MarkBassChat

[quote name='squire5' timestamp='1319378692' post='1413308'] I've just tested the pickups on my Washburn XB600 as the neck p/up seemed much weaker than the bridge.Readings from my multi meter are thus: Bridge p/up - 16.2.Neck p/up - No reading at all.My questions: Is the zero reading a sign that the p/up is duff?[/quote]No reading at all means infinity and it's exactly oposite to zero. Zero means shorted, infinity means open. I assume you have no readings (which means infinite resistance). [quote name='squire5' timestamp='1319378692' post='1413308'] If so,how can it give me an output through my amp?[/quote]The easiest way to check a pickup is to tap on it with a scewdriver. If you hear the sound in the amp, the pickup is OK and you are not measuring it correctly. [quote name='squire5' timestamp='1319378692' post='1413308'] And secondly,is the 16.2 (not sure what the units are.Kohms?) reading normal for what I believe are Active P/ups?Many thanks. [/quote]I don't think that this bass has active pickups. This bass has a preamp (active electronics) but this does not mean that the pickups are active. Especially that with an active pickup you cannot measure output resistance with multimeter (at least in a simple way). I can only guess that you have passive pickups. Mark

Mart, Of course you are right. This was a missunderstanding from my side. I wanted to show that there are piezo bridges for Warwick basses. But, as you mentioned, the piezo element is not under the saddle, it's immediatelly under the string. It was explained very well by Ikay. You can have either piezo transducer in acoustic style wooden bridge (this is used in some fretless basses), or separate piezo transducers for each string. I think that piezo film transducer will not work under the saddle because it's to delicate. I also want to add a piezo trancducer but I have a different bridge. I'm working on a piezo buffer for the bridge. Here: [url="http://www.thomann.de/gb/abm_3710g_p_single_bass_piezo_g.htm"]http://www.thomann.de/gb/abm_3710g_p_single_bass_piezo_g.htm[/url] is another example of a piezo transducer but again this is a different type than required. Mark

[quote name='Peaty' timestamp='1319206240' post='1411320']So does anyone know if it possible to get a piezo system under this type of bridge?[/quote]Yes, you can check it here: [url="http://www.warwick.de/modules/produkte/produkt.php?submenuID=14667&katID=00000014478&cl=EN"]http://www.warwick.d...000014478&cl=EN[/url] You may also type in google: "bass bridge piezo", or "Warwick bass bridge piezo". [quote name='Peaty' timestamp='1319206240' post='1411320']Can it be made to work or does it need to be under a more acoustic friendly design of bridge.[/quote]It will work with any solid body bass guitar. [quote name='Peaty' timestamp='1319206240' post='1411320']Is the bridge the best place to put a piezo sensor.[/quote]Yes, it is. [quote name='Peaty' timestamp='1319206240' post='1411320'] What sort of circuit is requited to convert the output to something that can go to a standard base amp.[/quote]You need a piezo buffer that converts high impedance of the piezo transducer to lower input impedance of a typical bass amp. You may also look for a preamp that mixes the piezo signal with standard magnetic pickups, like this one: [url="http://www.bestbassgear.com/bartolini-preamps-piezo-buffer.htm"]http://www.bestbassg...iezo-buffer.htm[/url] [quote name='Peaty' timestamp='1319206240' post='1411320']What brands/manufactures are worth looking for this sort of application.[/quote]See google results. Mark

Being frankly I wouldn't replace anything without checking what is wrong with the bass. Spector has a very loud preamp (TonePump or TonePump Jr). If this is not caused by an old battery, I would check pickups first - you can connect them directly to the output Jack, and then the preamp (is there a gain trimmer? is it in the minimum position?). If you cannot find anything, I would contact Spector for help. In several similar cases I've seen, only in one case the problem was related to the preamp (which couldn't be fixed). I all other cases there were problems with weak battery, broken wires, or output Jack. If you fail to find a solution, I would buy Aguilar OBP-3: [url="http://www.aguilaramp.com/products_preamps_obp3.htm"]http://www.aguilaramp.com/products_preamps_obp3.htm[/url] . In any case I wouldn't spend any money before checking which actually component failed. Mark

[quote name='geoffbyrne' timestamp='1318521894' post='1403298'] This amp cost me £22, Mark - is it worth it? G.[/quote]Yes, it is. The reverb spring alone is worth £22. So you have the rest of the amp for free. The only problem is that most probably you cannot fix it on your own. So you have to find out how to do it cheap. Check the output transistors as advised and then decide what to do later. Doesn't any of your friends know anything about electronics? I could guide him what to do. You could also start from providing the symbol of the amp and also looking for the schematic of the amp. You can also ask for advise (and the schematic) on this forum: [url="http://music-electronics-forum.com/f22/"]http://music-electronics-forum.com/f22/[/url] Mark

