I have my macbook hooked up to the monos in my dorm room but unfortunately one channel hums like the dickens. the two amps, when cables are hooked to them and they're on, hum a bit (especially the right channel) and when I rattle the cables, the cable noise carries to the speakers. It's eerie.
When I wave the ends of the cables near a florescent light fixture, crazy buzzing fills both channels. I use an adapter to hook the RCA's to the headphone out jack on a macbook and when I plug it in, the right channel has some audible hum. I built both monoblocks pretty much identically so I have no clue what the problem is. I do know this: When I plugged the same cable into a different (IBM) laptop, there was no hum problem.
What could I try doing? is it an impedance problem? I kind of have to play my music louder than usual to get past the hum and during quiet passages there's no helping it. anyway, thanks for the advice.
When I wave the ends of the cables near a florescent light fixture, crazy buzzing fills both channels. I use an adapter to hook the RCA's to the headphone out jack on a macbook and when I plug it in, the right channel has some audible hum. I built both monoblocks pretty much identically so I have no clue what the problem is. I do know this: When I plugged the same cable into a different (IBM) laptop, there was no hum problem.
What could I try doing? is it an impedance problem? I kind of have to play my music louder than usual to get past the hum and during quiet passages there's no helping it. anyway, thanks for the advice.
Hey soren,
sorry to hear about your amp woes. I can only think of a few things that might help...
- check the ripple voltage on all the power supply rails. a class A driver stage has only about 6dB of PSRR
- make sure the heater center tap is attached to a fixed low impedance potential (i.e. ground)
- make sure no cold solder joints on high impedance points like grids, etc.
is your mac-book connected to any outet? could be a ground loop hum ...
sorry to hear about your amp woes. I can only think of a few things that might help...
- check the ripple voltage on all the power supply rails. a class A driver stage has only about 6dB of PSRR
- make sure the heater center tap is attached to a fixed low impedance potential (i.e. ground)
- make sure no cold solder joints on high impedance points like grids, etc.
is your mac-book connected to any outet? could be a ground loop hum ...
Anatech,
Thanks for your post #39. Reassuring; I am at present sticking to 6L6EH.
Sorenj07,
I presume cables mean interconnects? Really sounds like something wrong with the cable screen, but then you say they work in a different set-up. Connectors either side firm and OK? It is certainly strange that you have cable noise in the way you describe. It seems to indicate a very high feed impedance, in other words no proper connection at the macbook side. Also when the cable ends pick up florescent lamp noise, do you mean open ends? That could be normal.
I am not familiar with the connectors on a macbook, but it certainly sounds that you may have a (mechanical) problem there somewhere. Without being there, it is difficult to give more specific advice ... (cable to another amplifier if available?)
Thanks for your post #39. Reassuring; I am at present sticking to 6L6EH.
Sorenj07,
I presume cables mean interconnects? Really sounds like something wrong with the cable screen, but then you say they work in a different set-up. Connectors either side firm and OK? It is certainly strange that you have cable noise in the way you describe. It seems to indicate a very high feed impedance, in other words no proper connection at the macbook side. Also when the cable ends pick up florescent lamp noise, do you mean open ends? That could be normal.
I am not familiar with the connectors on a macbook, but it certainly sounds that you may have a (mechanical) problem there somewhere. Without being there, it is difficult to give more specific advice ... (cable to another amplifier if available?)
unfortunately, reversing the cables made no difference, since the issue with cable noise and the florescent light happen when the RCA cables connected to the amp aren't connected to anything else. I guess I'll have to find a soldering iron somewhere and reflow a few joints. Is there a chance that the 500V B+ on the phase splitter, and the high B+ on the earlier stages is stressing the tubes' cathode insulation?
chased down the problem by unplugging various things from my laptop. i think i got a nasty ground loop to just one of the monoblocks, somehow, through the following path:
Right channel amp -> RCA cable -> headphone in on Macbook -> mini-DVI to DVI cable to external monitor -> grounded monitor plug -> amp ground
in any case, the monitor doesn't seem to really need a ground so I used some pliers to snap the pin off. It's a plastic-covered 19" that really shouldn't pose much of a threat. now, both amps are quiet as a baby. I'm happy!
