Bias issue

[daz]

Or not, i dunno. I just know this. I have 1 ohm resistors to read current and probe jacks and screwdriver adjustable pots on the back of the amp. I just rebuilt it putting in a new board and all components. All components are exactly the same except for different caps (tho same value) in the bias circuit. It's a dual bias with 2 EL34's, so there are 4 bias caps. 100uf/100v caps. The caps on the old board were some cheap no name ones, but for this i used F&T thinking they are the best so what the heck. They are literally double the size of the old ones which i figure has got to mean better quality or why make them so big.

So heres the issue. With the old board i could adjust the pots and the bias would very quickly jump up. With these new caps theres a delay and it takes a while before the bias starts moving. But heres the part that bothers me. Before when i turned on the amp and watched the meter it would start at the bottom from a couple millivolts as the PSU charged up and gradually move up to where i have it biased. Now when i do that as soon as i throw the standby the bias jumps instantly to 100 millivolts or more then starts moving down very slowly taking about a minute to get down to the safe range. What the heck could cause this? I'm not saying it IS the caps, but they are the only change over the old board. Any thoughts? And yes, the caps are oriented right, IE: + to ground.

[Enzo]

Here is your schematic, so we don't have to go hunt for it.

http://i24.photobucket.com/albums/c39/dazco/amp3.jpg

Your schemaqtic calls for 10uf, not 100uf, that might be part of it.


WHy would current ramp way up then slowly drop back down? Watch the voltage in the tube grids and see. If the voltage starts way low and takes a long time to get up, the tube current will take the same time to wind down.


Are you SURE the resistors are all the same? it is real easy to mount a 560k where a 56k belongs, or a 150k instead of a 15k. I have reached into a drawer of one value and found something else in my years more than once, so i always check. I have ordered new resistors, got them in with the right value on the packing slip and invoice, the right value on the envelope, but the wrong parts in the bag. APparently a mis stock at Mouser. Point is, it happens.


And if you suspect the new caps for some reason, try subbing the old back into one side.

It is also possible when you rebuilt things, you missed a ground connection in that area. Verify those.

[daz]

Holy $h|t ! This was on my part a monumental "space". I've spaced before but this takes the cake. How i did that i don't know, and no wonder they're bigger. Thats got to be why. What exactly would 100uf cause in the bias circuit in place of 10uf? I really don't understand at all how that works at all.
The resistors are the same, I've checked and I'm positive.

[52 Bill]

What exactly would 100uf cause in the bias circuit in place of 10uf?

The larger value caps will take longer to charge up through the resistors. This would cause your bias voltage to remain low for a longer time than the original caps, which caused the high idle bias current at start up.

[daz]

The larger value caps will take longer to charge up through the resistors. This would cause your bias voltage to remain low for a longer time than the original caps, which caused the high idle bias current at start up.

Thats what i figured as soon as Enzo pointed out my mistake. I mean, i wasn't sure WHY they caused that, but i figured they had to be the reason for it.