I had posted in the thread about KT90s that was discussing distortion.
http://www.diyaudio.com/forums/tubes-valves/76740-kt90s-anyone-usingem-4.html
About that time I realized the answer on distortion vs cathode current should be obvious.
Check the plate curves for the linear region. If we are up near the max plate dissipation curve the bias looks most linear. As we lower the bias the curves become more compressed as the grid bias goes further negative. Therefore, more current IS better.
This is the trend I've seen for most tetrodes and small signal Pentodes like 6AQ5.
Go for 70% max bias and don't worry about it. If you want your tubes to last longer, reduce the filament voltage 10%, reduce bias current below the 70% max point and expect greater harmonic distortion for class A operation PP.
Most of the distortion will be even order, and a lot will cancel as a result of PP operation.
http://www.diyaudio.com/forums/tubes-valves/76740-kt90s-anyone-usingem-4.html
About that time I realized the answer on distortion vs cathode current should be obvious.
Check the plate curves for the linear region. If we are up near the max plate dissipation curve the bias looks most linear. As we lower the bias the curves become more compressed as the grid bias goes further negative. Therefore, more current IS better.
This is the trend I've seen for most tetrodes and small signal Pentodes like 6AQ5.
Go for 70% max bias and don't worry about it. If you want your tubes to last longer, reduce the filament voltage 10%, reduce bias current below the 70% max point and expect greater harmonic distortion for class A operation PP.
Most of the distortion will be even order, and a lot will cancel as a result of PP operation.
Replacing transformers
Hi everybody. Sorry to interrupt your discussion. Yesterday I was testing a raw speaker that I freshly reconed and I used my Meng Mini 6P1 to perform the test. To do so, I connected the speaker on right channel and I left the left channel with no load (no speaker or resistor connected).
As I know now, I should not have do that, and the result is that I probably fried something in it. Burning smell and crunchy sound in left channel*. In the first place I was able to play at low volume but since I crank it up the crucnching sound came back and the volume drops.
*I think it's only in left channel, because as soon as I heard the crunching, I turned off the amp to spare my speakers but I'm not shure.
It's probably a burned transfo. So my question is: Which transfos I can use to replace them (output and power) Hammond mfg. or Edcor. What are the specs I need. I'm in North America, so my input is 110 Volts.
I'll probably take the occasion to upgrade the caps as well.
Thank you!
Hi everybody. Sorry to interrupt your discussion. Yesterday I was testing a raw speaker that I freshly reconed and I used my Meng Mini 6P1 to perform the test. To do so, I connected the speaker on right channel and I left the left channel with no load (no speaker or resistor connected).
As I know now, I should not have do that, and the result is that I probably fried something in it. Burning smell and crunchy sound in left channel*. In the first place I was able to play at low volume but since I crank it up the crucnching sound came back and the volume drops.
*I think it's only in left channel, because as soon as I heard the crunching, I turned off the amp to spare my speakers but I'm not shure.
It's probably a burned transfo. So my question is: Which transfos I can use to replace them (output and power) Hammond mfg. or Edcor. What are the specs I need. I'm in North America, so my input is 110 Volts.
I'll probably take the occasion to upgrade the caps as well.
Thank you!
I have a pair of Hammond 1609s that sound very good when driven by my 6P1P amp that is similar to the Meng Yue Mini.
I also have a set of Edcor 5K 30W transformers (I will change to load to present 10Kp-p), but haven't tried them yet for comparison. Both manufacturers are well spoken of, but there seems to be a slight nod towards the Edcor transformers from what I've read.
My understanding is that either should be an upgrade to the the transfomrers in the Meng Yue Mini.
In addition to the transformer you may have dammaged your output tubes so they should be tested and replaced if there is any doubt.
If you replace the transfomrers I'd like to borrow the originals to do some testing once you have them out.
I also have a set of Edcor 5K 30W transformers (I will change to load to present 10Kp-p), but haven't tried them yet for comparison. Both manufacturers are well spoken of, but there seems to be a slight nod towards the Edcor transformers from what I've read.
My understanding is that either should be an upgrade to the the transfomrers in the Meng Yue Mini.
In addition to the transformer you may have dammaged your output tubes so they should be tested and replaced if there is any doubt.
If you replace the transfomrers I'd like to borrow the originals to do some testing once you have them out.
OK, thank you for quick answer. The Hammond 1609 was them I had in mind. Do you think my power transformer might be damaged too? What voltage output I need? I was looking at the Hammond 269AX, do you think it will match the 1609s ? When transfos will be replaced, I will be glad to send them to you, as long as you pay for shipping.
Looking at the Meng yue Mini schematic I don't see a fuse (I recommend adding one).
However the 51 Ohm series resistors in the pi filter input of B+ should have helped protect the power transformer.