Right channel amp -> RCA cable -> headphone in on Macbook -> mini-DVI to DVI cable to external monitor -> grounded monitor plug -> amp ground
in any case, the monitor doesn't seem to really need a ground so I used some pliers to snap the pin off. It's a plastic-covered 19" that really shouldn't pose much of a threat. now, both amps are quiet as a baby. I'm happy!
Unfortunate new development: the right channel monoblock has always hummed a bit more, but recently, after letting it drop back down a little too quickly after looking underneath it, I saw the 5U4GB rectifier flash a bit of white from the tiny holes for the cathode at the top. Hum began to steadily increase until I switched it off. "Damn", I said. (loose translation). I switched out the 5U4GB from the other working amp and tried again. The amp started up OK, except I saw continual tiny sparks through the mica in what would be the region between plate and cathode. It didn't look healthy so I shut it down again.
At this point I should remark that the rectifier in this right channel amp has been making kind of a mechanical vibratey noise for a while.
What do I do? I ordered a couple NOS matched 5R4GYB's (straight glass envelope) to replace these 5U4GB's but I suspect that the problem is with the amp and not the rectifier. I built both pretty much exactly the same so I'm having real problems figuring out what's the matter. Even worse, I have neither a soldering iron nor a DMM at the moment, but I DID notice some soft, cloudy bluish glow inside the 6L6WXT's of the right amp. I haven't looked inside the left. If my 6L6's are gassy somehow (maybe the bumpy car trip to college did it), would it mess with the rectifier in this way? I'm hesitant to sub the left channel's 6L6's in because I don't have any tools to bias them at the 55mA point.
I will bug my parents to send my DMM and soldering iron (maybe there's a cold solder joint somewhere?) and maybe I can take a few resistor measurements. I worry that maybe my choke shorted and now the 5U4 is getting beaten up having to deal with the 100uF cap right after it. Help!
At this point I should remark that the rectifier in this right channel amp has been making kind of a mechanical vibratey noise for a while.
What do I do? I ordered a couple NOS matched 5R4GYB's (straight glass envelope) to replace these 5U4GB's but I suspect that the problem is with the amp and not the rectifier. I built both pretty much exactly the same so I'm having real problems figuring out what's the matter. Even worse, I have neither a soldering iron nor a DMM at the moment, but I DID notice some soft, cloudy bluish glow inside the 6L6WXT's of the right amp. I haven't looked inside the left. If my 6L6's are gassy somehow (maybe the bumpy car trip to college did it), would it mess with the rectifier in this way? I'm hesitant to sub the left channel's 6L6's in because I don't have any tools to bias them at the 55mA point.
I will bug my parents to send my DMM and soldering iron (maybe there's a cold solder joint somewhere?) and maybe I can take a few resistor measurements. I worry that maybe my choke shorted and now the 5U4 is getting beaten up having to deal with the 100uF cap right after it. Help!
another note: I popped the rectifier which had been in the left (working) channel back into the right channel and powered up that monoblock. Not as much sparking but there was definitely a lot of blue cloudiness in the rectifier which I hadn't seen when it was running in the left amp. The 6L6s' glow also was a lot more pronounced, with more bluish stuff inside the plate structure, and the blue on the inside of the envelope had a much more purplish tinge. I'm a bit confused. There was also a lot more hum. Could drifting bias cause these conditions?
edit: I'm thinking of picking a quad of new JJ KT77's to sub in, I think it might be worth it to run the output tubes a little easier/within their specs... 500VB+ 55mA per KT77 into a 6.6K a-a load should be all right, right?
edit: I'm thinking of picking a quad of new JJ KT77's to sub in, I think it might be worth it to run the output tubes a little easier/within their specs... 500VB+ 55mA per KT77 into a 6.6K a-a load should be all right, right?
My dear Soren,
You do have the troubles all working as a relay team!
