$3 Tube Amp BIAS PROBE Made in UNDER 30 MINUTES!

In this vid, I'll show you how to make your own cheap, DIY tube or valve guitar amplifier biasing probes for measuring plate current and plate voltage. Similar ready-made probes from Eurotube sell for much more, and they don't come with the multimeter: amzn.to/2Fgsu4r
WHAT YOU WILL NEED: a cheap multimeter, an old worn out tube, and an octal socket. If you play your cards right, you can get the multimeter for FREE, the old tube of course for FREE, and the socket for FREE, but if you have to buy the socket, they are available for under $3. I also used a bit of black tape, some solid core wire, and shrink tubing.
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[@TheGuitologist]

Check out our sponsor, Native Sons Goods. Use the coupon code GUITOLOGIST10 for 10% off your entire order. They sent me a couple of the new 3" straps to check out, which I'll do in another vid. www.nativesonsgoods.com/

[@Andy_Yates]

I've got a couple of their straps now with the hemp backer. They're beautifully made and have consistently looked even better in person (even with the quality photography they have). The guitars stay secure with that backer too so I'm not fighting neck dive

[@TheGuitologist]

@@Andy_Yates Thanks for supporting the sponsor, Andy. I agree, these are the absolute highest quality woven straps I've ever laid hands on, and I have a large collection of them!

[@Andy_Yates]

@@TheGuitologist Yeah I remember watching your video a while back on them. I was thinking they're a little spendy, but I took a chance. When it arrived, I was like, ok this is awesome. I get it now. I have 4 of them now and I kinda want them all haha. Everything in stores just feels cheap by comparison, even stuff that is comparably priced or more expensive

[@TheGuitologist]

@@Andy_Yates They are a luxury purchase for sure.

[@wayshot]

I just ordered two straps; Lefty and Eden. Been wanting a Lefty strap for a long time because of the nice pattern and colors. Besides, I am a lefty, so that one may be the ultimate strap for me :-)

[@fuzzmountain7583]

I know this is a 5 year old video but I still wanted today thanks for this great tip.

[@TinwhistleD]

Thank you Amigo! This is brilliant. I have a completely blown EL34 from a Second Hand, Orange Rockerverb 50 MK11 which I am trying to repair. Now I am going to construct what you have just described. All the best. Keith from Liverpool UK.

[@sdjgfashjasbfasd]

I watched your video this morning and decided to make this bias probe today. Works great!

[@Boffin55]

If you just solder a 1 Ohm resistor, across the probe wires, then if your meter fails (or isn't connected) it still works, and instead of measuring current, just put it on the Voltage range instead of current, you'll get current (as the voltage across 1 Ohm is the current flowing through it....) V = I . R (and R=1)

[@soldersucker1]

Thanks Brad soldered one up last night worked like a charm

[@BeardedBluesDude]

What a great idea. I'm gonna have to build one of these once I get to working on my old amp stash :)

[@glenwhatley4125]

I have a few of these i made a while ago. Great idea. I used the black plastic octal base that Antique Electronics Supply offers for a few bucks and the PC mount Belton Micalex tube socket that just has straight long pins. I bent out pin 8 on the socket then inserted the rest of the pins into their respective spots in the octal base. Then soldered a 1 ohm resistor into the base pin 8 along with a piece of stiff wire. Connected the other end of the resistor to the bent out pin from the socket. Then all you need is a volt meter to measure the millivolts across the 1 ohm resistor. The reading of course translates directly into milliamps. This way you don't need to keep the meter connected. If you have two of these adapters you can then merely move the meter to the other tube without disrupting the operation or even putting the amp in standby. You should put some shrink wrap on each pin and wrap the whole thing in some kind of tape to keep from electrocuting yourself accidentally. I suppose you could get the octal tube socket saver (for tube testers (also available from Antique Electronics) and Dremmel a slot in the side of it right where pin 8 is and add the resistor there, thereby automatically have the high voltage pins out of reach. Nice demonstration... Mars Amp Repair

[@brianyork5510]

I'm finally present when a notification came. I dug it Brad. Very useful.

[@TheGuitologist]

Thanks, Brian. Thanks for all the support!

