Tube Amp connections : 0V vs EARTH vs HEAT vs CURRENT

[merlin]

Any prediction on timeline?
Curious if there is a one line description of what you found needed updating. I would think that there isn't much changing in the world of tube designs these days.

I'm hoping to get it out later this year, keep an eye on my website for any update. There isn't much that changes in the world of valves, but there are always changes that can be made to my books! This one is getting quite an overhaul, with less useful stuff removed and whole new chapters on new subjects added...

 

[ccaudle]

keep an eye on my website

I know where that is, but a link in your "about" tab of your profile might help people find it.
https://valvewizard.co.uk/ for anyone who doesn't already have it bookmarked.

 

[systemtruck]

I’m ready to post some more specific information here, with one key question below. This is all a learning experience. This is for fun, but I’m being careful. I am aware I’m creating fire hazards for the time being.

I’ve finished re-housing / re-building from scratch a 1950’s Hi-Fi amp. It was a receiver unit that has a radio. I was interested in repurposing the amplifier / audio section as a guitar amplifier, putting that in a new enclosure situation, then leaving the radio section in the old chassis to utilize that separately some time in the future. So that’s what i did, but i also divided the power section from the audio section. So i have two small units, power, and audio. The radio stuff is still sitting in it’s old chassis, unused for now.

I followed the grounding advice given here in this thread, along with the filter/0V distribution described in the valvewizard article on grounding, implementing a heavy gauge bus rail for 0V. Everything seems to be working great. All noise is FAR far quieter, which is very exciting and a nice success.

The obvious complication when deleting a whole radio section of this unit is that I’m now missing that whole load of a bunch of tubes/circuits, so all B+ is going to be too high.

But before we get there… a side note… when deleting the radio segment, i pulled a 5.6K high wattage resistor, so i had that available. And it has a tap @ 100R, with 5.5K being second section, so a total of 5.6K.. This is indicated as R59 part A and B, in the posted schematic below.

As expected, when powering up my new system that did NOT have the radio 5.5k + circuit load on it, and with no mods to compensate for this, the main B+ voltage is of course too high. It settled around 410 DC where as it should settle at 360 DC. So I figured, let’s have fun and try hooking up that high wattage 5.5K resistor section loading B+ to 0V just to see what happens. I know it’ll get hot as hell and i might even damage things or start a fire, but a short trial might teach me something. Worst case i destroy my amp and use my fire extinguisher haha. This power resistor is pretty large, at 3 inches long and 5/8” thick, so maybe it can handle some short term heavier wattage.

So i added the two mods drawn in light blue below. The other mod is a 7W 125R resistor I happened to have on hand. Figured maybe it would help lower voltage, assuming I’d be too high still. And on first power up, I ended up with an absolutely spot on 360 DC B+ rail. Bizarre. This is pure luck of course, since this in no way resembles the whole radio circuit.

But yeah, the 5.5K to 0V gets hot. You wouldn’t want to touch it for more than a second, or even less. Also, the neighboring components are now quite toasty. The added 125R resistor is hot too, but not as much.

This is not a safe or permanent setup. But man, the sound of the amp is exactly as original, sounding incredible but now also very quiet, so i know the dummy load mods are functional to whatever degree.

What can i take from this? Can i simply but a much higher wattage 5.5K resistor as this dummy load and call it a day? 100W perhaps? Maybe the heat will be handled by the resistor and it won’t fry neighboring components? According to my understanding, my setup sends nearly 25W through this resistor, as well as around 65mA. That’s a lot of lost energy. However, i suppose that if I’m nailing the 360V DC with this, it perhaps means that the original radio circuit is burning exactly that same amount of juice? Obviously the power transformer already was designed to provide DC current to a whole host of more tubes, so maybe it has around that much to spare anyways, and would be the reason the voltage is now back to spec.

Below is a clearer sketch of the original power circuit, and the two mods in light blue that give a 360V DC B+ result. The original schematic is weird to trace, so this is much better.

