Questions about tube biasing

[ThreeChordWonder]

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?

 

[Ken Underwood]

But you are an engineer!

 

[ThreeChordWonder]

And an intelligent engineer asks questions if he's unsure about something.

 

[V-man]

1. There is no perfect setting. IMO getting the tubes as close to matched readings is the bigger goal with my older less-precise amps. biasing is like bath water. There is too hot and too cold. The former is dangerous to the amp, the latter is less pleasant to experience. Plate voltage also determines hot/cold as well, as no two Marshalls, esp old ones run identically, but in a typical range.

2. ALL tube amps require a load at all times AKA “the firearm is always loaded” should apply. If there are no power tubes installed, it is safe without a load. If the amp is on standby, it is presumed safe, but I would not. If the amp is on minimal volume (0-1) it is:far less risky but still possible to do damage.

 

[ThreeChordWonder]

As it happens, the currents I'm seeing are pretty much the same:

Low 35 mA
Mid 40 mA
High 32 mA

Maybe +/- 1 mA on each channel, but not worth moving the bias trim pot over.

How does readings look? Russian Mullard EL34s.

 

[Jon Snell]

Your guess at current is all very well but you must know the anode to cathode voltage to be able to calculate the overall dissipation ensuring you do not over run the valves.

Some like them hot some cold. Without getting too heavy with maths and algebra.
The maximum output from an EL34 is about 25 watts. So measure the anode voltage of the amp, for this exercise we will assume that to be 445 volts. Divide the 25 by the 445 then multiply by 0.7 the answer in ma. is the 70% plate load bias setting required for that amplifier. The 0.7 means 70% Hot anode / plate dissipation. Therefore you can use 0.5 (50%) as a Cool AB setting, 0.6 (60%) as a Warmer AB setting the one I would prefer. Or go for the 0.7 or even 0.75 (70 - 75%) if you like it hot.


Example: 25 / 445 X 0.7 = 39.3 ma. Bias​

The bias figure will depend on the anode voltage which will vary according to the current drawn by the valve. More current less volts, less current more volts. So you may need to do the calculation several times if large adjustments are made.

Example 2. 25 / 480 X 0.7 = 36.4 ma. Bias


Example 3. 25 / 410 X 0.7 = 42.6 ma. Bias​

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.


25 watts max o/p of the valve divide by plate volts 300 multiply by 1 = bias current requirement for class A operation. Example. 25 / 300 X 1 = 0.083 = 83ma. or 25 / 350 X 1 = 0.071 = 71ma.

Not rocket science - dead easy elementary stuff.
​

Hope that helps to explain how to bias an amplifier the correct way.

 

[2L man]

You should first test the bias range and mark trimmer(s) where 100% is. Possibly range never go to 100%?

Then play and adjust the bias for the sound You like and watch that tubes do not glow too bright! Then measure again what the bias currents came.

 

[ThreeChordWonder]

The currents weren't guesses, they were measurements.

My meters only measure milliamps so I have to either guess or infer plate voltages or back calculate. Basically, with no trim pots to adjust individual plate voltages anyway, they are what they are, and as long as the current readings are roughly the same for each tube and in the right ballpark, I'm okay, right?

I've no idea how the attenuator works either (the O50 has low (5W IIRC) medium (25W IIRC) and high / full power (50W) settings.

There's only one trim pot on the O50 and I believe that's there to balance the dual EL34s, not adjust the plate voltages, certainly not individually, as such.

 

[william vogel]

The method I use is to set a bias dissipation, say maybe start at 60%. I then run the amp with a dummy load and signal generator while I observe the output waveform. I attempt to ensure no crossover distortion occurs until after the power section begins to clip. What I generally find is that with EL34’s, if plate voltage is below 450, I’m usually able to set the bias below 70% dissipation (17.5 watts). 50 watt amps usually fall into this category. 100 watt Marshall amps usually always idle above 460 volts and it’s difficult to find a set of EL34’s that produce no crossover distortion before 70% dissipation. I use this method because I want the clean rms output to be truly clean. Crossover distortion introduces harmonics and I prefer them to be vacant before peak clipping occurs. I have been complimented many times for the sound of amps that I service and build and I believe this bias contributes to the richness of the sound.
If it turns out that the setting is anywhere between 60-75% dissipation, I run it. I’ve had no consequences whatsoever from this method. It’s very important to observe the output waveform. If you don’t have access to the equipment then bias to 65% PLATE dissipation. You must measure, record and subtract screen current from the cathode current to accurately calculate plate dissipation if you are using cathode current measurements.

 

[Ken Underwood]

And an intelligent engineer asks questions if he's unsure about something.

Engineer, electronic maybe but others maybe not, a competent electronic engineer would not ask questions like that

Using the word "Engineer" sounds impressive but without saying what type is not.

 

[StingRay85]

That's correct. Coincidently I'm also an engineer (chemical) but at least I know how to bias my amps. Just bias them to 30-35 mA, always good

 

[ThreeChordWonder]

Engineer, electronic maybe but others maybe not, a competent electronic engineer would not ask questions like that

Using the word "Engineer" sounds impressive but without saying what type is not.

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.

 

[william vogel]

Bias hum will occur without the input cable inserted. If it’s occurring after plugging in, it’s in your signal chain.

 

[Pete Farrington]

It may be a case of environmental magnetic fields and single coil pickups.
Have you tried it using guitar with humbucking pickups?

 

[ThreeChordWonder]

Single coils, humbuckers, P90s...

 

[ThreeChordWonder]

I swapped two of the ECC83s over in the preamp stage, replaced the mains cable and the hum has reduced significantly, but not gone away.

It gets much more pronounced, as you'd expect, with the gain turned up above 6 or 7, but also if the treble is turned up high, the tilt and presence likewise. It's there on all three power settings, but perhaps at its least worst on the middle one.

I'm not sure it's a 60 Hz hum, might be 120 Hz.

 

[Jethro Rocker]

Figure out plate voltage as per @Jon Snell above. Then apply the formula.
Biasing should be done as far as I know at full output but volume set to zero.
As for hum, the one studio I record in has horrid hum when his LED lights are on.

 

[XTRXTR]

Figure out plate voltage as per @Jon Snell above. Then apply the formula.
Biasing should be done as far as I know at full output but volume set to zero.
As for hum, the one studio I record in has horrid hum when his LED lights are on.

Should install independent ground for the studio equipment. Like 20 ft from house run a 8ft ground rod then separate all your studio equipment grounds at the panel to the new ground rod, leave the lights on the other panel.

 

[ThreeChordWonder]

25 watts ÷ 445 volts = 56 mA
56 mA x 0.7 = 39 mA

Which is about where I'm at.

 

[Jethro Rocker]

25 watts ÷ 445 volts = 56 mA
56 mA x 0.7 = 39 mA

Which is about where I'm at.

It doesn't have to be real precise as long as it isn't red plating or too cold.
Do you have 445V on the plates? I mean, 39 won't hurt it. Some people go 60 or 65 % some higher than 70.