[@soapboxearth2]

Thanks so much for all the great content ! Much appreciated! I started watching without the slightest clue about amps whatsoever but jow I'm proficient with head scratching... lol . I have learned a lot though. Also another fan of yours LowHeadRoom has been a huge help as I have completed my first build a 5e3. His tutorial was a Godsend. I built a current limiter , a cap discharge tool and a cheapo attenuator. Works well with up to about a 40 watt amp , bigger, and she smokes like a hash joint 😆. I saw this probe video and made one recently . I biased my 73 YBA1 today as well as a Hot Rod Deville I bought that was on the fritz (for about 200 usd. The IC caps were spent and had a bad power tube. Bought some Supertech caps , cheap.). works great. Now I'm going to gut it, sell the boards and build a Chinese kit in it. 5f6a with a plexi style channel mod. Another video I saw on yt. Also a very helpful dude. .. Anyway man I want you to know aside from the entertainment you have been a huge help to many people. Cheers!

[@hkguitar1984]

Super Useful DIY. If you own a tube amplifier you absolutely should learn how to check the bias. Great video, thank you.

[@themongoloid8944]

Sooooo glad you made it back!!!! We missed you man. !!!!!! I want you to get 200k subscribers by Black Friday. !!!!

[@TheGuitologist]

I think my channel is a bit too niche for that, but we'll see. Never say never, right.

[@yusdiy]

Thank you Brad, this is knowledge of the highest orda!

[@SeanKerns]

Nice tutorial! Bias probes are great.

[@bullwhipjohnson8247]

That's a pretty good tip. I've shorted out the ameter function on my Fluke a couple of times trying to prode bias current while the Amp is on.

[@jamesmccabe8736]

Thanks man. Good idea. I'm just getting into tube amp repairs. I really appreciate the videos.

[@TheGuitologist]

Thanks for watching James. I appreciate you more than you'll know. Best of luck with your repair endeavors! Keep learning every day.

[@Oobidoobi]

Cool project. Thanks Brad.

[@mischef18]

Great video bro. All the best

[@bobbaumeister5243]

Thank you very much Brad. This tool is clever and help us. Rock on!

[@JR-et5kx]

This video made me want to buy one premade but i learned how this thing works now so atleast i understand what im doing hahaha, awesome video !!

[@budandbean1]

Now this was cool, thank you Brad!

[@travix-1969]

I'm not sure why I didn't get notified of this video, I had the bell checked. Great video. Thanks.

[@TheGuitologist]

Thanks Travis. I appreciate you watching my videos, man.

[@thecascadian7401]

Built one today. Great video.

[@cosmicray007]

Great video !!!

[@ajm5150]

Thank you! Testing today 🤩👍

[@dennispennell]

Cool tip, great idea!

[@sarmtech9534]

Thanks for this Brad, building one today! I initially went to use a 6550 that was red plating for this project. When I opened the 6550, the grid connection pretty much fell off at the pin. No wonder it was red plating! :) Edit: I soldered the red lead to the amp side as that's the more positive of the two.

[@curtisprice9806]

I had to think a minute lol! Right...the output transformer (amp side) is the source of the 300-500+ volts that goes to pin 3 (plates) of both tubes or sets thereof(push/pull). Even when a voltage is negative such as bias voltage....the red lead goes to the less negative of the voltages. Example ...negative 40volts(-40) is more positive compared to negative 41 or 45volts(-41, -45)!!! Red lead goes to -40 point and the black lead goes to the -41 or -45 point.

[@user-oy5gu8wz2n]

Great idea!I'll make it too!

[@mikelucero6973]

Thanks I purchased one from nobsound and couldnt get it to work, by watching your video I realized what was wrong. One of the leads was not connected and this video helped me to solder where it needed to go. Bingo, amp is dialed in. Also thanks for showing a close up of the meter and the setting itself.

[@mobicus1]

Great hack! Thanks for this!

[@CraigHollabaugh]

Hey Brad, here's an interesting thing. I was checking out your 'STUFF I USE' on amazon, when I clicked on your JVC camcorder, all your other stuff variac, dmm, heat gun, contact cleaner, etc showed up in the "Customers who viewed this item also viewed" section. You have a real influence man. Thanks for all the content.

[@TheGuitologist]

Bezos knows what's up. He might be a lizard like Zuck, but he knows to listen when I make a suggestion. :)

[@CraigHollabaugh]

@@TheGuitologist If your fans all buy a variac, iso tranny and the Megantz book, then I'm OK with the lizards.

