Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Platefire on May 27, 2025, 04:53:02 pm
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I did this on one of my builds as a mod and as far as I can tell, it works OK. I always hated to open up an amp to test or set the bias. Even if you read the bias using external test point, you still don't know the plate voltage unless you open up the amp.
Of course we got bias tools like the RUBY BiasMaster to measure externally, plus I have a Military test adapter kit where you can plug the octal tester in the socket and plug the tube into the adapter and it has test points for ever tube pin to allow you read with a MM.
However with the extensive use of bias test points especially on new builds these days , it seems there would be some interest of having test points for plate voltage also! Attached is a layout for the hookup and the 1 Meg resistor tapped to the plate pin reduces the voltage at test point where there is no danger of getting bit from the test point and 1K resistor to ground allows you to set your MM to mV and get the voltage reading. You only need one test point per tube and you can use your ground lug for the bias test points to attach your MM ground for your voltage reading.
So---, am I missing something or why is this something not being used???
BTW-The Thread/discussion I did on this several years ago is here:
https://el34world.com/Forum/index.php?topic=23652.msg254484#msg254484
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Every million+ $ system I worked on monitored/(or TP's) ALL critical parts, even some useless parts......!
close to 40% of system problems were monitoring circuity gone bad :BangHead:
once you have an amp up n working, always go back and take readings everywhere, keep them with the amp, a future you or tech will thank you.
monitoring just the cathode V & I will tell you everything you need to know if you suspect something is wonky on the DC side
monitor away
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Back in the days when I could look for surplus bargains at Radar Electric's store on my lunch break, I bought a shit ton of test points. So far I have only used them for cathode current test resistors, but I have enough left over, so I could really rock out- plate voltage, screen voltage...to quote Jim Carey in "The Mask," --"Somebody STOP me...."
Nanny state folks would point out that a non-knowledgeable person could stick a piece of metal in the test point and get zapped....For that reason I plan to make a screw-on cover with suitable warnings.
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So---, am I missing something or why is this something not being used???
They're not used because they're not needed. If the amp has bias current test points it should come with a manufacturer's recommended plate current, which is all you need to know when adjusting the bias. Plate voltage doesn't change (much) with bias setting, so the test point would always give you the same reading, so it serves no purpose. The mfr could just write the plate voltage on the side of the amp if they thought you needed to know it.
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Yes, I see all your points! I have a Allen Accomplice with bias test points that on the chassis is written 25 mV for 6V6 and 35 mV for6L6's. So yow, he felt it was no need to get voltages for bias.
On the other hand, have you ever been in a playing situation where your amp did something crazy and your wondering what's going on. With these test points you could do a quick check with your MM without pulling the chassis to get a ballpark on plate voltage and bias. I don't know about you but I never had a amp tech following me around and I had to do my checking quickly on break. I did however sometimes carried a backup amp.
I guess it's just a personal thing that I will need to work my way through. When I first started building and converting amps I put a standby switch in everything. A lot of people laughed at me for that. Now days I leave most of my standby's on "play" and just switch the amp on or off!
On the other hand, if I ever build a new fixed bias amp again(getting kind of old), I would for sure put bias test points and think :think1: I would go to the extra mile to include the extra voltage test points also.
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Nanny state folks would point out that a non-knowledgeable person could stick a piece of metal in the test point and get zapped....
Sometimes the "nanny state" folks are correct. Yesterday I found my 2 year old granddaughter with a screwdriver in back of my stereo rig. God only knows where it was going to be inserted.
Since she clearly takes after me, it would have been the worst possible place!
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As I recall this circuit was worked out by PRR and the 1Meg resistor reduces the voltage to a point where even if you stick a screwdriver or piece of metal in the check point, it won't harm you. I was trying to find the original thread where this was discussed with PRR, but couldn't find it. But it was all important to me that the check point be safe from lethal DCV and I had to be satisfied of that before I installed the one I did. With PRR not being around lately, can't get him to verify it.
I don't think there is a way if the 1 Meg resistor shorted out that it would allow lethal DCV to pass??? So when resistors start to fail, don't resistance increase until it burns out and causes an open circuit? The danger would be if the resistor failed in reduced resistance and allow more voltage to pass but---that I think is unusual of non-existent
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Sometimes the "nanny state" folks are correct.
gonna disagree here
teaching "in the act" is the best option.
"what ya doing, making my stereo sound better?"
