PostPosted: 26 Jan 2017, 01:15
by HoggYBhoY
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So if i jumper R12 and L1 I can connect to a cab and I do not need to make it mono ?
what about C21?
Is L1 not needed for the power jack?

Re: Fender Mini twin amp mods and some help needed

PostPosted: 26 Jan 2017, 13:09
by allesz
R10, not R12, based on the poster schem.

L1 seems out of power circuit.

Re: Fender Mini twin amp mods and some help needed

PostPosted: 26 Jan 2017, 13:11
by allesz
Can't find C21 on da schem.

Re: Fender Mini twin amp mods and some help needed

PostPosted: 27 Jan 2017, 04:20
by HoggYBhoY
I don't think the schematic is 100% accurate. its not mine
if you look in the last picture it shows R12 C21 and L1
should i remove R12 and C21?

Re: Fender Mini twin amp mods and some help needed

PostPosted: 28 Jan 2017, 23:51
by Thomas_H
Nice Upgrade!
that speaker fits right in there.

Just found that someone crazy enough to change the circuit and use normal sized tubes...
http://profile.ultimate-guitar.com/Invader+Jim/pictures/gear/1173368/1352390
Image

Re: Fender Mini twin amp mods and some help needed

PostPosted: 29 Jan 2017, 09:31
by allesz
Ok, so it's possible that the schem does not match the board (and maybe the amp).
I would try tracing at least the small board before working on it.

Nice thube conversion Thomas. But the 9v battery option would be lost.
People are impressed when you plug this little thing in a proper cab, without even using a wall wart (vart? :scratch: ).

Re: Fender Mini twin amp mods and some help needed

PostPosted: 03 Feb 2017, 19:17
by HoggYBhoY
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ill try removing and jumping R12 and C21 as shown in this pic why else would they be there apart from making the signal suitable for headphones.
ill need to wait to get a new soldering iron its stopped working it was old :(
thanks for the pics of the tube mod id love one of them.

Re: Fender Mini twin amp mods and some help needed

PostPosted: 08 Feb 2017, 13:14
by allesz
Sorry for the iron.... always a sad moment when they die.
Bury it in the yard, next to the dog :lol:

Before soldering I would try a simple jumper: if the schem is correct you should be able, jumpering a couple of components (have you got an alligator clip?), to do the trick.
So, connect a speaker cable to the phones out and start fiddling around jumpering here and there (there are very few components), I bet something will pop out from the speaker at a point.