[@stu-nix]

Thinking of you, possibly in and around flooded areas. Hope the Lindsey Family stay safe.

[@timmyv694]

Thanks for this video :) I’m making one these for sure

[@alfatone2654]

Cool man, I'd love to see a part two where you actually measure the bias on a ( set of ) tube(s) that is the rated value in watts for a 6V6 GT and/or a 6L6 set and show us a how one might insert a variable pot at the bias resistor(s) to use either set of tubes on the same amp. Showing us Luddites in You Tube Land how to measure the rated wattage per matched set (V=IR or some variation thereof ) would go a long way to keeping us all subscribed, like our Uncle would. I've heard how one (you) cold set these hotter or cooler (whatever one might interpret from that terminology) and actually play us some Ramones some Keith Richards or some Wes Montgomery to demonstrate WHY you bias the tube set that way afterwards would be greatly appreciated. I know you can, so go the extra mile and keep us all happy...

[@hyperluminalreality1]

I made a bias probe in 1996 using an octal base from a blown Tesla tube with all 8 pins present, a ceramic clip in/lock ring style octal socket, 2 feet of zip cord, a dual banana plug and a 2w resistor that measured exactly 10.0 ohms on the same multimeter I would be using to take bias readings with. Still have the probe and use it all the time. The multimeter is long gone and so is the Beckman that followed it. Using a UT61E now. BTW, most all handheld Multimeters have the inputs spaced to accept a dual banana plug.

[@robertcalkjr.8325]

Thanks Brad! We can always use more test tools and equipment. The cheap test leads are usually the weak link that makes DMM's look bad. I personally do not trust cheap leads or wiring for anything.

[@robertcalkjr.8325]

Yep. I only buy test leads and probes from Probe Master now. And I have my own wire to make test leads with and I solder the alligator clips on. I made the leads for my DC bench power supply myself.

[@rkrasinski]

Please more of these video's 😎🤘

[@richclayton5785]

Liked the tunes too!

[@EM-df6mo]

Great video. I built something similar a few years ago. I like yours a little better.

[@tchip-bc1gp]

thanks , gonna do one !

[@IamtheFerryMan]

I was just aboit to drop $100on the Stewmac VHT probe kit. Thanks for this!!

[@rickya3877]

Very cool,thanks

[@michaelnorred1495]

Just as a note, instead of breaking the glass envelope, you can unsolder the pins and pull the tube wires out, then solder the jumber leads into the pins. Aslo If you have a tube with the tall sides from the pin base, you recess the socket into the base and make a little neater.

[@TheGuitologist]

Thanks. Only problem with the tall base is it extends the height of the unit, making it difficult to insert into some heads.

[@markhoskins]

That's a pretty good tip!!! :)

[@PooNinja]

home made tools from scraps.... Hell yeah that sparks joys that fixes toys :D

[@DetroitWrecker666]

Pretty cool!

[@sisajtegabre]

Great video, saving guitars from a trouble and saving them a lot of $.

[@williambock1821]

Thanx ,Brad!

[@cyb3r3vil30]

Got to say pretty nifty TFS

[@tele789]

Have you ever installed bias probe points on the back of an amp? That would be a great project.

[@jimhough6233]

Love the bias probe idea! Great riff with harmonica near beginning. That was you right!? Or what was that!?

[@PrinceWesterburg]

On Octal bases you can actually unsolder the pins and take the base off, there might be some cement in there thats hardened but its fairly easy.

[@kenhancock8931]

200mph duck tape is tha shit!! cheers brad

[@dirkayala5812]

Ya, so I know I'm getting alittle obsessed when my wife is starting to get bored of me watching ur vids!;} I been laid up on sick-leave,(Quickly on the mend now though), and I must have buzzed through 10-20 vids this last weekend and definitely shared channel with old bandmate/guitar player. Completely new to the game here but I'm gonna go look at an old Packard Bell Phonocord tomorrow for bout $20bucks as potential first real project. He says powers on & Tubes light up so I figure nuthin else, I can start the Parts-Stockpiling at very least, looked like original Mic was included too. Thanks again for the Inspiration to hopefully some fun times!:)...

[@thomasgwilliams7957]

Nice!

