Hoffman Amplifiers Tube Amplifier Forum
Amp Stuff => Tube Amp Building - Tweaks - Repairs => Topic started by: Electron Tornado on November 15, 2017, 10:44:55 am
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I'm working on an Epiphone Valve Jr that had a bad EL84. I am using this schematic for reference: http://www.muzique.com/news/images/Epi1.gif (http://www.muzique.com/news/images/Epi1.gif)
I tested the cathode resistor and cathode bypass cap. Both checked OK. I installed a new EL84 from JJ and decided to check the plate dissipation. Here's what I measured:
plate to cathode voltage: 328V
cathode voltage: 11V
cathode resistor: 220 Ohm
plate current = cathode voltage / cathode resistor = 50mA
plate dissipation = plate current * plate voltage = 16.1 Watts
That seems too high. The datasheet from JJ shows the maximum power as 12 Watts. It also shows the max plate voltage to be 300V. http://www.jj-electronic.com/en/el84-6bq5 (http://www.jj-electronic.com/en/el84-6bq5)
The JJ datasheet gives the typical plate voltage as 250V. Why is the EVJ using greater than 300V?
Why is the EL84 being run at greater than 12 Watts?
Did I make a mistake somewhere?
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"Everybody" ran the poor EL84 over-voltage. Anyway Typical is not Maximum. Design Center Max rating is 300V; this applies when you do not measure every radio you build. You can usually go 10%-20% higher if you "know" your voltage.
What is the G2 voltage?
> plate current = cathode voltage / cathode resistor = 50mA
That is Plate + G2 current. The datasheet shows that in roughly similar conditions, the tube may run 48mA Plate, 5.5mA G2. That is roughly 90% to plate and 10% to G2. 90% of 50mA is 45mA. 45mA * 328V is 14.76 Watts Pdiss.
~~15W is over 12W rating. We know the 6V6 is 12W Design Center but 14W Design Max, and Fender runs the new Champ 600 even hotter.
Also note that if you PLAY an SE amp LOUD it runs cooler. Don't stop playing!
Do you want to live forever? or get big sound for small bucks? How long is the *tube* warranty on a Epi Jr? (90 days!)
If you want to be kind to your tube, make R10 more like 500 or 1K 10W.
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Ditto
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If you have the opportunity get a 6P14P (better a 6P14P-EV) tube and store it
use it the next time you need to have a substitute for an over spec 6BQ5 (EL84) tube
https://www.hobbielektronika.hu/forum/getfile.php?id=21127 (https://www.hobbielektronika.hu/forum/getfile.php?id=21127)
Franco
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What is the G2 voltage?
G2 voltage to ground is 321V.
> plate current = cathode voltage / cathode resistor = 50mA
That is Plate + G2 current. The datasheet shows that in roughly similar conditions, the tube may run 48mA Plate, 5.5mA G2. That is roughly 90% to plate and 10% to G2. 90% of 50mA is 45mA. 45mA * 328V is 14.76 Watts Pdiss.
Right about here I need one of those smiley things that shows a light bulb coming on.
I'm still not quite sure I understand why these amps are running output tubes over spec.
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Running the tube hotter in a single-ended amp objectively allows more power output... if the rest of the circuit is set up to take advantage of it.
For that reason or others, many popular, valuable, "classic" designs have been running tubes out of spec forever.
Subjectively this is supposed to "improve" the sound (it does affect it somewhat, for better or worse), so they copy those designs.
All these years of "testing" by owners of these and other amps have revealed voltages above spec don't really matter, but power dissipation over spec does reduce tube life.
Because single-ended tubes actually dissipate less power while playing, and it's cheaper to replace just one tube, they've continued to consider this worthwhile.
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What is the G2 voltage?
G2 voltage to ground is 321V.
> plate current = cathode voltage / cathode resistor = 50mA
That is Plate + G2 current. The datasheet shows that in roughly similar conditions, the tube may run 48mA Plate, 5.5mA G2. That is roughly 90% to plate and 10% to G2. 90% of 50mA is 45mA. 45mA * 328V is 14.76 Watts Pdiss.
Right about here I need one of those smiley things that shows a light bulb coming on.
I'm still not quite sure I understand why these amps are running output tubes over spec.
It is common for tube amps made for guitar to go past max values.
Check out the AB763 Deluxe Reverb with 2 6V6. It is running them harder.
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Welcome to the Forum! EL84 voltage and plate diss are recurrent topics. You should get hits of you search the Forum. Note that many guitar amps run these tubes over spec; even way over spec. Traynor: over 410VDC!!!; Trainwreck about 330VDC.
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> why these amps are running ... over spec.
The little Briggs & Stratton lawnmower engine is a very conservative recipe for 3.75 horsepower at 3,500RPM. Probably run 1,000 hours that way. I have one from 1984, runs fine.
Go-kart racers take off the governor, rev them to 7,000RPM, get maybe 6 horsepower (with other mods). Probably not much over 100 hours before major failure. But you can do a LOT of 5-minute races in 100 hours of engine life. And the core is cheap enough to replace every few years. And the typical racer won't take the kart to the lawnmower shop, he will swap it out himself.
The specs for EL84 apply to home and especially car radios. Tube life should be l-o-n-g. Aunt Matilda is not going to open her Grundig radio and replace her own tube. Uncle Mortimer is not going to climb under the Citroen dash, remove the radio and replace tubes. They will PAY someone to do these nasty technical chores. *In that market*, long tube life is a benefit for users and thus for the tube factory (even though they'd love to sell replacements).
Guitarists learn to replace their own picks, strings, bridges, and tubes, zero labor charges. Hot-rodding the tubes makes some sense in that application, just like at the race-track.
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That’s a lovely analogy!
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410V in a Traynor - you're lucky. Both of my YGM-3s run the EL84s at nearly 450V. I use the 6P14-EVs and still use a Brown Box to run the wall voltage down 5-7%.
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My solution for EL84 Traynor Bassmate: 12V bucking tranny & B+ dropping resistor > about 280VDC. Note that this costs less than $20.