Biasing question for a B-52 ATX-100

[Rp3703]

I have a B-52 ATX-100 I am trying to bias with new Sovtek 5881/6l6WGC(23watt) tubes. I cannot find any literature on this amp but the method the manufacturers recommends for biasing their AT-100 is to pull the HT fuse and measure both sides for 120mADC. Unfortunately, my Fluke was only reading 0 using that method.
I am used to biasing tube amps using the 1 ohm resistor from pins 1/8 to ground and measuring across method, so I decided I’d try it that way. I measured 508V for my plate voltage. Biasing at 70%, I used the formula 70/100 X 23/508 = 32mA(or mV). When I clipped a 1 ohm 1/2 watt resistor from pin 8 to ground, I measured 98 mV. I turned the bias trim all the way down and the lowest it will go is 68mV.
Am I doing something wrong?

[sluckey]

I cannot find any literature on this amp but the method the manufacturers recommends for biasing their AT-100 is to pull the HT fuse and measure both sides for 120mADC. Unfortunately, my Fluke was only reading 0 using that method.

You're supposed to pull the fuse, set your meter to measure current (usually means move the red probe to a current jack and twist the knob to the correct position), connect one meter probe to one side of the empty fuseholder, connect the other meter probe to the other side of the fuseholder, IOW, connect the probes ACROSS the fuseholder, then turn the amp on. All the B+ current will be flowing through your meter if you do this correctly. Adjust the bias to set the meter to 120mA.

[Rp3703]

The HT fuse in this amp is connected to two spade connectors on the PSU board. I pulled the two connectors and clipped on my Fluke onto the spades. I only got a reading of 0 so either that method doesn't work for this amp or my fluke is not sensitive enough to read 120mA... I'm guessing.

[acheld]

I was just looking at a schematic for an AT-100, apparently the same amp though with SS rectifier, and other minor changes.   In that schematic, there are two fuses that could be mistaken as HT fuses.  Not sure if this applies to your amp (as I could not find a schematic for yours), but it would be worth checking which fuse you pulled.

-- posting the schematic for the AT-100 (NOT the ATX-100).

[sluckey]

Which Fluke meter? Show us a pic of that fuse.

[Rp3703]

It's a Fluke 1587. Working on the fuse pic.

[Rp3703]

Here's a pic of the HT fuse connection to the PSU board.

[Rp3703]

Here's a pic of the Fluke hooked up to the HT fuse connectors on the PSU.

[Rp3703]

I was just looking at a schematic for an AT-100, apparently the same amp though with SS rectifier, and other minor changes.   In that schematic, there are two fuses that could be mistaken as HT fuses.  Not sure if this applies to your amp (as I could not find a schematic for yours), but it would be worth checking which fuse you pulled.

While the majority of this amp is identical to the AT-100, the PSU is different. The fuses are in the same locations though. The Bias and 12V circuit are very different than the AT-100.

[sluckey]

Nice meter. Looks like you are doing everything properly. I would check the meter fuse.

[Rp3703]

Yup, the fuse was blown. Thanks for figuring that one out for me.

[sluckey]

I'm curious about your Fluke 1587. It's so way over the top for simple tube amp work. What line of work are you in that inspired you to get this meter? I have an 87V and my choice was based on 40 years of Fluke experience (probably about 20 years with the Fluke 87 models) during my career as a FAA radar technician. What's your story?

[Rp3703]

This is going to sound totally made up but I was walking down the street when I saw a black plastic case sitting under a bush. Inside the case was the meter along with all it's accessories, which must have been sitting there for a while since everything was wet from being rained on. I took it home, pulled it apart, dried it out for a few days and then put it back together. It worked. I never checked the fuse which has probably been blown since I found it. I'm guessing it got stolen out of someones truck and tossed into the bush when the person who stole it realized they had no use for it.

[Rp3703]

I ordered a new fuse to perform the bias measurement the way it was instructed by the manufacturer but I am still wondering if my second method of measuring mV across the 1 ohm resistor from pin 8 to ground was correct or not?

[sluckey]

I ordered a new fuse to perform the bias measurement the way it was instructed by the manufacturer but I am still wondering if my second method of measuring mV across the 1 ohm resistor from pin 8 to ground was correct or not?

That's usually correct. Do you know for sure that each socket pin 8 is actually connected directly to ground? Can you get to each tube socket to install a 1Ω resistor on each socket?

[Rp3703]

Pins 1&8 on all 4 tubes are wired together and running through a single 1ohm 5W resistor to ground.

[Rp3703]

Would measuring mV across the 1 ohm resistor be just as accurate as measuring mA across the HT fuse? I tried it and with the bias trimmed all the way down, I am getting around 129 mV. If I go with my 70/100 x 23/508 = 32 formula and multiply the 32 by 4, I should be looking for 128 mV, correct?

[sluckey]

That's correct. But using a single 1Ω only tells you what the total current is for four tubes. If you want to know if the tubes are matched you need a 1Ω for every tube (4 total).

[Rp3703]

The tubes were bought as a matched set. Should I be concerned that with the bias trim at it's lowest possible setting, I am still running a mV or two high?