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Old 18th March 2015, 09:30 PM   #1
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Default Modding an old practice amp - switches, pots etc

Cross posting this from the Amplifiers section as suggested...

First time poster and rookie tinkerer here, so apologies for any noob questions, breaches of etiquette, general stupidity etc.<br />
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I have a cheap early 90s solid state practice amp (made in Korea by Acrobat, apparently) which I'd like to try and upgrade, really just for the hell of it. I'm curious to see if a decent sound can be got out of it and even whether it could be made giggable in a small bar type environment. <br />
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The main things I'm doing are to replace all the pots (some of which are broken and probably not great quality to start with) and upgrade the massive 5" speaker to something more substantial. I'd also like to add some switches for overdrive and reverb. <br />
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Common to amps of this size, it's single channel but has an overdrive pot. What would be cool would be if I could set the overdrive to the desired level, then switch it in or out at will. I understand that if I just put a switch over the pot, it would let me switch between full distortion and a level set by the pot, which isn't what I'm after - I want to switch from a level set by the pot to clean.<br />
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The pot is labelled as 1 meg. Is there such a thing as a push-pull pot that would switch to maximum resistance (ie clean signal, presumably) when pulled out, then function as normal when pushed in? Or if not, is there some way I could put a 1meg resistor either on a push-pull pot, or a separate switch, to perform the same function?<br />
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Any advice would be greatly appreciated. In the fullness of time I'd also like to add a footswitch, but first things first eh...<br/>
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Old 19th March 2015, 07:11 AM   #2
JohnDH is offline JohnDH  Australia
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I wouldnt hold out too much hope of getting a truely great tone for your bar gigs. But it could still be a good project to mess with.

You may be able to find a 1M push pull. But if not, a seperate mini-toggle switch could do it. Take the wire from the centre lug of the pot and connect it to a centre lug on the new swifch. Then wire an outer switch lug back to the pot, so it reconnects the wire to where it was if flicked one way. Take the other outer connection back to an outer pot lug corresponding to max resistance.
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Old 19th March 2015, 05:56 PM   #3
JMFahey is offline JMFahey  Argentina
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A couple comments.

First, congratulations on trying to learn and practice.
Your project is as good as any other.

2) against what's "common knowledge" , duly fueled by parts vendors, in general "sound quality" does not depend on "parts quality" but on amp design/project.

You may replace a cheap 50 cents generic Chinese pot with a same resistance $35 sealed, Aerospace/Military rated Bourns pot and sound will be exactly the same.

Yes, if left unused for a coupled years in a damp garage or basement, the standard one will probably be scratchy while the Bourns found in a Vietnam cave since 1972 may still be fine ... but that's not the point.

So go get reasonable commercial quality parts, as sold by, say, Mouser, and call it a day.

3) the single biggest and most significative upgrade will be a larger better speaker.

Only point which worries me a little is that having such a small speaker it *might* be very low powered, say 4 watts (which is enough for a bedroom but nowhere else) .

Open it , if possible post some guts picture, and check whether it has a chipamp , TDA2030 or similar is very popular, although it mignt have some "mystery" korean chip.
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Old 20th March 2015, 11:41 PM   #4
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Thanks for the thoughts guys. Yes, I totally understand I'm not going to stick a screwdriver in this thing and magically end up with a Fender Twin, but I'm just curious to see what I can get out of it. Plus I'm of a lo-fi indie persuasion anyway so if I end up with something that sounds a bit odd that's not necessarily a drawback!

I'm definitely going to try the push-pull solution, although 1meg push-pulls seem virtually unheard-of in the UK so it might be a while before I get to try it.

Definitely getting a bigger speaker too. There's no indication of what wattage the amp is, but the 5" one is pretty blown now so I should think something more substantial would work.

Here are a couple of pictures of the guts and the front panel (partly disassembled). Will be able to get some better ones next week. Click the image to open in full size.
Click the image to open in full size.
Click the image to open in full size.
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Old 21st March 2015, 01:04 AM   #5
JMFahey is offline JMFahey  Argentina
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O)K.
Upside down but better than nothing
The PCB states TDA2006 , but it has better and pin compatible TDA2030 ... fine.

I'm surprised that there is no screw or spring clip pushing ittight against the "heatsink" (bit of chassis) , did you pull any?
We are talking around possible 12/15W RMS here, not bad if driving a good speaker.

Please measure voltage to ground on pins 1/2/3/4/5 .
Be careful not to let test probe tip slip and short 2 pins.

You don't have space there for a larger speaker, so consider building a new cabinet out of some chipboard or whatever (or modify some plastic or wood box you find suitable) to hold a future 8" to 12" speaker and either transplant the chassis there or use it as a "head" by adding a speaker out jack.