In a dreamy ideal world, i would have a toggle switch that allowed me to either engage this dummy load, or engage a connector that send this original B+ rail through 5.5K to the separate radio unit. Then I can power up the radio with this same power supply and audio amp. The radio tubes and indicator lights were all on their own 6.3VAC, and right now i just have one indicator lamp hooked up. The voltage is probably too high but i haven’t checked yet. I could install a drop down high wattage resistor there too.

One great perk of this setup is that the high voltage gets drained almost immediately after power down. Maybe a handful of seconds. That’s really handy!

 

[Matador]

What can i take from this? Can i simply but a much higher wattage 5.5K resistor as this dummy load and call it a day? 100W perhaps? Maybe the heat will be handled by the resistor and it won’t fry neighboring components? According to my understanding, my setup sends nearly 25W through this resistor, as well as around 65mA. That’s a lot of lost energy. However, i suppose that if I’m nailing the 360V DC with this,

The 5.6K isn't needed. Just increase the new 125R until your b+ is back down to 360V.

 

[systemtruck]

The 5.6K isn't needed. Just increase the new 125R until your b+ is back down to 360V.

Ok got it. Whenever i attempt to achieve that within PSUD2, it is difficult to get the voltage to lower that much. The load at the end of the line in PSUD2 is difficult to create so as to mimic reality, and i think that’s what is making it difficult to simulate voltage drop.
Any guesses as to what value and wattage will safely drop the rail 50 V (360 down from 410) in this spot where i have the 125R? Experimental shopping for power resistors really adds up in cost. I could get an adjustable power resistor though, high wattage. So maybe that’s the best attack, if i can just figure out a rough target value first. Maybe I’ll try installing the 5.5K there and see how low it drops it.

 

[ccaudle]

Any guesses as to what value and wattage will safely drop the rail 50 V

V=IR (Ohm's Law)
Rearranging R=V/I, so you need to know the circuit current, then you can calculate what resistor will drop 50V with that current. No need to guess, but you do need to measure.

 

[systemtruck]

V=IR (Ohm's Law)
Rearranging R=V/I, so you need to know the circuit current, then you can calculate what resistor will drop 50V with that current. No need to guess, but you do need to measure.

So measure the actual current draw of the whole DC rail of audio amp. Safety assumed, I guess the best way to do this is put the meter in line at the beginning of the B+ line somewhere? The reason I say this is that interrupting a 0V leg seems confusing since 0V is not actually returning anywhere except for the chassis point at input jack. Or can I in fact get a current reading between 0V of first cap and 0V of rectifier or something like that?

 

[ccaudle]

0V is not actually returning anywhere

Current always flows in loops, the current is getting back to the power transformer somehow. When the current does not actually return that is called an open circuit, and all the devices stop working.

Do not get complacent with the "0V" connection, when you break the connection to insert a meter, if there is power applied (or even power off but capacitors still charged) the high side of that connection will go up to the B+ voltage. Hopefully you know that, but in case you don't make sure you don't shock yourself trying to connect your current meter into your 0V connections.

If only the chassis is used for the return path, then figuring out when you can reliably interrupt the current to measure is possibly trickier than it is worth, so measuring on the high side would be the most reliable.

can I in fact get a current reading between 0V of first cap and 0V of rectifier

You should be able to, but that will be pulsed current, so might be hard to interpret. Your meter should average it and show the RMS current.
By the way, did you mistype? 0V of the first cap connects to the center tap of the transformer according to the schematic you posted, the rectifier connects to the transformer and to the high side of the cap, there is no "0V of rectifier" connection.

 

[systemtruck]

Current always flows in loops, the current is getting back to the power transformer somehow. When the current does not actually return that is called an open circuit, and all the devices stop working.

Do not get complacent with the "0V" connection, when you break the connection to insert a meter, if there is power applied (or even power off but capacitors still charged) the high side of that connection will go up to the B+ voltage. Hopefully you know that, but in case you don't make sure you don't shock yourself trying to connect your current meter into your 0V connections.

If only the chassis is used for the return path, then figuring out when you can reliably interrupt the current to measure is possibly trickier than it is worth, so measuring on the high side would be the most reliable.



