Bad readings using Weber bias rite kit. Help!

[kingsxman]

I just purchased a Weber bias rite kit (just the octal plug and wires). I have soldered it all together, checked and double checked connections (its put togther right. No shorts and everything appears to have connectivity to the proper pins).

I've got the outputs of the bias probe connected to my Fluke 77 meter inputs. (voltage input, 300 ma input and ground). My measurements arent what I'm expecting. (This is on a jcm 800 2205 by the way). I have all the knobs to 0. I flip on power, then standby.

My plate voltage is ready .044. This is in DC voltage mode. I "should" be reading around 440 volts. So maybe I'm just reading this wrong. I can take that. When I go to the current reading however i'm getting a readout of .008. WHen I change ranges I can get it to read 8.3...but at that point I'm not sure what I'm reading. I'm "assuming" its 8.3 milliamps. It looks like I should be getting around 31. If I change the bias pot a little I can get it to raise some..but I hestitate to turn it too much as I'm not sure I'm reading this right.

What the heck am I doing wrong? Its been so long (years) since i've used the current measurements on this meter its possible I'm just not reading it right.
Thanks

 

[tweber]

I don't bring out the full voltage because people would kill themselves. The .044V is a scaled down sample of the plate voltage. That's why you installed the two resistors between pins 3 and 8. It is a voltage divider. So, .044 if 440 VDC.

Something is wrong in your current circuit, though. Either the tube isn't drawing current (one or more wires not connected/not soldered/miswired, etc.) It is a sample across a 1 ohm resistor in series with the cathode to ground. It is a one to one voltage to current. .05 Volts is .05 amps (50 milliamps)

 

[kingsxman]

Excellent Ted! Thanks for the quick reply. (Ignore my post on your web sites forum).
I really think I have everything connected right. The amp I'm measuring this on is used.... just got it. It had bad power tubes (blowing fuses). I threw in a set of Svets (el34) and it worked. It was starting to "pop" however after 1 gig. I havent used it since it started to make noise thinking that it just needed to get biased since these tubes were thrown in and not biased. Is it possible that this low current is causing the popping? Could it be possible that the bias pot on this amp is just that far off?

I suppose I can grab one of my fender amps and test to see if its working right...but those are all 6l6. I think thats ok though...isnt it?

 

[Wakarusa]

Hey Ted!

Glad to see you've dropped in. This joint can always use more Hoosiers

Kingsxman: Might want to take a closer look at the blowing fuses -- were the old tubes the cause or a symptom?

 

[kingsxman]

Wakarusa, the old tubes were the problem. As soon as I threw in another set of power tubes the problem went away. These looked to be VERY old tubes and it just was shipped 2000 miles.

someone else mentioned that when measuring cathode current, you use the voltage funtion on the multimeter. This doesnt seem to make sense to me. To measure current, you need to break the circuit...i.e. using another probe socket (labed 300ma) on my mutlimeter.

I tried this just for fun (hoping I wouldnt break my meter) and got measurements of 53 v dc. CAn this be the correct way to measure this? Ted mentoined that his device scales down the voltage to a more "safe" level. COuld the current be doing the same thing?

 

[John Phillips]

You DO need to be measuring voltage, not current, to take the current measurement.

That's because what you are actually doing is measuring the voltage drop across a 1-ohm resistor, and inferring the current from that, not measuring the current directly.

It's a very bad idea to actually break the circuit and feed the entire cathode current out of the amp and through the meter - the length of the wires involved can result in the amp becoming unstable.

So, if you had your meter set to measure current, it was in parallel with the 1-ohm resistor, which would indeed cause spurious readings. Set it to read voltage (mV) and you should get the correct results.

If you're reading 53mV, that's WAY too hot - the absolute maximum current you want at a plate voltage of 440V is 40mA, and even that's a little too much IMO. I am not a fan of biasing right up to 70% of max. dissipation as has become fashionable... amps are more reliable and IMO sound better when they're set more conservatively.

 

[kingsxman]

I've got it! You guys are great. It took a bit to get through my thick skull....but I've got it now.

Your right, I am actually measuring voltage as a direct relation to current. 1 to 1. So...I went back and rechecked. 440 volts on 1 plate with a 52 ma measurement. I was able to dial that back to somewhere around 41. ..but my bias resistor was turned all the way one direction.

I checked the other tube with the kit and found a plate voltage of 450...and a 28 ma measurement. What the... why the big difference? These are SUPPOSED to be matched tubes. In any event, they arent the tubes I'm planning on using. Just some Svets that I borrowed to get the amp running.

I plugged it in and gave it a quick go round. The "crackling and popping that i was getting is gone. I'm sure that was from the tubes running too hot. I'm amazed I got a full gig out of it before the snap crackle and pop started.

Anyway, when I get another chance I'm gonna try my measurements again with the 15 ohm resistor connected.

Thanks for all your help. Ted, it might be a good idea to have a little more specific info on the instructions for the kit that discusses these relationships a little better. That being said, this is an EXCELLANT product. For the money, it cant be beat.

 

[John Phillips]

Swap the tubes around and see if the current imbalance goes with the tube or stays with the socket.

If it goes with the tube, your matched tubes... aren't.

If it stays with the socket, you've either got a bias imbalance (most likely caused by a slight DC leak through the coupling cap) or a damaged OT... luckily less likely. I'd take a bet on the tubes though.

The large plate voltage discrepancy is most likely due to the excess current in one side pulling the voltage down (via the resistance of the OT winding), not an OT fault which would tend to make it go the other way round, high voltage and high current together.

It's not unusual for Marshalls to bias correctly with the trimmer right at the 'cold' end, BTW.

 

[kingsxman]

Thanks again John. Excellant information and much appreciated.

 

[kingsxman]

FYI John. the issue was with the tubes. Bias readings followed tubes.

Thanks again.

 

[Matt Gordon]

Can you post where you got the tubes for future reference. It benefits us all. Thanks a bunch! MG

 

[kingsxman]

Matt, I got my tubes from mike a KCA. He was great to deal with and pointed me in the right direction. He is the one that suggested the weber bias rite and I am very happy with it.

His web site is:
www.kcanostubes.com

 

[Matt Gordon]

Originally posted by kingsxman
Matt, I got my tubes from mike a KCA. He was great to deal with and pointed me in the right direction. He is the one that suggested the weber bias rite and I am very happy with it.

His web site is:
www.kcanostubes.com

I was going to say, surely not Mike's tubes. Then I re-read and found that they were borrowed tubes, not KCA's supply, the unmatched set. Just wanted to see if there is a vendor that might need to match a little better. I'm pretty happy with everything I've gotten from Mike. Thanks for the reply! MG

 

[kingsxman]

You had me worried there for a second. I thought I might have seemed like I was dissing Mike....far from it.

Yep, "borrowed" set was unmatched. Mikes were matched perfectly.