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Millett "Starving Student" hybrid amp - Page 416

post #6226 of 6782

Probably where you measured current... 

Without unsoldering stuff you cant measure current directly in a running amp. you must measure resistance (with the amp off) and voltage across that resistor (with the amp on) and crunch the numbers. 

post #6227 of 6782

facepalm. i get it. ive got 0.551mA on one channel and 0.5mA on the other. my dmm is kind of crappy, might take it to the lab tomorrow and get the values from one of the nice fluke bench dmm's. everything seems to be in order, had a quick scare though.  thanks guys. when i order parts for the ccs, i will be getting trimmers to replace R5 and R11 as equalizer suggested, so that value of .051mA can be turned down a bit. 


Edited by kchapdaily - 10/8/12 at 8:29pm
post #6228 of 6782
Quote:
Originally Posted by marcus76 View Post

hi there,

nice to see theres still something going on in this thread!!

 

since my first try on the ssmh 12au7 has been hiding in a box for way over a year or even longer, last week i decided to redo it with some perfboard.

Mostly because my old build (not working) was just a mess of wires.

I ordered all parts (around 8 Euro's) and tried again, re-using the already wired pot/sockets/power switch and the rca's.

 

I fired it up at the weekend and luckily : no sparks and stuff, tubes light up fine, mosfets get warm,

so i decided to hook up a cheap mp3 player and some old sony headphones.

And then : NOTHING ... not even a hint of sound, noise or whatever, just pure silence :)

 

Btw, I wired my pot like beftus did in post #5326 (should be the same one) and the switched neutrik jack just to try if this might be the fault, on the other side, too. nothing.....

 

So i measured :

48V ---r13--- 44,7V

Zero V at both inputs and zero V at both outputs at r6 & r12

48,1 V at both Mosfet Pin 1 after r3 + r9

1,4 V at both sides of the tubes pin 3 + 8

21,6 V at the tubes pin 1 + 6 both sides , and 18,7 V at mosfet pin 3 both sides

 

Whats wrong ?

Any help is much appreciated :)

 

I'm away from home right now and can't compare to my amp, but your readings look quite alright to me. I bet your problem is a simple miswiring. I'd check the pot first. Did you try moving it while music was playing? Check the output caps and resistors (C3, R6; C5 R12). Check also the interstage coupling caps (C2, C4).

 

cheers!

post #6229 of 6782
Quote:
Originally Posted by kchapdaily View Post

facepalm. i get it. ive got 0.551mA on one channel and 0.5mA on the other. my dmm is kind of crappy, might take it to the lab tomorrow and get the values from one of the nice fluke bench dmm's. everything seems to be in order, had a quick scare though.  thanks guys. when i order parts for the ccs, i will be getting trimmers to replace R5 and R11 as equalizer suggested, so that value of .051mA can be turned down a bit. 


 So I'm not getting old and decrepit :) ! I did seem to remember that the current in the 19J6 build was ~.5 mA

 

Careful there, the trimpots will NOT change the current. The current is set by the Constant Current Source. For changing the current set by it, you choose different values for the two resistors associated to the transistors. For easy change of those resistors I suggested you use some form of socketing for them.

 

What the trimpots allow you to do is to change the tubes' cathode bias. As you change the value of the trimmer, the constant current going through it will cause the voltage drop across it to change. This effectively changes the grid to cathode voltage which in turn changes the operating point of the tube.

 

By measuring the voltage at the tube's plate while turning the trimmer you can set a voltage that is half the supply voltage, allowing the tube to swing voltages symmetrically... at least in theory :)

 

cheers!

post #6230 of 6782

ok thanks for the advice. will a 10% tolerance trimmer be ok? it feels high for such an important position

post #6231 of 6782
Quote:
Originally Posted by kchapdaily View Post

ok thanks for the advice. will a 10% tolerance trimmer be ok? it feels high for such an important position

 

10 percent is perfectly fine. You will, after all, be setting the plate voltage by measuring it while setting the trimmer so it doesn't matter if you have to set one of them one or two turns further more than the other one :)

 

 

cheers!

post #6232 of 6782

thanks! ill be ordering parts soon.

post #6233 of 6782

beerchug.gif

I wired c3 + c5 to the output, but not to the rest ....

Sweet music is playing for 20 minutes now, i got some 50hz buzz, but i think thats because of my laptop wallwart. Already had that with other equipment.

Thanks to the_equalizer  :)

Next : Casework

post #6234 of 6782
Quote:
Originally Posted by marcus76 View Post

beerchug.gif

I wired c3 + c5 to the output, but not to the rest ....

Sweet music is playing for 20 minutes now, i got some 50hz buzz, but i think thats because of my laptop wallwart. Already had that with other equipment.

Thanks to the_equalizer  :)

Next : Casework


Excellent! I'm glad that you're enjoying some music out of your build. It's always thrillilng to see those tubes glow and hear music coming out of the amp.

 

cheers!

post #6235 of 6782

Hi there,

beginner question again:

I'd like to put some switchable (dim/bright) tube led's and a power led into the circuit, would this be correct for the wiring (switch wiring lower right) ?

