Questions about tube biasing

Pete Farrington

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It doesn't have to be real precise as long as it isn't red plating or too cold.
Just to note that most valves won’t redplate until they’re overdissipating heavily, eg significantly above their absolute max / twice their design centre limit, .
So the absence of redplating shouldn’t be taken as an indication that the idle point is ok.
Valve life seems to be directly related to dissipation, so the colder they’re idled, the longer they’ll last.
If a big fat lively clean tone is desired then idling anodes in fixed bias at around 18W is probably helpful. But if using preamp overdrive, then the good linearity at high power output provided by hotter bias may not be particularly beneficial, and the reduction in valve life something of a drawback.
 

pedecamp

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I'm going to bias the tubes on my Origin50. I have the meters (two of them, side by side) with the right adaptors. Don't worry, it's a straightforward job on an Origin, and I'm an engineer so I know to keep my fingers out the way.

First question:

What are the ideal settings?

Second question:

I've always been told you should never run a tube amp without a speaker connected. Something to do with charge buildup in the tubes and / or power caps.

But...

Does this apply during biasing as well?
Watch this:

 

ThreeChordWonder

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Biggest problem I've got is that I can't find meters that will SAFELY* read plate voltage at a reasonable price. Cheapest I've found are about $110 each. If I need two to measure two tubes concurrently that's $220. My $40 for a dual ammeter unit, won't read plate voltages, so I can only infer or back calculate them from what I've seen or read online.

As far as I can tell from reading what's been posted above and watching online, the current readings I'm getting, which are more or less identical tube to tube, should be okay, and it doesn't seem like they're the cause of my hum/buzz when I plug in a guitar. The tubes aren't red plating either.

* I can find plenty of adaptors that will plug in between the tube socket and the tube that have alligator clips attached to the ends of wire tails. Having 400 to 500 volts on the ends of not very well protected alligator clips, however...
 
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Pete Farrington

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Biggest problem I've got is that I can't find meters that will SAFELY* read plate voltage at a reasonable price
I suggest to measure the HT voltage, eg at the output transformer centre tap OT CT.
At idle, that will be very close to the anode voltage, with a few volts, certainly well within any sensible margin of error.

We’re not dealing with guided missiles here, this stuff will fluctuate constantly eg with mains voltage etc, near enough is plenty good enough.
 
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Jethro Rocker

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Just to note that most valves won’t redplate until they’re overdissipating heavily, eg significantly above their absolute max / twice their design centre limit, .
So the absence of redplating shouldn’t be taken as an indication that the idle point is ok.
Valve life seems to be directly related to dissipation, so the colder they’re idled, the longer they’ll last.
If a big fat lively clean tone is desired then idling anodes in fixed bias at around 18W is probably helpful. But if using preamp overdrive, then the good linearity at high power output provided by hotter bias may not be particularly beneficial, and the reduction in valve life something of a drawback.
Good to know!
There was a member who was doing a test on an amp leaving idle current at 100% just to see. Last I heard it was over a year at regular playing with no ill effects. Granted it eats tubes faster.
 

ThreeChordWonder

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Figured it out.

Two voltage probes and two digital multimeters on order. Those and the dual milli-ammeter I've already got are getting nailed* to a board. The adaptors will be glued together and tack-epoxied to the meters. $60 or $70 for the lot. Including the $40 for the dual milli-ammeters and their adaptors.

* Brass nails, naturally. Don't want any stray magnetics :D
 
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V-man

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Biggest problem I've got is that I can't find meters that will SAFELY* read plate voltage at a reasonable price. Cheapest I've found are about $110 each. If I need two to measure two tubes concurrently that's $220. My $40 for a dual ammeter unit, won't read plate voltages, so I can only infer or back calculate them from what I've seen or read online.

As far as I can tell from reading what's been posted above and watching online, the current readings I'm getting, which are more or less identical tube to tube, should be okay, and it doesn't seem like they're the cause of my hum/buzz when I plug in a guitar. The tubes aren't red plating either.

* I can find plenty of adaptors that will plug in between the tube socket and the tube that have alligator clips attached to the ends of wire tails. Having 400 to 500 volts on the ends of not very well protected alligator clips, however...
I purchased a TAD Biasmaster back in the day when I was hopping around 6+ Marshalls. Possible overkill for those who have 1-2 amps, but such a great and convenient unit, albeit expensive AF.
 

william vogel

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^ I'm NOT prodding around inside the thing...

View attachment 107771
Just FYI but every time you plug a cord into the electrical receptacle you are very very close to 339.39 volts peak to peak if it’s exactly 120 volts AC rms. I promise you won’t get any closer inside the chassis. It’s really not a big deal. Go inside and take the measurements and write it down on the chassis or a sticky note and reference it later.
 

yladrd61

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BEng CEng MIMech E. So mechanical.