Replying from the hip (I have not back-checked), but is the power supply filter a choke input? It sounds like that from your 2nd last mail. In that case you may be unfortunate enough to have high voltage spikes generated every time a rectifier section "switches off". These can be 1 - 2KV, and it is advised that one connects a 10K resistor in series with a 22nF/1.5KV capacitor over the choke terminals to swamp this. Some of the signs you mentioned reminds me of over-voltage between rectifier filament-plate. This situation can be checked with a scope, but under rather special conditions. A suitable high voltage probe is necessary. It can also be that too much current is being drawn from the rectifier. The latter case is easily checked: What is your h.t. voltage?
You will need a meter in a hurry! (And respecting forum policy, you will realise that lethal voltages exist where you are about to measure!). I can't presume as a first approach that both your rectifiers are faulty - so, check carefully and quickly, and try not to have the works on for longer than necessary while you hunt.
Good luck!
Edit: Simple question, but you could also have the 6L6s' bias disappearing, judging by the reported increase in bluish colour. Again: Measure! ... and pronto, before you have real damage.
You do have the troubles all working as a relay team!
Replying from the hip (I have not back-checked), but is the power supply filter a choke input? It sounds like that from your 2nd last mail. In that case you may be unfortunate enough to have high voltage spikes generated every time a rectifier section "switches off". These can be 1 - 2KV, and it is advised that one connects a 10K resistor in series with a 22nF/1.5KV capacitor over the choke terminals to swamp this. Some of the signs you mentioned reminds me of over-voltage between rectifier filament-plate. This situation can be checked with a scope, but under rather special conditions. A suitable high voltage probe is necessary. It can also be that too much current is being drawn from the rectifier. The latter case is easily checked: What is your h.t. voltage?
You will need a meter in a hurry! (And respecting forum policy, you will realise that lethal voltages exist where you are about to measure!). I can't presume as a first approach that both your rectifiers are faulty - so, check carefully and quickly, and try not to have the works on for longer than necessary while you hunt.
Good luck!
Edit: Simple question, but you could also have the 6L6s' bias disappearing, judging by the reported increase in bluish colour. Again: Measure! ... and pronto, before you have real damage.
yeah, it is a choke input. Here's my PSU for lack of any scanning software:
Hammond 282X (500-0-500 200mA)
NOS (not so N any more) Sylvania 5U4GB
Hammond 193J (10H 200mA 79 ohms measured)*
*there are two .22uF 1KV caps; one connected to the DC output of the rectifier, the other to the first capacitor, and each connected to each other and then with a wire to PSU ground (a Y-shape across the choke).
(I'm not sure the rest of the PSU matters, but here goes: the cap after the 193J choke is a 100uF 330VAC, which is B+, then there's one more choke which feeds the 6SN7 drivers, and two separate RC's for the input and cathodyne splitter stages. )
Anyway, why would there be any type of over-voltage? Here's my idea: Some kind of voltage spike fried the little Y-network of caps across the choke and now they act as a straight wire, bypassing it. A cap-input filter would be putting out arund 700V across the OPT. Would a 700V (yikes!) B+ do something like this? Is it worth it to cut one end of this Y-network and see if things get back to normal?
Hammond 282X (500-0-500 200mA)
NOS (not so N any more) Sylvania 5U4GB
Hammond 193J (10H 200mA 79 ohms measured)*
*there are two .22uF 1KV caps; one connected to the DC output of the rectifier, the other to the first capacitor, and each connected to each other and then with a wire to PSU ground (a Y-shape across the choke).
(I'm not sure the rest of the PSU matters, but here goes: the cap after the 193J choke is a 100uF 330VAC, which is B+, then there's one more choke which feeds the 6SN7 drivers, and two separate RC's for the input and cathodyne splitter stages. )
Anyway, why would there be any type of over-voltage? Here's my idea: Some kind of voltage spike fried the little Y-network of caps across the choke and now they act as a straight wire, bypassing it. A cap-input filter would be putting out arund 700V across the OPT. Would a 700V (yikes!) B+ do something like this? Is it worth it to cut one end of this Y-network and see if things get back to normal?
as a matter of fact, ASC (the manufacturer) in another forum was reported to have rated their 330VAC caps at AT LEAST 660VDC so I think I'm OK in that respect. the full AC swing across the cap should translate to +-330VAC = 660VAC = 933V, so my measured B+ of around 500V should be all right.
sorenj07 said:
Sorenj07,
As a result of the mentioned "turn-off" spikes across the choke. When those are present and add to the instantaneous voltage over the rectifier plate - cathode, there could be more than the rated PIV.
