• WARNING: Tube/Valve amplifiers use potentially LETHAL HIGH VOLTAGES.
    Building, troubleshooting and testing of these amplifiers should only be
    performed by someone who is thoroughly familiar with
    the safety precautions around high voltages.

Anode current only swings positive

Hello, first post here please be gentle, also with the material i'm going to post i'm 100% sure i'll trigger someone on how I get the 600v (scary stuff).
Now I've built this nice chunk boi of an amplifier but it's the first time in a tube amp that I stick a current meter (well I use it more to know if I'm going to die or not) and it seems weird in the way it moves. If I play a 5hz tone the needle swings negative and positive respect to the idle point as expected, If I play music and crack up the bass it just swing positive and I dont get why, Is here some kind soul that has time to explain me how a class A amp works into a inductive load?

Yes I'm going to build all of this again with proper wiring, current limiter and everything this is just a test jig dont hate me and my crappy job (I know how to solder and do clean things, but if I'm testing you bet I'm gonna get my 1.99€ solder)

Little explanation about the OT: it's a 230v to 6v ups trasformer with a tap for the 220/230 switch (crappy upses use a relay between these 2 tap to regulate the output)
I'm using the tap as an ultralinear but it's far from it, the tap probably is at 5% maximum, so think it as a pentode config with the g2 to B+

The input OT it's a generic 9v 230v 1VA used to couple a car amplifier and get a few volts out to drive the grid.

The negative grid psu it's a switching thinghy I made time ago with a tl494 and a few spare stuff
 

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FalloNero

Member
2019-12-25 10:25 am
Rome
No power transformer? If so the thread will get nuked, as it's very dangerous without very specific safety considerations and against forum rules.

Well at least i'm being honest, I'm using an isolated inverter for that (cheap one, square wave 12v lead battery) filaments is separate, there is a second wire for that if u look closely.
I could draw a 600vdc psu and lie about it, nothing would change as the question/issue has nothing to do with how this monstrosity is powered.

Still I do understand the safety issue as i'm separating everything from mains (and will use some MOT to get around 1000vdc)

small OT, how can u talk about safety when u still need 600vdc at least 1A, that's gonna fry your guts isolated or not... my 2 cents :)
 

FalloNero

Member
2019-12-25 10:25 am
Rome
I like 600V ! I like people who arn't afraid of 600V. Just be careful!

I'm going to be completely honest, I've only shocked me once, WHILE IT WAS OFF
I guess capacitor bleed resistors are a must :)

And still I'm going to ask where is safety when you have the anode connection exposed on the GU81 and no matter how many isolation trasformers you have you're not playing with 6v6s or similar, there is no current limiter that's going to save you, this bloody tube can suck almost a full ampere :cool:

Still anyone knows why the anode current just goes up and not moving up and down as I think is expected?
Someone knows how to plot the loading curves for this case? I mean the gu81 is not really for audio purposes...
Maybe i'm saturating the OT? I've a few of those trasformer, maybe i can parallel them on the output and series on the HV side?
 

FalloNero

Member
2019-12-25 10:25 am
Rome

Just finished xmas lunch and I'm hardly breathing, I hope you guys had a good one too!

Can you please explain me why this makes the current just move up? I'm curious, I understand basically how everything works in DC but when you put an inductor in the mix stuff gets real bad real quick :)
The only purpose for an inductor for me is to block RF in a trasmitter (I made a 4mhz one with 2 6v6 in parallel, cathode modulated, got 40watt out of them and then decided to switch it off to not kill the tubes :p)
 

Sorento

Member
2008-03-12 9:59 pm
You say needle, so your meter is old school moving coil.
The moving parts have a mechanical resonance frequency in the single Hz and require some sort of damping such that the needle settles smoothly.
5Hz is below the resonance and the needle follows the signal swing.
Music is 50Hz + and hence above the resonance.
Now the damping means may not work symmetricly for up and down, e.g. faster up than down.
Which might explain what you see ...
 
I'm going to be completely honest, I've only shocked me once, WHILE IT WAS OFF
I guess capacitor bleed resistors are a must :)
?

I always put a high value resistor in series with an LED across the main cap to give a visual indication of a dangerous voltage level. It also slowly discharges the capacitor on power down.
 
You say needle, so your meter is old school moving coil.
The moving parts have a mechanical resonance frequency in the single Hz and require some sort of damping such that the needle settles smoothly.
5Hz is below the resonance and the needle follows the signal swing.
Music is 50Hz + and hence above the resonance.
Now the damping means may not work symmetricly for up and down, e.g. faster up than down.
Which might explain what you see ...

Yeah maybe true, but it's really not going even a millimeter under the idle current, I mean just for the weight and the spring effect of the needle coming back it should, look at the video, i'm passing big current on bass peaks :/

Ignore the loud hum the filter caps are pretty small, I'll be getting a batch of caps these days from a lot of atx psus :)
The meter reads volts on the 100ohm cathode resistor, so it idle somewhere 120mA @600vdc

here's the video, it's too big as an attachment, let me know if you can see it
http://rushallnetwork.com/ASD/tubeboi.mp4
 
I always put a high value resistor in series with an LED across the main cap to give a visual indication of a dangerous voltage level. It also slowly discharges the capacitor on power down.