You can be as creative as you wish, this was posted yestarday at SS Guitar:
Click the image to open in full size.

others commented on modifying dirt cheap yet nicely finished IKEA rectangular wooden cases meant to house plant pots; a week ago a Musician showed how he cut a combo such as yours and made a mini "Blackface Bandmaster" head , sort of, to drive an external 1 x 12".

The sky is the limit

EDIT:
read the full thread at SS Guitar, but just as a free sample, the Musician I mentioned started by replacing the speaker in his $20 Squier with a Jensen MOD 6" speaker which already improved it tremendously:

Click the image to open in full size.

and then went ahead with the full Monty:

Click the image to open in full size.

cute, isn't it?

and now completely exhilarated, he made a cabinet with .... you won't believe it .... :
Quote:
a 10" greenback and an 8" Jensen along with the 6.5" replacement Jensen
Click the image to open in full size.

for the full thread:

http://www.ssguitar.com/index.php?topic=870.0

Last edited by JMFahey; 21st March 2015 at 01:15 AM.
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Old 21st March 2015, 03:50 PM   #6
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Many thanks for all of the above - really helpful.

Yeah, I unscrewed the heat sink in the process of manoeuvring the pcb out of the cab so I could make a start on replacing the pots (half done at the moment). The pics were taken to remind me where everything goes.

The pics are perhaps a bit misleading as I've measured up and you could just about get an 8" speaker in there - or to really max things out, even a 10x8 elliptical (if such a thing exists). I'd quite like to keep the existing cab as it's nice and compact (well, compact anyway).
Quote:
Originally Posted by JMFahey View Post
Please measure voltage to ground on pins 1/2/3/4/5 .
Be careful not to let test probe tip slip and short 2 pins.
Aaaaaand that would be the limit of my technical knowledge reached. Sorry! Where are the pins you're referring to and why am I doing this?

Thanks also for the Ssguitar link. That looks like a forum I may find myself spending rather a lot of time poking around in...
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Old 22nd March 2015, 08:38 PM   #7
JMFahey is offline JMFahey  Argentina
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Quote:
Originally Posted by marksturdy View Post
Yeah, I unscrewed the heat sink in the process of manoeuvring the pcb out of the cab
OK, that was worrying me.
Be aware that besides the grease, there is also a thumbnail sized piece of insulating mica there, don't forget it.

Quote:
The pics are perhaps a bit misleading as I've measured up and you could just about get an 8" speaker in there - or to really max things out, even a 10x8 elliptical (if such a thing exists). I'd quite like to keep the existing cab as it's nice and compact (well, compact anyway).
Well, there is such a thing as elliptical 6"x9"speaker, unfortunately most of them meant for car radios, not really suitable for guitar.

The Jensen MOD 8 in 4 ohms would provide most bang for the buck, even better than the MOD 6 used in the example above.

Quote:
Aaaaaand that would be the limit of my technical knowledge reached. Sorry! Where are the pins you're referring to and why am I doing this?
Sorry for not being clear, I meant the TDA2030 pins, measure each of them with the black probe touching chassis/ground and the red one the pin, on the 20V DC scale (if there's more than 20V DC switch to 200VDC scale, but I doubt it).
Click the image to open in full size.

The reason?

OK !!!!! OK !!!!!!!! I'll CONFESS !!!!!!!!!!!
Click the image to open in full size.

that will let me know what kind of circuit you have and estimate actual power out, ok?????

Last edited by JMFahey; 22nd March 2015 at 08:41 PM.
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Old 23rd March 2015, 12:25 AM   #8
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Haha, thank you. I should be able to get those readings later in the week.

I had been wondering about an Eminence Beta 8 as a replacement speaker. Is 4 ohm the way to go then? No markings whatsoever on the current 5" that might indicate what it is.
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Old 24th March 2015, 01:54 PM   #9
shanx is offline shanx  Canada
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I am surprised a bit that the amp does not have a switch already in the circuit to go from clean to overdrive.
JMFahey, you did a effects loop mod on a practice amp posted on other forum which I thought was VERY cool, useful option. Simple but effective!
On the SP 10 (head version) you posted pictures of, gave me the idea of pulling the Overdrive switch off the board and rewiring so that a relay could be operated through a front switch or remote foot switch. If Markstudy's amp works like most, I think it would be possible.
Sadly we don't have a reference schematic on this amp...but maybe more pictures and a bit of digging around?
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Old 24th March 2015, 05:56 PM   #10
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More photos / hopefully voltage readings to follow tomorrow. If the existing circuit had facility to add a switch in that would be the dream - while I'm happy to put in a push-pull, it's proving an absolute nightmare to find one of the correct specification...
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