Reallly bad bias problem :(

[ACDCG400]

Hey guys i need some quick help :( im building a dumblesque amp in a weber halle chassis.  its not really working, but i think its because i miswired the relay.  the huge and pressing issue is that the bias cap exploded, and the one i replaced it with is also heating up a lot.  i checked the polarity and they both arent/werent backward. i disconnected the trem from it and am tempted to just make it cathode bias till i can diagnose the rest of the amp. any thoughts?

[RicharD]

>they both arent/werent backward.

It's a neg supply so it's supposed to be wired backwards (so to speak), + to ground.

[sluckey]

the bias cap exploded, and the one i replaced it with is also heating up a lot.

Is the diode installed backward?

The diode cathode (band) needs to be connected toward the PT and the cap positive must connect to ground. If these are connected properly then you are probably hitting the cap with way too much voltage. We need a schematic to be more specific.

tempted to just make it cathode bias till i can diagnose the rest of the amp. any thoughts?

Fix the bias. It's a simple circuit, but extremely important. Don't put any output tubes in it until the bias is correct.

[ACDCG400]

just checked and i did install both the bias cap and diode as u guys said. i was hoping itd be that. heres a scan of the exact bias schematic im using.  i also have a 6g11 trem in there that i mated to it, although its currently disconnected.  

[stratele52]

Cap explode = too much voltage on it.

Where from the power transformer you take your ac bias voltage ? A bias winding or on the high voltage winding ?

Is the 470 ohms resistor is enough ? How much ac voltage you have before and after the resistor ?

[sluckey]

You have the second cap installed backwards! Just remove it. If your reason for using 2 caps is to increase the voltage rating then you must connect them in series.

See pic...

[stratman_el84]

Are you fusing the B+ between the HV center-tap & ground? This method of fusing the B+ is fairly common and seen in many earlier Marshalls. If so, the bias circuit will see a massive over-voltage when the B+ fuse blows and lifts the HV center-tap from ground. This will also occur if there's a cold solder joint, poor wiring, or any other problems causing the current path through the fuse to ground to not be low-resistance. This will leave the bias circuit as the easiest ground path for the B+ circuit.

I've seen it happen that a bad power tube causes the HV center-tap fuse to blow, which then damages the bias circuit. The bad tube & blown fuse gets replaced, only to discover that now the bias is bad. Confusion then reigns...was it a bias problem that blew the tube, or was it a bad tube that killed the bias, or something else altogether?

Strat

[ACDCG400]

Slukley, i just looked at the amp and co e to find out, i did have the cap in right :( but they were in parallel, im going to change that now.

stratele52: its coming from a 50v tap, and i believe it is. it did bias up previously, before drifting and sparking and boom :(

stratman_el84: i just remembered one of the power tubes arched and i saw a carbon trace from pins 2 and 3 so youre right! ill check the HT fuse with a meter this time -_- and my b+ is fused that way, should i change it? and to be honest, idk, i think a bad tube since i used the amp for a while with the bias scheme and it was fine.  ill change the way its fused to the standard 100watt marshall way.  im starting to run out of 6l6's -_- one of my cats broke mine and the other is shit from arching, ill throw in some shit 34's when i redo this. any other thoughts?

[alerich]

I just remembered one of the power tubes arched and I saw a carbon trace from pins 2 and 3 so you're right!

Plan on replacing that socket if you have not already done so. I suppose you could clean it enough to remove that conductive path but I would change the socket.

[ACDCG400]

i cleaned it and replaced all that u guys told me to and it isnt exploding!! THANK YOU!. im just not getting any sound now, but its a hell ova step in the right direction.  i put in my shit 34's (that are new) and i see the screens blue inside. im not sure if they are drawing too much current, or are biased wrong (plate voltage is 457 and bias is currently at 37ma. i wired in an input jack at the input of the pi and got nothing, while this did something before. so the pi tube is either shot or something else down the line is shot. im gonna take a lil ss amp i have lying around and use it to fix the pre amp so i can rule that out.

[stratman_el84]

Yes, I would change the the way it's fused as you suggested. Either that or risk another bias-circuit blowout the next time a B+ fuse blows for some reason. I also strongly concur with changing the arced tube socket.

Keep in mind that if you switch from 6L6s to EL34s you may likely need to change the voltage divider resistors around the bias pot to adjust the bias adjustment range to accommodate the EL34s.

Strat

[stratele52]

ACDCG400 your voltage are good. I did not see the first time youre cap reverse like slukey write.

[ACDCG400]

im definitely going to change it and the socket that shit the bed. THANK YOU GUYS! i really appreciate it