[NikZ]

This is Penchum's opinion and ciphercomplete's opinion about the small shield and the large shield M8161. Hope it helps.

Little Dot MKIII Tube Rolling - Page 111 - Head-Fi: Covering Headphones, Earphones and Portable Audio

 

[raekwonse]

Just to let everyone know, Yen is selling the CV131 right now which is the civilian version of the M8161. They have the large shields. However I think the M8161 is military spec, and is higher quality.

I actually found a pair of M8161's with the large shield off an international seller on ebay. Not matched, but hopefully they will be ok.

 

[SemiAudiophile]

Quote:

Originally Posted by raekwonse /img/forum/go_quote.gif Just to let everyone know, Yen is selling the CV131 right now which is the civilian version of the M8161. They have the large shields. However I think the M8161 is military spec, and is higher quality. I actually found a pair of M8161's with the large shield off an international seller on ebay. Not matched, but hopefully they will be ok.

u mind giving a link or the name of the seller? i want to get the ones with the large shields too.

 

[SemiAudiophile]

received the m8161s today the ones with the small shield. they sound pretty good to me so far. kind of similar to the RCAs but with a bit more detail and texture. btw, yen is also selling the amperex 5654's. he calls them Amperex 5654W-PQ. PQ standing for premium quality. it's not listed though, so you'll have to contact him directly to order a pair. i have a pair on the way.

kind of debating whether i should get the CV131. Yen claims that they were designed and manufactured in the same factory as the m8161's and it's just the civilian equivalent. but if it's lesser quality, i think i'll just wait...

oh yeah, does anyone know if they sell the deoxit in department or electronic stores? i noticed that the pins were pretty dirty on mine too.

 

[ScottieB]

I got deoxit and deoxit gold in a 2-pack at Radio Shack.

 

[Dept_of_Alchemy]

Quote:

Originally Posted by SemiAudiophile /img/forum/go_quote.gif received the m8161s today the ones with the small shield. they sound pretty good to me so far. kind of similar to the RCAs but with a bit more detail and texture. btw, yen is also selling the amperex 5654's. he calls them Amperex 5654W-PQ. PQ standing for premium quality. it's not listed though, so you'll have to contact him directly to order a pair. i have a pair on the way. kind of debating whether i should get the CV131. Yen claims that they were designed and manufactured in the same factory as the m8161's and it's just the civilian equivalent. but if it's lesser quality, i think i'll just wait...

The Mullard large shield CV131 sounds really close to the M8161 in my opinion, from the factory codes it looks like they were made in the same factory, so maybe they're actually the same tube just labeled differently.

 

[SemiAudiophile]

hmm...i read from earlier post that we need to change the resistor from 62ohm to 120 ohm if we were to upgrade the power tubes to the 6H30PI's?

will there be any long-term side effects or damage if we don't do the mods and use them anyway? i want to upgrade but i've no experience with modding electronic components or soldering. is there an easier method to mod that newbies can do?

 

[ScottieB]

^ I'm not sure, but learning to solder is very very easy. I did it to fix my HDAM (for the Zero) and was functional enough to do the real thing in about 30 minutes. Do some research and then practice a few times and you'll be fine.

I'm not sure about your question, but I wouldn't advise using them without the mod.

 

[SemiAudiophile]

so how do you go about doing this mod (the one with the switch that lets you select the ohms)? i'm completely clueless as to what kind of parts i'll need and where to put them. if someone could put up a tutorial with pictures will be plenty helpful.

oh...nvm. i found some instructions on pg. 46 (after searching through this extensive thread). it seems complicated though...

 

[eboy2003]

Just report the story of my continuing mod for MK IV and MK III...

Edit: Not mean to hijack the tube rolling thread, I moved my MK IV mod to MK IV SE review thread here, since the mod is not necessarily related to the tube rolling requirement and more MK IV specific.

And I finally take time to open my MK III to do the similar heater resistor mod. The MK III case is a lot tighter and I have to unsolder the RCA connector wires in the back panel to pull the board half way out, and then unsolder all wires on the board to pull it out.

Oh my god, now I know why my first set of 6H30Pi-EH burned out on MK III, look at those nasty burn marks on the resistors of power RC filter section, the two resistors are completely discolored due to overheat. On the back, one of the 150ohm plate resistor for 6N6P/6H30Pi is also burning out. Can you even read the band as 150ohm? All bands are showing black now... This really sucks, I just wish I fully pull out the board and inspect it before replace the 68ohm cathode resistor to 120ohm for 6H30Pi (BTW, MK IV uses 150ohm instead of 120ohm). I thought the heater voltage is the only problem causing the death of my 6H30Pi, but now I suspect lots of power resistors are underrated and overheat ultimately caused the problem. I'm very disappointed by little dot, they owe me two sets of 6H30Pi. Well, at least I feel lucky I found this before putting 6H30-DR on my MK IV.

