Hi All,
So moving on from my last post where I wanted to Bias and Balance this 829b FU29 based amp, I now move on to my next problem fix / tweak.
So, I managed to get a good balance / bias on on the output tubes by measuring voltage drop across the 1ohm anode / plate resistors, I am running 32ma per anode (specs seem to state max of 40ma zero signal anode current), so in theory about 74ma total per tube. This seems to work well, the sound is great and everyone is happy. There is no red plating.
Now, when I ran this thing originally without messing around I noticed a whistle / whine / high frequency sound that kind of came and went, but mostly came once the system was warmed up. I put it down to poor balance between each half of tube, but now this is fixed, it still seems to be there, coming and going. It is very annoying, not massively loud but sounds a bit like one of those anti-cat devices that people put in their gardens.
I used a spectrum analyser on my phone and when the high pitch whine kicks in, it is comes in at pretty much 12.6khz, very pronounced on the analyser graph.
It appeared to only come out of one channel, and I moved around the preamp JAN WE 5670 tubes but not entirely sure if it followed. I have a feeling it might be one of the output tubes?
I notice one has more of a blue iridescence than the other - not much really. The camera shot attached actually makes the blue look a lot brighter than it really appears, it is quite a dark glow on the inside of the tube that cannot be seen by the eye unless all lights are pretty much off...
Does anyone have any idea if this could be a more fundamental problem with the system? I.e. Transformers or one of the resistors / caps in the unit?
I have on the way two NOS Westinghouse 829b's and two Raytheon 5670's. I have also ordered 4 * 1uf Mundorf evo oil caps to replace the coupling / output caps as I see that someone with far more knowledge than me has:
SM2YER Goran's Homepage
I am hoping that these upgrades will fix the issue. In the meantime I will try damper rings on the 1st stage 5670's and clear some cabling away from the case. I wonder if tube covers would help?
Also, could this be a form of RF interference or feedback within the system? Grounding issue perhaps?
So in my head the possible cause I could think of are as follows:
- Microphonic 1st / 2nd stage tubes (replacements coming)
- Gassy output tubes (replacements coming)
- Interference from an outside source, computer, lights etc
- Grounding problem (speakers grounded to chassis?)
- Internal wiring problem
- Bad caps / resistors - Originals are WIMA's but I heard sometimes these are fake? (Mundorf output caps coming)
- Dry Solder joints
- Oscillations?
- Internal RF feedback
Any thoughts or advice are really welcome - It is a great sounding amp and really packs a punch when opened up a little.
Many thanks!
So moving on from my last post where I wanted to Bias and Balance this 829b FU29 based amp, I now move on to my next problem fix / tweak.
So, I managed to get a good balance / bias on on the output tubes by measuring voltage drop across the 1ohm anode / plate resistors, I am running 32ma per anode (specs seem to state max of 40ma zero signal anode current), so in theory about 74ma total per tube. This seems to work well, the sound is great and everyone is happy. There is no red plating.
Now, when I ran this thing originally without messing around I noticed a whistle / whine / high frequency sound that kind of came and went, but mostly came once the system was warmed up. I put it down to poor balance between each half of tube, but now this is fixed, it still seems to be there, coming and going. It is very annoying, not massively loud but sounds a bit like one of those anti-cat devices that people put in their gardens.
I used a spectrum analyser on my phone and when the high pitch whine kicks in, it is comes in at pretty much 12.6khz, very pronounced on the analyser graph.
It appeared to only come out of one channel, and I moved around the preamp JAN WE 5670 tubes but not entirely sure if it followed. I have a feeling it might be one of the output tubes?
I notice one has more of a blue iridescence than the other - not much really. The camera shot attached actually makes the blue look a lot brighter than it really appears, it is quite a dark glow on the inside of the tube that cannot be seen by the eye unless all lights are pretty much off...
Does anyone have any idea if this could be a more fundamental problem with the system? I.e. Transformers or one of the resistors / caps in the unit?
I have on the way two NOS Westinghouse 829b's and two Raytheon 5670's. I have also ordered 4 * 1uf Mundorf evo oil caps to replace the coupling / output caps as I see that someone with far more knowledge than me has:
SM2YER Goran's Homepage
I am hoping that these upgrades will fix the issue. In the meantime I will try damper rings on the 1st stage 5670's and clear some cabling away from the case. I wonder if tube covers would help?
Also, could this be a form of RF interference or feedback within the system? Grounding issue perhaps?
So in my head the possible cause I could think of are as follows:
- Microphonic 1st / 2nd stage tubes (replacements coming)
- Gassy output tubes (replacements coming)
- Interference from an outside source, computer, lights etc
- Grounding problem (speakers grounded to chassis?)
