Hi
I have a Ming Da headphone amp, now obsolete, that I have a few questions on.
I modded this amp a couple years ago, but then stuck it in a closet. Pulled it out this weekend, and played with it.
It is designed for one 12ax7 in front, then a pair of 6H6N tubes that drive a pair of output tranny's, single ended. The 6H6N each have their two sections tied together in parallel, so one tube is for one channel.
Mods I made were to lower the heater voltage which was a bit high, around 7V, and to rewire the 6H6N to a 5687 tube.
Back when I did this, I knew very little about tubes. Now I know just enough to be dangerous.
Last night I figured out I only had 1/2 of the heater wired up, so I fixed that and thought it sounded pretty good.
I traced out a schematic, and in looking at it I saw I didn't have a resistor on the cathode. So the cathodes on the 5687's are floating, except there is a 100uf cap from the cathodes to ground. Even I knew this didn't seem right.
Found some info here that was useful, like 10k load on the output tubes:
improving a mc66ae
And looked at the tube calcs here for some help in figuring things out
5687 tube calcs
I also found some pics that show a 330 ohm cathode resistor, and someone else that said it was 320 ohms, so I think that is what ming da put in.
I swear I didn't remove any resistors, but it is what it is lol.
I also think B+ is 290V or around there.
Plugging in those numbers into the 5687 tube curve, seems like ming da was going for a -5V cathode voltage, and 15ma of plate current.
I was thinking of going for -6V with a 430 ohm cathode resistor, at 14ma of plate current.
Does this make sense?
TIA
Randy
I have a Ming Da headphone amp, now obsolete, that I have a few questions on.
I modded this amp a couple years ago, but then stuck it in a closet. Pulled it out this weekend, and played with it.
It is designed for one 12ax7 in front, then a pair of 6H6N tubes that drive a pair of output tranny's, single ended. The 6H6N each have their two sections tied together in parallel, so one tube is for one channel.
Mods I made were to lower the heater voltage which was a bit high, around 7V, and to rewire the 6H6N to a 5687 tube.
Back when I did this, I knew very little about tubes. Now I know just enough to be dangerous.
Last night I figured out I only had 1/2 of the heater wired up, so I fixed that and thought it sounded pretty good.
I traced out a schematic, and in looking at it I saw I didn't have a resistor on the cathode. So the cathodes on the 5687's are floating, except there is a 100uf cap from the cathodes to ground. Even I knew this didn't seem right.
Found some info here that was useful, like 10k load on the output tubes:
improving a mc66ae
And looked at the tube calcs here for some help in figuring things out
5687 tube calcs
I also found some pics that show a 330 ohm cathode resistor, and someone else that said it was 320 ohms, so I think that is what ming da put in.
I swear I didn't remove any resistors, but it is what it is lol.
I also think B+ is 290V or around there.
Plugging in those numbers into the 5687 tube curve, seems like ming da was going for a -5V cathode voltage, and 15ma of plate current.
I was thinking of going for -6V with a 430 ohm cathode resistor, at 14ma of plate current.
Does this make sense?
TIA
Randy
The 6N6P heater is 6.3V, with no center tap.
The 5687 heater is 12.6V, and has a center tap.
Are the 5687 heaters wired correctly for this?
For a 6.3V heater supply, pins 4 and 5 should be tied together, and the heater supply should be connected to pins 4+5 and pin 8.
--
The 5687 heater is 12.6V, and has a center tap.
Are the 5687 heaters wired correctly for this?
For a 6.3V heater supply, pins 4 and 5 should be tied together, and the heater supply should be connected to pins 4+5 and pin 8.
--
Per the datasheet, you can connect the heaters on the 5687 in parallel with 6.3V.
I connected pin 8 (CT) to ground, and pins 5 and 6 to 6.3V heater power.
Before I fixed it, I had pin 5 to 6.3V, and 6 and 8 to ground :/
I connected pin 8 (CT) to ground, and pins 5 and 6 to 6.3V heater power.
Before I fixed it, I had pin 5 to 6.3V, and 6 and 8 to ground :/
I don't think that's right. For 5687...
Pin 6 is the cathode of triode no. 1, so connecting your heater supply there is going to fry that triode section.
For using a 5687 with a 6.3V heater:
1) Tie together pins 4 and 5 and connect them to one terminal from the 6.3V supply.
2) Then connect pin 8 to the other terminal of the 6.3V supply.
Is your heater supply 6.3V AC or 6.3V DC?
--
Pin 6 is the cathode of triode no. 1, so connecting your heater supply there is going to fry that triode section.
For using a 5687 with a 6.3V heater:
1) Tie together pins 4 and 5 and connect them to one terminal from the 6.3V supply.
2) Then connect pin 8 to the other terminal of the 6.3V supply.
Is your heater supply 6.3V AC or 6.3V DC?
--
oops, I meant pins 4 and 5. Sorry about that.
It's already connected, and didn't fry a tube.
Heater supply is DC.
I made a CRC using the resistor to drop down the voltage a bit. I'll check tonight to see what it really is under load.
So pins 4 and 5 are tied together, and go to 6.3V.
Pin 8 is grounded.
It's already connected, and didn't fry a tube.
Heater supply is DC.
I made a CRC using the resistor to drop down the voltage a bit. I'll check tonight to see what it really is under load.
So pins 4 and 5 are tied together, and go to 6.3V.
Pin 8 is grounded.
So I screwed up, there is a 330 ohm cathode resistor installed, I just put it on my schematic, oops :/
I did measure B+, it was 245V, so I had to redo my tube curves. Based on my new curves, looks the 330 was as close as you can get in a standard value for the 4V curve at a load of 10k load resistance.
Found it was easier to markup the curves with acrobat, and attaching a picture to show what I did.
Even though it turned out I didn't have to make any changes, it was good since I learned a little more about tube amps and how to figure out the curves.
I did measure B+, it was 245V, so I had to redo my tube curves. Based on my new curves, looks the 330 was as close as you can get in a standard value for the 4V curve at a load of 10k load resistance.
Found it was easier to markup the curves with acrobat, and attaching a picture to show what I did.
Even though it turned out I didn't have to make any changes, it was good since I learned a little more about tube amps and how to figure out the curves.
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