How do they look? One burnt badly and one OK? Both OK?
The Meng Yue Mini uses a Voltage Doubler circuit to generate B+. If you change to another transformer you will need to rework the power supply.
The Hammond 263AX is only rated for 250VCT at 115mA. That is only 28mA per tube at full load on the transformer along with the triodes pulling another mil or two. I think the Hammond 263AX will be fully loaded and run hot if you use it.
Figure you need 36ma times four for the output tubes, plus a few mA for the driver. You need about 150mA at 250V+. The Hammond 270CAX might be a better choice. 500V CT @81mA gives you half wave of 250V @162mA. That at least gives you a little margine so the transformer does not run so hot.
I've got the 270FX with is reated 550CT @173mA (mine is only labled 150mA). Half wave rectified with the CT at ground I can get over 250mA out of it at 280V. This is too high fo rthe 6P1, but I'm abusing my 6P1Ps-EV.
Speaking of which, which tubes are you running? Do you still have the Chinese 6P1s or are you using Russian 6P1P/B-/EV Tubes?
However the 51 Ohm series resistors in the pi filter input of B+ should have helped protect the power transformer.
How do they look? One burnt badly and one OK? Both OK?
The Meng Yue Mini uses a Voltage Doubler circuit to generate B+. If you change to another transformer you will need to rework the power supply.
The Hammond 263AX is only rated for 250VCT at 115mA. That is only 28mA per tube at full load on the transformer along with the triodes pulling another mil or two. I think the Hammond 263AX will be fully loaded and run hot if you use it.
Figure you need 36ma times four for the output tubes, plus a few mA for the driver. You need about 150mA at 250V+. The Hammond 270CAX might be a better choice. 500V CT @81mA gives you half wave of 250V @162mA. That at least gives you a little margine so the transformer does not run so hot.
I've got the 270FX with is reated 550CT @173mA (mine is only labled 150mA). Half wave rectified with the CT at ground I can get over 250mA out of it at 280V. This is too high fo rthe 6P1, but I'm abusing my 6P1Ps-EV.
Speaking of which, which tubes are you running? Do you still have the Chinese 6P1s or are you using Russian 6P1P/B-/EV Tubes?
None of the transfo have apparent damage. I may let the power transfo in place and try replacing the the output ones so I will have no mod to do. And yes I replaced the original tubes by the Russian 6P1P/B-/EV
As you can notice, I'm a newbie in the amplifier DIY, I used to build and design speakers and crossovers but tubes and electronics are new to me. For now I have only basic knowledge but I'm very well at soldering and to follow schematics. So, if I decide to go along replacing all the transfos, including the power one, will you be able to do provide me the changes I will have to do on the power supply? And where to install the fuse to protect the power transfo ?
Thank you.
Thank you.
Even if you want to change the power transformer, I'd recommend changing the output transfomrers first, and making sure everything was working properly again before changing the power transformer.
One thing you will have to be careful of in changing the OPTs is phasing of the fedback signal. If wrong, it is easy to correct by swapping the two transformer to plate connections at the tube sockets.
Adding the fuse is pretty straight forward. It goes in series with the hot wire from the power cord. Do you have a three wire plug with green ground? If not, you might want to change your power cord at the same time. Good safety precaution.
Once you have it back up and running we can discuss power transformer/power supply changes if you choose to go that route.
I'll have to do a power budget for the filaments to make sure the replacement transformer can handle all the tubes.
I just noticed another safety issue with the power supply. There is no bleeder resistor across the B+. I recommend adding a 100K 1/2W resistor across each 220uF/200V capacitor in the input of the power supply. These resistors are a safety device to bleed the charge off the storage capacitors when power is turned off.
The time constant of the two resistors in series, in parallel with the bulk capacitors is effectivly 200K * ((220/2)+ 2*180uF)=47 Seconds. It will take six times that to discharge the capacitors 99%, so figure 5 minutes from the time you turn off power till the circuits are dead.
It is best to use shorting clips with a 22K 5W resistor to discharge the capacitors once you unplug the power cord to work on the amp. This only takes anbout 25 seconds to discharge the caps. Once they are discharged the safety resistors keep them from self charging back up. Be sure to remove the 22Kresistsor before plugging th e amp back in.
One thing you will have to be careful of in changing the OPTs is phasing of the fedback signal. If wrong, it is easy to correct by swapping the two transformer to plate connections at the tube sockets.
Adding the fuse is pretty straight forward. It goes in series with the hot wire from the power cord. Do you have a three wire plug with green ground? If not, you might want to change your power cord at the same time. Good safety precaution.
Once you have it back up and running we can discuss power transformer/power supply changes if you choose to go that route.
I'll have to do a power budget for the filaments to make sure the replacement transformer can handle all the tubes.