G. In 90% of cases it's rather the transistors that are shorted and only in 10% (or less) you can blame the bridge rectifier. It can be asily checked with a cheap multimeter and I advise you not to replace it without checking it first. I was trying to suggest to take the amp to someone who knows how to check the transistors and the brigde. The problem is that if the transistors are shorted, most probably few more components have failed (e.g. driver transistors or resistors). And even if you replace the output transistors (only), the amp may still not be working. This would be a waste of time and money. It looks like not very complicated problem but still some technical knowledge is required. Can you make a photo of the output transistors (on the heatsink), maybe some problems will be visible? Mark

[quote name='deanbean502' timestamp='1318455141' post='1402550'] short on transformer windings [/quote]No, only the internal fuses blow. It means that this is either the bridge rectifier or output transistors. Most often this is caused by shorted output transistors (as already mentioned by Subthumper). Can be easily fixed but you need to have some tools, meter and parts to be replaced. Mark

I was asking MoonBassAlpha whether he is sure about the switching power supply in MB150 (and not you whether you know what amp you have ). This amp is quite old and it was manufacured in several versions. I'm refering to the latest version: Series III. This one didn't have switching power supply (and most probably the older ones also didn't have it). Take a look at the power supply in this amp (see attachment). It's just typical power supply. The power amp uses class H topology (switched rails) - this is mentioned in the manual and also on Wikipedia (about Gallien-Krueger). Regarding the problems that you have I'd advise to identify whether the problem is with the preamp or the power amp. In each amp you have SEND/RETURN effect loop. So you can for example connect SEND output from one amp to RETURN input in the other amp. In this way you will be able to identify if the problem is related to preamp or power amp. You can also connect both preamps to external mixer and see what signal levels you get. In the TE I would check whether the pot that was replaced was linear or logarithmic. This is a typical mistake when replacing pots. I think that in GP12 both pots (input and output) were linear but different values: 500k and 50k. And the results could be exactly as you describe. Of course there may be several other reasons (e.g. old valve in the preamp). Mark

[quote name='MoonBassAlpha' timestamp='1317987811' post='1396977'] I'm pretty sure my luthier has an MB150. When I was last over there he told me some interesting things about it. His is an old one, so this [b]may not apply to yours. [/b] They have a switched-mode power supply that can be problematic. He is an electronic engineer by trade and took it to a fformer colleague who is a guru. After searching out the schematics, he said it's a surprise it works. The design looks like it was done by an inexperienced grad and had some fundemental design flaws. Short of it is, he substantially redesigned it for him and it is now working fine. [/quote]Are you sure that it was MB150? It does not have switch mode power supply. What is special about this amp is that it has +/-32V and +/-60V double power supply rails which are switched to the higher one when the signal if high enough. This is called Class-G (you can read about it here: [url="http://en.wikipedia.org/wiki/Electronic_amplifier"]http://en.wikipedia.org/wiki/Electronic_amplifier[/url] ) and it is quite popular way to get more power with less heat. So the rails are switched but the amp does not have switching power supply (which works completely different). This still can be difficult to fix if the amp fails. Mark

[quote name='Quoth'd' post='1354797' date='Aug 28 2011, 11:25 PM']I have already replaced the speakers (last post was a follow up from a month or two ago) and am having the same problem. Contour is off & all tone controls are set mid way.[/quote]So you already spent a lot of money and the problem is still there. The proper way to do it is to find out what is wrong and only then start spending money :-). [quote name='Quoth'd' post='1354797' date='Aug 28 2011, 11:25 PM']Running a tone generator into it produces a clean wave at 100hz & 1khz. The sine wave from the bass is good both at low and high frequencies. The problem only seems to occur when playing low frequencies (bottom string) through the amp.[/quote]As I said, you need to verify the amp in test environment (generator->amp->dummy load->oscilloscope). If you get clean sinewave at full power (I repeat: at full power), there is nothing wrong with the amp. You may test it at 40-80 Hz. You have to understand that signal from bass guitar is not a sinewave. And tone controls modify the signal even more. Also I'm not sure whether middle position in GK corresponds to "neutral" frequency response. It seems to me that some GK amps are exceptions to this rule. Have you already verified the amp with another cabinet (more powerful than the one you have now)? If none of this ideas help, I think it's time to take the amp to a tech. He will know how to verify the amp. Mark

