# Bad power tube, filter cap, resistor?

#### dan40

##### Friend of Leo's
10 ohms will work fine but they need to measure exactly 10 ohms to produce accurate measurements. Check them with your meter to be sure they are exactly 10 ohms and don't forget to subtract the resistance of your meter leads. To find out the resistance of your leads, just touch them both together at the tips.

#### peteb

##### Friend of Leo's
He is using 470R screen resistors.
V4 has 1.1mA of screen current.
V5 has 8.6mA of screen current.
Screen current is around 11% of plate current.
Both screen currents are consistent with the plate currents.
He is using 470R screen resistors?

the schematic in post 11 shows 100, measured slightly lower.

what I used is written by the OP as this:
V4:
Voltage across the 100R screen resistor: 0.49 V
Plate voltage (pin 7): 325.7 V
Screen voltage (pin 9): 293.1 V

V5:
Voltage across the 100R screen resistor: 4.02 V
Plate voltage (pin 7): 303.2
Screen voltage (pin 9): 289.4
When the OP writes “voltage across the 100R screen resistor”

I only know how to interpret that as a 100 ohm screen resistor.

why do you say 470R?

#### Ten Over

##### Tele-Afflicted
He is using 470R screen resistors?

the schematic in post 11 shows 100, measured slightly lower.

what I used is written by the OP as this:

When the OP writes “voltage across the 100R screen resistor”

I only know how to interpret that as a 100 ohm screen resistor.

why do you say 470R?
Look at this:
Powered off measuring voltage (in-circuit) of the V4 & V5 100R screen resistors gives me: 464R and 465R respectively (same as when I first had it built).
Now this makes no literal sense, but it gives some clues about how he is viewing things. He obviously measured resistance and not voltage and we will just let that one slide. He is identifying the screen resistors as "100R screen resistors" probably because that is how they were named on the schematic or something that he was referring to. He doesn't mean that the actual resistors on his amp are 100R, he is just specifying the location. The values that he measured made it abundantly clear that he actually has 470R screen resistors.

The real clincher is that the voltages are within the realm of possibility with 470R screen resistors and they are not with 100R screen resistors.

#### joulupukki

##### Tele-Meister
To make matters worse ... I forgot that I actually changed the screen resistors from 100 Ohm to 470 3W resistors, which is how it's currently connected. Sorry for any confusion. Where does that leave the state of the amp for when I get new power tubes? Not entirely sure.

I switched them to the 470 3W resistors after a recommendation from another forum member (https://www.tdpri.com/threads/my-18w-tmb-kit-build.1102251/post-11419421) and also after studying some more modern schematics of the 18w builds over at 18watt.com.

#### joulupukki

##### Tele-Meister
10 ohms will work fine but they need to measure exactly 10 ohms to produce accurate measurements. Check them with your meter to be sure they are exactly 10 ohms and don't forget to subtract the resistance of your meter leads. To find out the resistance of your leads, just touch them both together at the tips.
I have 2 of them that are very very close, mostly measuring 10.0 on my multimeter (sometimes 9.9 and sometimes 10.1). Touching my DMM leads together gives me a reading of 0.

#### joulupukki

##### Tele-Meister
Got a new set of matched tubes today and the amp is back up and running with these measurements:

Just as an experiment, I took the old tube that was *not* red-plating before and put it in V5. It seems like the power output is more closely matched with this configuration, but I don’t know if it’d be better to keep it like this or keep both new tubes in (just inexperience here).

New tube in V4, old tube in V5:

Which way would be best for tube longevity … or does it matter?

#### andrewRneumann

##### Tele-Afflicted
I suppose one could just use one 1R resistor as the jumper between the two cathodes. No terminal strip required… just a little more math to subtract out one current from the total.

#### joulupukki

##### Tele-Meister
I suppose one could just use one 1R resistor as the jumper between the two cathodes. No terminal strip required… just a little more math to subtract out one current from the total.
You’d be measuring V4’s cathode current, right? …and then where do you measure the total current? I must be tired.

#### King Fan

##### Poster Extraordinaire
Are you concerned your MPD is over 100%? Usually in cathode bias I'd say no big deal, but that EL84 thread suggested they can be touchy. Not my area, don't take my word for it.

#### joulupukki

##### Tele-Meister
Are you concerned your MPD is over 100%? Usually in cathode bias I'd say no big deal, but that EL84 thread suggested they can be touchy. Not my area, don't take my word for it.
No. Not concerned about that anymore really. Just wondering if it’s better to have the power tube’s plate dissipations be closer together comparing V4 and V5 or if it’s better to use matched tubes in this case.

#### andrewRneumann

##### Tele-Afflicted
You’d be measuring V4’s cathode current, right? …and then where do you measure the total current? I must be tired.

The total would be reflected in the voltage drop across the 130R shared cathode resistor.

#### joulupukki

##### Tele-Meister
The total would be reflected in the voltage drop across the 130R shared cathode resistor.
For kicks I added a 10 ohm resistor (because I don’t have a 1 ohm) as the jumper between the cathode pins of V4 and V5.

I’m getting a reading of 401.3 mA across that resistor, which means, 40.13 mA after dividing by 10. The voltage across the 128.4 ohm shared cathode resistor is currently 10.53 VDC.

