Twincam Posted May 11, 2016 Share Posted May 11, 2016 Changed the main filter caps in my wem dominator bass 25 as one was burst. Has had hardly any effect on the noise. Has tightened the sound up a bit but think I preferred the knackered caps haha. But the amp does sound great I think. Noticed a slight red plating on a valve so will replace the cathode bias resistor and the cap next. Back to the buzzing Im not sure where next to check I've had a look at the earth points and they seem fine. Maybe tidy the wiring although it's pretty much as it left the factory is a very clean amp never messed with (till now). Valves maybe (all original mullards), I messed about with the three pre amp valves swapping them about and checking the channels I noticed one of the pre valves sounded a bit different when swapped about. No difference to the buzz. So maybe the power valves? Going to check the jacks too. Any other suggestions? I would like to record with the amp and it would just be way to much noise for that at present. Quote Link to comment Share on other sites More sharing options...
3below Posted May 11, 2016 Share Posted May 11, 2016 My next port of call would be adding a centre tap / humdinger to the heater circuit (assuming it does not have one). Straightforward to implement. [url="http://www.valvewizard.co.uk/heater.html"]http://www.valvewizard.co.uk/heater.html[/url] Quote Link to comment Share on other sites More sharing options...
Twincam Posted May 11, 2016 Author Share Posted May 11, 2016 [quote name='3below' timestamp='1462994298' post='3047719'] My next port of call would be adding a centre tap / humdinger to the heater circuit (assuming it does not have one). Straightforward to implement. [url="http://www.valvewizard.co.uk/heater.html"]http://www.valvewizard.co.uk/heater.html[/url] [/quote] Unless I'm mistaken and I could be but it has one already. The heater wiring really needs looking at and tidying the more I look at it. I am sure it's the bias though since one valve is red plating slightly. And its recommend to change the resistor to a higher value one. Quote Link to comment Share on other sites More sharing options...
Twincam Posted May 11, 2016 Author Share Posted May 11, 2016 Success I've re did the heater wires. So much quieter now and almost there, actually I've known some modern ss amps that are worse! Still needs the bias sorted hopefully it will quieten it that little further. And then it's serviced and sorted. Learnt a lot more than I knew previously , feel like I've accomplished something Quote Link to comment Share on other sites More sharing options...
mikeydee Posted May 12, 2016 Share Posted May 12, 2016 If it is the amp I am thinking of it is a class a output stage with two valves. I would expect that if one of the valves is red plating then so would the other as they are connected to the same cathode bias resistor. Reasons why this wouldn't be the case are the valves are not will matched or perhaps if the input coupling caps are leaking. Worth checking for dc on the grids? Lot of strain on the cathode resistor and cap. If that is dried up then the amp will hum. By the way hum and buzz are different sounds and mean different fault conditions. Sound clips can really help. Quote Link to comment Share on other sites More sharing options...
Beer of the Bass Posted May 12, 2016 Share Posted May 12, 2016 Regarding the red-plating, a good quick check would be to swap the position of the two EL84s. If the same valve still red-plates, it may well be a valve issue. If the red-plating stays with the same socket, it could be a poor connection on the socket or one of the issues Mikey points out. Quote Link to comment Share on other sites More sharing options...
Twincam Posted May 12, 2016 Author Share Posted May 12, 2016 [quote name='mikeydee' timestamp='1463038001' post='3047930'] If it is the amp I am thinking of it is a class a output stage with two valves. I would expect that if one of the valves is red plating then so would the other as they are connected to the same cathode bias resistor. Reasons why this wouldn't be the case are the valves are not will matched or perhaps if the input coupling caps are leaking. Worth checking for dc on the grids? Lot of strain on the cathode resistor and cap. If that is dried up then the amp will hum. By the way hum and buzz are different sounds and mean different fault conditions. Sound clips can really help. [/quote] Yes I forgot that it shared the bias. Regarding humming and buzzing I don't like either term as one persons buzz is another's hum. But in this case it was definitely a 60hz buzz or is that a hum? I've ordered new components and a few different size resistors and will check the bias accordingly when they arrive. 1 Quote Link to comment Share on other sites More sharing options...