[@chrisf6216]

Thank you for sharing/posting - awesome! Once upon a time, I might have bought the GT Tube kit which includes that fancy widget shown here. I think I paid more than $50 for it... and no DMM included.

[@Eric-uf8zx]

I cracked open some matched NOS WE Black Plate 300Bs for this, they will make superior probes.

[@whitneyschuster2439]

...damn near gave me a heart attack 🤣

[@BradBartram]

Wrapping the whole assembly in duct tape reminded me of 90% of the stuff my father used to make. And just to mention, if you don't have a tube or don't want to go through reclaiming the base, the same places that sell sockets also sell empty bases for not more than a buck or two each.

[@TheGuitologist]

I very nearly filled the cavity with epoxy instead of using the tape. Would have been prettier.

[@BradBartram]

@@TheGuitologist Glad you didn't. There's a certain charm for stuff like this if it's wrapped in duct tape.

[@TheTrueVoiceOfReason]

Now, if you'd have wrapped it in electrical tape and then friction tape after that, it would match the vintage look of all those vintage amps. :)

[@TheTrueVoiceOfReason]

@@goodun6081 By the time that would happen, you'd have to rebuild at least the test lead connections, at which time it would be a complete re wrap anyway. And Duct tape gets sticky over time when it is handled like that. Besides, you won't be leaving it in circuit for any appreciable amount of time anyway. And its only aesthetic.

[@valvenator]

@@goodun6081 I think you misspelled "Messy Booger" :))) ................FLEX!

[@LastChanceRiders]

That was awesome I've always wondered how this works. I bought a premade probe exactly like the one you just showed me how to build. But now I know exactly how it works. By the way we would love to sponsor a few of your videos. What do we need to do to make that happen? ( John - Guitar player )

[@TheGuitologist]

Your probe could be measuring the current at the cathode, which is safer because no high voltage, but you can't get a plate voltage reading. Also, any current measurements taken at the cathode will be adding in the screen current to the plate current, so this is way more accurate. As for sponsorships...I'm always looking for sponsors. Email me and I'll send you a rate sheet: bradlinzy at gmail

[@tomfoolery2082]

I got that same meter at home depot for nx to nothin but it does a good job,, yep gona do it thx.

[@richardweinberger2756]

This is some classic Guitology, for sure !! If I wanted to make an adapter for a 6BQ5 could I use the same concept only with the other type of socket arrangement? You can still use your cheapie meter for other tasks by plugging in another set of leads. Very wise. I have a few of those myself, if you buy something for 99 cents they give you the meter for free. Thanks, Harbor Freight :)

[@keithfields1178]

Curious, how did you get solder to stick to the original stems from the tube as they are normally coated for heat dissipation and solder won’t stick to them? With the plate current measurements, are they the values needed to adjust the overall biasing required by the tube and controlled adjusting the reostat provided for biasing? Is the current a more accurate value for biasing than observing and adjusting voltage?

[@marcuscarrozza732]

If I made one of those I would be 💥⚡🔥😶‍🌫️😵. I just don't trust myself doing that . I know me and I am afraid of electricity. But it's great for those who are confident enough to make one . Thanks for the do it yourself video.

[@kgsalvage6306]

I appreciate your idea! It took a little more than 30 minutes though. Even with everything on hand. Unless you're in 2X speed.

[@TheGuitologist]

So you got one made?

[@kgsalvage6306]

@@TheGuitologist I actually made one the day you posted it. I had everything here. I had an octal plug that I used instead of breaking a tube. I put female banana plugs right on the test socket so I can use banana jumpers. I think I'll make one for 9 pin minis, EL84 ect. Thanks again for the video.

[@kevingriffin8854]

Hey Brad. Youve got this funky japanese bass. Ive got the same one. But just the body. Do you know what brand? Also what do you think of the pickups? Would it be worth hacking them into another bass?

[@paulcargo4233]

Purt neat.

[@chkao59]

Can I use the BV1.0 line not the 16awg line? Thank you.

[@francisangrignon5914]

Hey ! Great video ! Question for you, is it possible that I need to change the position to be mesured depending on the tube or the circuit? Or position 3 should do it ? Thanks ?

[@TheGuitologist]

Use the position shown in the video and that'll work for all tube types.

