Paraphase adjustment measurement
Can someone please point on the picture where to put the multimeter probes to check AC balance after changing driver tubes - these are the guts of a Yarland FV34C but it's probably the same circuit as the Mini. What values are expected (I understand equal levels of AC to both power tubes in the channel, but how many V approx?)
Is it sufficient to play a steady 1KHz tone from the CD or is some more 'sophisticated' input required? (I posted this also on Yarland thread but this one seems to be much livelier)
Can someone please point on the picture where to put the multimeter probes to check AC balance after changing driver tubes - these are the guts of a Yarland FV34C but it's probably the same circuit as the Mini. What values are expected (I understand equal levels of AC to both power tubes in the channel, but how many V approx?)
Is it sufficient to play a steady 1KHz tone from the CD or is some more 'sophisticated' input required? (I posted this also on Yarland thread but this one seems to be much livelier)
You want a sine wave going into the amp, CD would be fine, or an MP3. A 1Khz tone should be OK. Best to use dummy loads instead of speakers, if you must use your speakers just turn up the volume a bit but not too much to hurt your ears or speakers.
The pot only adjusts the AC audio signal to ONE output tube, not both. The idea is to use the pot to adjust the AC voltage to one output tube to make it the same as the AC voltage to the other output tube. So the overall AC voltage is not critical, what you are aiming for is to make them the same. The ac voltages you measure should be steady and could be anywhere from 2 volts to 6 volts roughly, depending on the voltage of the input signal and the setting of the volume pot.
Where to measure - at the coupling caps or at pin 2 of each output tube (assuming they are EL84), so red lead to pin 2 or coupling cap, and black lead to ground. Its handy if you have a black test lead with a crocodile clip on it, you can attach it to a ground point before connecting mains power, and then you only need to hold the red probe, much safer. Make sure you set the multimeter to AC not DC volts. Beware of high voltages inside the amp. One slip of the probe can toast your amp (or yourself).
The pot only adjusts the AC audio signal to ONE output tube, not both. The idea is to use the pot to adjust the AC voltage to one output tube to make it the same as the AC voltage to the other output tube. So the overall AC voltage is not critical, what you are aiming for is to make them the same. The ac voltages you measure should be steady and could be anywhere from 2 volts to 6 volts roughly, depending on the voltage of the input signal and the setting of the volume pot.
Where to measure - at the coupling caps or at pin 2 of each output tube (assuming they are EL84), so red lead to pin 2 or coupling cap, and black lead to ground. Its handy if you have a black test lead with a crocodile clip on it, you can attach it to a ground point before connecting mains power, and then you only need to hold the red probe, much safer. Make sure you set the multimeter to AC not DC volts. Beware of high voltages inside the amp. One slip of the probe can toast your amp (or yourself).
Can someone please point on the picture where to put the multimeter probes to check AC balance after changing driver tubes - these are the guts of a Yarland FV34C but it's probably the same circuit as the Mini. What values are expected (I understand equal levels of AC to both power tubes in the channel, but how many V approx?)
Is it sufficient to play a steady 1KHz tone from the CD or is some more 'sophisticated' input required? (I posted this also on Yarland thread but this one seems to be much livelier)
I set mine using a spectrum analyzer. I guess tube and transformer variation is so great, the DC measurement method is just not as close as it should be. Put 1K in at about 1W output and adjust for minimum harmonics. Easy.
Hi all,
Can anyone help me?. My mini 6p1 has been in use for about 2 years, it’s ok but my Rogers Cadet3 sounds much nicer. So any how i found this thread and noted what was said about transformer volts rating and not having any chassis grounding.
Well I had a look and the transformer is indeed 220v my mains supply is 240v holy smoke batman!!. I’ve put 50R Wirewound Resistor on primary which dropped it down to about 226-223 much better, but ht is still up in the 280+volts has some thing cooked with the long term volts abuse? and more importantly what should I be looking for. Any help will be much appreciated.
Cheers Pat
Can anyone help me?. My mini 6p1 has been in use for about 2 years, it’s ok but my Rogers Cadet3 sounds much nicer. So any how i found this thread and noted what was said about transformer volts rating and not having any chassis grounding.
Well I had a look and the transformer is indeed 220v my mains supply is 240v holy smoke batman!!. I’ve put 50R Wirewound Resistor on primary which dropped it down to about 226-223 much better, but ht is still up in the 280+volts has some thing cooked with the long term volts abuse? and more importantly what should I be looking for. Any help will be much appreciated.
Cheers Pat
I use a small dropping resistor between one lead of the transformer and the heater string to keep my heater voltage close to 6.3V (but not below 6.1V) with old transformers that put out over 7V on the 6.3V winding with US mains that now are running about 10% higher than they did in the 60s.
In the case of the Stancor T-8401 this worked out to 0.47Ohm with two 6P1Ps and one 6N1P.
The standard used to be 115V +/- 10% and now it runs more like 125 +/- 5%.
In the case of the Stancor T-8401 this worked out to 0.47Ohm with two 6P1Ps and one 6N1P.
The standard used to be 115V +/- 10% and now it runs more like 125 +/- 5%.