I just noticed another safety issue with the power supply. There is no bleeder resistor across the B+. I recommend adding a 100K 1/2W resistor across each 220uF/200V capacitor in the input of the power supply. These resistors are a safety device to bleed the charge off the storage capacitors when power is turned off.
The time constant of the two resistors in series, in parallel with the bulk capacitors is effectivly 200K * ((220/2)+ 2*180uF)=47 Seconds. It will take six times that to discharge the capacitors 99%, so figure 5 minutes from the time you turn off power till the circuits are dead.
It is best to use shorting clips with a 22K 5W resistor to discharge the capacitors once you unplug the power cord to work on the amp. This only takes anbout 25 seconds to discharge the caps. Once they are discharged the safety resistors keep them from self charging back up. Be sure to remove the 22Kresistsor before plugging th e amp back in.
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Thank you for all those good advices. I'll first change the OPT's as you suggest. My power cord have a ground on it, when I received the Mini P1, the ground tab of the socket was not connected to the amp frame, I corrected it since then. There's a fuse in this socket so I guess that's the one missing on the shematics and the same you was expecting to see.
Make sure of the dimensions of the OPT before ordering one, as it has to fit under the cover.
Gary
Gary
Oh, and the fuse is usually built into the line socket. It's a part of the IEC socket, and there is normally a spare fuse in there as well. It's not shown on the diagrams on this thread, as they are not official diagrams.
If you haven't already read :
"On Line Tube Learning for newbies...."
and
"Safety Practices, General and Ultra-High Voltage"
Please do so before undertaking any electrical changes to the amp. Even though the Meng is a low power amp, it has potentially lethal voltages present and you need to be well aware of the risk associated with HV before working on the amp.
"On Line Tube Learning for newbies...."
and
"Safety Practices, General and Ultra-High Voltage"
Please do so before undertaking any electrical changes to the amp. Even though the Meng is a low power amp, it has potentially lethal voltages present and you need to be well aware of the risk associated with HV before working on the amp.
In looking at the Hammond transformer, I think the Filament supply will be insufficient.
You need 4 6P1P-EVs at 500ma each, and two 6N2Ps (or 6N1Ps depending on schematic) at 500-600mA per tube for a total of 3.0-3.2A. The Hammond transformer is only good for 2.5A.
Best to stick with the original transformer unless a more suitable one can be found.
You need 4 6P1P-EVs at 500ma each, and two 6N2Ps (or 6N1Ps depending on schematic) at 500-600mA per tube for a total of 3.0-3.2A. The Hammond transformer is only good for 2.5A.
Best to stick with the original transformer unless a more suitable one can be found.
Thank you guys for your advices. TheGimp, I will read the documentation you suggest and rotaspec, the dimensions of the OPT is almost the same so it will probably fit well. I just ordered the transfos from Newark. Now, I just have to wait, they expect to send it march 24th. Woah! I hope they'll get them earlier. Anyway I have some speakers construction in progress to fill my spare time. I'll let you know as soon as I get the output transformers.
You might want to check around on the web. I got mine in stock at Antique Electronic Supply in about a week. I think they were less than $50 each plus shipping. I can't find the receipt right now.
Hi, I can't cancel my order from Newark but I still ordered a set of 1609 OPT from Antique Electronic Supply. I'll put the other set of OPT in my other Meng Mini that is still fully functionnal. It will be an upgrade.
Did AES confirm that they had them in stock?
I thought about getting a set of 8K p-p transfomrmers to compare to the 10K ones to see if I can tell any difference.
It looks like power will go down and distortion up (from the distortion vs power graph). To reduce distortion to minimum it looks like 11K p-p would be best. Power would also be max there.
I wonder if the plots were made with resistive loads on the plates, and represent ideal values or were they made actual transformer coupled outputs and realistic loads?
I thought about getting a set of 8K p-p transfomrmers to compare to the 10K ones to see if I can tell any difference.
It looks like power will go down and distortion up (from the distortion vs power graph). To reduce distortion to minimum it looks like 11K p-p would be best. Power would also be max there.
I wonder if the plots were made with resistive loads on the plates, and represent ideal values or were they made actual transformer coupled outputs and realistic loads?
My tubes finally came. Now to remember where I left off! Actually, in a jam as I can't do correct THD measurements as my brand e-mu seems to be defective. Waiting on feedback from them. Anyway, starting with the output, Russian tubes, LED CVS , tweak the bias, B+, add grid isolation. Then on to the input. Backwards of everyone else I guess.
This project is starting to last forever. Now I have my parts, I had to send my brand new e-mu 1616 back to Creative as it was defective from stock. I can't make the baseline distortion and noise measurements I want before I start. With how fast they seem to work, looks like a three week delay. Stopped my speaker projects too. About all I can do is build the led boards.
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