Millett "Starving Student" hybrid amp - Page 430

Not sure if I understood you correctly.

So in my amp I simply connect pin 4 and 5 to the ground, right?

And one more question. My amp is almost finished. What pre-cautions I should take before plugging anything into it? 

The only thing I can think of is to measure both AC and DC voltages on the output.

Quote:

Originally Posted by ProTofik 

Not sure if I understood you correctly.

So in my amp I simply connect pin 4 and 5 to the ground, right?

 

And one more question. My amp is almost finished. What pre-cautions I should take before plugging anything into it? 

The only thing I can think of is to measure both AC and DC voltages on the output.

Yes, tie pins 4 and 5 to ground.

Yes, you want to turn the amp on, with no headphones connected to it and measure DC volts on the output. If you have your meter hooked up while measuring the output you'll see up to three spikes, as the tube heater, power supply and output caps interact while the tubes warm up. After that, the DC volts should dissappear from the output.

If you measure AC volts, hum and or noise can show up as low voltages AC at the output, even with the volume turned fully down.

Congratulations on trying a 12SR7 build and be sure to post pics of your build with those cool octal tubes.

cheers!

Hi guys, I went thru last night and shortened/tidied up my wiring a fair amount. I have a question. I get an audible hiss that does not change volume with the pot. So no volume I have the same amount of hiss as full volume. It's in amp, not input, because it happens weather or not a source is plugged in. It's not horrible but it's noticeable. When music is playing you really can't here it, but it still bothers me it's there. I've got everything grounded to my metal chassis nicely. I do not have the resistors R3/R9 for the FETs soldered to the FET pins directly, rather they are inside the chassis a few inches away. Could these be the problem, or is a little hiss in these amps just normal? Basically does anyone have the 12au7 build with zero hiss? This is my first Tube amp so I'm new to all of this . I'm going to start the mini max kit tonight so when that's done I'll be able to compare but I would still like to know about the hiss I'm hearing.

Thanks Trav

Trav,   how is the pot grounded? Most of the hiss issues I've had were pot grounds...

I have the ground wire braided/twisted around the signal wires to the chassis ground in the same spot as all my other grounds, minus all of my direct chassis grounds from the dc jack, headphone jack, and inputs.

MOSFETs can actually oscillate quite easily.  It may very well be that what you hear is oscillation and indeed, the resistors need to be soldered to the pins.  That was a warning from Pete Millett in the very first post of this thread, I think.   That's just a guess, of course - only you can try it out and tell us for sure.

As for Vixr's question, I don't think he's talking about the ground wire of your signal input.  Rather, the volume pot body itself must be grounded to the amp for many pots.  I don't think it would be a problem simply because you have the signal input ground twisted with the L & R leads. Although, two ground wires would be better. 

It's hard to tell what you mean by "hiss."  Power supplies and lack of grounding can be a source of hum or ripple - that's usually around 60 - 180Hz.  Hiss is normally a much higher frequency noise and often results from a slightly unstable output - not enough resistance on the output of an opamp, for instance.  Oscillation can often be mistaken for exagerrated highs and a sometimes accompanying lack of bass. 

Thanks guys. I'll try moving the resistors and hopefully that'll do it. The pot is indeed grounded straight to the chassis and all my jacks/inputs are grounded to the chassis as well. I ground off the paint in all those places so they have good grounds. Now that I think about it, I have a question. My rca inputs are grounded to the chassis and then I have a wire going from the rca ground to all other grounds in the amp. They all connect to the rca ground/chassis at the same point. That in essence is my star ground point. Is this a bad thing and do I need to isolate the rca's instead as well as my headphone jack? In other words every single input/output jack is grounded to my chassis. I ground down all the paint around all jacks/inputs to make a good ground connection. But maybe that's why I'm getting hiss? I thought that would be a good thing when I was drilling the holes in the chassis. Hopefully with some more experimenting I can quiet it down some more. Because besides the hiss it sounds great. Tom got the mini max in and ready to be built tonight. Nice kit sir! Although I broke one of the tube sockets last night trying to drill it out. Got one out successfully, the other broke. So ordered new ones from you already  Darn things are fragile.

Thanks Trav

awesome...I used the copper clad and just soldered every ground straight to it...

Quote:

Originally Posted by vixr 

awesome...I used the copper clad and just soldered every ground straight to it...

Yea I used a hammond metal enclosure so I figured I could just ground everything to the chassis. Thanks for the help man. 

Quote:

Originally Posted by tdubl07 

Hi guys. I fixed my hiss. I'm thrilled. I ran the ground of the first big capacitor after the switch back to the dc jack directly instead of tying into all the other grounds and that did it. And it sounds better to boot!

This is how I fixed the hiss in my build too. Power decoupling capacitors should not to connected to the star ground. They have to be connected directly across whatever they decouple. Reservoir caps work the same way: they essentially decouple what is after them from what is before them.

I originally had all the capacitors in my PSU connected to a star ground, and it was noisy. I rebuilt the PSU with the ground going from one cap to the next, and the noise was significantly lowered.

You did a great job on the case work. I like how you maintained the same color scheme trough out. It looks clean and professional. 

Thanks for the kind words Kim. Chilling out listening to my new hiss free amp as I type this. My case is nice and simplistic, definitely not as cool looking as yours though! 

Quote:

Originally Posted by tdubl07 

Tom got the mini max in and ready to be built tonight. Nice kit sir! Although I broke one of the tube sockets last night trying to drill it out. Got one out successfully, the other broke. So ordered new ones from you already  Darn things are fragile.

 

Thanks Trav

Shipped them out Friday!  Yeah, drilling the sockets can be a pain sometimes.  I've tried the screwdriver-prying method, too, but the gold-pin sockets seem a lot more "fixed" than the silver ones.  The silver-pin ones we used for the SSMH could be pryed open without damage - most of the time.

Quote:

Originally Posted by Grumus 

I have a question. I just ordered all the parts for the 19J6 build because I got two tubes for 12 dollars. But if they don't work or I need to replace them in the future I might have to rebuild it for 12AU7 if I can't find any 19J6s. Will the mounting holes for the tube sockets line up between the 9pin and 7pin? I don't want to wreck the case if the mounting holes aren't the same. Should I just buy the parts for 12AU7 and then drill and build everything according to that from the start?

A 9-pin socket is bigger than a 7-pin one. So no, the mounting holes will not line up.  If you are simply punching holes for the tubes in the top of the case, as in with a PCB- or perfboard-mounted sockets, then the same hole that the tube pokes through can very easily accomodate a 9-pin tube.  1" is a good diameter that will work for both.

If you're mounting the sockets to the case, you won't have an issue, either.  Drilling out the hole to accomodate a 9-pin tube socket will "clear out" the 7-pin socket's mounting bracket holes.  This is easily shown comparing the dimensions of the most-used new-manufacture ceramic sockets:

(7-pin socket pic from vacuumtubesinc.com)

and this one:

(9-pin socket pic from vacuumtubesinc.com)

Here are the comparative dimensions:

9-pin:
22mm hole
28mm on-center mounting holes (2)

7-pin:
16mm hole
22.5mm on-center mounting holes (2)

As you can see, drilling out the hole for the 9-pin socket in the future would leave 1/4mm of the 7-pin bracket mounting holes on either side of the socket hole (22mm hole for the 9-pin socket vs. 22.5mm on-center mounting holes for the 7-pin).  That's not enough to even worry about.  You will also have to drill new mounting holes for the 9-pin socket, but the socket's top rim, bracket, etc. will cover any overage.