Wrong Voltages on Speedball & Suggested Uses for Vegemite

[IdiotoftheYear]

I have just replaced the transistors on my small Speedball board with MJE-5731A's, facing the opposite way to those provided with my kit (as instructed on page 5 of this thread https://forum.bottlehead.com/index.php?topic=5989.60 ). The voltages are now reading something like this:

OA 171V
IA  169V
IB  169V
OB  171V

No sound until I turn up the pot all the way, and even then it's extremely quiet.

How have I buggered up this time? How can I fix it?

[Paul Birkeland]

Never, ever, ever, ever listen to an amp that doesn't pass its voltage checks.  The voltage checks tell you it's not working.  Listening to it is extremely risky to your safety and to the survival of whatever you have plugged into the Crack. 

Now that I got that out of the way, why did you replace the MJE350s?

Is the 12AU7 glowing?

[IdiotoftheYear]

1. Replaced the MJE350s to dissipate extra heat required in order to run 12BH7 with a R1 of about 120Ohms to get an output voltage of about 70-80V. Haven't changed resistor value yet.
2. Yep 12AU7 is glowing

There's a reason why (many, actually) I'm called "idiot of the year"

[Paul Birkeland]

Do you have glowing LEDs on the socket?  (If not, just leave the LEDs alone for now)

Idiocy is often the most expedient avenue toward learning.

[IdiotoftheYear]

Yes thanks. All LEDs glowing nicely.  Actually, better double check.

Edit: Confirmed - Lovely glowing action going on where it should be.

[Paul Birkeland]

Could you post some photos of your build? 

I just noticed that your OA and OB voltages are higher than their IA and OA counterparts.  Did you remove the boards when you swapped the transistors?  IA and IB go to terminals 2/4, while OA/OB go to 1 and 5.  Those could well be wired in backwards.  If they are not, you may have a loose black wire in the amp. 

If none of that is the issue, it would be good to know that this amp did work 100% at some point.

-PB

[IdiotoftheYear]

My memory was wrong. Here are the correct voltage readings on the small SB board rounded to the closest integer:

OA       169V
IA        173V
B-A/B       0V
IB         173V
OB        169V

The amp was working perfectly until this morning, which is when I swapped the transistors.

[IdiotoftheYear]

Images

[Paul Birkeland]

Can you take the mounting screws off the board, flip it up, then grab a photo of the bottom?  Did you buy those transistors from a reputable seller?  (eBay and AliExpress are not considered reputable)

[IdiotoftheYear]

Bought the transistors from a random Chinese eBay seller.  Images attached. My soldering certainly won't win any beauty contests.

[Paul Birkeland]

Bought the transistors from a random Chinese eBay seller.

[IdiotoftheYear]

 

You can't blame a man for trying to avoid buying from Mouser, which would entail spending $24 to ship less than $4 of parts? Can you?

Actually you can. Lesson learned expediently.  No trains were harmed this time, which is a blessing. 

[Thermioniclife]

Hi I.O.T.Y. Are you outside of the USA? I ask because Mouser charges $7.99 shipping and I get it in : 2 days, they are in Texas and I'm in New York, That's a long haul. I love Mouser and use them almost exclusively. 

[IdiotoftheYear]

Hi I.O.T.Y. Are you outside of the USA? I ask because Mouser charges $7.99 shipping and I get it in : 2 days, they are in Texas and I'm in New York, That's a long haul. I love Mouser and use them almost exclusively. 

I come from a land down under

[Paul Birkeland]

https://au.rs-online.com/web/p/bjt-bipolar-transistors/1844907/

You'll have to do a side by side of the real ON vs. what you got if that ends up being the problem.  To be honest the ones you have look off to me for some reason, but I don't have any ON MJE5731A here to compare to.