Help with analog meter driver wanted...  - Page 1
Forum section: electronics / projects

author: Audiofanman
author_rank: Contributor
timestamp: June 26, 2025, 11:19:13 am
content: I built KT120 (Tube) monoblocks and love them.I did make a few modifications from the original design, one of which was customizing aDC 0-15V meter (https://www.amazon.com/dp/B06XXM74SC?ref_=ppx_hzsearch_conn_dt_b_fed_asin_title_2)by removing the original internal resistor, adding a pot bla bla bla... 1mA full deflection meter now in parallel with the 10Ω cathode sense resistor reads mA pretty accurately.After having it run hard for a few months, I noticed one of the PS filter caps was leaking.Measuring B+ while using KT88's (Instead of Kt120's) shocked me to find that I was way over the published and previously measured voltages.Im guessing KT88's didnt load down the PT as the KT120s would have...Recapped the PS with much higher voltage caps, and problem solved (for now)...Got me thinking on how I might add a selector switch that could use the existing meterto measure B+, B++ etc and heck feeling adventurous measuring negative voltage for fixed bias.Guess Im building a custom old school MultimeterI then imagined my life in the distant future, trying some tube swapping and knowing the voltages at various points without having to bring to the bench, flip upside down remove bottom panel.. oohh la la!This project has been going on for months now. Im new to EE, New to LTSpice, New to PCBwayNew to everything but learning day to day...The attached and zipped LTSpice project is probably the 10th time I though I was done with any number of circuits leading to each.At least 3 times I took the the results into kicad, created pcbs,waited weeks only to realize I forgot something or decided on a better approach. (Guess everyone has been there!)EE isnt a joke. For such a simple circuit, the enormity of it all is nuts. Deciding on components, researching datasheets, models, pcb fab oh my !Im glad I didn't choose a real challenging project. My hats off to all those EE folks out here who do this every day!!!The last production board I made, I was able to verify that the 100v and over divider and voltage follower circuit worked perfectly.My requirements for this design;1) high impedance or enough so, that the amp wouldn't know it was there.I figured as close to 10meg would be a good starting point. Would mimic a DVM while connected and shouldn't introduce any problems.2) 100% analog or as close as I can get.The screen on a tube is where the signal is taken to be amplified from and any noise added to it would certainly be a problem.3) Very low power and current.4) The ability to adjust various stages of the circuits so the output is reasonably accurate.The meters I use right now are 150Ω, 1mA full deflection. The boards may be used in the future for movements with different specs so 20turn pots of the correct value could be easily adjusted or swapped...The circuits consist of one circuit for measuring under 100V and is a divide by 10.At up to (or down to?) 100v of negative input, 10v of positive voltage is produced at the inverting opamp.That 10v would need to run through 10k pot to supply 1mA for a full deflection on the meter.(I will have to print a new scale for the meter face plate as its graduations are 0- 150 now)The High voltage measurement side 100v-600v is a resistor divider of 100 to 1,600v (max) to 6V and the Opamp acts only as a voltage buffer.So 600 V in, 6v to the pot for 6k to give 1 mA..This last meter scale would need to be printed so full deflection equals 600v.I believe for the most part, I have reached those goals.The circut works well in LTspice, and at least the 100v-600v section has beenpreviously been on a PCB and worked perfectly.Id like the next pcb delivery to be the one that is actually used in the amps,Can anyone see any obvious things I should be aware of ?Comments, recommendations and suggestions would be greatly appreciated.