But this "turn-off" spike thing does not occur as regularly as some design guidelines indicate. Many power supplies do not have them. They are more prone in a silicon diode rectifier case. On the other hand, a C and R are inexpensive enough as insurance. It is difficult to predict (even calculate) accurately to what extent such spikes will occur; it depends on the circuitry.
The two 0,22uF caps should take care of the matter, but some prefer the mentioned R-C directly across the choke. There was some comment on this matter some time ago on another thread; my experience is not thorough, but it seems a real enough threat to merit and inexpensive R-C, as said.
But to repeat, in your case an overload (even momentary) is also a very real possibility.
sorenj07 said:
Not quite the way it works, 330VAC is signless, (absolute value) does not equal 660VAC, but does equate to 466Vpk, however there is usually a coefficient they multiply this by to get the dc rating and that can range from 1X - 2X. This has a lot to with internal construction, particularly the lack of corona discharge issues at relatively low dc voltages as compared to ac, dielectric losses and all sorts of other things I know very little about.
I'm conservative, if the part does not have a specific dc voltage rating I try to avoid running it more than 10% above the equivalent Vpk voltage.
I agree with you though, I doubt that a 330VAC rated ASC running at 500V is going to be an issue and are running in the range I think reasonable for these. (I've had no problems at 450V - 500V with ASC caps.)
I'll bet those 6L6WXT are getting gassy and that is where some of your problems are coming from. As Johan has indicated there may be a number of other issues still plaguing your amps. A scope would be handy.
You could get a lot of help by joining the local tube circle (NNETG) - if interested ping me through my site.
A technique that helps if you suspect you are beating up your rectifier tubes is to install a silicon diode in series with the plate lead. You still get the soft turn off characteristics and warm up delay of the tube rectifier, but you also get some considerable extra PIV margin by virtue of the PIV rating of the diodes. I use 1KVPIV diodes minimum and have not lost a rectifier to arcing since I did this. In critical applications you can install equalizing resistors across the diodes and rectifiers - I have never needed to do this.
(Technique makes 5U4 live at >1KV ouch!)
(Technique makes 5U4 live at >1KV ouch!)
Hi Sorenj07,
What can I say? Never run capacitors above their rated voltage.
You must look at peak voltages, rms voltage has no meaning for the dielectric. Peak voltage may punch through. BTW, I don't care how well regarded the part is. It is only rated for "X" voltage.
No offense Kevin, I don't agree with you here.
There are a number of things happening here. Once you overheat a tube, out gassing may occur from the metal components, or a track may get burned into the mica. Doesn't take much. If you had a problem with the bias or a connection, this could ruin an output. Unbalanced current in the outputs will create a hum. You may have damaged your rectifiers.
Removing the choke will probably help destroy a rectifier. Consult a tube manual for the maximum rated capacitance. You will find that 100 uF is well beyond any reasonable expectation. Do not do this. What I would try first off is to replace your 0.22 uF caps, or at least disconnect them. That is a no cost option.
Can you post a schematic of your power supply, or have you done that earlier?
-Chris
What can I say? Never run capacitors above their rated voltage.
You must look at peak voltages, rms voltage has no meaning for the dielectric. Peak voltage may punch through. BTW, I don't care how well regarded the part is. It is only rated for "X" voltage.
No offense Kevin, I don't agree with you here.
There are a number of things happening here. Once you overheat a tube, out gassing may occur from the metal components, or a track may get burned into the mica. Doesn't take much. If you had a problem with the bias or a connection, this could ruin an output. Unbalanced current in the outputs will create a hum. You may have damaged your rectifiers.
Removing the choke will probably help destroy a rectifier. Consult a tube manual for the maximum rated capacitance. You will find that 100 uF is well beyond any reasonable expectation. Do not do this. What I would try first off is to replace your 0.22 uF caps, or at least disconnect them. That is a no cost option.
Can you post a schematic of your power supply, or have you done that earlier?
-Chris
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.