heater elevation not working

[mxrshiver]

on this modded '64 Showman, the heater elevation i installed is mysteriously only giving me 0.056VDC.

i've got the transformer wires running directly to the pilot light at right, then twisted wire out to the heaters from there. a 250R humdinger with the legs attached to either side of the winding, and the wiper to the middle of the elevation voltage divider, seems to work great and eliminates hum with a small adjustment. heater voltage is running a little hot at 3.35V/3.3V, so i might drop it with a couple 0.1R resistors, but can't imagine that's the issue.

the elevation divider comes directly from the B+ at the end of the rectifier diodes, which consist of one 1N4007 and one fast switching UF4008 in each leg, with 10nF 2kV caps bridging each one, as well as a 22nF/2.7K hash filter at the transformer wires. filter cap is two 80uF 450V's in series with 220K balancing resistors across, then a choke to the next stage. B+ measures about 460V.

the divider itself is a 390K 2W metal oxide from the B+, then a 43K 1W metal film to the power supply ground, with a 33uF 63V cap in parallel with the 43K for additional filtering (overkill, i know, but with what i had on hand it was 33uF or 220uF). the cap is definitely attached with correct polarity, and tests with no leakage up to 200VDC on my Heathkit C1. i also replaced the divider resistors, cap and humdinger just in case - same result. i was aiming for an elevation of 45V...

anyone know why i'm only getting a few 10's of milivolts?? 🤔🤔🤔

[acheld]

You write very well, but it's hard for me to visualize w/o a schematic.  Any chance of posting yours?

[sluckey]

Look at the schematic and you'll see the PT filament winding has a center tap that is internally grounded. You must break that internal ground in order for your elevation to work. Not worth it IMO.

     https://el34world.com/charts/Schematics/files/Fender/Fender_showman_ab763.pdf

[mxrshiver]

here's the schematic for everything mentioned. forgot to add the .056VDC measurement at the middle of the heater elevation divider.

sluckey ur the mvp ✨️ i had not noticed that note! however, part of the reason i didn't notice it is because there actually seems to be a heater center tap brought out on an orange wire that was grounded with the red/yellow high voltage ct. my meter doesn't do well below half an ohm or so, but it and either heater lead measure 0.2 ohms to each other, same measurement between the two leads. is it possible that there is some sort of internal grounding of the heater ct in addition to it being brought out on the orange wire? what would be the purpose of this, just to have double insurance for the ground connection? and in order to undo it, i'm guessing i'd have to open up the transformer? main purpose of adding the heater elevation is to cater to a nice cathode follower someone added that's got about 210V on the cathode, but i can always bump down the cathode resistor if elevation is infeasible.

the heater wires would be a pain to desolder, but i did disconnect the high voltage wires just to confirm resistance readings, and i found something REALLY strange, at least to me. with the positive multimeter lead attached to any of the high voltage leads and the negative to any of the heater leads, the reading is infinite resistance, as expected. but reverse them, with positive to the heater and negative to high voltage... and it reads 284.5K ohms! is this normal, or indicative of impending failure? the amp performs great by all indications, whisper quiet with the B+ only a few volts above schematic and rock steady. if there was really a leakage path between the high voltage and heater windings, i'd expect some sort of issue under load...

[kagliostro]

To know if there is an internal CT in addition to the external CT is simple

Disconnect the 2 wires plus the CT you see, then connect your meter to the CT you see and measure if there is continuity between it and the transformer bells

Franco

[sluckey]

The orange wire is probably connected to a shield inside the transformer and should be connected to ground. It has nothing to do with the heater winding.

with the positive multimeter lead attached to any of the high voltage leads and the negative to any of the heater leads, the reading is infinite resistance, as expected. but reverse them, with positive to the heater and negative to high voltage.

Remember the filament CT is grounded. This hi/lo meter reading is being measured through the bias circuit which is fed from a tap on the high voltage leads. The bias diode is responsible for the hi/lo readings (AKA front to back ratio).

[mxrshiver]

with the measurements i took (i'm away from the bench until this evening), either side of the high voltage winding and its center tap were disconnected, though the bias tap was still connected. the orange wire presumed to be a shield was also disconnected. i will remove the two heater wires again if needed, but they're such a PITA and that lil humdinger does not put up with unsoldering/resoldering and unbending/rebending its legs very well, so i'd like to avoid it if i can.

if the heater ct is internally grounded and the orange is connected to the transformer shield, shouldn't that mean they're connected to the same place internally? that would gel with my readings since, with the orange wire pulled, it still measures 0.2 ohms to either heater leg in circuit. i'm also not sure what else 'internally grounded' would mean... if it wasn't brought out on the orange wire, what would it be connected to internally to make it 'grounded'? would this be in reference to the physical connections from transformer to chassy via the bells and bolts?

as for the 284.5k ohms reading, the polarity does make sense with the polarity of the bias circuit. but the resistance amount makes no sense... the bias circuit has 470R, the diode, a 10k pot and a 27k from the tap to ground. the heater elevation circuit has 125R thru one half of the hundinger and 43k to ground. add all those up and it's about 80k, a far cry from 284k.

i'm operating under the assumption that the mounting of the transformer does not ground the bells, and therefore shield and internal heater ct. it seems that the only connection for both is that orange wire. a major part of what's informing this assumption is the fact that, since i assumed the orange wire was heater ct only, i had it taped off when operating the amp, and everything functioned well except the heater elevation, including the humdinger, which i understand would not function if there was another path to circuit ground for the heaters besides the humdinger.

buuuuuut if the orange wire is the only path for that 'internally grounded' heater ct, and it was taped off... why wouldn't the elevation circuit work?! i need a coffee...

[sluckey]

if the heater ct is internally grounded and the orange is connected to the transformer shield, shouldn't that mean they're connected to the same place internally? that would gel with my readings since, with the orange wire pulled, it still measures 0.2 ohms to either heater leg in circuit. i'm also not sure what else 'internally grounded' would mean... if it wasn't brought out on the orange wire, what would it be connected to internally to make it 'grounded'? would this be in reference to the physical connections from transformer to chassy via the bells and bolts?

The Heater CT and the orange wire are both connected to the shell/core of the PT and when the PT is bolted to the chassis everything is mechanically/electrically connected to chassis ground.

as for the 284.5k ohms reading, the polarity does make sense with the polarity of the bias circuit. but the resistance amount makes no sense... the bias circuit has 470R, the diode, a 10k pot and a 27k from the tap to ground. the heater elevation circuit has 125R thru one half of the hundinger and 43k to ground. add all those up and it's about 80k, a far cry from 284k.

You are assuming the forward biased diode has a resistance of zero ohms. That's wrong for your simple meter check and that's why the resistance doesn't add up. The clue is the hi/lo resistance reading. You're lucky that your meter puts out enough voltage to forward bias that diode through the approx. 80K series resistance. Every meter will probably show a different resistance reading. Nothing to get hung about.

i'm operating under the assumption that the mounting of the transformer does not ground the bells, and therefore shield and internal heater ct. it seems that the only connection for both is that orange wire. a major part of what's informing this assumption is the fact that, since i assumed the orange wire was heater ct only, i had it taped off when operating the amp, and everything functioned well except the heater elevation, including the humdinger, which i understand would not function if there was another path to circuit ground for the heaters besides the humdinger.

That's a wrong assumption. The only way to get your humdinger or elevation to work will be to totally unbolt the PT and isolate it from the chassis.

buuuuuut if the orange wire is the only path for that 'internally grounded' heater ct, and it was taped off... why wouldn't the elevation circuit work?! i need a coffee...

You've been told the orange wire has nothing to do with the heater C/T. Either accept it or go home, disconnect the orange wire, and prove it to yourself. An easy check is to simply measure heater voltage with reference to chassis. If you still measure about 3vac from each heater wire to chassis, then the C/T is still connected to chassis.

I believe you will have to abandon the elevation idea. May as well pull the humdinger pot too because it cannot function as expected as long as that internal C/T is grounded. If that pot is a small low power trim pot it probably won't last much longer.

[mxrshiver]

my meter reads little to no resistance forward biasing these diodes out of circuit (Fluke 87), so i guess i figured it'd be the same in circuit, but wasn't considering what the meter had to do to forward bias it enough to get no resistance out of circuit, and how that'd be a tougher job in circuit with 80K of series resistance. that makes sense, thanks!

i don't doubt anything i've been told, i'm just trying to get a more complete understanding of what's going on here. unfortunately my knowledge and experience at this point is mostly practical and results-oriented, which leads me to hangups on more basic principles, as evidenced by my lack of understanding of multimeter function - i'm trying to change that.

i've been confused by the dual assertions that the heater center tap is connected to the shield, bells, orange wire, and mechanically to the chassy by mounting... and also has 'nothing to do' with the orange wire. but regardless of phrasing, i definitely understand now that there's no way to lift the heater ct besides taping off the orange and mechanically isolating the transformer from the chassy, which i could do with some nylon shoulder washers but it probably wouldn't be worth the effort. so it does seem that the orange wire is provided as dual insurance for that mechanical ground connection for the PT and heater CT?

besides that, the only question in my mind is how the itty bitty humdinger was working at all with a separate direct heater ground reference - hum definitely increased at the outer rotation limits and was at its lowest just off center. so, it sounds like you'd expect a lot more dissipation thru the humdinger with the existing ground reference in the PT? i'd love to keep it as it seems to be working and has been for 5-6 hrs of hard playing time, but if it'd be risking overdissipation or instability then it's not worth it of course.

[sluckey]

What's the power rating and ohm value of the pot? For example, a 250Ω pot connected to 6.3v will dissipate...

     P = E²/R = 6.3*6.3/250 =  0.16W. Double that to 0.32W for safety factor.

Is your pot rated for 0.32W or higher? Many are rated for 0.5W. Many others are rated for only 0.25W.

Even though your humdinger should sorta work if the wiper is grounded and the C/T also connected to ground, I believe it would work much better if the C/T was not grounded. I don't know this for a fact as I've never tried both grounded. But I know even without trying that you cannot elevate that humdinger with the heater C/T grounded. And you do too!