Extending the service life of an EL84.

[MarkT]

Is it possible to mod the EL84 circuit to extend the tubes service life without modifying the amps Power Supply?

[stratomaster]

Increase R71 to 150ohm 5-10W

Increase R105/106 to 1k 3-5W, or add a series 470-1k resistor before the split to R105/106.  I prefer the latter.

[tubeswell]

Put the EL84s in your microwave on high for a few seconds until you see the getter flash, then immediately stop it. But make sure it’s the getter flash. 😉


(Tis the season to be jolly)

[PRR]

EL84 is the cheapest readily available power tube on the planet.

Is EL84 cost really eating your budget?

Buy good brands. Don't drop-slam the amp.

[stratomaster]

If the voltages on the schematic are to be believed, then the tubes are idling north of 15w each.  There's no need for that even with inexpensive tubes. 

EL84 datasheets also often lists max screen of ~250v and max plate of ~300v.  The EL84M (TAD datasheet) lists 320v and 420v respectively. 

So the answer is do what I recommended above and use EL84Ms.  You'll still be biased nice and hot, and would be operating much closer to the published specs. 

[kagliostro]

If you aren't interested to extend the life of a specific pair of EL84 tubes (as an example because you like the specific tone of it), but want to extend the life of the power tube on that amp, swap tubes from EL84 to 6P14P-EV

6P14P is the Russian version of the EL84 tube and EV means that it is the military version of the tube

E = Long Life

V = Vibration Proof / High Reliability / Military Grade

The EV version can also afford a higher B+ voltage

See attached files

Franco

[AlNewman]

Can you show a schematic of the whole amp?

[acheld]

Based on my long experience with Blues Juniors, EL84's don't last long when biased hot, as with all Fender BJ's prior to the BJ IV.

When biased more conservatively (eg 60% in fixed bias) most known brands last a very long time.   Have also had great experience with the BJ IV circuit (eg, proper cathode biasing) -- tubes are definitely lasting longer.

I have not heard a difference in sound quality with conservative bias -- others may differ on that point.

Within reason, I don't worry too much about voltages.   Current is what kills these tubes.

[jjasilli]

Take a look at my avatar photo.  To me, the purpose of DIY is to blow (your own) things up.  So far I've been moderately successful.   


However, for safety, bullet-proofing, see How to Service Your Own Tube Amp, by Tom Mitchel.  As mentioned in a prior post this is more applicable to big bottle tubes.  E.g., 3X 1n4007's to ground from power tube plate pins.

[acheld]

Is it possible to mod the EL84 circuit to extend the tubes service life without modifying the amps Power Supply?

Yes, very simple mod.  R71 can be increased in value -- I'm guessing in the range of 150R (maybe 140R if you can find such) -- but you do need to check your bias.

My personal preference is to use a separate cathode bias resistor for each cathode -- since each resistor is passing half of the current a shared resistor, it is dissipating 1/2 the power and thus can be rated smaller, and it seems like the availability of lower rated (and physically smaller) resistors has a wider range of values. 

For example, in the schematic, R71 is 120R @ 6 watts.   If you split the cathodes, equivalent values would be 240R @ 3 watts on each cathode. 

I've had good luck running EL84's with about 270 - 300R, of course depending on plate voltage. I aim for about 90% max plate dissipation.   Lot's of people bias these amps at 100% -- cooler is your friend when it comes to tube longevity.

And by the way, the aim is not to keep the same tube "forever."   Where's the fun in that??  No, all we're trying to do is to achieve a reasonable service life with good tone.   

[MarkT]

Based on my long experience with Blues Juniors, EL84's don't last long when biased hot, as with all Fender BJ's prior to the BJ IV.

When biased more conservatively (eg 60% in fixed bias) most known brands last a very long time.   Have also had great experience with the BJ IV circuit (eg, proper cathode biasing) -- tubes are definitely lasting longer.

I have not heard a difference in sound quality with conservative bias -- others may differ on that point.

Within reason, I don't worry too much about voltages.   Current is what kills these tubes.

I helped a guy with a 1st generation Blues Junior that was running Amperex EL84's. The socket traces had lifted of the board it was so very hot.

Cheers!

PS the original schematic is a Trace Elliot Velocette Twin 10.

[MarkT]

Can you show a schematic of the whole amp?

[Brownie]

120 ohm cathode resistor is what a Vox uses and they are almost Class A.  When I do my "18W", I'll use 150ohm 10W and 7189 pentodes.

[acheld]

120 ohm cathode resistor is what a Vox uses and they are almost Class A.  When I do my "18W", I'll use 150ohm 10W and 7189 pentodes.

Probably will work, but it is always best to check your bias voltage and plate current.

[sluckey]

Vox AC-15 uses 130Ω. I prefer 150Ω.

[HotBluePlates]

... EL84 ... extend the tubes service life without modifying the amps Power Supply?

The best way to extend the life of an EL84 is to get the plate & screen well below the voltages shown on that schematic.  Like ≤300v.

... a Trace Elliot Velocette Twin 10.

I remember those amps seemed really cool in the magazine ads.

I can't remember ever seeing one in-person.  Apparently, they didn't set the standard for what an EL84 amp should be (haven't they been forgotten outside of bass-amp use?).

[AlNewman]

Maybe I'm wrong, but looks like you're running about 16.5 watts/tube plate dissipation.  That'll burn up tubes, especially 4 at a time.

You could increase the cathode resistor, and possibly bring down current, but you'll be chasing your tail as your voltages will increase nearly as fast as your current decreases, you're already above "max voltage", whatever that means.

I've had good luck putting in low value, high wattage dropping resistors in the B+ line, in your situation you could probably use 2, 1/side.  Also, you could probably increase your screen resistors to 1.5k/screen, dropping screen voltage to or below plate voltages, without interfering with preamp voltages.

You could drop say 30V on your B+, with 2 150R resistors, and then increase the value of your cathode resistors to lower current and save your tubes.

[MarkT]

... EL84 ... extend the tubes service life without modifying the amps Power Supply?

The best way to extend the life of an EL84 is to get the plate & screen well below the voltages shown on that schematic.  Like ≤300v.

... a Trace Elliot Velocette Twin 10.

I remember those amps seemed really cool in the magazine ads.

I can't remember ever seeing one in-person.  Apparently, they didn't set the standard for what an EL84 amp should be (haven't they been forgotten outside of bass-amp use?).

Gibson bought TE for penny's on the dollar.
I bought the amp at a mom & pop music store fire sale. (They never opened the box. Mom gave it a dusting every now and then.)
I haven't used it for more than a decade. It's not a workhorse like a Fender. The only way to transport it is in a road case and a roadie.
I'm going to cobble a head box and tiny 2-10" cab and wire it so it don't blow it up when I'm noodling Train Kep A Rollin' for the neighborhood every day.
After all I'm almost too old to rock & roll, but too old to die.

Cheers!

[jjasilli]

I say it's about tone.  If I like the tone, then the tubes may be doomed.  But I also like lower voltages.  As said above  that's the way to get non excessive cathode bias / plate diss. Short of replacing the PT, this can be done with a bucking tranny and/ or power resistor to drop  B+ voltage.  Or convert to fixed bias.