Setting BIAS with light bulb limiter?

[Ricker24]

Hey folks,

Always keeping safety in mind, I want to use the light bulb current limiter I built at all times when my amps are on the the bench. However when it comes to setting bias on the power tubes doesn’t it throw things off? I watched an Uncle Doug video on setting bias - he used the limiter with a 250W bulb and took all the bias measurements as accurate - without mention off the limiter impacting things. However, when I used my limiter with a 200w bulb it was devaluing my plate dissipation measurements (vs. just plugging into the wall) by anywhere from 20%-40%, with the amount of devalue going up as I made bias hotter - which would make sense, and likewise the bulb burned brighter as I upped the bias current on the tubes.

Just wondered if maybe I am overlooking something? I plan on getting as 250w bulb to lessen impacts like this but it won’t make up for differences I was seeing. I suppose you could eventually create a chart for how much to devalue bias measurements given a specific current limiter , but as folks have mentioned the light bulb is a variable resistor - so it’s a not as constant and impact will vary depending on current.

Thanks,
Rick

[martin manning]

Most people use a much smaller wattage bulb, 60-100W, and only use it for the first start-up, or if the amp has some unknown fault, blowing fuses say. I would not set bias with the limiter.

[nworbetan]

The two biggest things you're overlooking are:

1) Your amp circuit is probably different from the example you saw on youtube, and the longer list of differences, the more you should expect your results to vary from the example.

2) Uncle Doug isn't the worst technician, but he's by no means an authority. If you learn things from him, great! I'm glad he's helping you progress in this hobby. But sometimes he needs to simplify things to make them fit into a video, and sometimes he's just incorrect. It happens to everyone.

In this case I'm leaning towards Uncle Doug being wrong. But to be honest I've been so broke the last few years (as I've been getting back into the swing of electronics work) that I haven't even built a light bulb limiter yet.

Here's something you can do to verify what you're doing: With your amp plugged into your limiter, measure the voltage at the power input jack. Is it less than the voltage at the wall? If it is then the light bulb in series with your amp is dropping voltage and your bias measurements and plate dissipation are going to be inaccurate when you plug into the correct wall voltage.

[wpaulvogel]

You can’t accurately set bias with a current limiter in series. There’s no reason to use a limiter at this point. The limiter WILL cause a voltage drop.

[thetragichero]

But to be honest I've been so broke the last few years (as I've been getting back into the swing of electronics work) that I haven't even built a light bulb limiter yet.

I've spent far less building a light bulb limiter than i have on replacement power/driver transistors let alone fuses

[Ricker24]

Ok. I think these comments confirmed what I was thinking - if you want accurate bias measurements you pretty much need to ditch the light bulb current limiter and be VERY careful. The limiter definitely has it’s place when first bringing the amp back up after a mod, or its use use as safety device when it won’t impact what you are doing is generally a good idea, but it gets in the way of accurate bias measurements. Just a little surprised Uncle Doug didn’t mention it when he was taking measurements to three or four digits of accuracy on vintage amps and acting like it was providing very accurate bias calculations through the light bulb limiter. I suppose he was emphasizing safety, and I definitely like his cat and dog.

Thanks for responses.

[Leo_Gnardo]

I definitely like his cat and dog.

Looks like you got the message. Uncle Doug's a fun watch occasionally when I get bored - let's see how someone else does it - especially when they have a talented cat to inspect the work and the packaging. But Doug's way off base from time to time and this is one of those times. There's no reason to have the light bulb limiter in circuit when setting bias.