"Ya G-pa, I want to see how it works.."
cool, lets do it correctly, 1st........
always reward... with knowledge, that which will kill you. by 8 they will be soldering, 10 they will be taking DC voltages, 14 they will be designing IED's (so i've heard) :icon_biggrin:
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How about installing a little digital voltage/amperage display module, they're cheap. Sluckey's site shows him adding one to his variac. Here's some examples from eB@%. Prices are high b/c I had it restricted to U.S. sellers only. If you can wait/hope on getting trade from China, you can pick one up for much less.
https://www.ebay.com/sch/i.html?_nkw=digital+voltage+display+500V&_sacat=0&_from=R40&rt=nc&LH_PrefLoc=1 (https://www.ebay.com/sch/i.html?_nkw=digital+voltage+display+500V&_sacat=0&_from=R40&rt=nc&LH_PrefLoc=1)
Now I'm thinking--dang, that might look really cool. I bet the metal heads would love it if they could slam a huge power chord and watch the B+ sag...
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Cool but why not watch B+ with something like this https://www.ebay.com/itm/355581019345
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or, keep it Analog;
https://www.daliborfarny.com/
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(https://media.tenor.com/nL7sUhLt9MsAAAAM/%D0%BC%D0%B8%D0%BC%D0%B8%D0%BC%D0%B5%D1%82%D1%80-%D0%BE%D1%87%D0%B5%D0%BD%D1%8C-%D0%BC%D0%B8%D0%BB%D0%BE.gif)
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But ...shouldn't that meter go up to 110%....? :l2:
or, keep it Analog;
https://www.daliborfarny.com/
YES! Nixie tubes! I love it.
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the Radar system I worked on in the Navy was the 1st "digital" computer controlled radar, all the servos n synchro's were replaced with digital, the "display" for bearing, speed, etal were Nixies, the drivers were 5 vdc IC chips that latched 90V pulsed
nothing like being BIT by an IC chip!!!
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If you're going to measure externally, you may want to set it up to adjust externally.
I have set some of mine to measure at the back panel similar to the above, and to adjust bias I remove the back cover to adjust a standard pot on the top. But, I also include a lock on the pot to avoid accidental movement. You can get either one of these from Mouser:
P0700-8
ACCPOTDBB173190100
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Wow! Some kind of brain storming going on here :icon_biggrin: That's good!
I don't know how practical having a constant view of plate voltage or any other readings you would want to display continually? I don't usually look at my amp when I'm playing other than turning it on or off. Only exception is if it's behaving strange, I'll take a closer look. Sometimes when I'm getting ready to shut her off, I'll glance at the power tubes for any trace of red plating and maybe do a quick touch and feel to see how hot the tubes are. Maybe if I did have a DCV read out handy, I would be checking more often. I never have carried a MM around with me to gigs, so without that my test point would be pretty useless and a display would be best for a super fast check.
My thoughts on installing a DCV digital, analog or steam display :icon_biggrin: it seems like you would have to plan for it in your new amp build to locate it in a spot you could have easy access visually(front) and also that it wouldn't be close to anything that would cause interference with the amp operation. I have played a many location where my amp was up against a wall or furniture with no easy access to back of amp.
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If you go to Antique Electronic Supply and look for tip jacks, you'll find a photo of these installed on an amp with both the bias current and plate voltage test points. Looks pretty neat.
Rob Robinette's bias calculator says to measure Plate to Cathode voltage, and not just the Plate voltage. If that's the case, then would this Plate Voltage reading method not be appropriate?
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If you go to Antique Electronic Supply and look for tip jacks, you'll find a photo of these installed on an amp with both the bias current and plate voltage test points. Looks pretty neat.
Rob Robinette's bias calculator says to measure Plate to Cathode voltage, and not just the Plate voltage. If that's the case, then would this Plate Voltage reading method not be appropriate?
we mainly measure cathode to anode votage for cathode biased amps. with fixed bias amps the cathode is usually grounded so measuring anode to ground is fairly accurate.
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Here is my test point for DCV. Guess I need to go all the way and do one for the other 6L6GC
Question??? Why does my picture of my test point rotate vertical when the picture I copied from was horizontal?? I've been having that issue a while :dontknow:
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I see it vertical but if I click on it it opens horizontal
Franco
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Cwazy Computers :dontknow:
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As to investigate I give a different cut to the image as to see what it will happen
Seems that the image is shown depending on his resolution aspect ratio (large x tall)
Franco
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Thanks! Don't know how you accomplished "His Resolution Aspect Ratio" but almost afraid to ask :icon_biggrin: