Bias Meter Build - Benchtop / Rack Mount / Overkill

cooljuk

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In keeping with the trend of my variable isolated power supply build, I'm crafting myself a fancy bias metering tool.

I've repaired my 10 year old and discontinued Weber Bias Rite about a dozen times. I've also never really loved it. It has some inherent flaws...

I can only see voltage OR current at a time, and only for one tube at a time. I hate flipping around the rotary dial and switches while adjusting. I want my hands free. It's also a little non-backlit LCD screen that I can't see from a distance. The leads are only two feet long and not long enough to put the meter somewhere practical, nor route the leads away from sitting across the glass of cooking hot tubes. The probes are tall. The probes are conductive copper. The socket portion of the probes keeps coming off with the tubes. That thing is going to give me a nasty shock, at best, one day. So....

Transformer shunt method is great. No problem with that! I'm well aware of it. ...but what I wanted more than a way to just bias an amp, is a tool to observe plate voltage and current draw on four octal or 9 pin tubes, at the same time. I want to see all of that data clearly, at a glance, from a comfortable distance, while I work on an amp. I also wanted easily replaceable probes / leads, isolated and non-conductive probe bodies, longer leads, something extremely rugged that will last me a very long time, and a plate voltage range up to 1kv and a current draw up to 200mA. With my bench meter being 19" rack mount, I'll make this the same.

Of course, nothing like that exists. ...yet.




I messed up and for 1kV AC meters instead of DC. I'll have use for them in something else. New ones on the way.


Here's my basic idea for a layout. Voltage and current above each other for each probe. Food Neutrik 4 pin jacks for the probes on the right and power switch on the left.







It's going to be an AWESOME MONSTER!
 

cooljuk

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These are the octal probes I'm building. Bases and sockets, I'll wire them internally and drill a hole on the side of the socket for the lead to exit. I think I will leave on the metal flanges, at least for now until they get in my way, as an easier way to lift the probes straight out of the amp's tube socket, or to hold it in the socket while I remove a tube.


Here's that pretty little NOS gem that photo-bombed in the background (1965 Jensen C12Ns in the WAY background).

Blackburn plant gold! ...no, better than gold!
















Already running into my first build problem!



The meters are about 2mm too tall to fit in the case, without cutting away sections of the upper or lower flanges. I think that's what I'll do. Unfortunately, this rack case front panel is a thick section of aluminum sandwiched against a thinner section of steel, and I'll be cutting away a good amount of it in pretty fine shapes. I think I'll ask my local CNC machinist for a hand. ...and maybe laser would be better? Either way, I'm going to try to hand this part off. I don't want to Dremel it.

I did get these backup meters (on the top), in case the others didn't fit. They are voltage and current on one meter. The problems with those are that they are short a digit, they are much smaller 8 segment LEDs, and the voltage range is limited to 600v, not the 1kV I want.


Ok, that's all for now!

Crazy or awesome? Both, maybe?
 

JaxLPGuy

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Ok, gonna have to watch this.

Sort of reminded me of when Radio Shack used to have a tube tester.
 

Soul Tramp

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Well that's a lot of work!

So it would seem you don't use the cross-over notch to adjust bias. Hmmmm?? :rofl: :rofl:
 

cooljuk

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Well that's a lot of work!

So it would seem you don't use the cross-over notch to adjust bias. Hmmmm?? :rofl: :rofl:
Well, both, really. :)



The downside to that, for me, is that it doesn't say anything about the individual tubes. I like the idea of watching how all the tubes behave as I bring the whole amp up or adjust the bias. Of course, I'm open to being totally wrong about all of that. Certainly, you know more about it than I do and I'm happy to learn. Maybe I could probe on the individual tubes and not the output transformer? I do have a four trace scope that does some fancy math and could be fun to watch just the differences and such.

While we're chatting - maybe you can enlighten me on something regarding the notch method - when I use my ears and / or eyes on the scope, I get that notch nearly gone, but not totally, and get a big clean sound I like. ...but then I do the math and I've got a class A/B amp biased at 95% plate dissipation or something crazy like that and have to bring it back down. So, I end up with all my amps biased at about 70% and wishing I could go higher (and I know some do, maybe I should? I use excellent tubes).



This is a good shot of as clean as can get the notch before clipping. This is on a plexi. I feel like I should be able to get it a little less "notchy" but that's 70% dissipation. Think I'm doing something wrong or do I just need to embrace the notch a little more?



The "wobbly" on the peaks is just due to my needing to recap the power supply of that scope. It does the same on a clean square wave direct from a function generator, after the scope warms up a bit. Just haven't had the time and patience to take that thing ALL the way apart.
 

Soul Tramp

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My mentor wrote the book on x-over notch biasing (literally). My experience has been the same as yours. When you bias an amp to sound its best, the tubes are boiling their life away and on the edge of detonation. But it sounds spectacular. Like you, I bias around 75%.

What spooks me most about biasing at 90%+ is the risk of a component drifting to a degree that red-plating at high volume occurs.

I've found biasing individual tubes to be more of an intellectual exercise than something that actually improves overall sound quality. Especially when you consider the waveforms on either side of the P/I are somewhat asymmetrical (don't forget Leo Fender and his purposeful design of an unbalanced P/I).

Biasing individual tubes who have more value in a hifi amp, but even those don't do it.
 

cooljuk

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I'm always down to read a good technical manual about old gear and music-related gear so feel free to share a link or title to your mentor's book!

I did a Dynaco ST-70 rebuild that ended up having individual bias and balance per pair, so I was able to bias each tube individually. I think it would have been nice for cutting down the hum if it had not been for my PT which actually had some vibrations and probably needed a baking and re-varnishing. Triode made a toroidal transformer that was also beefed and could run KT88s better. That was my plan. When I moved across the country I gave that amp to a good friend, though. If I did it over, I'd rebuild a pair of MKIIIs.

I'm not really interested in biasing the tubes individually in any amps. More just watching how they all behave and seeing if they are matched well enough. If one starts climbing, I'd like to catch it fast, as I've been playing with lots of old tubes. In fact, I should probably get / build a curve tracer. I've been plotting traces in a spreadsheet manually. In the meantime, this will let me sort of do that at a glance as them come up and just watch for anything funny going on, as well as seeing hot they match up. Just biasing one or even one side leaves me wondering how that load is distributed.
 

cooljuk

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Here's what I'll use for my 9 pin probes. I got a set of "socket savers" that I can tap into. Securing one of the pins after I cut it for a current meter will be interesting. I'm not yet sure how that will work. Initially, I'm thinking "bend it and epoxy it."

 


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