[tomb]

Looks good so far!

 

[TomForshaw]

Thanks - I just wanted to congratulate on the quality of the kit and documentation by the way - it's truly excellent.
 
I'm going to leave it where it is for today because I want to savour the build. 

 

[TomForshaw]

Progress! Drilled out the tube sockets - was a bit scary but not as bad as I'd imagined. Glue them back together they seem to be fine. They haven't fallen apart yet...
 
Sorry about the shoddy pictures...promise better ones when it's a bit lighter
 

 

 

 

 
...done!
 
I'm fairly sure that's everything, apart from the rectifiers of course. Hoping to get it tested over the weekend...
 
Edit: Gratuitous tube shot...
 

 

[Gotez]

Clean up your desk
 
Why did you decide to add the rectifiers last?
 
BTW: I would like to buy extra tubes for the Minimax, how important is it that they are matched? Few ebay seller provide matched tubes! I am testing the 12FK6 tubes now, and plan to try the 12FM6 tubes tonight.

 

[TomForshaw]

Haha it's got pretty hideous lately! Since I spilled about a million washers/lock nuts the mess seems to have defeated me...
 
Not sure if you can see it but I'm not actually using the rectifiers - instead I'll be using a big SMPS from a printer which I found as the plug-in AC supply won't work over here. Instead you can see one of the positions is jumpered - I've added one of the little filter caps to try to tame any potential high frequency noise. My Dad has handily informed me that the ripple rejection of the 317 is still around 60dB at 50kHz (though I'm not sure what the switching frequency actually is) so I'm hoping the capacitors will do the rest. 
 
I'm hoping to get it boxed up and biased over the weekend. Yay!

 

[jdkJake]

I would like to buy extra tubes for the Minimax, how important is it that they are matched? Few ebay seller provide matched tubes! I am testing the 12FK6 tubes now, and plan to try the 12FM6 tubes tonight.

Without starting a religious war, matching is nice to have, but, probably not essential. IMO, it really depends more upon whether the tubes are of the same construction and same manufacturing lot. If you can get a matched set without spending a fortune then I would do it, otherwise I would not worry about it too much.

As for tubes, I generally prefer to stay away from eBay. Especially for millett tubes. Both Beezar (http://www.beezar.com/) and Radio Electric (http://www.vacuumtubes.net/) will sell you all the matched pairs you can handle at a fair price and stand behind the sale. I have dealt successfully with both and can recommend either.

 

[Gotez]

Quote:

Quote:

I would like to buy extra tubes for the Minimax, how important is it that they are matched? Few ebay seller provide matched tubes! I am testing the 12FK6 tubes now, and plan to try the 12FM6 tubes tonight.

Without starting a religious war, matching is nice to have, but, probably not essential. IMO, it really depends more upon whether the tubes are of the same construction and same manufacturing lot. If you can get a matched set without spending a fortune then I would do it, otherwise I would not worry about it too much.

As for tubes, I generally prefer to stay away from eBay. Especially for millett tubes. Both Beezar (http://www.beezar.com/) and Radio Electric (http://www.vacuumtubes.net/) will sell you all the matched pairs you can handle at a fair price and stand behind the sale. I have dealt successfully with both and can recommend either.

 
I have dealt with Beezar and I very content with the service. I have purchased two sets of valves there. I would however like to try different brands and makes. I will check Radio Electric. Thank you for the advice.
 

 

[the_equalizer]

So, a bit of a strange question... about a year ago, when I built my MiniMax, I just did not feel like cutting the pot's shaft to proper length. Now, it's starting to become irksome to see the knob kind of 'hanging in the air'.
 
Is there a high risk of damaging the pot (or some other component. Maybe the relay?) if I use my Dremel to cut the shaft with the amp assembled  (I'd remove the tubes, of course) ?
 
cheers!

 

[MrDavis]

Yes, it can be done.  The directions are right here: http://www.diyforums.org/MiniMAX/MiniMAXwiring.php
 
Its on the bottom of the page.  Item #10 under Assembly.
 