[quote name='Quoth'd' post='1353916' date='Aug 27 2011, 11:02 PM']The output from my bass has a symmetrical sinewave[/quote]I doubt it. A symmetrical sinewave sounds like cheap keyboard. Can you scope it and post a photo? [quote name='Quoth'd' post='1353916' date='Aug 27 2011, 11:02 PM']so I know it's being induced somewhere in the amp. It gets tricky as if I use a tone generator then I get a symmetrical sinewave coming out of the amp.[/quote]The proper way to check the amp is to use a tone generator and a dummy load. The signal looks like this because the input is not sinusoidal wave and maybe tone controls are not set to the middle position. The contour filter has to be disabled. Before you start replacing speakers you have to verify the amp. If it's OK, you can verify the amp with external cab (you can borrow it instead of buying new speakers). Mark

I changed my mind and I agree with SilentFly. At least I understand the problem with 2 transducers. Maybe he couldn't explain it well (for me) but if you imagine a piezo transducers as a voltage source with some impedance, one piezo is feeding the other through its impedance. And these two transducers have their source impedance connected in series (this info was missing). So on the output you get V/2 voltage. And there are no two potentials (at the same time). One voltage is with one transducer, and the other is when you connect two transducers (or more) in parallel. However, in real world it's not a problem. Piezo transducers have such a strong signal that it is a common practice to connect them in parallel. The problem with the voltage divider is solved by FET buffers (separate buffer for each transducer). So if you want to do it correctly, you have to make a buffer for each transducer. But this is a huge cost. So you may solder transducers in groups (each 6) and then mix the groups. I'm sure SilentFly knows how to do it. Mark

I cannot argue with the Thévenin's theorem . Today I installed a piezo bridge in a bass guitar (4 transducers connected parallel). And the signal was so strong that I had to use a voltage divider on the output . Otherwise the bridge was overdriving the preamp. Of course an active buffer will sound better because it causes that very high output impedance of the transducer "is seen" by the following preamp stage as a low impedance (which e.g. allows to use longer wires). Mark

I had a similar problem with my Warwick Thumb. And I had to replace the pot. It was not that expensive. MEC part are expensive but not that much as listed by Lemmy. You can check it directly at Warwick shop: [url="http://shop.warwick.de/index.php?cat=c93_Potentiometer.html"]http://shop.warwick.de/index.php?cat=c93_Potentiometer.html[/url] What costs 68Euro (and this is not £70) is a module (pot + printed circuit board + sockets). The pot itself costs (only ) 18 Euro. This is very far to £70. And I bought this pot (but I don't remember where) for 8-10 Euro. Of course you need to unsolder the pot from the printed board (which is not that easy because the is double-side board). I did this without any problems (but you may need a hot air gun). Anyway, since the parts are so expensive, you have to be sure that the pot is causing problems. Mark

Chuck, With most problems that you have I would go to a luthier. But regarding the sound of G string (especially with treble boosted), I think that the gain of the preamp is set to high and you just overdrive the preamp. Can you check whether there is a little trimmer in the preamp, and if there is one, just turn the gain lower. If not, decrease the gain of the amp (and increase volume). Mark

Dan, I'm not sure what you mean by piezo-mics. Is it a set of piezo transducers e.g. for a drum kit, or is it for a guitar? Connecting piezo transducers in parallel is widely used by piezo bridge manufacturers. If you look at this: [url="http://www.shopping.com/Graph-Tech-Graph-Tech-Ghost-LB-63-Floyd-Bridge-with-Piezo-Pickups-Gold/info"]http://www.shopping.com/Graph-Tech-Graph-T...ckups-Gold/info[/url] you will see that 6 piezo transducers are connected in parallel and the output goes to a preamp. There are two conditions when you do it: the wires have to be very short (15 cm max), and the input impedance of the preamp has to be very high (e.g. 10 MOhms). Otherwise it will sound bad. There are no such problems like described above (one transducer vibrating cause another transducer vibrate), or voltage drops down. Voltage divider is when you have two components in series and here you have 6 components in parallel. Make a search on piezo buffers or Ghost piezo bridges. Some companies make buffers where every piezo element has a separated input (e.g. Ghost - for a MIDI preamp) but ,as you can see with other Ghost bridges, it is not required. Just two days ago I installed a piezo bridge in my colleague's guitar and it sounded great - almost like an acoustic guitar. But remember what is needed: short wires and high impedance buffer. Mark

Simple test is to use it with another power supply. If the problem stays the same, it needs to be fixed. In this case it requires some manual skills since it uses surface mount components. Mark