You should be able to, but that will be pulsed current, so might be hard to interpret. Your meter should average it and show the RMS current.
By the way, did you mistype? 0V of the first cap connects to the center tap of the transformer according to the schematic you posted, the rectifier connects to the transformer and to the high side of the cap, there is no "0V of rectifier" connection.

Ah yes, that’s what I meant! Meant to say 0V of transformer.
And yes I already consider 0V to be just as dangerous as the B+ since it’s all the same signal finding its way home.
I’m glad you mentioned about the power being pulsed because I forgot about that factor in this context. I’ll just go ahead and measure the B+ rail current draw. I believe my Fluke DMM will handle it fine but I’ll double check the max voltage.

Thanks for the tips, I’ll report back. One other question.. with said 5.5k high watt resistor sitting where it currently is, from B+ to 0V, the B+ is perfect 360V as the amp was designed. Should I measure current draw as it stands now? Or should I delete the 5.5k resistor, and let the B+ sit high at 410V and measure current like that? I do worry about over voltage somewhere but maybe it’s fine.

 

[ccaudle]

Or should I delete the 5.5k resistor, and let the B+ sit high at 410V and measure current like that?

You can measure with the 5.5k removed, or measure with, and then subtract the current through the resistor from the total to get the current of just the circuitry. At 360V the current in the resistor would be 360/5500, or 65.4mA, so you can just subtract that from the total you measure with the resistor in place.
That might be more accurate, possibly the circuit draws slightly more current at a higher voltage, and current at the desired voltage is what you want to know.

 

[merlin]

I guess the best way to do this is put the meter in line at the beginning of the B+ line somewhere?

Never use an inline current meter if you don't have to, it's just an invitation for mistakes. Instead measure the voltage across your 125R resistor.
Current = Voltage / Resistance.
Now you know the current.

You could even measure the voltages across each resistor in the power supply, allowing you to calculate the current at each branch in the circuit, and you can plug those numbers into your PSUD model.

 

[systemtruck]

Never use an inline current meter if you don't have to, it's just an invitation for mistakes. Instead measure the voltage across your 125R resistor.
Current = Voltage / Resistance.
Now you know the current.

You could even measure the voltages across each resistor in the power supply, allowing you to calculate the current at each branch in the circuit, and you can plug those numbers into your PSUD model.

Well that’s inticing and a heck of a lot easier than desoldering and resoldering major power connection points!

Can I work backwards from the final preamp stage and measure drop across its resistor to determine current, then measure drop across resistor/stage prior to that and then subtract that other (final) stages current to get its resulting current too? And keep working backwards? Is that how it works? Since it’s all in a line, following it backwards I’m guessing each successive current draw is included in the one before it?
I suppose if I can get separate current draws per stage, i can then add a “constant current tap” at each filter in PSUD2 to create each actual measured current draw at each stage?
If that’s the case, I can fully recreate my system and just start increasing resistor values at the 125R location until my first rail reads 360V.

 

[merlin]

Yes exactly!

 

[systemtruck]

Yes exactly!

Great thank you for this help! It made things so much easier.
I went and measured all voltages, with the dummy high watt mod still in place.
It doesn’t quite balance out perfectly at the early stages around that dummy load, and I’m assuming some of this is because the resistor values are not spot in and because heat dissipation is killing some energy too. Maybe the resistors even have as much as 20% tolerance. That would probably explain the issues in totaling things up.
But overall this gives me a great picture to work with. I’ll go ahead and create current loads within PSUD2.
Also, i had incorrectly drawn the dummy load placement in previous drawing here. This V2 drawing is correct, and. It shows where the old 5.5k placement used to be. But in the mod, the 5.5K branches off in between the 125R and 100R. This was my mod that i just tried for fun, and happened to give a solid 360V at B+.
So when recreating this in PSUD2 with all my downstream current loads in place, I’ll adjust the 125R upwards until it achieves the same voltage drop when I delete the 5.5K, and then I’ll just add said new higher resistor value to the 100R that follows it so they can become one resistor. I’ll make sure to calculate wattage of the total new resistor and get one that’s at least 4X the power rating.