[@szaki95]

Alternatively you can use an octal socket relay if you don't have a salvageable tube on hand.

[@p_mouse8676]

The Guitologist. How is that soldering iron doing so far, using it on a pretty regular basis (daily?) ?

[@stoneyvision108]

I have one of the eurotubes bias testers that doesn't work on some amps others it works on. I'm guessing this will not work on my peavey valveking gen1 but will on my 5150.Just like the euro bias probe. Would I be correct?

[@RaviNewfarm]

Yupp, that works, just remember this thing may be lethal. You have 300-400 volts exposed in several places here, and even if you have tuned off the amp but have charge in the B+ caps you could get lethal shocks out of this. May be easy to get in touch with B+ here when you insert or pull the probe, and the other hand on the chassis to steady things since there is a bit of force required to pull or insert the probe. Could easily happen when changing over to do the other tube.. I would have made sure everything was insulated, e.g with a separate lead for the anode voltage measurement and fully shielded probe ends for the meter side, or by to inexpensive meters and build the whole thing in a box.

[@feffryjeffjeff]

Will that work on EL34 's

[@TheDogPa]

I think I'd prefer a connection that lets me put the leads into a female socket, not have a male one exposed, but great video and glad to see you here today. Can you do me a favor? Explain to Henning Pauly how to do an ad that won't make me change the channel....LOL!

[@BuhdaPunk]

what about voltage, what is the max voltage the meter needs to handle?

[@lowheadroom]

Hey Brad, didn’t you find some octal sockets with pins that you can use in place of the broken tubes? Also great video as always man

[@Dustyfingers]

What Fluke model do you have? I think my Fluke 83 iii only goes down to 400ma.

[@michaelanderson5560]

Thank you - this is very helpful! Can you clarify the setting and connection for the plate voltage?

[@TheGuitologist]

If your amp is NOT cathode biased, you can just hook a second meter to measure plate voltage from one of the plate leads coming off the probe to chassis ground. that will measure your plate voltage. If your amp is cathode biased, you'll have to open the amp and measure from plate to cathode.

[@michaelanderson5560]

Thank you again!

[@ThomasACarlos]

I'd like to see someone (maybe you?) build a probe for an EL84, 9 pin tube. I like the idea of the probe sold through Tube Depot, the Bias Scout. It utilzies a 1 ohm resistor coming off the Cathode pin and a Voltage divider coming off the plate. So one probe allows you to measure (stepped down) plate voltage (in millivolts) and cathode current. Unfortunately, I cannot find a kit for this.

[@rangerdoc1029]

Sweet! I need all the money saving tips I can get. Did you ever find a better display for the variac? Mine lost its magic smoke from running it too low. I haven't been able to find one with separate power & input.

[@TheGuitologist]

There is a hack for these. I've been meaning to hack it and make a video of that. Thanks for reminding me.

[@BradBartram]

@@TheGuitologist I just upgraded my variac and added the meter along with the hack referenced. It works great and is pretty easy to do.

[@TheGuitologist]

@@BradBartram I keep meaning to do mine, but just keep putting it off. Maybe I'll put it on the short list.

[@martinkk2943]

So how do I bias the tube with that?

[@mattliebenau9083]

Cool. Good tip. Maybe a dumb question but if you're working with paired tubes would it be worth it to make 2 testers?

[@TheGuitologist]

If you want to take the time to make a second one, it'll save you time in the long run, but you can do it with just one probe.

[@AMSOfficial79]

so... biasing is as simple as using the homemade probe and using the bias pot(s) on my amp to tune the bias to my amp's recommended settings?

[@theguitaramptech]

I love all you videos Brad, and I like this your probe design, but it can be very dangerous. You hinted at it in your video, but I think a "Red Alert Warning" may have been more appropriate. You will have the full plate voltage of around 450V (700V on a MusicMan) on your probes. You could have made your probe safe by putting in a 1ohm metal resistor in the cathode path and measure set your meter to volts. 35mv isn't going to hurt if you touch it.

[@TheGuitologist]

I hear ya, and thanks for watching. But... Measuring at cathode is less accurate. Also, if I put a "red alert warning" on this video, what level warning belong on my regular videos where I'm prodding around inside the chassis?