- Internal wiring problem
- Bad caps / resistors - Originals are WIMA's but I heard sometimes these are fake? (Mundorf output caps coming)
- Dry Solder joints
- Oscillations?
- Internal RF feedback
Any thoughts or advice are really welcome - It is a great sounding amp and really packs a punch when opened up a little.
Many thanks!
Attachments
If you tap the valves with a Biro, they should stay quiet. If they are microphonic, they will ring. My bet is that there is no grid stoppers fitted on the output valves or the power supply is unstable. I find the optimum anode current is 25mA per anode, they run too hot otherwise. The rubber bands on the glass have no effect unless the valve is microphonic.
I have repaired many due to overheating and other problems.
I have repaired many due to overheating and other problems.
Thanks for your reply.
I do notice that tapping the first stage tubes results in a chime / thud, but it doesn't carry on reverbing / resonating, it doesn't seem to set off the squeal either.
I will readjust the tubes down to 25ma per anode.
With regard to the grid resistors, is this something I should check for and add if necessary?
Cheers,
Rich.
I do notice that tapping the first stage tubes results in a chime / thud, but it doesn't carry on reverbing / resonating, it doesn't seem to set off the squeal either.
I will readjust the tubes down to 25ma per anode.
With regard to the grid resistors, is this something I should check for and add if necessary?
Cheers,
Rich.
So adding approx 1k resistors to each grid (4 of them) might help?
I have loads of simple 1/4watt resistors knocking around so can add something. Thing is I don't know if there is something there anyway, however I suppose if I am only adding a 1k resistor it wouldn't cause a massive problem if there was already something there. I could parallel a bunch to get the value I want and bring up mere 1/4 watt resistors to something more like a watt - not that they would need to dissipate that sort of power??
Some of the schematics out there seem to think that there is a 3.3k resistor in line with the grid, however based on my experience with these supposed schematics of the M828a they are about as accurate as my attempts at painting the mona lisa.
I will look at dropping bias current and adding grid resistors tonight and see if that kills off this squeal.
Also, new tubes and output caps should arrive soon so will add those too.
Cheers for assistance - so many things can be done to incrementally improve on this amp.
I have loads of simple 1/4watt resistors knocking around so can add something. Thing is I don't know if there is something there anyway, however I suppose if I am only adding a 1k resistor it wouldn't cause a massive problem if there was already something there. I could parallel a bunch to get the value I want and bring up mere 1/4 watt resistors to something more like a watt - not that they would need to dissipate that sort of power??
Some of the schematics out there seem to think that there is a 3.3k resistor in line with the grid, however based on my experience with these supposed schematics of the M828a they are about as accurate as my attempts at painting the mona lisa.
I will look at dropping bias current and adding grid resistors tonight and see if that kills off this squeal.
Also, new tubes and output caps should arrive soon so will add those too.
Cheers for assistance - so many things can be done to incrementally improve on this amp.
Hi, well I think the high pitch squeal that I had went away when I got rid of a plasma TV I had in around the area. It may have been some sort of interference from the PSU on the TV that managed to set up some sort of resonance inside the amp.
I did add resistors to the inputs of the tubes, that didn't make any difference. I was using an isolation audio transformer as well on some equipment I connect to it that causes some Nasty ground loops - that works really well. Ground loops are killer on these things and some equipment doesn't play nice. A simple audio transformer works for killing any hiss from ground loops.
I did also put a filtering IEC socket on the thing, thinking that would help but it didn't. Since the plasma TV is gone, no more squeals! It might be worth you taking the amp out somewhere "clean" to see if that gets rid of it, if it does, then you know its the environment around it that's causing it...
Good luck and let me know how that goes!
I did add resistors to the inputs of the tubes, that didn't make any difference. I was using an isolation audio transformer as well on some equipment I connect to it that causes some Nasty ground loops - that works really well. Ground loops are killer on these things and some equipment doesn't play nice. A simple audio transformer works for killing any hiss from ground loops.
I did also put a filtering IEC socket on the thing, thinking that would help but it didn't. Since the plasma TV is gone, no more squeals! It might be worth you taking the amp out somewhere "clean" to see if that gets rid of it, if it does, then you know its the environment around it that's causing it...
Good luck and let me know how that goes!
Thanks! I'll start tinkering around with the basics, maybe reseat the tubes and try to check for oscillation and move the whole amp to a different place.
In my case, it seems that the whine appears after a while, probably after the amp has warmed up enough - usually takes 10 minutes or so. The whine is not that loud. You can hear it while there is no music going through but it is really annoying - very hard to "unhear it" when you notice it.
In my case, it seems that the whine appears after a while, probably after the amp has warmed up enough - usually takes 10 minutes or so. The whine is not that loud. You can hear it while there is no music going through but it is really annoying - very hard to "unhear it" when you notice it.
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