On the side note, MK III does not use regulated voltage for 6AK5 like it does on MK IV. So the heater resistors are needed for both 6N6P and 6AK5.

Be careful MK III owners, do not leave your MK III on for more than a few hours, MK III definitely is not designed for 24x7 burn-in.

 

[Henmyr]

Quote:

Originally Posted by eboy2003 /img/forum/go_quote.gif snip... Be careful MK III owners, do not leave your MK III on for more than a few hours, MK III definitely is not designed for 24x7 burn-in.

That's not good news. But I've been using my LD MKIII for +8 hours lot's of times. I have not opened it up to see if there is a heat problem, but I have not had any problem with it yet.

 

[Penchum]

Quote:

Originally Posted by Henmyr /img/forum/go_quote.gif That's not good news. But I've been using my LD MKIII for +8 hours lot's of times. I have not opened it up to see if there is a heat problem, but I have not had any problem with it yet.

Well, before I did the resistor change for 6H30 family tubes, I was running my MKIII for 8 hours, then a cool down to room temps, then more 8 hour sessions and so on. I did this for at least 1000 hours total. When I took her apart for the mod, I had no signs of heat related problems. I think what we see in the above pics, is all related to running two sets of 6H30's (until dead) without doing the resistor mod.

If anyone running the stock power tubes (6H6N family) is worried about heat problems, it is easy to unscrew the back two screws and look at the resistors with a flashlight. The best advice still stands as stated way earlier in this thread; "If you want to move to 6H30 family of power tubes, you MUST do the resistor mod first!"

I've been running my "resistor modded" MKIII, using 6H30PI-EH power tubes for at least 750 hours total so far. I have not had any heat related issues so far. The same goes for my MKII modded as well.

 

[eboy2003]

Quote:

Originally Posted by Penchum /img/forum/go_quote.gif I think what we see in the above pics, is all related to running two sets of 6H30's (until dead) without doing the resistor mod.

This is not true. I did the 120ohm cathode resistor mod on MK III before putting on the 6H30Pi-EH. One of the 6H30Pi died after about 1 month. Admittedly I have left amp on 24x7 for several days burn-in, which LD might not designed for that with higher current of 6H30Pi.

If you follow the thread and know my story, I got lucky and exchanged the 6H30Pi from Yen and purchased the MK IV non SE. But the same thing one 6H30Pi died after 4 weeks on MK IV. I opened my MK IV and found one channel's plate and cathode 150ohm has some burn signs. Then I measured the heater voltage and found it's about 6.9V and reported this to Mr. Yang at little dot. He admits the transformer is designed for 110V AC and suggesting adding an 1ohm resistor in series for 6H30 heater when use 120V AC. I think the combined higher than spec heater voltage (which cause even more heat) and the overheat of the resistors contributed to the death of 6H30Pi on both MK III and MK IV. But I will not ask Yen for changing another set of 6H30 since this is really not his tubes fault.

FYI, MK IV has changed the two 180-200ohm stock power resistor to 5W cement ones, so for MK III with 6H30 mod, it's better to change those resistors as well. I highly recommend also change the underrated 150ohm/150ohm (MK IV), 150ohm/120ohm (MK III) plate and cathode resistors to 2W. If you want to, you can use 150ohm as cathode resistor in MK III as well, MK III and MK IV circuit is mostly identical, just MK IV change coupling capacitors which improves the sound.

 

[Pricklely Peete]

Eboy, what pin is the heater voltage on again ? I have a set of 3W 1.1ohm resistors ready to go...I'm thinking pin 3 but want to make sure. I don't have a meter to measure RMS voltage so I'm guessing 1.1 ohm will drop the heater voltage into a better ballpark range (line voltage here averages 125-127V 60hz).

My plate resistors have no heat related damage...go figure. I'm also running a NOS set of DR's but used a pair Kiwame 1% 3W 121 ohm for that grid resistor mod. If I have some correct value 3W Vishays on hand I will swap the plate resistors out as well, might as well if I have the amp apart.

Thanks,

Peete.

PS Hi Pench !!!

 

[eboy2003]

The heater pin for 6H30 is pin 4 and pin 5. You can see 6H30Pi datasheet here.

MK III also uses unregulated AC for 6AK5 heater, so you might also need to put a resistor in. I haven't tried that yet, but rough calculation is about 3-4ohm for 6AK5. 6AK5 heater pin is pin 3 and pin 4.

The MK III plate resistors (150ohm for 6N6P/6H30, 33K for 6AK5) are not so high quality metal oxide resistors, from the size my guess it's only 1/2w, I highly recommend change those to 2W ones and use metal film ones if possible.