Or is there a better point to hook up the tube led's?

 

Need to check for the proper resistor values.

Thanks

post #6236 of 6782

This LED wiring looks sensible to me, however, if you're using the original stock PSU I'd be wary about adding three LEDs, as I know that this may be quite tight on current (I remember people commenting that two LEDs plus the amp was tight). So if your power supply will handle it (what is its rated current?) then great, go for it.

 

I'm planning on doing something similar as I've almost removed all of the noise from my build now (big update to my thread coming up :))

 

Cheers


Edited by Goobley - 10/12/12 at 2:10pm
post #6237 of 6782
Quote:
Originally Posted by marcus76 View Post

Hi there,

beginner question again:

I'd like to put some switchable (dim/bright) tube led's and a power led into the circuit, would this be correct for the wiring (switch wiring lower right) ?

Or is there a better point to hook up the tube led's?

 

Need to check for the proper resistor values.

Thanks

 

What type of switch is that? I've never seen one that works that way. Usually, the middle pins are the common, and the connections are made in the same row, like so:

 

The red lines correspond to the pins that are connected together internally when the switch is either up or down. Note that this is inverted in the case of a sliding switch. But either way, you should simply use your multimeter and check the continuity between each pins, at each switch positions. You can't go wrong this way. Even people with experience do this as a good practice, just to make sure the switch is working properly.

 

You don't even need a dual pole switch for your application. A single pole will do just fine:

 

A more efficient way to power LEDs in the SSMH is to use the voltage across the tube heater. Connect one of them to pin 4 of each tube, or both to just one tube if you want to use a switch. If you have a double pole switch, then you can use it to connect one LED to one tube. Since the voltage of the tube heater is only 14v, you won't have has many volts to drop across a resistor to power the LEDs. This means less wasted power.


Edited by KimLaroux - 10/12/12 at 5:16pm
post #6238 of 6782

Well thought with regards to the switch, I just assumed that marcus had checked it and that was how it was (I believe I may have seen one with this sort of pinout before). It is of course very true that one should check things with a multimeter, particularly if it's just something that you've dug out of your parts bin (the other day I spent several hours changing caps and op amps when i actually had a dodgy contact in the power switch which would have shown had I used my multimeter).

 

Interesting idea about connecting it to the heater, it certainly sounds like less power loss, but wouldn't the extra current draw across the heaters affect the voltages. Also IIRC the heaters are pins 4 and 5 and in my build are just under 13V, pins 3 and 8 are the cathodes and measure under 1.5V.  I just checked the diyforums PCB build of the original 19J6 build and they definitely dropped from 48V for the LEDs using a large 2.4k 2W metal oxide resistor. Whilst I am not sure of any implications of connecting the leds to these tube pins I'd be very wary of doing so for fear of messing up the biaising or something. I may be talking crap in this last paragraph, so if anyone with more knowledge than myself would care to contradict and correct me I feel that it'd be a useful learning experience for all involved. :)

 

Cheers

post #6239 of 6782

Oh you're right, it's pin 4! I confused myself there. Thanks for clarifying that, I edited my post to correct this. In my build I actually connected them to the source of each mosfet. It was easier that way.

 

Many people in this thread built the SSMH with LEDs connected across the tube heaters. I did it myself, without issues. I'm not entirely sure it actually is more efficient, as the additional 20mA used by the LED is an additional 20mA that passes trough the MOSFET. This means that instead of dropping the voltage across a huge resistor, the voltage is dropped trough the MOSFET. But I'm not sure if this is the case, or if it's the heaters that will adjust so that less voltage drops across them... it's confusing. Anyone else can explain what happens when we add an LED across the heater? Else I'll have to add this test to my to-do list, for next time I open my amp.

 

Many DIY amplifiers connect tube LEDs across the tube heaters. Just take for example the Bijou and the SOHA II. But then these don't use the heaters as current sources to bias a mosfet, like the SSMH does. They actually have dedicated power rails just for the heaters.

post #6240 of 6782

Hi,

I have the stock PSU with 48V 380mA, after moving it to another outlet, away from my laptop PSU, its perfectly quiet. But its not in its case yet...

For the switch, its a "miyama ms 169" and I found it to be odd that it didnt worked the way I thought it should, like KimLaroux said.

I tested it for continuity and it seems to work like this... had it in my parts box, so i thought why not...

Interesting idea to use the voltage across the heaters (which is 18,7V in my build)

I have to look into that, escpecially for the wiring of the switch, i'm a bit confused about it. Best way seems to use this dual pole switch to connect one LED to each heater.

I'll wait if someone else chimes in about efficiency, since I really like the idea of using the heaters, but as Goobley pointed out, I dont want to mess up something.

I'm quite happy right now I got it working, so i might do some casework before :)

Btw. here's what it looks like so far :

 

The wiring is still quite long and messy, but as I dont know how/if it would fit in my case, I thought having some extra lenght might be a good thing.

Case is some old metal printer switch box, where the base slides in to the top part..

cheers

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