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/>
First time poster and rookie tinkerer here, so apologies for any noob questions, breaches of etiquette, general stupidity etc.<br />
<br />
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 />
<br />
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 />
<br />
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 />
<br />
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 />
<br />
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/>
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.
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.
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.
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.
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.
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.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
An externally hosted image should be here but it was not working when we last tested it.
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:
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:
and then went ahead with the full Monty:
cute, isn't it?
and now completely exhilarated, he made a cabinet with .... you won't believe it .... :
for the full thread:
http://www.ssguitar.com/index.php?topic=870.0
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:
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:
An externally hosted image should be here but it was not working when we last tested it.
and then went ahead with the full Monty:
An externally hosted image should be here but it was not working when we last tested it.
cute, isn't it?
and now completely exhilarated, he made a cabinet with .... you won't believe it .... :
An externally hosted image should be here but it was not working when we last tested it.
for the full thread:
http://www.ssguitar.com/index.php?topic=870.0
Last edited:
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).
Thanks also for the Ssguitar link. That looks like a forum I may find myself spending rather a lot of time poking around in...
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).
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...
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.
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.
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).
The reason?
OK !!!!! OK !!!!!!!! I'll CONFESS !!!!!!!!!!!
that will let me know what kind of circuit you have and estimate actual power out, ok????? 😉
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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.
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.
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?
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?
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...
Well, it may be a simple enough thing. If the Overdrive Pot is a 1 Meg, we pretty much need to figure out if its acting as a variable resistance (in a feedback loop of op-amp using diode clipper) or varying the signal as an attenuator (volume control) before the fixed clipping circuit. So depending on the case it may be simple enough to wire in a separate switch. Even a push/pull switch is a separate switch assembly piggy backed on a standard pot, so there is no reason not to go with a standard pot, and just wire the switch in the circuit. We'll see if we can figure it out from pictures, and maybe a description of how your amp controls work now.
Just guessing, but 1 Meg normally would be a feedback pot for the opamp gain, as most solid state amps don't use 1 Meg for volume, more like 10 or 20 K for that.
As promised, some better shots of the guts:
Pots l-r are reverb (10k), lo (100k), mid (100k), hi (100k), volume (10k) and overdrive (1m).
And the underside:
An externally hosted image should be here but it was not working when we last tested it.
Pots l-r are reverb (10k), lo (100k), mid (100k), hi (100k), volume (10k) and overdrive (1m).
And the underside:
An externally hosted image should be here but it was not working when we last tested it.
Ok... having replaced all the pots and connected the thing back up, it seems the treble pot is now an AM radio. It didn't do that before. Bad soldering?
Hi Markstudy, It's possible that you have a bad soldering on the treb pot, it may be open circuit on one of the pads. Or if you powered it up without bolting the board back into the chassis, you may not have a good signal ground.
Note you should always have that power amp screwed on the heatsink if you are running it, (with the insulator in place). It could blow quite fast if not heatsinked.
Thanks for the pics. It's not a bad PCB, given the fact that there are components values printed on there, instead of R1, R2 , C1 C2 etc
The pic showing underside of board: you notice that the 1M Overdrive pot has the middle and the right hand side pads connected? that means it is setup as variable resistance. Good.
Likely it is varying the resistance of the negative feedback loop of the op-amp responsible for ''overdrive''.
To do a temporary test, solder wires to the 1M pot, one to the left pad and another to the middle pad (while looking at the underside of the board) and on the other end of the wires solder in a simple on -off switch. With the switch open circuit, the overdrive pot will work normally. With the switch closed, it should short the pot's resistance to zero, as if the overdrive is set at minimum position. Let us know how it goes.
If you get that result, then you are on the ''right track''.
Note you should always have that power amp screwed on the heatsink if you are running it, (with the insulator in place). It could blow quite fast if not heatsinked.
