Bias an amp using external test point?

[fossilshark]

Hi all sorry for the noob question. I got a Peavey 3120 in the shop that i absolutely cannot find the schematic for online. I got a fresh set of EH EL34s in it and now I need to bias it.

On the back there is a set of red and black test points for the bias voltage and a pot to set the voltage. My question is: how to I convert the voltage into the correct current? The tubes are matched at 40ma.

The output tube sockets are on a board that could potentially have the 1ohm resistors on each tube that I could get the current from but it is upside down and very hard to get at. And like i said i do not have the schematic.

Any advice on how to proceed?

[Platefire]

Go to peavey.com and go to support. I have had Peavey send me schematics in years past for free.
If those power tubes are set up with 1 Ohm resistors from from cathode to ground on the test points, you shouldbe able to set your multi-meter to Mv and the reading you get will be Ma

[fossilshark]

the test points are currently reading 45v. I do not think they are set up with a 1ohm resistor.

[AlNewman]

With the amp off, what's the resistance between the 2 points?

Do you have pics?

[fossilshark]

Resistance is about 15k between the bias terminals. The cathodes are going directly to ground.

How do I bias an amp without 1ohm resistors on the cathode?

[SEL49]

On the back there is a set of red and black test points for the bias voltage and a pot to set the voltage. My question is: how to I convert the voltage into the correct current? The tubes are matched at 40ma.

Those two test points simply monitor the bias voltage sent to the control grids. You cannot convert that voltage to current.

What makes you think the tubes are matched at 40mA?

[fossilshark]

They are all marked 40ma matched set from tube depot.

[AlNewman]

If you can find the B+ to the OT, you can measure plate dissipation across that and the plates of the power tube.  You'd have to measure the resistance of the OT primary, then measure voltage drop to find current, and then multiply with the B+ voltage.

[HotBluePlates]

Hi all sorry for the noob question. I got a Peavey 3120 in the shop that i absolutely cannot find the schematic for online. I got a fresh set of EH EL34s in it and now I need to bias it.

On the back there is a set of red and black test points for the bias voltage and a pot to set the voltage. My question is: how to I convert the voltage into the correct current? The tubes are matched at 40ma.

The output tube sockets are on a board that could potentially have the 1ohm resistors on each tube that I could get the current from but it is upside down and very hard to get at. And like i said i do not have the schematic.

Those two test points simply monitor the bias voltage sent to the control grids. You cannot convert that voltage to current.

I have a few of these bias probes for situations like this.  There are some available for octal (with the plate on Pin 3), and some for EL84.

[Merlin]

Those two test points simply monitor the bias voltage sent to the control grids. You cannot convert that voltage to current.

What a strange design choice

[tubeswell]

“Go to peavey.com and go to support.”

^What Platefire said^

[shooter]

What a strange design choice

while they are very helpful with questions and schematics, sometimes I'd wonder if they hired Gibson designers that were fired

[fossilshark]

I emailed Peavey, who knows if theyll get back to me.

OK so i think i have a plan.

https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

My only question is: is the current measurement i get from that for all 4 tubes? The tubes i got are all marked with "40ma" so I need to make sure every tube is running at 40ma right?

[tubeswell]

I emailed Peavey, who knows if theyll get back to me.

OK so i think i have a plan.

https://robrobinette.com/How_to_Bias_a_Tube_Amp.htm

My only question is: is the current measurement i get from that for all 4 tubes? The tubes i got are all marked with "40ma" so I need to make sure every tube is running at 40ma right?

With no signal, each half of the primary sees the DC tube plate current for the output tubes on that side of the primary.

But (to digress) if there’s a signal, the plate current for all the output tubes goes through the entire primary during the entire Class A signal cycle* (albeit that for half of the tubes, the current is ‘positive’ and going in one direction, while for the other half of the tubes, the current is ‘negative’ and is going in the other direction- which amounts to the same overall flow of current).


