[tomb]

Quote:

Originally Posted by sandbasser /img/forum/go_quote.gif Hi Guys - The MiniMAX Case temp is ~ 100F directly above the LM317, ~ 87F above the BJTs. I have NOT really measured the temp of the heatsinks themselves... I'm trying to avoid taking everything apart unless it's absolutely necessary and it's tough to get a reliable reading shooting the laser in the vent holes. Initial setting of the bias was 50ma... I have NOT rechecked it yet - the amp has significantly less than 100 hours on it. The Triad wallwart (a 750ma unit) is ~ 102F. Room temp is ~ 72F. (I have a MicroTemp Laser Thermometer.) I guess those temps are probably OK... But, the reason I asked is that I'm wondering if the tightness of the heat sink screws matter. I didn't tighten them real tight when I installed them. Does tightness have any effect on temp??? Daveve - Thanks for the comments on listening... I am definitely enjoying listening. I currently have a set of Denon AH-D950s, Grado SR-80s and Allesandro (Grado) MS-1s (with full MS-1000 conversion). I'm in the market for a better set of phones. I'm going to go out this weekend and try some. Recommendations anyone??? (great sounding phones for MiniMAX) Thanks,

Actually, the heat sinks need to be tight up to the point of cutting into the Bergquist pads - which doesn't take much. As long as you have a snug fit and the lock washer is compressed, the Bergquist "flowable" substance should take care of the rest.

Just as you say, those temps seem quite alright to me. One thing to check - the volume knob should get warm (assuming you're using a metal one) to the touch after the amp has been on for 3-4 hours. This varies considerably with room temps, though. During the summer next to a bright window the knob may get warm within 1 hour, but takes a lot longer at 72 deg. and less without external influence (sun through a window).

 

[tomb]

About the listening ... if you can get your hands on a recording of Chesky's Demonstration Disc, that can get you started with telling the differences in sound quality. Better yet is the Focal JM Lab Tools CD - it has specific frequency tones all the way down to 20Hz and up to 20KHz. Playing that through a MiniMAX with different headphones and comparing to some other amps (no names!) can be an ear-opening experience.

Also, it may sound silly to some, but a pair of KSC-75's are just unbelievable with a MiniMAX. I think they are better than many of the lower level Grado's - at least with a good amp like the MiniMAX. Another great suggestion - other than Grados and you're already on that path - are the Sennheiser HD25-1's. The MiniMAX does well with both the HD580 and HD600, too, but those are probably more expensive. On the other hand, I've seen some places lately that sell the HD25-1 minus accessories for ~$100. That's a steal.

 

[sandbasser]

TomB -

Thanks for the information on discs and suggestions on phones.

 

[acvtre]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif ...Better yet is the Focal JM Lab Tools CD - it has specific frequency tones all the way down to 20Hz and up to 20KHz. Playing that through a MiniMAX with different headphones and comparing to some other amps (no names!) can be an ear-opening experience. ...

That's cool, where can I find it?

 

[acvtre]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif ...Better yet is the Focal JM Lab Tools CD - it has specific frequency tones all the way down to 20Hz and up to 20KHz. Playing that through a MiniMAX with different headphones and comparing to some other amps (no names!) can be an ear-opening experience. ...

That's cool, where can I find it? I'm not going to buy a rapidshare account for sure.

 

[tomb]

Yeah, it looks like rapidshare may be the only place to get it. Just Google "JM Lab Focal Tools CD." You'll get a bunch of responses, but they all go to rapidshare. Still, it looks like it's free, but be sure to check for viruses, etc. I got mine through the News Groups on GigaNews (alt.binaries.sounds.lossless). Maybe I shouldn't have said that, but I don't think Focal's selling it, anyway - can't find it anywhere on their website. There's my confession, though.

 

[ujamerstand]

I've just finished building my minimax. Tuned it to the recommended settings. All I can say is... wow, this stuff is really good! The distortions and clipping I would get before I hooked the amp up all but disappeared. In addition, the amp revealed details that I did not noticed before. Except for some minor flaws like microphonics from one tube, and some scratching noise when I turn the knob, (which is barely noticeable when the music is on) It made my listening experience much more enjoyable! Thanks for making it possible tomb, you really made my day!

 

[sandbasser]

I don't know how this compares with the 'JM Lab Focal Tools CD'; but the Bink downloads are certainly easier to get

Here's a linky.

Michael Knowles: Extras

 

[tomb]

Quote:

Originally Posted by ujamerstand /img/forum/go_quote.gif I've just finished building my minimax. Tuned it to the recommended settings. All I can say is... wow, this stuff is really good! The distortions and clipping I would get before I hooked the amp up all but disappeared. In addition, the amp revealed details that I did not noticed before. Except for some minor flaws like microphonics from one tube, and some scratching noise when I turn the knob, (which is barely noticeable when the music is on) It made my listening experience much more enjoyable! Thanks for making it possible tomb, you really made my day!