Pat:
If you check back to where I started this thread, you will see where I had 283v on the anodes of the output valves and the sound was poor. With the original Chinese 6P1 valves, anything over 255v caused at least one of them to go into runaway (hence the poor sonics). I actually had the cathode voltage on one tube reach over 20 volts at one stage as the self-bias tried to rein in the increasing current.
56 ohms dropped the mains enough that I got 251v at the anodes and the valves stabilised.
If you are indeed getting over 250v on the anode pins (ref ground), increase the dropping resistor, either at the mains side (check your filament voltages first and if they are right then leave the mains resistor alone) or the two (usually) 51 ohm resistors that form part of the first HT filter.
That should improve the sound a lot. Other sonic improvements were detailed in the rest of the thread, but the next biggest is the conversion from paraphase to cathodyne phase splitter.
I've sold my Meng, so I can't offer too much more help.
Just got my scratch-built EL84PP amp running and it sounds fairly good. Just a little PS buzz in one channel to fix, and sort out NFB values and it will be ready to put in a nice case.
Gary
If you check back to where I started this thread, you will see where I had 283v on the anodes of the output valves and the sound was poor. With the original Chinese 6P1 valves, anything over 255v caused at least one of them to go into runaway (hence the poor sonics). I actually had the cathode voltage on one tube reach over 20 volts at one stage as the self-bias tried to rein in the increasing current.
56 ohms dropped the mains enough that I got 251v at the anodes and the valves stabilised.
If you are indeed getting over 250v on the anode pins (ref ground), increase the dropping resistor, either at the mains side (check your filament voltages first and if they are right then leave the mains resistor alone) or the two (usually) 51 ohm resistors that form part of the first HT filter.
That should improve the sound a lot. Other sonic improvements were detailed in the rest of the thread, but the next biggest is the conversion from paraphase to cathodyne phase splitter.
I've sold my Meng, so I can't offer too much more help.
Just got my scratch-built EL84PP amp running and it sounds fairly good. Just a little PS buzz in one channel to fix, and sort out NFB values and it will be ready to put in a nice case.
Gary
Hi All,
Got my Meng X1 6P1 6AQ5 the other day. There was some damage in shipping: the cage had come out of its fittings and the cage posts had scraped away paint over several areas on the surface. Both wood pieces had been knocked loose. When I plugged it up, all the tubes lit up and it sounded very nice...for about 15 minutes. Then it started having (at first) transient drop-outs in volume followed by continuous drop in volume and severe distortion. Now...when I turn it off and let it "rest" for an hour or so, then turn it back on, it repeats the above story: sounds fine for 10-15 minutes, then starts dropping out here and there, then fades to very low muffled/distortion....
Is this maybe a tube problem? I imagine they got quite a shaking on the trip. They all light and no cherrying, and when I tap each while on I can't detect any that are microphonic. I didn't pay for shipping insurance (exorbitant), so...now I guess I'm screwed.... Any thoughts?
Got my Meng X1 6P1 6AQ5 the other day. There was some damage in shipping: the cage had come out of its fittings and the cage posts had scraped away paint over several areas on the surface. Both wood pieces had been knocked loose. When I plugged it up, all the tubes lit up and it sounded very nice...for about 15 minutes. Then it started having (at first) transient drop-outs in volume followed by continuous drop in volume and severe distortion. Now...when I turn it off and let it "rest" for an hour or so, then turn it back on, it repeats the above story: sounds fine for 10-15 minutes, then starts dropping out here and there, then fades to very low muffled/distortion....
Is this maybe a tube problem? I imagine they got quite a shaking on the trip. They all light and no cherrying, and when I tap each while on I can't detect any that are microphonic. I didn't pay for shipping insurance (exorbitant), so...now I guess I'm screwed.... Any thoughts?
The problem with trying to diagnose this problem is that the X1 is a different beast to the Mini's that we have (or had, in my case). It has 8 6P1s in what I assume is parallel push-pull. That could be a blessing in disguise here, in that you can pull half the tubes out of the power stage to try and narrow the problem down. The difficulty is that we don't have a schematic to know which to pull.
Is the problem with yours confined to one channel, or both? If it's one channel it makes things easier as you have a reference side that works, so you can swap tubes from there into the bad side (keeping track of which ones so you can discard the bad one). Start by swapping the WHOLE set from one side to the other, and if the fault transfers there is a bad tube among them.
If it is on both channels, then either there is a MAJOR tube problem that is causing the power rails to drop, or a power supply problem itself, and to diagnose any further will REQUIRE a schematic and some test gear.
Careful here, Starne ...
As said above, but you might cause further damage while trying to find the problem. First check immediately whether either of the power tubes begin to glow red on anodes or screen grids. If so, cut power immediately. (This would mean lack of bias on the power tubes or internal shorts.) If you have any sort of multimeter (preferably two!), connect to the power tube bias and h.t. and watch. (Necessary safety there!) Having done that and perhaps other immediate voltage checks, one then knows that no tubes are probably in danger of self-destruction and one can then proceed per the above advice. Particularly power tubes operated without bias for only minutes will cause loss of emission.