Twincam Posted May 12, 2016 Author Share Posted May 12, 2016 More issues Now the sound isn't right at all a bit of distortion to a lot depending on string that been struck. Tried my girlfriends mustang bass athrough it and the sound was terrible. Tested the valves in either socket one looks like it's red plating slightly but also has a scratch or a crack in the glass. So looks like a pair of valves are needed. Quote Link to comment Share on other sites More sharing options...
3below Posted May 12, 2016 Share Posted May 12, 2016 It will be worth it when sorted There is something very satisfying having kit that can be self repaired, something that will increasingly be less possible. Quote Link to comment Share on other sites More sharing options...
mikeydee Posted May 12, 2016 Share Posted May 12, 2016 Sounds like a great project by the way. Inn terms of sounds we could do with some reference tones but my take is that. . . . hum is 50 - 120hz and sounds like hmmmmmm Buzz is a mixture of frequencies and sounds like buzzzzzzzz often picked up from airborne sources like fluorescent tubes, dimmers and fridges but also picked up on mains like my dimmer light at home Hiss is high and sounds like hisssssssss And then there is Clicks Pops Crackle That's enough Quote Link to comment Share on other sites More sharing options...
HowieBass Posted May 12, 2016 Share Posted May 12, 2016 (edited) I think I read somewhere that the frequency of the hum identifies it as coming from either rectified or non-rectified sources (100Hz or 50Hz) which can help you track down the problem. My old WEM Westminster 15 suffers with a hum problem; I suspect some of it comes from the AC supply on the pre-amp valve heater circuit (so without modification it'll never be that quiet) and possibly also some from the wiring arrangement. EDIT: You might get some help from this WEM Owners Club website http://wem-owners.com/ Edited May 12, 2016 by HowieBass Quote Link to comment Share on other sites More sharing options...
Twincam Posted May 12, 2016 Author Share Posted May 12, 2016 I just hope new tubes sorts it. After the new caps, cleaned the valve sockets, tidying the ac heater wires, checking the grounds, new power amp tubes (still to come) and new cathode bias resistor and cap (hopefully arriving tomorrow). My current knowledge will be exhausted. Should I of replaced the rectifier diodes? I actually paid top whack for the amp as it is in great condition. Quote Link to comment Share on other sites More sharing options...
HowieBass Posted May 12, 2016 Share Posted May 12, 2016 From what you've said it sounds like you've done pretty much everything you can, replacing the usual components that can suffer through ageing. I'm aware that some valve amps will be prone to hum because of their design (with an AC supply to the heater circuit as I mentioned). Hopefully some valve amp/WEM experts can assist here... Quote Link to comment Share on other sites More sharing options...
mikeydee Posted May 12, 2016 Share Posted May 12, 2016 Yep UK mains hum is 50hz and will occur from pickup on previous amp valve heaters. This minimised through twisted pair wiring. 100hz noise is what is left on the power supply after rectification and smoothing. In a push pull or class ab amp which is most amps with multiple valves this imperfection is cancelled out by the push pull circuit. In a class a it is left there. The cathode bypass caps job is to filter it out and on the first preamp too. Replacing that cap is inexpensive and worth doing Quote Link to comment Share on other sites More sharing options...
Twincam Posted May 13, 2016 Author Share Posted May 13, 2016 [quote name='mikeydee' timestamp='1463091773' post='3048616'] Yep UK mains hum is 50hz and will occur from pickup on previous amp valve heaters. This minimised through twisted pair wiring. 100hz noise is what is left on the power supply after rectification and smoothing. In a push pull or class ab amp which is most amps with multiple valves this imperfection is cancelled out by the push pull circuit. In a class a it is left there. The cathode bypass caps job is to filter it out and on the first preamp too. Replacing that cap is inexpensive and worth doing [/quote] Never thought about the pre amp bias. Well if the power amp bias and the tube replacement doesn't work then I guess there's another area to look at. It's starting to give me nightmares. Quote Link to comment Share on other sites More sharing options...