Great idea!, I'm planning something nice with separate switches for filament, grid bias and HV with current limiting HV mosfets (I mean just to prevent I poke a hole in the anode if something on the grid goes bad)

And maybe a decent isolation trasformer for the mains input. Realistically speaking I'm relying on the chassis to ground (once my granpa told me, keep the sparks inside and hope for the GFCI to pop, I live in Italy, if you pass more than 10mA to ground that boi's gonna pop) and a better mounting/shielding for the tube :)
 

Parafeed813

Member
2012-08-15 8:24 am
First: you can try to bend the rules all you want, but forum rules are very clear: no discussion showing schematics directly connected to the mains.
Many people are only googling images, find a schematic and think it's ok. Next they build it, touch a grounded pot and find themselves being electrocuted...

About the amp part: you're using a power transformer as single ended output transformer.
That means that there is a significant dc current going through it, magnetising the core.
Add bass frequencies and the core is loaded even more, making it saturate.
You need a transformer with an air gap.
If this transformer is not welded together, you can try to remove the lams and restack with a gap.
 
First: you can try to bend the rules all you want, but forum rules are very clear: no discussion showing schematics directly connected to the mains.
Many people are only googling images, find a schematic and think it's ok. Next they build it, touch a grounded pot and find themselves being electrocuted...

About the amp part: you're using a power transformer as single ended output transformer.
That means that there is a significant dc current going through it, magnetising the core.
Add bass frequencies and the core is loaded even more, making it saturate.
You need a transformer with an air gap.
If this transformer is not welded together, you can try to remove the lams and restack with a gap.

You're right, too many people on google going for it without proper knowledge you dont even know how much I've planned to limit damage in case something went wrong, I'll try to edit the schematics (you know really what's going on there but not everyone needs too)

Yeah these transformer are the cheapest thing I've held in my hand I think just a few hits and it's going to delaminate, what should i do to air gap it? I know that MOT have like little pieces of metal that everyone remove to get more death out of them
 

PRR

Member
Paid Member
2003-06-12 7:04 pm
Maine USA
www.diyaudio.com
......If I play music and crack up the bass it just swing positive and I dont get why... how a class A amp works into a inductive load? ...

An ideal class A amp, not clipping, the current does not change.

A *real* A amp, the tube (or BJT or FET) turns-on easier than it turns-off, so at high level the average current rises a small amount (5%-10%). This is discussed in the standard references such as RDH.

*When clipping*, the current is not set by bias but by loading. There is an ideal load for an ideal amp which causes no current change. With real parts we can match the expected small rise and get a bit more rated power. If the load is too low for the bias, current will rise; if too high current falls. (Note that speaker impedance is all over the place, as is our music, so the real trend is uncertain.)

Push any R-C coupled amp too hard and it grid-blocks, current falls.

None of this is worth dying for.
 
An ideal class A amp, not clipping, the current does not change.

A *real* A amp, the tube (or BJT or FET) turns-on easier than it turns-off, so at high level the average current rises a small amount (5%-10%). This is discussed in the standard references such as RDH.

*When clipping*, the current is not set by bias but by loading. There is an ideal load for an ideal amp which causes no current change. With real parts we can match the expected small rise and get a bit more rated power. If the load is too low for the bias, current will rise; if too high current falls. (Note that speaker impedance is all over the place, as is our music, so the real trend is uncertain.)

Push any R-C coupled amp too hard and it grid-blocks, current falls.

None of this is worth dying for.

Wow your answer is great, I would listen to you all day, this such things are not teached here in university (i'm almost done with and yes eletronic engineering), they slaps you the classic "class a bad not efficient" and that's it, never tought that infact current should be the same! more voltage flowing in the load, less in the tube and viceversa.
So basically I'm clipping :p
I was wondering how much I'm pushing out, is there a way to calculate how? every loadline I've tried to trace I get pretty strange numbers... then for "low" plate voltage (i'm right at the knee for a pentode, is better if I go with a triode config? I''m aiming for power, not really stellar but to justify a bit the 100w+ heaters, I'm not expecting 100watt of audio mind you)
I'l search those references your talking about and give them a look
I' always angry to know more, thanks you!

OT: About the schematic, the POWER GROUND is NOT connected to chassis, everything is floating, input trasf, output trasf, grid bias is referenced to power ground but not the chassis, the 12v dc psu inside has the negative and earth post separated, the chassis is at earth potential, any issue with the amp apart massive current able to create a hole in the plate will hopefully catched by the chassis popping the ground fault breaker, I'll probabl6 put one in the device itself, not all homes here do have one.
 
Lines on paper are not the whole answer.

Why don't you *measure* the power output? Monitor for clipping. Monitor DC currents for clean and clipped.

I'll rig up something with a dummy load and an oscilloscope hooked up to it. It's an old single trace so I'll have to switch between input and output, but I'm pretty sure input side is clean, it comes from a bridged 100watt car amp I use to power the subs.

I'll let you know asap, running this tube does heat up my room after a while :p
Thank you for your knowledge and time.
 

Sorento

Member
2008-03-12 9:59 pm
An ideal class A amp, not clipping, the current does not change.

That's hard to believe, without momentary plate current changes there could not be any output signal whatsoever ...
What you have in mind is that the *average* plate current does not change.
And this is what happens when the O.P measured the plate current with a moving-coil meter:
At 5Hz the needle follows the momentary current swings to some degree.
With a musical programme which contains only frequencies 50Hz and above (even with heavy bass) the coil damping does not allow the needle to follow the fluctuations and hence the meter integrates and shows just the average current, which indeed does not change much.