Glad to hear it!

About the microphonics - if it's one of my tubes, I'll replace it, of course. Microphonics is something that a tube tester can't check.

About the scratchiness in the volume knob - this is an artifact of the tubes and Millett circuit. Some tubes do it, some don't. Just a guess, but I think the distortion is a reaction in the tube grid/plates to the increased signal. Black Gates on the output seem to exaggerrate the condition, too. My guess is that the BG's are so fast, the tubes can't keep up when the volume knob is adjusted quickly. Most of the time, this only happens with the high-gain 12AE6's, but I'm not sure if that holds in every case.

I once ruined an RK27 by flushing it with alcohol (it removed all the shaft grease) in an attempt to remove the scratchiness. It didn't work. That MAX had Black Gates and every one I've built since with BG's has had a similar phenomenon, although it can happen sometimes with ES's, too. Unlike the conventional wisdom with other pots and amps, however - I can guarantee you it's not the Alps pot!!

 

[tomb]

Quote:

Originally Posted by sandbasser /img/forum/go_quote.gif I don't know how this compares with the 'JM Lab Focal Tools CD'; but the Bink downloads are certainly easier to get Here's a linky. Michael Knowles: Extras

Ah - very good! Yes - I bet those will work well. As I said before, it's very interesting comparing a MAX/MiniMAX and headphones with others on those 50Hz tones and below.

 

[ujamerstand]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif About the scratchiness in the volume knob - this is an artifact of the tubes and Millett circuit.

The scratchiness actually disappeared, I'm guessing the problem corrected itself after the tubes have been used after a while. The microphonics only occurs on one side, and since the amp doesn't move around, it doesn't really bother me all that much. Perhaps later I'd get a second set of a tubes just to play with.

 

[Firestorm252]

hiya.
Long time lurker, first time poster so apologies if my first post is about a problem

Finished getting mine put together (after a long on-off building process).
Started getting everything setup and biased and found my right tube bias isn't budging from 23.5V, but after some testing I found my RA1R trimmer is bad. I've got everything else zeroed in except for that value, also the DB's were set to <40mv first thing when I applied power.

I'm off on a tangent from my real problem. The right channel isn't working, but the weird part is that when I apply audio only to the right channel and turn it up there is audio. It's just full of static and extremely quiet, as in I hear it at 50% volume or higher when left channel is already too loud at the 9 o'clock position. Flipped the 12FM6 tubes and same effect and also tested with a pair of 12AE6 tubes and same.

I read another user, adn6244, had an issue with the bad tube bias trimmer, but I don't believe they had an issue with that channel being faulty. And as I understand the circuit, a high tube bias shouldn't have such an effect on the sound unless I'm misreading it which is very much a possibility.

Also, after letting it run while zeroing in the DB bias I noticed that the QB8R heatsink is dead cold compared to everything else.

Current knowns:

• All components are in the proper places and orientation
• Checked and reflowed all solder joints
• Left channel is fine, right channel is not.
• QB8R (2SC2344) heatsink remains dead cold.

I'm inclined to say that QB8R is bad or that it's not receiving enough signal. Are there any ways to check for a bad NPN transistor or whether a component behind it is bad? Am I going down the wrong troubleshooting path somewhere?

Thanks for any help ahead of time. It may be a bit before I can reply as I'm squeezing some more troubleshooting in between university capstone projects and tests.

 

[tomb]

Quote:

Originally Posted by Firestorm252 /img/forum/go_quote.gif hiya. Long time lurker, first time poster so apologies if my first post is about a problem Finished getting mine put together (after a long on-off building process). Started getting everything setup and biased and found my right tube bias isn't budging from 23.5V, but after some testing I found my RA1R trimmer is bad. I've got everything else zeroed in except for that value, also the DB's were set to <40mv first thing when I applied power. I'm off on a tangent from my real problem. The right channel isn't working, but the weird part is that when I apply audio only to the right channel and turn it up there is audio. It's just full of static and extremely quiet, as in I hear it at 50% volume or higher when left channel is already too loud at the 9 o'clock position. Flipped the 12FM6 tubes and same effect and also tested with a pair of 12AE6 tubes and same. I read another user, adn6244, had an issue with the bad tube bias trimmer, but I don't believe they had an issue with that channel being faulty. And as I understand the circuit, a high tube bias shouldn't have such an effect on the sound unless I'm misreading it which is very much a possibility. Also, after letting it run while zeroing in the DB bias I noticed that the QB8R heatsink is dead cold compared to everything else. Current knowns: All components are in the proper places and orientation Checked and reflowed all solder joints Left channel is fine, right channel is not. QB8R (2SC2344) heatsink remains dead cold. I'm inclined to say that QB8R is bad or that it's not receiving enough signal. Are there any ways to check for a bad NPN transistor or whether a component behind it is bad? Am I going down the wrong troubleshooting path somewhere? Thanks for any help ahead of time. It may be a bit before I can reply as I'm squeezing some more troubleshooting in between university capstone projects and tests.