Total: 82 mA (10.53 / 128.4)
V4: 40.13 mA
V5: 41.87 mA (82 - 40.13)

Plate to Cathode Voltage:
V4: 318.4 V
V5: 318.1 V

Watts:
V4: 12.7773 W (318.4 * 0.04013)
V5: 13.3188 W (318.1 * 0.04187)

Dissipation:
V4: 106.5% (12.7773 / 12)
V5: 110.99% (13.3188 / 12)

I assume this is how you’d calculate it using that method. This is with the one new tube in V4 and the old tube in V5.

With both new tubes, the same measurements and calculations are:

Cathode Resistor Voltage: 10.31 VDC
Total: 0.080296 A
V4: 42.3 mA
V5: 37.996 mA

V4: 317.8
V5: 319.7

Watts:
V4: 13.44294
V5: 12.1474

Dissipation:
V4: 112.02%
V5: 101.23%

Last edited:

#### andrewRneumann

##### Tele-Afflicted
For kicks I added a 10 ohm resistor (because I don’t have a 1 ohm) as the jumper between the cathode pins of V4 and V5.

I’m getting a reading of 401.3 mA across that resistor, which means, 40.13 mA after dividing by 10. The voltage across the 128.4 ohm shared cathode resistor is currently 10.53 VDC.

Total: 82 mA (10.53 / 128.4)
V4: 40.13 mA
V5: 41.87 mA (82 - 40.13)

Plate to Cathode Voltage:
V4: 318.4 V (318.4 * 0.04013)
V5: 318.1 V (318.1 * 0.04187)

Watts:
V4: 12.7773 W
V5: 13.3188 W

Dissipation:
V4: 106.5% (12.7773 / 12)
V5: 110.99% (13.3188 / 12)

I assume this is how you’d calculate it using that method. This is with the one new tube in V4 and the old tube in V5.

Your almost there. You have measured the cathode current which is the sum of the current coming in from the plate and the current coming in from the screen. You need to subtract out the screen current. You can simply deduct a simply percentage like 5% or 10%. But you have individual screen resistors, so why not measure the voltage drop across those. Convert it to current using ohms law and then subtract it from the cathode current. What you’re left with is plate current and that’s what you use to calculate plate dissipation.

#### joulupukki

##### Tele-Meister
Holy smokes this is a lot of math for a Saturday evening.

Did I do this right?

With the 2 new tubes:

Screen Resistor Current:
V4: 1.804 V and 464 Ohms = 3.89 mA
V5: 1.687 V and 465 Ohms = 3.63 mA

Plate Currents:
V4: 42.3 - 3.89 = 38.41 mA
V5: 37.996 - 3.63 = 34.366 mA

Watts:
V4: 12.2067 W (0.03841 x 317.8 plate-to-cathode voltage)
V5: 10.9868 W (0.034366 x 319.7 plate-to-cathode voltage)

Dissipation:
V4: 101.72% (12.2067 / 12)
V5: 90.8% (10.9868 / 12)

#### nickmm

##### Tele-Holic
18 watt call for a 150 ohm cathode bias resistor . You are running a 130 ohm.

Plug your numbers in the bias calculator to see what you dissipation would be with a 150 ohm.

Those tubes are right on the edge.
It's not uncommon for modern EL84s to red plate in old amps.
Simple fix is to up the cathode resistor a step.

#### joulupukki

##### Tele-Meister
18 watt call for a 150 ohm cathode bias resistor . You are running a 130 ohm.

Plug your numbers in the bias calculator to see what you dissipation would be with a 150 ohm.

Those tubes are right on the edge.
It's not uncommon for modern EL84s to red plate in old amps.
Simple fix is to up the cathode resistor a step.
I’ll give that a shot next week. I’ll have 150 and 200 ohm 5w resistors I can experiment with. This is from a Mojotone 18w TMB kit which only came with the 130R.

#### nickmm

##### Tele-Holic
I’ll give that a shot next week. I’ll have 150 and 200 ohm 5w resistors I can experiment with. This is from a Mojotone 18w TMB kit which only came with the 130R.
Yes I've had to up the bias resistor on my 60's Vox AC30 due to it consistently red plating JJ and Ei's.
Original Mullards were fine but they rattled.

You should also be able to source a 180 ohm.

#### joulupukki

##### Tele-Meister
One thing that’s confusing me is how power tubes should be “matched” but in this case, I’m assuming it’s the differing resistances of the output transformer secondaries that is causing the difference in plate currents between the two power tubes (when I had the matched pair installed)? Isn’t the point of having matched power tubes so that they will work at about the same power? …but then it’s normal to have different resistances on the output transformer. Odd.

#### nickmm

##### Tele-Holic
One thing that’s confusing me is how power tubes should be “matched” but in this case, I’m assuming it’s the differing resistances of the output transformer secondaries that is causing the difference in plate currents between the two power tubes (when I had the matched pair installed)? Isn’t the point of having matched power tubes so that they will work at about the same power? …but then it’s normal to have different resistances on the output transformer. Odd.
Probably matched within 20% and that 2 watts is enough to red plate one.
Mark the bad one and pick the closest of the ones you have.
Is it the position or the tube red plating?

#### joulupukki

##### Tele-Meister
Probably matched within 20% and that 2 watts is enough to red plate one.
Mark the bad one and pick the closest of the ones you have.
Is it the position of the tube red plating?
Well, I have the new matched pair but it seems like the one older one (older one is only like a couple weeks old) paired with one of the new ones gives a closer reading on the dissipation. Fortunately neither combo is red plating, that I can see.

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