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Millett "Starving Student" hybrid amp - Page 426

post #6376 of 6780
Quote:
Originally Posted by tomb View Post

Could be a bad tube, have you switched them to see if the problem follows the tube?

 

Did switched it but the left channel still won't sound up.. I suppose my tubes are okay, thankfully. 19J6 tubes are very rare these days :(

 

Quote:
Originally Posted by Goobley View Post

Right, it's fairly easy to work along the signal path in this amp. There is of course a risk of shock when working on a live amp, so all the usual caveats... I'm not responsible for any harm you may do to yourself or if you set fire to your house... but honestly it's not a very dangerous amp (I did get a decent belt off one of the my big decoupling caps a while ago whilst trying to discharge it).

 

Carefully power up the amp with case open and the innards accessible.

 

First of all, stupid question do both the heaters light up?

 

When following these instructions if any step doesn't read like it should report it here so we can try and work out the problems.

 

Now let's check the power supply path. With your DMM on VDC you should see 48V at the power input to the amp and going to pin 2 of each mosfet. You should also have a CRC pi filter with about 45-47V on the other side of the 2k resistor. Then check the voltage at the top of R1 and R7, they should be the the same as the voltage you measured after the pi filter. After this measure the voltage on the tube cathodes (pins 3 and 8 on 12a_7s and pin 7 on 19J6s) it should read around 2V from memory, but the most important thing is that both tubes read the same.

You can also check the heater voltage now (between pins 4 and 5 on a 12a_7 or pins 3 and 4 on a 19J6), it should read either around 13V or 19V.

 

Now for a signal path check:

 

I recommend connecting a cheap source that you're not fussed about and then play some music or test tones at a decent volume from your source (test tones are best due to them being constant power). Set your DMM to AC 20V or similar then measure the power on both RCA input jacks (or just both channels of whatever input jack type you're using). Left and right should have the same voltage, if not you're problem is before the amp, if its the same move down the signal wires to the pot. I turn the pot to about 50% and then measure the voltage on the wiper (with the source still playing). Once again the left and right channels should measure the same (I recommend turning the pot half way because they are rather imbalanced in the first 10-20%). 

 

Now you want to measure the AC voltage on the tubes plates (pins 1 and 6 on 12a_7 builds pins 1 and 2 on 19J6  builds) you should now see your amplified voltage and it should be very similar across both channels (allow a bit for valve differences). If it is the same continue this process if not go the note a. Next I would ask you to measure the AC voltage on the other side of the coupling capacitors C2 and C4, it should be the same as on the other side. Now, set your DMM to DC 20V and measure the voltage at the mosfet gates (in the same place as you took your last AC measurement). Once again they should be the same and about 16-17V for 12a_7s and 24V for 19J6s. If this is the case then switch back to AC and measure the output of the mosfet, where it connects to coupling capacitors C3 and C5. Once again report any difference you find here. If everything is still the same then check the voltage on the other side of these capacitors. If you still haven't found your problem then I would say to check your headphone jack for damage, beyond that I'd be at a loss.

 

note a: So if your not getting the same voltage out of your tubes then check the plate voltage (same place as the AC signal but in DC this time) it should read around 24V for both channels. If it's not an issue and your tube heaters are correctly powered then I'd reckon that your left channel tube is damaged. To test this swap the left and right tubes around, the faulty channel should now have switch over (only swap tubes with the amp powered down!)

 

Good luck with your debugging,

Chris

 

 

Yes, both of the heater lights up.

Will do the debugging as soon as I can get my DMM.. Lost it somewhere at home.. 

Anyway, suppose I did get a broken Caps, I have to replace it on both channels right?

And, sorry for the basic question, am I going to break the amp if I prod the DMM sticks at the wrong points? I'm a bit bad at reading schematics and deciding on where to prod. Hahaha. If it's the AC measurement I suppose it's safe, it's the DC measurements I am afraid with. 

post #6377 of 6780
You won't break things with a voltmeter unless you short across multiple wires with one of the multimeter probes. We can give you some ideas where to look if you post a picture of the inside of your amp.
post #6378 of 6780

Just make sure your DMM's probes are not in the A socket, as this socket creates a short between the probes. Always leave the probes in COM and V.

 

I know it's a noob thing, but I've seen people shorting circuits using their DMM with the probes in A instead of V.

 

Also, it's been shown that cheap DMM can have an influence on the circuit. In V mode, the DMM has an impedance between it's probes. Cheap DMM can have relatively low impedance, which, when placed in parallel with high impedance circuits, will modify the electric characteristics of the circuit. One such circuit is the MOSFET bias. If you try to measure the voltage at the gate, chances are your DMM will change the bias and therefor not show you the actually bias the MOSFET operates at. You won't damage the circuit, but your measurements will be meaningless. 

post #6379 of 6780

Does anyone know where I can buy the 19J6 tubes under 10ea

also

is there a way to have dual outputs on this amp so that me and a friend could use it or would I need double the components?

and is there any Parts lists for this project with part numbers from mouser?

post #6380 of 6780
Quote:
Originally Posted by cj208 View Post

Does anyone know where I can buy the 19J6 tubes under 10ea

 

 

19j6 are basically extinct. Building an amp around them now is a very bad idea. 

