EL84 safe filament voltage range

[mikey99]

I have an EL84 based amplifier with two EL84s in each channel.
Both tubes on one channel filament measures 6.75 , the other
channels filament measures 7.35. These measurements were
taken with a good quality digital multimeter after the amp warmed
up for about 5 minutes.

Is the 7.35 filament voltage too high ? I think each channel
filament is probably driven from a separate winding on the
transformer which might account for the voltage difference.

On the channel with 7.35V filament, the heater on one of the EL84
glows about 50% brighter than the other EL84 in that channel .
This brightness difference is noticable at the top of the tube.
Both are RCA 6BQ5/EL84. Could the difference in brightness be
just a characteristic of the tube ?

 

[jaymanaa]

That 7.3 is way to high, and probably the result of higher wall voltages these days. Get it down with a dropping resistor and your tubes will thank you. Excessive filament voltage is thought by many to be the number 1 cause of premature tube failure.:thumbsdn:

 

[mikey99]

Good idea on the dropping resistor. Any ideas what value/wattage
could be used ? I measured the resistance of the filament on
one of the EL84 = 2 ohms cold although this probably changes as the
tube heats up. I would think a small value resistance would be needed.
If I wanted to drop 10% of the voltage then I'd need something like a
.25 ohm resistor.

That 7.3 is way to high, and probably the result of higher wall voltages these days. Get it down with a dropping resistor and your tubes will thank you. Excessive filament voltage is thought by many to be the number 1 cause of premature tube failure.:thumbsdn:

 

[Squidward]

What happens when you swap the tubes?

Also, what's the B+ in relation to what it should be on the schematic, etc.? Maybe a dropping resistor on the primary might be in order.

 

[jaymanaa]

Good idea on the dropping resistor. Any ideas what value/wattage
could be used ? I measured the resistance of the filament on
one of the EL84 = 2 ohms cold although this probably changes as the
tube heats up. I would think a small value resistance would be needed.
If I wanted to drop 10% of the voltage then I'd need something like a
.25 ohm resistor.

.37, but by math is usually bad. and yep, you better check the B+ too.:yes:

 

[mikey99]

B+ looks okay ...schematic calls for 330 VDC, it measures 316 VDC

On the tube glowing brighter issue , if I move the tube to the other socket
in the same channel the tube glows brighter the same way. This tube
is a NOS tube I purchased recently. Its also an RCA 6BQ5 but
maybe not the same vintage as the ones already in the amp.

 

[JonL]

Some tubes just appear to glow differently. Must be the geometry of the filament, how much is visible, etc. Some look very, very bright; some it is hard to tell they are even working.

 

[Tom Bavis]

To drop 1V at .76A, you need 1.3 Ohms at .76 watt. That's per tube - half that (and twice the power) if it's easier to use one resistor per pair. I'd use 2 watt resistors (5W for a pair) to give some margin (lower the temperature...).

 

[seta1]

Hi,

Up to about 6.7 volts is fine. Much above that is a problem. I do not believe that this thing of increased line voltages is the issue in all cases. Back in the 60's, when I started building amplifiers and such, my line voltage, as measured with both a Simpson 260 and a Heath VTVM was 121 volts AC. At my house now, my line voltage is 119 VAC. as measured with another Simpson and my Fluke meters.
I also worked for an electric utility company for a couple years and we are still using generators and equipment today that were in place in the 60's and before. The big thing I see today is a much wider variation in voltages relative to where you live then I did in the 60's and 70's. So it is more a regional thing then a thing of geee, power was only 110 volts in 1962, but oh my God it is now 120 volts everywhere, our amps are going to go up in a cloud of sparks and smoke.
You have something wrong with either your meter, your power transformer or you have unusually high line voltage. Maybe the transformer was wound improperly from the beginning. Maybe there are some shorted windings in the transformer. The bottom line is that you need to get that filament voltage down below 6.6 volts (preferably to 6.3 volts). The first thing I would do is measure your line voltage at your wall outlet. I have been in areas where the line voltage approached 130 volts. I live in an area where it varies from 115 volts to 120 volts with it normally settling around 119 volts. Next, try another meter. Your meter could be off.
If your wall socket voltage is above 125 volts, you will see higher then normal filament voltages. But, if you can verify that your line voltage is at or below 120 volts and you have these high filament voltages, you will need to add resistors in the filament circuit to reduce this voltage. Something like a 1 - 2 ohm resistor at around 2 watts for each output tube should work. Calculations will say a 2 watt is overkill, but in my experience, using 2 watt resistors will keep things cool and reliable for many years.

 

[mikey99]

Thanks for all the advice. Today I retrieved several .47 ohm 2W resistors
from my parts box. Started trying to figure out where in the circuit to place
one of these, then I suddenly realized there is more involved here. There are
several other tubes in this receiver being driven from the filament winding.
SO I would need to place the resistor in the circuit where it would drop the
voltage for all the tubes filaments driven from this winding. This would also
affect the resistance value required.

Another thing I noticed in the schematic is each filament winding (there are two)
has a Hum Balance pot in the circuit. I'm wondering if adjusting these pots
affects the filament voltage at all. The pot is connected across the filament winding ,
so if it was open might explain the higher voltage.

 

[mikey99]

I added the .47 ohm resistor and it nicely dropped the voltage on the EL84's down to 6.5 VAC.

Unfortunately the resistor is only 2W and is running pretty much at its max rating , so I need to hit parts express and order a 5W resistor, among other things