[quote name='motoxross' post='1162950' date='Mar 15 2011, 03:13 PM']Can anyone give me advice on why my SWR Working pro 400 may be blowing fuses, normally when first switched on. My set up is Warwick Corvette $$ in to a Boss TU3, EBS multicomp, SansAmp bass driver, SWR Working Pro 400, SWR Goliath Senior cab. The only thing I can think of is the the order in which the equipment is turned on. Does this affect amp fuses?[/quote]You are joking, right? There may be several reasons but none of them is related to the order in which the equipment is turned on. You have either incorrect fuses (check with the manual), or they are FAST instead of SLOW, or there are shorted output transistors in the amp and it has to be fixed. Another possible reason is shorted power supply. Mark

[quote name='Nostromo']Hi Mark, . . . . is that your amp ? If so, have you ever had any problems with yours ? . . . . .[/quote]No, it's not mine but I heard of several guys who do not have any problems with this amp. [quote name='Nostromo']For info, I've just found a couple of old obscure posts (90's) saying that some of the Rumble Bass Amps had duff valve sockets and that otherwise unfathomable problems could sometimes be resolved simply by upgrading the valve sockets ?[/quote]I wouldn't believe in everything what is on the internet. This is a high power amp and the valve sockets are the components that get a lot of heat. And they sometimes can even get desoldered. On your place I would check how the power valves sockets look like and whether they ar still soldered correctly. The airflow in this amp could be a little bit stronger (what about a more powerful fan?) but first you have to fix the amp. You could compare the airflow of the fan in this amp with a fan in similar amp - Ampeg SVT-CL (I have somewhere data for this fan). And yes, in SVT-CL the fan directs airflow directly over the power valves. This may be an important difference. [quote name='Nostromo']Havent checked out whether pre or power side problems as I dont know how to do that ?[/quote]This is simple, next time when you have problems with the amp (so maybe right now) just plug a bass into POWER AMP INPUT socket on the rear of the amp. There will be still noise if the problems are caused by the power amp. But if the are caused by a preamp (this is less likely), there will be no noises. You can also plug a Jack-Jack cable into EFFECTS SEND and RETURN - often dirty sockets cause strange nioses. Mark

Can you post a photo of the amp from the back? I assume that the fan is working. Have you checked whether the problem is with the preamp or the power amp? Mark [attachment=74790:FenderRumble2.jpg]

[quote name='Nostromo' post='1160421' date='Mar 13 2011, 04:07 PM']Psycho Bass Guy who is a regular contributor to the "Vintage Amps Bulletin Board" appears to be extremely knowledgeable on these amps, (unfortunately he's not in the UK) and he advises me that my Rumble Bass head probably needs a complete recap, including the small coupling caps, and probably could do to have all the critical resistors changed, too.[/quote]I don't think that this is a good advise. It is possible that some caps need to be replaced but I never do it, or advise other people do it, if I don't know the exact reason of the problems. When you have a problem with a car, you do not start replacing all parts, but you look for the source of problems. Exactly the same is with amps. Usually, problem is caused by a few components and you should first find out which components need to be replaced. Replacing everything is the worst possible advice you can give to someone. Your problems look like related to cold soldering points (posibly related to heat - read: valves). In such cases I also do not advise to resolder everything - you have to find out exactly what needs to be fixed. ANd only then you can think whether you need to replace some caps. In your case it would be quite easy to determine (at least) whether the problem is related to the preamp or to the power amp. Have you checked it? You can simply test it by plugging in the bass into the power amp in (when the problems appear again). I could fix it but I'm also not located in UK. Try with StevesAmps. Mark

[quote name='StevieD_FenderP2009' post='1157306' date='Mar 10 2011, 11:53 PM']I tried that over a month ago just so i had one in time but haven't had a reply. It really sucks.[/quote]This is strange because I asked them for the diagram and got it after 3 hours. Send me your e-mail and I will forward the diagram to you. This is for Vol+Vol but this is easy to change it to Vol+Blend. Mark

On their Web site: [url="http://www.spectorbass.com/accessories/tonecontrol.html"]http://www.spectorbass.com/accessories/tonecontrol.html[/url] it says that the preamp comes with a wiring diagram. Even if you hadn't purchased the preamp from Spector shop, you can still e-mail them and ask for the diagram. Mark

You have three options: 1. Find original 16 Ohms speakers. They are difficult to find but if you search for Trace Elliot service in UK, you will find a guy who most probably will be able to help you. When I contacted him a year ago, he had horns used in TE 1225. 2. Buy old 8 Ohms speakers and convert them to 16 Ohms (sevral companies can do it - and it's not expensive). 3. Fit 8 Ohms speakes but then you will not be able to use an extension cab. I would personally go for the option #2. Mark

I would also fix it with WD40 (it helps in most cases). If not, just write down the symbol of the fan and search it in google. You will know what is the price. Mark