Metropoulos Forum

So the time has come for me to remove the 1 ohm resistor to ground. What is the best and most accurate method on measuring bias without the resistor?

Bias probes I think. Why you want to remove resistors? ~40mV drop is very small and inaudible. By the way, some probes use exactly the same method with resistors:)

I was told it affects the tone.I would like to try it without to hear. Plus i feel line its a crutch i want to get over.

I guess a bias prob is in order.

Have you tried with and without to hear if it sounds different ?

You have more than 450 volts in power section, the 1 ohm resistor takes less than ~0.04 Volts... If you can hear that difference than you are a super man. Really

There are numerous reasons it may affect tone aside from voltage drop. Not least the effect of interrupting the ground.

Even for the non cathode biased amp? Can you give some examples please? I heard that some say that it is effects the tone, but no one gave the reasons, so I thought it can change some thing only because of the voltage feedback to the grid for the cathode biased stages...

You mean that the method of 1 ohm resistor is inaccurate because of the plate and the screen current that runs trough it, meaning that it's hard to tell accurately how much the current goes through the plate, or there is a sonic impact that it creates?

So how did people bias befpre the bias probes?

Wow, you are really going for it here, I think it's great! Here is Robert explaining how to do it:
http://forum.metroamp.com/viewtopic.php?f=28&t=23950&
So after you read Robert's post, here is how I do it. I love this method. First put a buss wire where the 1ohm resistor used to be on the power sockets. With the amp turned off, I measure the resistance of the OT between the HT fuse (or center tap) and pin 3 of V5 and V6. You will probably get from 13 to 17 ohms one either side. If you get say 15ohms at pin3 on V5 and 17 ohms on pin3 of V6, use the V6 reading for the formula. Now turn the amp on and measure the plates. With the amp still on put your multimeter probes back onto the HT and pin3. Watch out because you have some very high voltages on those probes, it is best to use alligator clips here. Formula: 17.5(typical el34 or 6ca7 dissipation) decided by plate voltage X 17(this number is the OT's resistance you measured) X 2(for 100W amp, but for 50w amps don't multiply times two) = target number. with the plates around 410 you should get a target number of about 1.5 volts at pin3 V6 or V5 with the formula. Keep rechecking the plates and adjusting the formula because they change a bit as you bias. Try biasing beyond the target a bit say 2.0, or try biasing slightly colder to see how that sounds.
Neikeel posted this in another thread, it's a good read:
http://www.aikenamps.com/index.php/the- ... on-biasing

At Aiken amps there is no mentioning that 1 ohm resistor have a sonic impact in the grid biased push-pull amps...

Strat78 wrote:Wow, you are really going for it here, I think it's great! Here is Robert explaining how to do it:
http://forum.metroamp.com/viewtopic.php?f=28&t=23950&
So after you read Robert's post, here is how I do it. I love this method. First put a buss wire where the 1ohm resistor used to be on the power sockets. With the amp turned off, I measure the resistance of the OT between the HT fuse (or center tap) and pin 3 of V5 and V6. You will probably get from 13 to 17 ohms one either side. If you get say 15ohms at pin3 on V5 and 17 ohms on pin3 of V6, use the V6 reading for the formula. Now turn the amp on and measure the plates. With the amp still on put your multimeter probes back onto the HT and pin3. Watch out because you have some very high voltages on those probes, it is best to use alligator clips here. Formula: 17.5(typical el34 or 6ca7 dissipation) decided by plate voltage X 17(this number is the OT's resistance you measured) X 2(for 100W amp, but for 50w amps don't multiply times two) = target number. with the plates around 410 you should get a target number of about 1.5 volts at pin3 V6 or V5 with the formula. Keep rechecking the plates and adjusting the formula because they change a bit as you bias. Try biasing beyond the target a bit say 2.0, or try biasing slightly colder to see how that sounds.
Neikeel posted this in another thread, it's a good read:
http://www.aikenamps.com/index.php/the- ... on-biasing

Yea man. I don't want to be so handicapped with this 1 ohm thing. Not that the 1 ohm this is bad. It is awesome. Just trying to make the jump.
Thanks for the info phil. Always awesome to hear from ya!

Haze13 wrote:At Aiken amps there is no mentioning that 1 ohm resistor have a sonic impact in the grid biased push-pull amps...

I feel ya man. I am trying to do my own experimenting.

Bust this Myth!