 

Just build it for 12au7. Really cheap 12au7 come in around $10 each, you have tons of options for tube rolling, and future availability should be very good thanks to the high popularity of the tube. 

post #6381 of 6780
Quote:
Originally Posted by Goobley View Post

You won't break things with a voltmeter unless you short across multiple wires with one of the multimeter probes. We can give you some ideas where to look if you post a picture of the inside of your amp.
Quote:
Originally Posted by KimLaroux View Post

Just make sure your DMM's probes are not in the A socket, as this socket creates a short between the probes. Always leave the probes in COM and V.
I know it's a noob thing, but I've seen people shorting circuits using their DMM with the probes in A instead of V.
Also, it's been shown that cheap DMM can have an influence on the circuit. In V mode, the DMM has an impedance between it's probes. Cheap DMM can have relatively low impedance, which, when placed in parallel with high impedance circuits, will modify the electric characteristics of the circuit. One such circuit is the MOSFET bias. If you try to measure the voltage at the gate, chances are your DMM will change the bias and therefor not show you the actually bias the MOSFET operates at. You won't damage the circuit, but your measurements will be meaningless. 

I just opened it again and just realized that there were 2 stray cables. 1 from the left channel and 1 from the right channel. It's quite a mess.. Should I post the images here or can I email you instead?
Just 1 question first, is it correct that the number 4 pin from the tube goes to the ground? It seems that the problem is the soldering from previous owner.


Edit: Resoldered the stray cables. Still got the crackles on left. Strangely, sometimes the crackles dissapear for like 2 secs, then it come back.
Btw, I also get humm noise on both channels, and the loudness of the humming depends on the position of the pots.

I'll make the photo of the amp as simple as I can so that you can help me easier wink.gif
Edited by cpdelc - 2/6/13 at 9:59am
post #6382 of 6780

Very inspirational thread! Right now I have HD598 phones and would like to build an amp. Will this amp be future proof for higher impedance phones and will it suffice sound wise further down the road? 

post #6383 of 6780
Quote:
Originally Posted by Grumus View Post

Very inspirational thread! Right now I have HD598 phones and would like to build an amp. Will this amp be future proof for higher impedance phones and will it suffice sound wise further down the road? 

Future proof? Sure, until you meet Mr. Woo out on a fine afternoon stroll that is
High-Impedance? 600 ohms will still yield hearing damage
Recommend? Indeed

Cheers!
post #6384 of 6780
MSSH is now my workhorse; however, a friend has convinced me to look into building an all-tube amp (possibly based on the common 12a_7)

Any rec's for a schematic/build?
post #6385 of 6780
Ahshan06 thanks for the reply! I think you just sold me on my first DIY amp:)
post #6386 of 6780
Grumus, this amp is actually strong enough to drive my high impedance beyers.. Go for it! Just be cautios that the 19j6 build can be troublesome, as the tube is very rare atm.. wink.gif

Btw guys, here is the pics of my amps guts. A bit messy though.. pic (Click to show)

Edited by cpdelc - 2/9/13 at 12:00am
post #6387 of 6780
I've built a MSSH to drive my DT880's and although I've only used it for 15mins or so as its not boxed up yet, they do sound good. I'm still looking at whether or not its possible to drive them directly from the speaker terminals of a power amp as well as I think that this may give them the voltages they need to really sing.
post #6388 of 6780

I tried asking some questions in the stripboard thread but I think it's a bit dead right now so I'll try here! Before I order parts and get the iron hot I need abit of help. I will build the 12UA7 version.

Source: Laptop

Phones: Sennheiser HD598 (50 Ohm)

 

 

1. Is there any advantages to using a ground plate and point to point wire it over a strip board? As I see it strip board is a sure way to reduce the length of wire used. Of course I will wire the rest as tidy as I can to my ability to reduce noise. Probably do a preliminary CAD design and place the parts out so I get a better feeling of it.

2. I mostly sit at the computer when I use my headphones, which includes gaming, movies and music(=long hours). Can I use this amp for long sessions at the time since it gets hotter than 19J6 builds?

3. What should I be looking for in heat sinks. What values do you recommend since I want to use it for long sessions at a time?

4. Would it be feasible to hide the heat sinks inside the build with openings to let air in and out? (mainly for aesthetics)

5. I have heard turning the amp on or off could release DC voltages straight to the phones damaging them. Is this true? If so is there a way to build in protection for this? I want the amp to sit pretty on my desk and just be turned on and off by anyone without problems.

 

Appreciate any answers! Oh any particular advice to a first time builder? Looking forward to the build so much I am dreaming about it beyersmile.png

 

Edit: stripboard thread is alive!


Edited by Grumus - 2/14/13 at 10:19pm
post #6389 of 6780
Quote:
Originally Posted by nikongod View Post

19j6 are basically extinct.

There are plenty of them on eBay now.
post #6390 of 6780
Quote:
Originally Posted by Captn Dave View Post


There are plenty of them on eBay now.

At $15 - $25 a piece ... and I wouldn't call 6 listings "plenty." wink.gif

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