Pipelines, subsea production systems, oil and gas. 30+ years.

The point was I know enough about amplifiers not to poke my fingers where they don't belong and to read a meter. I never pretended to be an expert on the things, which is why I asked the questions I did.

As it turned out I've got 32 to 40 mA across both plates, depending on which power setting its on. The tube vs tube readings are pretty much cock-on identical, so I didn't need to adjust the bias pot, which, AFAIK, is the only adjustment possible on an O50 without getting the soldering iron out.

I'm still getting a lot of mains hum from the thing. It's as silent as the grave when just plugged into the mains, but when I connect up a guitar, directly or through my pedalboard, it hums. We don't have any floursecents in the house, all LED bulbs now, and no electric motors anywhere nearby either. I've tried changing rooms, different outlets, etc. It diminishes when I touch the guitar strings though. The only things I can think of that are left before I send it to a pro (expensive - a new head might end up cheaper) are the rectifier tube (which is virtually new) or that the mains cable ground wire is bad, so instead of the amp grounding me, I'm grounding the amp.
Are there any dimmers on any of your nearby light switches ?
 

ThreeChordWonder

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About 30 feet away, but a lot of smart switches in the house. They might be the cause, I suppose.
 

ThreeChordWonder

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Just FYI but every time you plug a cord into the electrical receptacle you are very very close to 339.39 volts peak to peak if it’s exactly 120 volts AC rms. I promise you won’t get any closer inside the chassis. It’s really not a big deal. Go inside and take the measurements and write it down on the chassis or a sticky note and reference it later.
Yes, some things from my university electrical engineering classes, nearly 40 years ago now, still stick, but for DC voltages the potential difference between 450 volts and ground is still 450 volts, not +/- 120 x 1.414 (square root of 2).

Prodding around inside an amp with no idea of what one is prodding strikes me as rather dangerous, and if there's a safer, contact free (or at least one or two steps removed) method, I'll take it.
 

william vogel

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Yes, some things from my university electrical engineering classes, nearly 40 years ago now, still stick, but for DC voltages the potential difference between 450 volts and ground is still 450 volts, not +/- 120 x 1.414 (square root of 2).

Prodding around inside an amp with no idea of what one is prodding strikes me as rather dangerous, and if there's a safer, contact free (or at least one or two steps removed) method, I'll take it.
The calculation for rms Is peak to peak x .707/2. Or peak to peak = rms x 1.414 x 2. This is why the 120 volts looks much less than 450. I agree that if you know nothing about what lurks inside to stay clear but I also believe making adjustments with minimal knowledge and blind assumptions is equally dangerous for the equipment. Setting bias without a known plate voltage is pure speculation. You can access the voltage measurement outside the chassis from pin 3 of the socket and that will keep you outside. I understand your safety but don’t agree with blind adjustments. My suggestion is to become knowledgeable about the equipment and Practice safety protocol or leave servicing the equipment to a qualified person. I believe you can have qualification to make adjustment but you’ll have to lose the fear. There are insulating gloves available to offer protection.
 

ThreeChordWonder

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As it happens, I didn't adjust anything. The plate currents looked about right, within a milliamp of each other, and so Ohm's law tells me the plate voltages should be about the same too, assuming the impedances of the bought as "matched" tubes were about the same. The tubes weren't red plating, so I left everything alone, the "if it ain't broke don't mend it" mantra.
 

Pete Farrington

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assuming the impedances of the bought as "matched" tubes were about the same.
The currents you've measured shows that the 2 EL34s are conducting similarly.
But even if they weren't, their anode voltages at idle would still be very close to each other.
Because both anodes are tied to the HT supply node via the low resistance of the OT primary winding, eg about 50ohms each.
So one or both valves would have to be drawing massive fault current to drop any appreciable voltage across such low resistance.
Hence if the anode or cathode currents aren't too dissimilar, the anode voltages will be identical (to within any reasonable margin or error).
 

ThreeChordWonder

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The currents you've measured shows that the 2 EL34s are conducting similarly.
But even if they weren't, their anode voltages at idle would still be very close to each other.
Because both anodes are tied to the HT supply node via the low resistance of the OT primary winding, eg about 50ohms each.
So one or both valves would have to be drawing massive fault current to drop any appreciable voltage across such low resistance.
Hence if the anode or cathode currents aren't too dissimilar, the anode voltages will be identical (to within any reasonable margin or error).
That's what I thought.

Thanks:cheers:
 
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If looking for the Class A requirement, that's 100% plate dissipation. The figure to multiply by would be 1 but check the valve is operating within recommended plate voltages. Here is an EL34 example with 300 volts on the plate / anode.


I 'm curious jon: is it normal in your experience to bias a tube operating in a class A amp to 100% dissipation? I've generally used lower figures .
 


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