Good luck!
As said above, but you might cause further damage while trying to find the problem. First check immediately whether either of the power tubes begin to glow red on anodes or screen grids. If so, cut power immediately. (This would mean lack of bias on the power tubes or internal shorts.) If you have any sort of multimeter (preferably two!), connect to the power tube bias and h.t. and watch. (Necessary safety there!) Having done that and perhaps other immediate voltage checks, one then knows that no tubes are probably in danger of self-destruction and one can then proceed per the above advice. Particularly power tubes operated without bias for only minutes will cause loss of emission.
Good luck!
From looking at close-up pics of the PCB in the X1 Mengyue, I'm pretty sure it uses the same cct diagram as the one discussed in this thread, see page 1 for a schem, except PPP. The cathode resistors are probably 2 x 260 ohm in series as per later models.
A friend bought a standard Mengyue the other day, I fitted a mains earth and checked voltages. Running on 248V mains here, heaters measured around 6.7V, B+ around 320V, around 20V across the 520 ohm cathode resistor load, Plate dissipation within spec but around 300V across the valve means 50V over spec on the max plate voltage. Seems to be running fine so far (a couple of weeks).
A friend bought a standard Mengyue the other day, I fitted a mains earth and checked voltages. Running on 248V mains here, heaters measured around 6.7V, B+ around 320V, around 20V across the 520 ohm cathode resistor load, Plate dissipation within spec but around 300V across the valve means 50V over spec on the max plate voltage. Seems to be running fine so far (a couple of weeks).
Thank you all very much...
...being rather retarded in the electronics area
, maybe the safest thing for me to do would be to just buy all new tubes...?
In checking around on Ebay, I see that they are not very expensive.
Does anyone have any guidance in that direction: 1) what kind of tubes to buy (brand, variants), and 2) do either sets need to be matched?
As I may've stated earlier: the amp sounds great for about 10 minutes, then starts dropping out, then goes to low, muffled distortion. Turn it off and let it "rest," then it repeats that narrative.
No cherry plating. Puts out a considerable amount of heat--is that normal?
Again, thank you all for your suggestions. I really do appreciate it.
...being rather retarded in the electronics area
, maybe the safest thing for me to do would be to just buy all new tubes...?In checking around on Ebay, I see that they are not very expensive.
Does anyone have any guidance in that direction: 1) what kind of tubes to buy (brand, variants), and 2) do either sets need to be matched?
As I may've stated earlier: the amp sounds great for about 10 minutes, then starts dropping out, then goes to low, muffled distortion. Turn it off and let it "rest," then it repeats that narrative.
No cherry plating. Puts out a considerable amount of heat--is that normal?
Again, thank you all for your suggestions. I really do appreciate it.
When it 'goes to muffled' try cranking the volume all the way up (& down & up several times) so check if it's going to 'break through' and get to normal * undistorted level - I once had a poor solder joint on one of the coupling caps so when as the amp heated up it was not conducting well but a stronger signal made it through.
Lots of heat is normal for tube amps.
Another possibility is loss of bias current through the output tubes.
This sounds like your bias is going low for some reason. You will get low output, and veiled distorted sound if the bias drops considerably (1/4 or less than design). I'd check the bias pots if it has any. Make minor changes to see if they have bad spots in them.
If you feel compitent to take measurements the following might shed some light on the subject, but you will need a schematic and you will need to identify some components and take measurements.
CAUTION!!! UNPLUG the amp and make sure the amp has had time to discharge the HV Power supply caps before doing the following!
If minor adjustment of the bias pots doesn't make any difference, it will be necessary to make measurements of the cathode resistor values and write them down, then power up the amp and take voltage measurements across the cathode resistors and calculate the bias current. (I=V/R) Measure the voltage across the cathode resistors when the amp first starts working, and again after it starts to drop out. If the values are significantly different (>10% change) then it looks like a bias issue. This can be caused by several factors including the bias pots (if present), loss of screen supply, with the screens going towards ground, and possibly other causes.
Where are you in the USA? If you are lucky, there might be someone nearby that could assist you.
Another possibility is loss of bias current through the output tubes.
This sounds like your bias is going low for some reason. You will get low output, and veiled distorted sound if the bias drops considerably (1/4 or less than design). I'd check the bias pots if it has any. Make minor changes to see if they have bad spots in them.
If you feel compitent to take measurements the following might shed some light on the subject, but you will need a schematic and you will need to identify some components and take measurements.
CAUTION!!! UNPLUG the amp and make sure the amp has had time to discharge the HV Power supply caps before doing the following!
If minor adjustment of the bias pots doesn't make any difference, it will be necessary to make measurements of the cathode resistor values and write them down, then power up the amp and take voltage measurements across the cathode resistors and calculate the bias current. (I=V/R) Measure the voltage across the cathode resistors when the amp first starts working, and again after it starts to drop out. If the values are significantly different (>10% change) then it looks like a bias issue. This can be caused by several factors including the bias pots (if present), loss of screen supply, with the screens going towards ground, and possibly other causes.
Where are you in the USA? If you are lucky, there might be someone nearby that could assist you.
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