*But (to digress further) when in Class B, half of the tubes are not conducting for under half of the signal cycle - just so everyone’s clear on that LoL.

[acheld]

My only question is: is the current measurement i get from that for all 4 tubes? The tubes i got are all marked with "40ma" so I need to make sure every tube is running at 40ma right?

No.    I looked all over TubeDepot's website to see if they would define their tube matching set, but no luck.  What I assume it means:   At X voltage and Y plate dissipation, the cathode current is 40mA.    We don't really know what X and Y are, just that the selected tubes use the same bias at those settings.

Your voltage and your intended plate dissipation (most likely) will not be the same as TubeDepot's test procedure, and thus the "40mA" has no meaning for your amp. 

While RobRob's biasing instructions are correct, I find them confusing. And there is risk measuring high voltages in a live amp when you're not familiar with the procedure. 

Personally, unless you're ready to dig into the amp's circuit, I'd suggest asking Peavey for their recommended biasing procedure for this amp.

[fossilshark]

I see. This is frustrating BUT I have to learn how to do this.

So the current value that i need to set my tubes to: how do I figure that out.

I am very comfortable with high voltage measurements and digging into an amps circuit. Does anyone have any good resources I should read up on?

Everything I seem to find (admittedly aimed at guitarists and not technicians) just says "measure the 1ohm resistors" and I now have 2 amps on the bench that need a retube and dont have 1ohm resistors to measure off of.

Should I just buy one of those bias meter things?

[fossilshark]

I found this. I think this may be the answer to my problems. Anyone have any experience with this?

[AlNewman]

May be simpler to buy a bias meter, but you still need to know what your current should be.
You pick a power output you want your plates to dissipate at, then calculate using ohm's law based on the plate voltage and current.
Usually in fixed bias, you want your plates to dissipate at 50-70%.  I find 55-60% is a good number.  So if EL34 is 25W max, 60% would be 15W/tube.
15W at say 450V plate voltage would be 33mA, but it's all dependent on your particular amp, and the voltages change depending on how much current your tubes are drawing.

[fossilshark]

Ahhh i see thats not too bad. I will order the bias meter and follow your instructions. Thank you!

[shooter]

you can save a bunch of money and simply solder 1 ohm resistors cathode to ground under each PA tube, always there, no hunting up the tool, no "disruption" to the circuit pulling tubes, inserting tool..... just measure V-drop across the R and you have tube current


matching is another "cork-sniffing" concept, since once you add signal all the "matching" goes right out the window

[fossilshark]

shooter, I would do that except the PCB would make it difficult to do that and I dont want to muck up my buddies amp.

[HotBluePlates]

I found this. I think this may be the answer to my problems. Anyone have any experience with this?

See my earlier post.   

Unlike the 1Ω resistors at output tube cathodes, the Eurotubes Bias Probe measures only plate current.  It also tells you Plate Current & Plate-to-Cathode Voltage (and also Watts of Plate Dissipation, if you pay for that option) without pulling the chassis.

It is unusual for any amp to give you all the figures needed for a dissipation calculation on external meter jacks (often the plate-to-cathode voltage is absent).


They are expensive for a hobbyist (especially if you buy more than one).  A tech who is going to pull the chassis anyway may perceive them as unnecessary.  But I've made good use of mine for a number of years now.

[mresistor]

Doug does sell a bias checker tool in his store  in the tools section.  I have one and use it occasionally.   It's for octal tubes.

[rafe]

15 years or so ago, I made one from a broken octal tube and an octal socket hardwired to a Simpson Ma meter .....still works fo me .....lots of biasing videos on you-tube ...Euro-tubes has a good one so does Uncle Doug. Some basic math needed. I picked up a 4 octal tube contraption from Doug on his shop clear-out  ...I am going to give it a try ...has the 0ne ohm resistors on the sockets. Looks well-made and could be an early prototype    lol