1 Ohm Plate Resistors?

XTRXTR

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Curious if anyone has plate resistors installed in series between plate Pin 3 and CT on their builds?
The point is to be able to measure the current via voltage setting on the meter just like the cathode. I would put in test points on either side of the resistors on the top of my DIY chassis with the bias pots accessible without opening chassis. Gives the ability to measure plate current directly while dialing the dual neg voltage bias pots.

What watt rating should I consider if I do this?
AC can swing at what Vpp for the V^2/R power calc?

El34 data sheet says 2x95mA Ia at max signal so I^2R. But at R=1 Ohm: I^2R = V^2/R = P in watts.
.095^2x1 = .095^2/1 = .095^2 = 0.009025 watts = 9mW
But if I look at the Max cathode current 150mA then just state that as a worse case that the plate current is the same as cathode current to build in some error for the watt rating. Then 0.150^2 = 22.5mW. Thus the .25 watt plate to CT series resistor is .25/.0225 = 11 times the operating power needed for EL34.

What about if I run some other tube that can run higher currents later on when I forget that I have a 1/4watt resistor in that position?

KT66 200mA max cathode current
KT77 200mA max cathode current
KT120 250mA max cathode current
6550 175mA max cathode current
5881 140mA max plate current same as 6L6

Okay good enough 250mA max .250^2 = 62.5mW is 4 times the Max operating power. That is within the 4 times the size general rule. And if more current flowed it would likely mean a tube failure is happening. So 1/4 watt would act as protection to the OT if it failed open as it should. And I always run tubes cool 55% to 60%.

What do you think? Build it in? yes , no , why not?

Discuss
 

Pete Farrington

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plate resistors installed in series between plate Pin 3 and CT
A 1ohm resistor between the anode and OT CT would be in parallel with the primary winding, pretty much bypass it :)
I think I know what you mean though, ie a 1ohm resistor between the anode and its OT primary connection?
A current sensing resistor in series with the anode isn’t an idea I’m keen on, because the anode circuit is a massive source of EMI and lead dress there is critical. Hence I don’t think it’s a good idea to probe an output valve anode, unless absolutely necessary. So making the circuit path more convoluted than necessary doesn’t seem right.
It seems easy enough to use the voltage drop across the primary resistance, or cathode current - screen grid current, if cathode current is considered imprecise?
 

XTRXTR

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@Pete Farrington
Thanks for clarifying yes I meant between pin 3 and the OT primary leg connection.
After I posted I read somewhere that measuring the drop from CT to P3, and of course the DC resistance to determine the plate current is the most accurate to calculate dissipation.

The shunt method is too risky for me and I don't like doing it. No need to be unnecessarily risky.

With one less test point than I initially proposed (4) I can get Va, DCR of the CT to Pin 3 leg, and Vdrop across the same DCR with three test points. Connect the test point directly to the socket Pin 3 lug with a good solder job. Use the other lug hole for the OT leg wire connection.

With a cathode resistor I would still need the plate voltage. I might as well calculate the plate current with the voltage drop from CT to P3. Plus I can determine if there is a DCR drift over time, unlikely as it may be. I'm installing a dual balance bias circuit so I could intentionally make it unbalanced and then reverse the unbalance to the other side once in a while.
 

Pete Farrington

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I suggest to avoid probing the anodes of output valves. It is easy to unwittingly induce oscillation by doing so. Then the reading is invalid because the amp is no longer idling.
For any reasonable analysis of our requirements, idle VDC at the anode = idle VDC at the OT CT.
Given the magnitude of the HT voltage, if anyone thinks that the voltage differential between those 2 nodes isn’t negligible, then I’m at a loss how to proceed with them :shrug:
For our purposes, I think cathode current is fine. Dues to screen grid current, anode current will be 5-10% below that.
Which I perceive as a good thing, it allows for a bit of drift up, high mains etc.
If perfect accuracy is required, simply measure and subtract the screen grid current.

Yes, OT shunt is unnecessarily hazardous, and due to the OT primary still being in circuit, will read maybe 5-10% (maybe even over 10% with cheap meters and 100W OTs) below the actual anode current.
So to my thinking, it has absolutely nothing to recommend it.

OT voltage drop means probing the anode, so I’m not keen on it, plus the winding resistance has poor temperature stability, need to take the resistance measurements when all warmed up.
If no cathode current sensing resistors ate feasible I use that method, but to eliminate the potential for oscillation, with valve in the phase splitter socket removed.

Hope that helps:)
 

XTRXTR

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@Pete Farrington
Okay you have convinced me, once again, good sound logic🙂 If I introduce oscillation the only way I might know that is by having a scope on it at the time.

My initial idea was to give myself more accuracy than I have in the past 45 years, provide access outside the chassis to bias pots and test points with just a DMM. Most helpful on the fly during a gig if I blow a tube. I keep extra sets with me - they have their own comfy case with the DMM, power, preamp tubes and fuses.

So instead I will do pretty much the same as I have for decades with the exception that I provide cathode current sensing resistor test points, a ground, an HT OT CT test point. As you said its usually 1 volt or so off of the plate higher at idle which builds in even more error on the side of caution. And I won't have to open the chassis and worry about dropping something in there or any of the other☠️wild possibilities I imagine will happen, with the added comfort of outside access to the dual bias pots.

Thanks for your input
 

sct13

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I have built them in...and it seems to work OK ....

But keep in mind its not the most accurate method....but all my builds have them, never had any issues
 

alpha al

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Older Sunn Model T amps have a 47 Ohm (or 50 Ohm) plate resistor for each of the four 6550's.. I have measured voltage drop across those (very carefully) in order to calculate plate current.
 

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