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Bias set too low?

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8/25/1999 5:18 PM

Bias set too low?

I've been measuring my bias current on my Fender 64' Bassman and while I have not put the 1ohm resistors to ground so I can measure my current draw (need a stronger iron), I have been able to measure the negative bias voltage.

It's -61 volts and it seems like it should be -54 volts on the schematic.

Another thing is that I measured the supposedly 27K 1 watt resistor and it measured closer to 22K!

Anyway, I would imagine for my amp that anything lower than 27K should give me a less negative voltage anyway.

I guess too negative a voltage cannot hurt anything but once I get the resistors in, I will be able to tell a lot more.

8/25/1999 5:21 PM
SpeedRacer	When your bias is that negative, your output tubes will be biased too '"cold", or what is called overbiased. No damage really from this, but your tone will suffer.

8/25/1999 6:13 PM
Aron	Thank you! That's what I thought and have read. I am going to mount a bias pot in there but first I will have to either borrow a powerful soldering gun from someone or buy a Bias Probe type of thing. I am just imagining how much better this amp will sound when it is correctly biased. Aron

8/25/1999 6:13 PM

quote:
""I have not put the 1ohm resistors to ground so I can measure my current draw (need a stronger iron)""

Since pin 8 is already connected to chassis ground, you can avoid having to solder directly to the chassis. Just disconnect the wire from pin 8 and leave the other end still attached to the chassis. Then solder one end of the 1 ohm resistor to the wire and the other end to pin 8.

Hope this helps.

8/25/1999 6:21 PM
Aron	In the back of my mind I knew I could do this. I guess I was trying to be extremely meticulous on this amp. Ahhh but what the heck. OK. Aron

8/25/1999 6:25 PM
R.G.	Mouser has 1 ohm 3W 1% wire wound resistors for about $0.70. Part number 71-RS2B-1.0

8/25/1999 6:31 PM
Aron	Thanks R.G., I went down to the local store and found 5 watters and I measured with my meter to find exactly 1 ohm. Aron

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