3below Posted May 13, 2016 Share Posted May 13, 2016 Could be worth looking at the grounding layout. Merlin's book is a useful guide, online chapter here http://www.valvewizard.co.uk/Grounding.html. Some grounding techniques are subtle (certainly would not have been obvious to me) e.g. only connecting one end of shielded cables. Quote Link to comment Share on other sites More sharing options...
luckydog Posted May 13, 2016 Share Posted May 13, 2016 (edited) ^ +1 what Mikey says about replacing those small coupling caps on g1 of both output valves to fix redplate on one valve in that circuit. Shouldn't redplate, that's worth sorting and chances are the rest of the hum might go away too. Be sure to be use correct voltage rating of those replacement caps or err on the generous side - HTH ! LD Edited May 13, 2016 by luckydog Quote Link to comment Share on other sites More sharing options...
Twincam Posted May 13, 2016 Author Share Posted May 13, 2016 [quote name='luckydog' timestamp='1463122925' post='3048717'] ^ +1 what Mikey says about replacing those small coupling caps on g1 of both output valves to fix redplate on one valve in that circuit. Shouldn't redplate, that's worth sorting and chances are the rest of the hum might go away too. Be sure to be use correct voltage rating of those replacement caps or err on the generous side - HTH ! LD [/quote] It's a shared cathode bias. So one valve must be duff as the other is fine. Quote Link to comment Share on other sites More sharing options...
luckydog Posted May 13, 2016 Share Posted May 13, 2016 Twincam, the caps which couple to the grids (not the cathodes !) can go electrically leaky and increase the bias significantly on just one valve...... Those caps are between the anodes of the preceeding stage and the grids of the output valves, and typically have a couple of hundred volts of dc across them. If they become leaky by only a tiny current, that's enough to increase bias and cause redplating, typically in only one valve. As you say, if that cathode circuit has a fault, it affects the bias of both valves, and they would both tend to redplate. In case of mismatched valves, or even if one is stone dead or missing, one will be biased hotter, but not hot enough in principle to redplate, though you did say it was slight.....?! If the redplate persists with new valves and is always in the same socket, suspect the grid coupling caps ! HTH! LD Quote Link to comment Share on other sites More sharing options...
Twincam Posted May 13, 2016 Author Share Posted May 13, 2016 (edited) [quote name='luckydog' timestamp='1463141112' post='3048942'] Twincam, the caps which couple to the grids (not the cathodes !) can go electrically leaky and increase the bias significantly on just one valve...... Those caps are between the anodes of the preceeding stage and the grids of the output valves, and typically have a couple of hundred volts of dc across them. If they become leaky by only a tiny current, that's enough to increase bias and cause redplating, typically in only one valve. As you say, if that cathode circuit has a fault, it affects the bias of both valves, and they would both tend to redplate. In case of mismatched valves, or even if one is stone dead or missing, one will be biased hotter, but not hot enough in principle to redplate, though you did say it was slight.....?! If the redplate persists with new valves and is always in the same socket, suspect the grid coupling caps ! HTH! LD [/quote] I get you. I've tried swapping the tubes the red plating follows the valve so I don't think there's an issue there. But could be something else to look into. The list is growing! Update soldered in the new power amp bias resistor and cap. No relating at all and the sound is slightly better. There is still the distortion and crackling every now and then, output is still odd. Anyhow that's one more job down. Get the pre valves tomorrow I hope maybe the power tubes too if not Monday I will know if it needs more work. Edited May 13, 2016 by Twincam Quote Link to comment Share on other sites More sharing options...
Recommended Posts
Join the conversation
You can post now and register later. If you have an account, sign in now to post with your account.