$14.95 octal base bias tester kits

[Steve A.]

At $14.95 each (plus s/h) from The Tube Depot these are very tempting. You do have to assemble them yourself but if you have more time than money they are a great bargain. Of course with a price like that you need to supply your own DMM...

TubeDepot Bias Scout Kit

If you want to build your own from scratch here is a link to the PDF file with the assembly instructions and bill of materials. (You do want to have octal tube sockets and plugs which will work together properly.)

http://site.tubedepot.com/pdf/bt-bias-scout.pdf

Steve Ahola

P.S. If you have alternate sources for reasonably priced bias testers please post them here. (I am not shilling for The Tube Depot- just passing on what looked to me like a good bargain.) Even better would be a source for the matching octal sockets and plugs...

[soundguruman]

A bias tester saves a lot of time and high voltage risk...it's probably a great idea. Even for $50.

[Prairie Dawg]

Really. Having 450 volts flopping around loose gives me the willies. I had a test socket with breakout points for all 8 pins and tried that one time. Bam! as the chef says. Never again.

[guitician]

I'd just put the plate voltage safely on a well insulated SPDT switch to the DMM.

[big_teee]

At $14.95 each (plus s/h) from The Tube Depot these are very tempting. You do have to assemble them yourself but if you have more time than money they are a great bargain. Of course with a price like that you need to supply your own DMM...

TubeDepot Bias Scout Kit

If you want to build your own from scratch here is a link to the PDF file with the assembly instructions and bill of materials. (You do want to have octal tube sockets and plugs which will work together properly.)

http://site.tubedepot.com/pdf/bt-bias-scout.pdf

Steve Ahola

P.S. If you have alternate sources for reasonably priced bias testers please post them here. (I am not shilling for The Tube Depot- just passing on what looked to me like a good bargain.) Even better would be a source for the matching octal sockets and plugs...

It shows to be $19.95 when I look at it at TubeDepot.
Weber has a similar kit for $16.
Weber Mod Kits

PS: I prefer test points on the amp.
T

[Tom Phillips]

Really. Having 450 volts flopping around loose gives me the willies. I had a test socket with breakout points for all 8 pins and tried that one time. Bam! as the chef says. Never again.

I had the initial same thought. However, checking out the posted construction instructions for this particular bias probe you will find that the high voltage is internally divided down 1000:1. Therefore, the HV isn't actually present on the leads brought out. If the plate voltage is 450V it appears that you will measure 450mV at the meter leads when using the setup that allows you to measure plate voltage vice the idle bias current. Then you just do the math to calculate actual plate voltage.

Tom

[Steve A.]

It shows to be $19.95 when I look at it at TubeDepot.
Weber has a similar kit for $16.
Weber Mod Kits

The price went up $5 since I posted that link- I shoulda jumped on it right away...

The Weber kits are temporarily out of stock. Also I read that they are fairly high which means that you will need to pull out the chassis for some amp heads.

PS: I prefer test points on the amp.

As in pulling out the chassis? The purpose of a bias probe is to eliminate the need to pull out the chassis. However if you are servicing the amp you want to pull the chassis out anyway. No problem for hand-wired amps but some of the pcb amps can be tricky. The bias probes would be great if you are selling tubes over the counter and want to throw in a quick bias check for free to determine if the amp does need to be rebiased (for $!) Actually I was thinking more of using the probes to check a friend's amp at a jam with no filthy lucre changing hands.

I have always used the transformer shunt method if there is no 1.0 ohm resistor between the cathode and ground. BTW the resistor does not need to be exactly 1.0 ohm (as long as the value is stable); we have hand-held calculators these days so you can punch in the exact value of the resistor into the formula to get the correct bias reading.

I was under the impression that the transformer shunt method was more accurate than measuring the current at the cathode. Is that true?

As for adding 1.0 ohm resistor to someone else's vintage amp... I think that I will pass on that! BTW I added all sorts of test points to one of my amps and I thought that the extra wiring might have introduced added noise and parasitics. In any case I decided that it really wasn't worth the effort.

Getting back to the Tube Depot I would think that if you are already ordering tubes from them there will be no additional shipping charge for them to throw in the bias tester kits. So unless you are already ordering something else from Weber the costs could be about the same- whenever they get them back in stock.

Thanks for all of the suggestions- except for the PM that suggested something that is anatomically impossible.

Steve Ahola

P.S. Anybody remember SwampAmp? Back in the 90's he was selling bias testers that he put together himself for a reasonable price- at least until he disappeared leaving several people holding the bag having paid for items that they never received.

[Tom Phillips]

…
I was under the impression that the transformer shunt method was more accurate than measuring the current at the cathode. Is that true? ….

True since the shunt method is measuring just the plate current and the cathode resistor method is measuring the sum of plate and screen current. The accuracy of the measurement is dependent upon the specs of the meter of course. Having said that, I think that the error is insignificant and either method is fine for normal guitar amp service.