author: jwet
author_rank: Frequent Contributor
timestamp: June 26, 2025, 02:32:22 pm
content: The circuit is mostly fine. Here are a couple of comments.Things that could use addressing-AC input should have fuse- you likely know this but its importantOp-Amp should have by pass caps of 100 nF or so at V- and V+The input Z of bias input is about 1M which is no problem likely but your stated goal was 10MThat main summing node in the bias circuit has very high impedance, suggest put a 100 pF or so cap from main node- don't make it to big, it has to get charged through big R's=slowSimplicationsIt looks like you have a 5v supply elsewhere- you could do all this with 5v single supply techniques and avoid that AC SupplyAs above, you can buy an off the shelf AC to +-15v supply pretty cheap especially at these low currents.Don't get hung up on the 10M input- DMM's are a ultra general purpose instrument. Think in terms of light loading in the context of the circuit. You Input Z >> circuit Z. Your meter probably has a DC resistance around 150 ohms. If you add 9850, to give you 1 ma with 10v input, you have a 10K input Z. Can the nodes you're probing drive 1 mA at 10v (10k). Back in the day, a Simpson 260, the standard multimeter of the day, was 50000 ohms/volt which mean it used a 200 uA meter movement. No amplifiers, no tubes, no fuss,If you want to experiment and have future plans for circuits then carry on with amp circuit.Have Fun

author: UnijunctionTransistor
author_rank: Frequent Contributor
timestamp: June 26, 2025, 04:03:21 pm
content: The 600 volts may be too much withstanding voltage for the series R2 and R3, if you are using conventional 1% resistors.I would use at least 4 series resistors if you will be using THT devices, at least 5 if you will be using SMT ones.You know, Scheiße happens.

author: Audiofanman
author_rank: Contributor
timestamp: June 26, 2025, 04:39:22 pm
content: Jwet,I planned on a fuse for the transformer primary, and as far as off the shelf dual rail 15v power supply, I see most of them as the switching type. I would like to remove as much possibly for the introduction of noise...As far as the 10meg impedance, im not hung up lol, just what I thought was a good target ;-)The 100-600 volt supply has plenty of overhead to drive the meter on its own, however the negative bias to the grid may not. Like an opamp, the grid on a tube has very little current flow, and negative voltage for the fixed bias supply pretty much is one diode, a few resistors and potentiometer to adjust from -0 to -60v. Again, for my own sanity I preferred not to directly drive anything as I hoped for as much isolation as I could.I will look into the V- and V+ bypass caps !! Ty!UnijunctionTrans,The (2) 1% dropping resistors chosen were picked specifically for their rating of well over the 600v.. its been month or two, so don't remember exactly.But thank you!

author: Terry Bites
author_rank: Super Contributor
timestamp: June 26, 2025, 07:42:57 pm
content: This is the classic way to drive a meter.

author: Audiofanman
author_rank: Contributor
timestamp: June 26, 2025, 08:44:21 pm
content: Terry Bites,Hmm,,, today I learned lol...The way I see it is I have one stage after the opamp, all of the EE would have to be managed before the opamp you show...I like your way as it means even less overall current,In my circuit, 600v = 1mA and the circuit is designed to go as low as 100v as it is now, that's 160 microamps..I did not know an opamp can operate in the sub 1mA region accurately enough.Time to learn even more LOl.. at least for now, I may keep it as it is. BUT I do like what your saying at least for the next version..

author: Audiofanman
author_rank: Contributor
timestamp: July 01, 2025, 03:18:17 pm
content: In my schematics I had 4 outputs leading to one meter....I never though through the actual act of selecting only one of the 4 outputs and how that would be done...I could do something like a 2 deck, 2 pole 4 position rotary switch,So both the input and output are switched together but wondered if there was a more economical way,those switches sure are spendy...I though a simple 4 position rotary and a 4 diodes would prevent back feeding anything back to circuits not selected but this now changes everything...Now wondering how someone would bias an opamp so a .3 to .7 volt drop on the output (Depending on what diodes I chose) could be compensated for.I guess id like to offset or bias the opamps but the term offset and opamp always leads to the opamps internal offset imperfection and nulling it.Can anyone point to how this might be done with a non inverting unity gain follower, and also an inverting unity gain ?Again, this EE stuff is fun and NOT fun lol....

author: Audiofanman
author_rank: Contributor
timestamp: July 01, 2025, 04:01:29 pm
content: I believe the problem was made more difficult than it had to be, I realized I could put the diode before a current limiting pot and make up for the loss by adjusting it a little..And I think I partially gured out the original question, but will wait to see if anyone chimes in ...