A15mk2 no sound! Help needed!

Hello!

I just finished building my Madamp A15mk2 but I get no sound of it. I've double checked all the connections but everything seems to be fine... : /
When the amp is on the tubes glow like they should. Power tubes get so hot that I can't touch them. Is that normal?

When speaker is connected I can't hear even a buzz or hiss.

Also all the voltages I measured are too low:
C29 at positive leg should be 298v, and I get 233v
C28 at positive leg should be 290v, I get 209v
C26 at positive leg should be 262v, I get 178v

Voltage at V5 pin 4 should be 288v, I measure 202v.
At pin 3 voltage should be 282v, I get 178v.

R3 should be 145v, I get 81v
R4 should be 1,2v; I get 0,7v
R9 should be 154v, I get 92v
R28 should be 228v, I get 131v
R21 should be 156v, I get 87v
R38 should be 181v, I get 121v

Voltages at power transformer are fine.

Here are some pictures of my amp:
[ATTACH=CONFIG]10475[/ATTACH][ATTACH=CONFIG]10476[/ATTACH][ATTACH=CONFIG]10477[/ATTACH][ATTACH=CONFIG]10478[/ATTACH][ATTACH=CONFIG]10479[/ATTACH]

Could it be that one of the tubes are broken?

I hope somebody can help me!
Thank you!

Hi again!

Thanks for all the replies!
I figured out that the problem was three shielded wires which had shorted with the shielding. Now I have replaced those wires and with 8ohm speaker connection I think the amp works like it should.
When I try to play through my 16ohm cabinet which is connected to 16ohm speaker connection it only gives very low sound even when I max the volume. At low volumes I can't even hear the sound. Very strange problem : O

EDIT: I've also now verified that the problem is not in switching jacks or in effects loop. Could the problem be in OT? I was struggling to get the solder stick in those four 16ohm connection wires. I still don't think that the problem is in that solder connection. I've redone that connection two times now to be sure it's okay. Are those OT wires covered with some kind of covering under the cloth shielding? Before I soldered that connection last time I scratched all the wire heads with a knife to be sure there is no covering left.

Hi!

I finally solved the problem which was a shorted speaker cord -.- I still have one problem left! V6 power tube runs very very hot and the V5 tube is alot cooler (still hot). Also the voltage at V6 R52 is only ~7mV and at the V5 R51 voltage is ~35mV. There is noticeable hum when the amp is on. Could anyone help me?

Tomfinland

Hi Nils!

The bias trim changes the quiescent current only like 1mV. Swapping tubes didn't change anything so the tubes are fine. Everything else seems to be fine but V6 runs like boiling hot and the quiescent current is only ~7mV. Also the current at V5 is a bit too high, like 36mV. When I measured the voltage from pin 2/3 at VR9 to ground, the voltage was 44-50v.

Tomfinland

Zitat von Nils;337341

Hi,

That's weird.

How do you figure it's running "boiling hot"? The quiescent current of only 7 mA (the 7 mV over the 1R resistor translate to a current of 7 mA) would indicate the contrary, the tube running super cool. Does the tube show red spots on the anode or something? Are you absolutely sure your measurements are correct?

Did you mean minus 44-50V, or did you really measure plus 44-50 Volts? Both is pretty impossible and indicates that the wiring of the bias circuit is wrong. The best idea would be to post some good closeup pictures of the amp.

Nils

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Hi,

It's so hot that my hand would burn in couple of seconds. Ofcourse it's not immediatly hot, but it gets very very hot. And yes I've measured those many times.

And I mean plus 44-50V, or atleast my multimeter said that : D Am I measuring right? Positive meter probe on VR9 middle leg and negative meter probe in star ground.

Anyway here are two pics!
[Blockierte Grafik: http://i1241.photobucket.com/albums/gg512/sublike/DSC_0366a4.jpg]

[Blockierte Grafik: http://i1241.photobucket.com/albums/gg512/sublike/DSC_0366a3.jpg]

Tomfinland

Zitat von Nils;337358

well that should be pretty normal. You cannot touch the heated power tubes without burning your hand immediately. If you can do that with the other tube it means it might not be heated properly. Did you measure the heater voltage on both sockets (Pin 7 against pin 2, AC!!!)?

The way of measuring is correct. Either your DMM reads bull, or there's definitly soething wrong with the wiring.

Hard to tell from the pics; where do the two wires from the bias pot end on the power board? It looks as though the left one ends et the junction of R20 and D1. Also, it looks like (could be the perspective) the C33-Cap's right terminal is touching the 47µ's ground wire, which it mustn't !

I can't really find any other irregularity... sorry. Maybe someone else will see somthing. I recommend you retrace the bias supply and compare it with both the layout and the schematic.

Nils

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Hi Nils!

First of all thank you! You have been very helpful!
Ok, the tube temperature is maybe then ok. And whoopsie!! I just noticed that it has been v5 which is hot. You were right!
The v6 draws only 7mV and is cooler. V5 draws 36mV and is hotter.
Heater wire voltage is 6,5V at every tube.
Those wires from bias pots end where they should. It only looks wrong in that picture. Did you mean C31's right terminal with C33's right terminal? In picture it looks like the C31 is touching C26 ground wire but that is also the perspective.
I have to check all bias circuit connections. I hope this problem is solved some day : P

Tomfinland

Okay, I have no idea what I've measured yesterday, because now I got -18 - -24 volts for bias voltage... It's quite lowish tho.

Also today I noticed that when the bias trim is all the way in counter-clockwise direction (bias voltage = -18v) the quiescent current at V6 is ~8mV and at V5 the current is ~36mV. When the bias trim is all the way in clockwise direction (bias voltage = -24v) the quiescent current is ~4mV at v6 and ~37mV at V5.

Does someone know what causes that?

PROBLEM SOLVED!
Finally the problem was a shorted shielded wire. The "A" wire in schematics. Now the amp is working and quiescent current is set at 34,7mV.
Many thanks for all the people who helped me!

Tomfinland