…As for adding 1.0 ohm resistor to someone else's vintage amp... I think that I will pass on that….

I concur. Since I’m a test equipment freak who likes playing with vintage test equipment almost as much as guitar amps, I deal with that by using a clip on DC current probe. A very useful tool although not practical for most people to own.

Cheers,
Tom

[Steve A.]

Since I’m a test equipment freak who likes playing with vintage test equipment almost as much as guitar amps, I deal with that by using a clip on DC current probe. A very useful tool although not practical for most people to own.

I started using a clamp-on AC ammeter in 1984, starting off with the analog Amprobe model and then going digital in the late 80's. So how accurate are the DC clamp-on ammeters? Not necessarily for guitar amps but for other DC circuits as well (like the current drawn from the battery when you turn off you car.)

Steve

[Prairie Dawg]

Just my own opinion but I do not like being anywhere near plate voltage or having it anywhere near me. It's a safety thing, particularly where you have no control over the person who's building or using the device. it's just one more problem waiting to happen. JMHOYMMV.

Steve, Amprobe made a nifty line of recording voltmeters and ammeters that were just the ticket for tracking what a device was doing over the course of a day as it cycled on and off. I acquired one of them a few years ago and it answered the question of "What is MidAmerican doing with the power at different times of the day?" The answer was "A lot of stuff that you wouldn't ordinarily notice."

[Tom Phillips]

...So how accurate are the DC clamp-on ammeters? Not necessarily for guitar amps but for other DC circuits as well (like the current drawn from the battery when you turn off you car.) Steve

The specs for the HP428B that I use include
Current Range: 1 mA to 10A full-scale in nine ranges.
Accuracy: ± 3% of full-scale ±0.15mA
The 428B is a specialized test instrument which uses very fancy circuitry to achieve its performance. It also provides an output to allow a scope to be used to monitor the waveform of AC current. Neat!
Tom

[Steve A.]

Just my own opinion but I do not like being anywhere near plate voltage or having it anywhere near me. It's a safety thing, particularly where you have no control over the person who's building or using the device. it's just one more problem waiting to happen. JMHOYMMV.

With this being a kit you could certainly omit the HiV lead (which is cut down to 0.1% of the actual voltage but shit does happen.) And I would only use it myself. Actually I had picked up some of the octal plugs to build my own bias probes years ago but never got around to it. Speaking of B+ I got very unreliable readings from several of my DMMs which were otherwise very dependable. I think that you have an AC component on the plates which might be confusing the "bad" DMMs. And I think that I had one or two that were actually fried- did not give good readings on any of the scales after that. So I now check B+ only when I really really have to- and of course use my Fluke.

Steve, Amprobe made a nifty line of recording voltmeters and ammeters that were just the ticket for tracking what a device was doing over the course of a day as it cycled on and off. I acquired one of them a few years ago and it answered the question of "What is MidAmerican doing with the power at different times of the day?" The answer was "A lot of stuff that you wouldn't ordinarily notice."

I could never get one of my bosses to order one of those. It would have been great to figure out what exactly was going on with a furnace or AC 24 hours a day.

Thanks!

Steve Ahola

[big_teee]

With this being a kit you could certainly omit the HiV lead (which is cut down to 0.1% of the actual voltage but shit does happen.) And I would only use it myself. Actually I had picked up some of the octal plugs to build my own bias probes years ago but never got around to it. Speaking of B+ I got very unreliable readings from several of my DMMs which were otherwise very dependable. I think that you have an AC component on the plates which might be confusing the "bad" DMMs. And I think that I had one or two that were actually fried- did not give good readings on any of the scales. So I now check B+ only when I really really have to- and of course use my Fluke.


I could never get one of my bosses to order one of those. It would have been great to figure out what exactly was going on with a furnace or AC 24 hours a day.

Thanks!

Steve Ahola

Before I retired from My Telephone Comm Tech job we used a Fluke accessory probe a lot.
Similar to this one.
Fluke 617727 i410 AC/DC Current Clamp : Amazon.com : Automotive
It's too big for amps, but handy for what we used it for.
We also used a small current probe that fit on a O-scope.
T

[Tom Phillips]

...Fluke 617727 i410 AC/DC Current Clamp : Amazon.com : Automotive
It's too big for amps, but handy for what we used it for...

Also the spec lists "AC and DC accuracy: +/-(3.5% of reading + 0.5A)" That +0.5A part won't do for making a bias adjustment.

[big_teee]

Also the spec lists "AC and DC accuracy: +/-(3.5% of reading + 0.5A)" That +0.5A part won't do for making a bias adjustment.

I think the ones we used wre more accurate than that.
It was too big anyway for amps.
We used them to chase foreign currents on Isolated grounds, in Telephone office switch rooms.
The small current probes we used on the tektronix scopes would have probably worked.
It's been many years since I ran one.
T