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Old 20th October 2006, 06:38 PM   #1
Trout is offline Trout  United States
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Default Adding Bias Check Points Question

I have a question about adding bias check points to my homebrew amp.

Basically, I already have an adjustment pot in place, But I am only able to determine a -V reading at that point.

Snooping around a bit I see that a very common way to add test points is by adding a 1 ohm resistor to the power tube cathodes.

I drew up how I think it goes, But my question is 2 fold,
#1, Can I just leave these resistors in place and add test point jacks for my meter.
#2, Is the attached schematic correct or workable?

The MV reading on the meter should represent the MA reading within a couple MA?

Bias Points

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Old 20th October 2006, 06:48 PM   #2
Stocker is offline Stocker  United States
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mind the power rating of the resistor. At least double whatever the numbers say will be dissipated by it.

high voltage bites even from a distance, mind what you are doing !

beware of system instability. Simulate if possible, measure if not.

I don't know too much about toobz but unless something crazy is going on, that looks like it might be ok. If you lose too much power in that resistor, try a lower value and maybe an x10 opamp to scale.
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Old 20th October 2006, 07:27 PM   #3
Trout is offline Trout  United States
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The Resistor rating was another question, I have seen on a few guitar bias instruction sites saying 3W and others saying 6W.

I know there are a few bias tools out there, $75.00-$200.00 depending on brand and model.

What I gathered is basically its just a couple sockets set up to go between the tube and chassis.Then Measure MV across the cathode resistor.

I figured it easier to just add it to the unit with some suitable jacks to accept my multimeter probes.

I am fairly good at building the things, But I have learned to well to ask lots of questions rather than burn up a perfectly good build.

Here's pictures of the unit, Its pretty much a work in process.

Prototype KT77 Amp

The whole reason for this is because I let a bud borrow the unit while his amp was out for warranty service, He ran it fairly hard for about 2 hours and blew a fuse.

I had a 2A fuse in the unit which is basically borderline, Original vintage schematic calls for 3A.
I have run the amp for 20-30 hours but never near wide open continuous for more than an hour plus like he did. ( To hard on the ears).
I figure perhaps its biased a tad hot for the KT77's and would like to get a better handle on where it is. Since (Ducking from Poobah) I still have not gotten a scope, I really can not opt to set it based on crossover distortion.

I realize there is a happy zone that spans several ma and would like to be able to monitor the setting occasionally to get better life from the tubes.

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