Thanks for the pics. It's not a bad PCB, given the fact that there are components values printed on there, instead of R1, R2 , C1 C2 etc
The pic showing underside of board: you notice that the 1M Overdrive pot has the middle and the right hand side pads connected? that means it is setup as variable resistance. Good.
Likely it is varying the resistance of the negative feedback loop of the op-amp responsible for ''overdrive''.
To do a temporary test, solder wires to the 1M pot, one to the left pad and another to the middle pad (while looking at the underside of the board) and on the other end of the wires solder in a simple on -off switch. With the switch open circuit, the overdrive pot will work normally. With the switch closed, it should short the pot's resistance to zero, as if the overdrive is set at minimum position. Let us know how it goes.
If you get that result, then you are on the ''right track''.
Ah, the bolts - the one thing I skipped when fastening the thing back up. I'll tighten them up and see what happens.
Didn't skip that though!
Indeed - that's been very useful when sourcing pots.
Thanks - I will try this (and the chassis screws) (and getting the volt readings as previously suggested) next week. Sounds promising!
I should explain that I only really have chance to work on this on Wednesday evenings, so if I'm ever slow to report back then that's why. I'm really appreciative of the advice on this board so far.
But just a minute - wouldn't shorting the pot to zero mean maximum overdrive, not minimum? Or am I missing something?
Hi Markstudy
Great, it sounds like you have things under control. For the Overdrive pot, the middle pad is the wiper, so just imagine turning the wiper clockwise , it is going toward the side of the resistance that it is shorted to, away from the other side pad that it is not shorted to, that is the varying resistance increasing towards maximum. If we turn counterclockwise to minimum the wiper is decreasing resistance down to zero.
If you close the switch you will install, it will be zero ohms across those points, and as if you turned the pot fully counterclockwise, (but leaving the pot as in any position you set it).
If you look at a Tube Screamer schematic, the overdrive circuit here is likely the same configuration. Increase that pot resistance and you get more gain for the overdrive, decreasing it decreases the gain. Note at minimum there is another resistor in series with the pot on the board, that sets the minimum gain- and that is usually below the ''clipping threshold'' of the overdrive.
Have a read of this article by RG Keen for an idea of how the TS works. If you look at a lot of amp designs in solid state you likely will run into that basic configuration.
The two 1N4002 glass diodes that are on the far right side of your board (set diagonally, top view) are probably the clipping diodes.
The Technology of the Tube Screamer
Good weekend reading before you tackle next steps..
Cheers
Great, it sounds like you have things under control. For the Overdrive pot, the middle pad is the wiper, so just imagine turning the wiper clockwise , it is going toward the side of the resistance that it is shorted to, away from the other side pad that it is not shorted to, that is the varying resistance increasing towards maximum. If we turn counterclockwise to minimum the wiper is decreasing resistance down to zero.
If you close the switch you will install, it will be zero ohms across those points, and as if you turned the pot fully counterclockwise, (but leaving the pot as in any position you set it).
If you look at a Tube Screamer schematic, the overdrive circuit here is likely the same configuration. Increase that pot resistance and you get more gain for the overdrive, decreasing it decreases the gain. Note at minimum there is another resistor in series with the pot on the board, that sets the minimum gain- and that is usually below the ''clipping threshold'' of the overdrive.
Have a read of this article by RG Keen for an idea of how the TS works. If you look at a lot of amp designs in solid state you likely will run into that basic configuration.
The two 1N4002 glass diodes that are on the far right side of your board (set diagonally, top view) are probably the clipping diodes.
The Technology of the Tube Screamer
Good weekend reading before you tackle next steps..
Cheers
Id agree with shanx, that the pot is probably in the feedback loop. So min resistance = max negative feedback - = min gain = cleanest. In turning to min, its likely to be bypassing the diodes in the feed-back loop, also equating to the cleaner sound. As the pot resistance increases, less neg feedback, more gain, and the non-linearity of the diodes start to control the feedback and create a soft clipping.
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