Quote:

So, a bit of a strange question... about a year ago, when I built my MiniMax, I just did not feel like cutting the pot's shaft to proper length. Now, it's starting to become irksome to see the knob kind of 'hanging in the air'.
 
Is there a high risk of damaging the pot (or some other component. Maybe the relay?) if I use my Dremel to cut the shaft with the amp assembled  (I'd remove the tubes, of course) ?
 
cheers!

 

[the_equalizer]

Thanks for pointing that out! It's been so long since I read those directions that I didn't recall that fragment at all.
 
cheers!

 

[TomForshaw]

Hooray! It's alive!
 
Did the wiring on Friday - biased it today. I'll get some images up - but it sounds great.
 
As in really, really fantastic! I'm blown away by how natural it sounds - it's astonishingly good. My favourite thing I've built.
 
Also, there's no obvious problem using the DC supply - no noise or any nasties like that. I love it!
 
 

 

[tomb]

Quote:

Hooray! It's alive!
 
Did the wiring on Friday - biased it today. I'll get some images up - but it sounds great.
 
As in really, really fantastic! I'm blown away by how natural it sounds - it's astonishingly good. My favourite thing I've built.
 
Also, there's no obvious problem using the DC supply - no noise or any nasties like that. I love it!
 
 

Glad to hear it!  Looking forward to your pics!

 
 

 

[TomForshaw]

Running the tubes at 13.5V with the buffer voltages at 110mV (so 50mA?) and it's sounding fantastic
 
Had a bit of a scare when measuring the buffer as I misread the instructions and measured relative to ground rather than the correct reference point. I measured rather more than ~30mV...turned it off pretty quickly! Once I'd figured out where I was actually supposed to measure it from though they were both at 27mV.
 
I was wondering, would it be possible to make the amp run as class AB with the buffers wound all the way back, or would I still need to burn my headphones and ruin my hearing to make it come out of class A? I'm not sure what the usual amount of power is when listening...
 
The thing that really grabbed me is the extension at both ends - I was expecting a nice top end, but the bass is fantastic too. I'm using Alessandro MS1s and the bass really sounds great. The positioning is really something too - can't see me ever getting rid of it.
 
It's a lovely sounding thing though. I wasn't expecting it to sound as noticeably better as it does (if that makes any sense...). Here it is playing with my DAC - seems like a great combination:
 

 

 
As you may be able to see, I haven't put the nut into the headphone jack - it seems quite stiff and I'm reluctant to really go for it and wrench it down as the socket is plastic. Any other builders found it to put up a bit of a fight?

 

[tomb]

I'm not sure what's going on with your headphone jack.  Sometimes the front plate has to be tourqued on a bit, depending on the height at which you soldered the headphone jack and volume pot.  That's somewhat of a variable whereas the case screws do not vary at all.  If that's the situation, it could be torquing the headphone jack a bit, causing the inside threads to deform slightly, making the ferrule difficult to screw in.  Try loosening the front plate screws so that the front plate can wiggle a bit, then see if that makes it easier to screw in the ferrule.
 
Everything else sounds normal from your description.  As for power/Class A/Class A/B, the whole point of most DIY headphone amps is to run them in Class A.  You get the best sound possible from the circuit, because the transistors never have to switch on to respond to a musical peak (they're always on), causing switching distortion.  The amount of Class A needed depends on the headphone load - but 50ma on the MiniMAX is what it's designed for, heat rejection-wise.  It's tough for commercial amps to utilize Class A because of energy standards or simply not wanting to make the investment in heat sinks and cooling.
 
As for your headphones - they're only going to draw whatever power they need, not any more than that.

 

[TomForshaw]

Thanks Tomb.
 
I was just inquisitive to see if I could hear a difference between class A and AB, but I guess it'd need to be insanely loud to get enough current flowing to pull it out of class A. I'll try loosening the panel a bit. It goes in a little way, just not all the way down - though it feels like it would if I gave it the beans with a spanner or something.