[@theguitaramptech]

What you're saying is true. There are life-hazards all through a tube amp. This one just made me feel a little uncomfortable. What I really like about your videos is that they are real. You show the dead-end trails we all follow, the mistakes we all make, until you solve the problem. I like your honesty

[@gerthalberg9735]

Hi Brad, from the look of your solder joints you’re using lead solder. What temperature do you set your iron to??

[@uncled39]

Stun

[@nickzambrano]

I can't find an output transformer for a Peavey 6505+ other than the 300 dollar mercury magnetic ones. Does anyone know where I can find one for a reasonable price? Tube amp doctor is out of stock right now.

[@Iam_Dunn]

Hardest part for me was finding an octet socket. Pretty hard to find wayyyyyyyyyy up here in Northern Ontario. So, I bit the bullet and ordered a cheap one from RipOffexpress.... oops I mean Ali.....not so express... LOL :)

[@isodoublet]

Amazon has them.

[@kennygoody5862]

Dude i wasn't getting notified of your new videos.

[@TheGuitologist]

I know. KZbin has turned me off.

[@graxjpg]

I was just thinking I hadn’t heard you say fillies and fellers in a little while, where’s my shitpost?? Also, did you make that background music?

[@gbass7328]

Good job but that way all the external wiring and the meter are all at above 450vdc. Just remembered the output dc resistance/winding voltage drop method. That's what I'll be using on amps without a cathode resistor.

[@DavidImrie]

Just completed one of these.. took the heartache out of smashing a valve. Thanks. I just realized the one I used had no physical pins for 1&6 so I won't be able to use it for some like el34 I think

[@TheGuitologist]

Here are a couple alternatives: www.tubesandmore.com/search/node/octal%20base

[@DavidImrie]

@@TheGuitologist oh neat. Thanks. I'm based in the UK but I'll try hunt some out for V2.

[@seanwingate5734]

You should do a review of the Stage right Tube amp. Maybe mod it.

[@TheGuitologist]

If that's that Monoprice thing, it's already been overdone, I think. I thought about getting one to mod, but there were already a lot of vids on those up. If someone wants to send me one, I'll do it, otherwise, I'll pass on buying one to review.

[@jjsant3250]

Using the meter in line can be inaccurate unless it is verified otherwise. Who knows with that thing (which I also have.) Even a 1ohm in the cathode channel with that meter, in voltage mode, could be questionable. A 10ohm may be more accurate but would require sliding the decimal in your head. Tapping in the plate channel removes the screen factor which is good but the cathode avoids the high voltage. Lots of tradeoffs.

[@whitneyschuster2439]

why is using the meter in-line inaccurate if you have a decent meter? i thought this was basically the only way to directly measure current, isn't that what they're designed for?

[@Bob.Silverstein]

I am curious - why don't people expose a probe point for Pin 3 as well, so that it's possible to directly measure Plate-to-Cathode voltage in cathode-biased amps? I know that for fixed-bias, the cathode is grounded, so your plate voltage tells you all you need to calculate dissipation. But for some amps (e.g. Champs, Tweed Deluxe), you'd need to make a separate measurement of the cathode voltage and subtract it from the Plate voltage. Thanks for posting this video!

[@peterdavies5358]

Hi, a bit slow here in the UK but I just got told, honest.

[@cherokeejeep5382]

Quick question, years ago my friend showed me how to make a bias probe similar to this but he soldered a 1 ohm resistor I think on pin 3 between the red and black leads and read the millivolts? Would this work or is my memory failing? Thanks 😊

[@TheGuitologist]

That's only going to be as accurate as the resistor. If the resistor isn't exactly 1 ohm, the calculations will be off. Better to measure plate current directly.

[@philipdecatanzaro1822]

For the price of test sockets this is a more viable alternative. I've been seeing stupid prices on EBay for them.

[@gabrieltudor6165]

Would you use that cheap multimeter to monitor the plate voltage? I have one and i want to self bias my amp, but i`m afraid that the thing will blow up or something when messuring 450V

[@TheGuitologist]

That meter isn't rated for that, I don't think. But it'll measure the current just fine.

[@TheGuitologist]

You could try it. Worst that could happen is you brun up a cheap meter or blow the internal fuse.

[@BuhdaPunk]

If you post the measurement I can cad something so it can be 3d Printed.

[@frustro4323]

just made one