I'm a bit confused. Why would you expect perfect sound out of the right channel if you can't get the tube to bias? Essentially, that means the signal is getting either zero or very little amplification. Hence, low to no volume, staticky and noisy sound. Actually, that confirms that the right tube may be OK - you just can't get it biased. Replace the trimmer (if you're absolutely certain it's bad) and then get the tube biased.

As for QB8R running cold, it depends on where you've set the DB bias. If you only checked to make sure that it's <40mv while attempting this troubleshooting, it's quite possible that it's not biased at all (IOW, "OFF"). That means it's not going to develop any heat without a music signal. In that area of bias voltage/current, there's very little difference between ON and OFF with the transistor's current bias.

You know the buffer isn't going to burn up at this point, so get the tube biased and go from there. Attempting to listen to sound in the meantime is going to send you off on a tangent - as you noticed.

BTW, re-check the trimmer. Some tubes may bias in only the last five turns of a trimmer. If you've purchased the Murata or Vishay trimmers, they've got a clutch mechanism that prevents damage from over-turning. So unless you melted the heck out of it, chances are it's still good. Many people will come to the conclusion that the tube and/or trimer may be bad when it's just that they haven't really turned it 20-25 turns, which is what it may take for some tubes.

Also - to be perfectly honest, the output transistors are some of the more robust pieces on the board. People can install them backwards and power them up for quite awhile before discovering their mistake - then still have the transistors survive and everything be OK after turning the transistors around. I'm not at all saying you've put QB8R in backwards - only that those things are very seldom damaged/go bad.

I can't say for certain that's what's going on, but check some of these things I mentioned and come back and tell us what you found.

 

[Firestorm252]

Quote:

Originally Posted by tomb /img/forum/go_quote.gif I'm a bit confused. Why would you expect perfect sound out of the right channel if you can't get the tube to bias? Essentially, that means the signal is getting either zero or very little amplification. Hence, low to no volume, staticky and noisy sound. Actually, that confirms that the right tube may be OK - you just can't get it biased. Replace the trimmer (if you're absolutely certain it's bad) and then get the tube biased. As for QB8R running cold, it depends on where you've set the DB bias. If you only checked to make sure that it's <40mv while attempting this troubleshooting, it's quite possible that it's not biased at all (IOW, "OFF"). That means it's not going to develop any heat without a music signal. In that area of bias voltage/current, there's very little difference between ON and OFF with the transistor's current bias. You know the buffer isn't going to burn up at this point, so get the tube biased and go from there. Attempting to listen to sound in the meantime is going to send you off on a tangent - as you noticed. BTW, re-check the trimmer. Some tubes may bias in only the last five turns of a trimmer. If you've purchased the Murata or Vishay trimmers, they've got a clutch mechanism that prevents damage from over-turning. So unless you melted the heck out of it, chances are it's still good. Many people will come to the conclusion that the tube and/or trimer may be bad when it's just that they haven't really turned it 20-25 turns, which is what it may take for some tubes. Also - to be perfectly honest, the output transistors are some of the more robust pieces on the board. People can install them backwards and power them up for quite awhile before discovering their mistake - then still have the transistors survive and everything be OK after turning the transistors around. I'm not at all saying you've put QB8R in backwards - only that those things are very seldom damaged/go bad. I can't say for certain that's what's going on, but check some of these things I mentioned and come back and tell us what you found.

ah, thanks for the clarification
I'm completely new to tubes, so I assumed the bias would not be along the same path as signal and would not cause the issue. Assumed, without checking the schematic, that the bias would be hitting the heater and just causing it to run hotter.

I guess I've some more reading to do on how they work

As for the trimmer, I believe it is bad. I removed it, tested it, turned it 15x one way measured, reset, and turned 15x the other way. got no change in value at all. But I'll give it another go with 20+ turns when I get back from campus in a day or so.

As for the heatsink, I turned up the bias to ~80-90mv on each channel and left it for a bit to heat before I made the temperature observation on QB8R.

Thanks for the help

 

[tomb]

OK - sounds like you did adequate testing on the trimmer. Also, if you ran the bias up to 80 - 90mV, then you definitely should've gotten heat from QB8R.

Sounds like we need some pics.