Lastly, I 100% want to add a drain resistor permanently in place. It’s very reassuring to have the B+ get drained after power-off, and incredibly convenient for working on the amp moments after power was applied. I’ll select something high so that it doesn’t load things down, and i know it’ll take a little bit of time to drain. I wonder where the best place is for possible noise/hum issues caused by it? I’m splitting hairs though, I’m sure the amp is noisier than anything imposed by a drain resistor.

 

[Matador]

That seems a bit odd: once everything is charged and stable, the current through the 50 ohm resistor R66 (160mA) should be the same as the current through the 125R resistor (unless Kirchoff lied to us!). Also, the current through R59A should be the current through the 125R minus the current through R59B, but it isn't. So either:

a) The voltage readings are incorrect (seems least likely)
b) There is an additional current drain (16mA) on the 388V node that isn't drawn on the schematic
c) The 125R is actually a ~112R resistor

If I had to make a wild guess, c is correct, because the current through R59A is about the same as that through R66 (~160mA) minus the current through R59B (~60mA), leaving ~100mA through R59A.

IF i'm correct, then you need to drop the same voltages with ~60mA less current than you have going through your 125R and 50R, which means you need to go from 396V down to 370V with ~100mA of current, which is ~270 ohms. If you want to keep the 50R in place, then the 125R needs to be increased up to ~220R (with a 5W rating). If it was me, I would just go with two, 150R's (replacing the current 125R and 50R).

 

[systemtruck]

That seems a bit odd: once everything is charged and stable, the current through the 50 ohm resistor R66 (160mA) should be the same as the current through the 125R resistor (unless Kirchoff lied to us!). Also, the current through R59A should be the current through the 125R minus the current through R59B, but it isn't. So either:

a) The voltage readings are incorrect (seems least likely)
b) There is an additional current drain (16mA) on the 388V node that isn't drawn on the schematic
c) The 125R is actually a ~112R resistor

If I had to make a wild guess, c is correct, because the current through R59A is about the same as that through R66 (~160mA) minus the current through R59B (~60mA), leaving ~100mA through R59A.

IF i'm correct, then you need to drop the same voltages with ~60mA less current than you have going through your 125R and 50R, which means you need to go from 396V down to 370V with ~100mA of current, which is ~270 ohms. If you want to keep the 50R in place, then the 125R needs to be increased up to ~220R (with a 5W rating). If it was me, I would just go with two, 150R's (replacing the current 125R and 50R).

Each time i measured DC, i let the voltage settle down to the point where it stayed in place for at least 10 seconds. It should be really close to final stable voltage, but perhaps some give or take is contributing to some variation in math here. I just don’t want to let this circuit be powered up all that long, because i doubt that 5.5K resistor is meant for 25W and I’d rather keep that thing alive while i do all this testing. It sure is hot as hell.

But i think it’s mostly just the resistors being a good bit off in tolerance, maybe even 20% is what they used. I didn’t measure any of the higher watt ones yet, but i did measure a couple downstream lower watt ones and they weren’t impressively to spec.
I have some room to experiment with extra filtering too, like I could add one stage. I could tack on a capacitor after a solo resistor just to further filter the ripple, which will affect voltage drop I believe. So I’ll experiment in PSUD2 once i have the current draws all in place.

Fun stuff! I’m not concerned with precision for this particular project. I just want to safely operate this re-housed amp after deleting the radio segment. I’d also like to be able to connect the radio unit to this unit’s power section, so I’ll probably end up designing something that can be switched or patched or whatever is safe. Obviously that will impact the structure of the sections in question here.

 

[Matador]

Oh, you asked about the bleeder resistor to ground. A typical value is 470K for supplies around 400V. It can be placed anywhere on any of the supplies you measured. It's high enough value not to disturb the voltage values you just worked to establish, but it's also low enough to drain the supplies in a reasonable time (< 30 seconds).

Most people who work on tube amps have a special bleeder clip, which is a 10K, 10W resistor with a set of alligator clips, that can be clipped into the B+ supply to discharge it quickly.