Crack & Speedball - Seeking advice re: no signal when powered on

[Laudanum]

The hum could be a tube or an iffy ground connection.   It could also be caused by other things but Im better served letting others help you with that.  The "hiss" could also be a tube (tube rush) but again, could be other causes.  Couldnt hurt to check the solder joints again, probably focusing on the grounds.  Always a good idea to have some spare tubes as well.  They dont have to be the highly sought after tubes, just spares.  Always good to have spares to troubleshoot a possible problem tube and to get you back up and running in case a tube is problematic or goes south.  Fortunately, many of the 6080/6AS7 and 12AU7 types can be had for pretty cheap.  Others will chime in with more possibilities and better, more specific advice.

[rburrows]

The tubes being at fault had crossed my mind. Maybe I'll order some spares (like you say, they're good to have around in any case) and check against those before meddling with the circuit more. Thanks! -Robert

[rburrows]

I have some tubes on order and will report back as to their effect on the left channel hiss.

Another thing I observed in the running circuit is that though the 8 speedball LEDs are lit, the 2 LEDs wired to the input tube are not. The voltage readings at their terminals seem exceptionally low (see complete listing below). Is this normal or could they be blown? If something is wrong, what are the risks/detriments to running the amp like this? The amp sounds great right now, but perhaps I'm slowly destroying it or missing out on what it's actually capable of?

1:   46
2:  172
3:    0
4:  173
5:   50
6:    0
7:   83
8:    0
9:   83
10:   0
11:   0
12:   0
13: 173
14:   0
15: 191

20:   0
21: 210

a1:  49
a2:   0
a3: .56
a4:   0
a5:   0
a6:  46
a7:   0
a8: .24
a9:   0

b1:  45
b2: 173
b3:  84
b4:  49
b5: 173
b6:  83
b7:   0
b8:   0

[Laudanum]

I dont have the speedball upgrade yet so I dont know what the voltages are supposed to be.  But, I think the original 9 pin LED's are supposed to still light up.  You need better advice than I can provide.  But I know that Doc often mentions double checking to make sure that the right resistors and transistors are installed on the right boards.  Double check that if you havent and then, I guess, hang on for better help.  Im sure it will be forthcoming.  It's been slow on the board, understandably with the holiday.  Im sure more Bottleheads will be checking in soon.

[dubiousmike]

Another thing I observed in the running circuit is that though the 8 speedball LEDs are lit, the 2 LEDs wired to the input tube are not. The voltage readings at their terminals seem exceptionally low (see complete listing below). Is this normal or could they be blown?

FWIW - I damaged one of the LED's on my input tube while installing my speedball, and it actually resulted in a voltage that was way too high at A8.  Here is my trouble shooting thread that discusses, among other things, how you can measure resistance across an LED, causing it to light up, and the measurements you should see (quickly jumping to infinite resistance), if your meter doesn't have an LED setting.  (See http://www.bottlehead.com/smf/index.php/topic,1825.0.html )  I found that if I set my meter to 200 or 2000 ohm sensitivity, my undamaged LED's would light up (while the damaged one did not).  If I set it any higher, none of them would light up.

You might try this with your amp powered off, in a dark room just to make sure your LED's are actually bad before you contemplate replacing them.  It seems to me that if your LED's were bad at A3 and A8, you would be seeing unusually high voltages, like I did.  Then again, I'm not an engineer and this is really just lay speculation.  Best of luck!

[rburrows]

Hello again,

After some tube rolling I have found that the left channel hiss is always there, but its volume is subtly changed by different tube combos. I haven't investigated any further on this front.

As for the LEDs, I performed some diode checks on all of them to get a sense of their relative behavior.

On the big speedball board: the LEDs on the edge of the board do not light, but read .955. the inner ones light but give no readings.
On the two smaller boards: the LEDs closer to the black MJE350 unit read 1.2 with no light, and other ones light but give no readings.
On the input tube:  A3 LED reads .245. If I reverse the probes (ground on the non-metallic side on the LED) I get .228. For A8, I get the audible tone from the multimeter, and nothing when reversed. Given the cramped layout here, I'm measuring at the terminals and not directly on the LED leads. Not sure if that matters. No lights in any case.

All speedball LEDs light up when the amp is on. It is just the input tube LEDs that do not.

To get more reference points, I also tried Hopeful's method of using the low ohmeter settings, but I couldn't get any LEDs to light up or report values this way.

[Doc B.]

Not sure if this was mentioned before, but check the value of the R1 resistors on the small PC boards.

[rburrows]

reading 235 ohm for each R1 resistor on the small boards, so those seem OK.

Does it make sense that if the bias LEDs were dead and/or improperly soldered, the voltages could be mostly right and the amp could pass a good signal (but perhaps not its best) to the headphones? I recently re-measured voltage with different tubes, and they are closer to the manual now than my previous listing, though still a bit low:

1:  55
2:  171
3:  0
4:  171
5:  64
6:  0
7:  90
8:  0
9:  95
10: 0
11: 0
12: 0
13: 173
14: 0
15: 191

20: 0
21: 210

a1: 63
a2: 0
a3: .572
a4: 0
a5: 0
a6: 54
a7: 0
a8: .243
a9: 0

b1: 54
b2: 173
b3: 90
b4: 63
b5: 173
b6: 95
b7: 0
b8: 0

Thanks,
Robert

[Doc B.]

The LEDs usually either work right or don't work at all, but I suppose they could be partially functional. I would double check that the small metal can transistors on the small PC boards are 2n2907s and the ones on the big board are 2n222as.  If nothing else is obviously wrong I would maybe go ahead and change the LEDs on the A socket to see if it makes a difference. I might also ask the seller to verify that he saw the proper voltages at pins A6 and A1 and A8 and A3 when he built the unit with the stock tubes. It would help to know if the deviant measurements have always been there or if they developed after shipping.

[rburrows]

So I replaced the bias LEDs on the input tube. It was a little tricky with all the other circuitry in place, but with patience I got everything set. Now they light and the voltage readings are in line with the manual.

The sound quality has bumped up a couple notches. It was good even with the broken LEDs but I sensed it wasn't running at peak performance. The left channel hiss is also gone

[Laudanum]

Excellent!  Glad to read that you've got it sorted out.  Enjoy.

[dubiousmike]

Very happy to hear you got her up and running!