Bias current too high

[JoeCon]

Gents,

OK only my second amp with a fixed bias supply so bear with me.
It's a Marshall type supply HT into 220K - Diode - 15K to 10K pot with a 47K to ground.
El34's 25W/385 plate x.70% = 45ma. Lowest I can go is 85 ma which is over 100%.

So changing out the 15K is the way to go?? Increasing that resistor will reduce the bias voltage/current...YES??

Thanks...

And this is in a VOX AC50 type amp I'm working on if that makes a diff...

[sluckey]

I would remove the power tubes until this is resolved.

Are you using a bridge rectifier for the B+? If so, you will need to use the bias circuit found in the JCM-900 amps.

[JoeCon]

Steve

Yeah I will. But old full wave type recto. And never worked with EL34 s before either.

[sluckey]

What voltage swing do you measure on pin 5 of the output sockets when you adjust the bias pot end to end? You normally need approx. -30 to -45 for EL34s.

Decrease that 220K to increase the voltage swing on pins 5. This will cause the cathode current to decrease.

EDIT... I just noticed you are using a 10K pot. Change that to 25K or even 50K.

[JoeCon]

Steve

Thanks I'll try that. Yes my voltage was on -20 to -25 volts...

[M Fowler]

I use those 10k CTS pots in everything just decrease that 220k to say 150k.

[Stevem]

A 20 turn Cermet trimmer is the way to go, solder 3 leads to it ,insulate the leads with silicone sealer and use the same to glue it to the chassis some where or make a small pcb to mount it on .
Never install any 34 type tube before confirming that you can get -46 volts to show up on pin 5 of the socket.
With a 6L6 tube you want to see -54.

[JoeCon]

Mark, thanks I'll change that resistor out.

Steve, that's a lot more than I have but good to know.

Ok back to work.....

[Stevem]

What? These type of trimmer cost like a buck more than a good pot does!

[JoeCon]

Steve

Sorry I meant more VOLTAGE than I have....

Where you get those cermet pots?

[Smokebreak]

I use those 10k CTS pots in everything just decrease that 220k to say 150k.

Ditto. You can just parallel in R over the 220K till you get in range, then fix it.

[Phil_S]

Where you get those cermet pots?

I'd look on eBay. This is the kind of thing that $2 should get you several. If you buy them from a place like Mouser, they'll still be $2 (or whatever; these are a low cost item), but you'll also pay $10 for shipping. If not $2, then $5, but not $12 or $15 total including shipping. Look for a USA seller. From the Pacific rim, it could take a month to get delivery.

Edit: I remembered this place, where you can find a selection of things you might use for dirt cheap and they only charge actual postage, which can be very low for a small order. Here's the trimpot stock: http://www.surplus-electronics-sales.co ... x&cPath=64 Didn't see any cermet, but this other stuff will work.

[sluckey]

... I just noticed you are using a 10K pot. Change that to 25K or even 50K.

Of course the 10K will work. But the reason I suggested changing the pot is because a 10K pot working with a 47K resistor will have a relatively narrow adjustment range. In your case you only got a 5v swing. Marshall was fond of 22K or 25K which gives a wider range. And the 50K pot will give a wide range that will usually allow swapping between EL34, 6L6, KT?? without having to replace fixed resistors to get the proper bias swing.

[M Fowler]

Slucky I understand that and in many builds I use 25K or 50K like you suggested but I figured I would pipe up about that 10K pot since he already had one probably already in a dedicated chassis hole

Mark

[dorrisant]

Ive been using these recently...

www.ebay.com/itm/121005352097