Do I need to change my Cathode Resistors? Class A content...

[guitarspaz]

Here's the deal, I just picked up a used Trace Elliot Speed Twin C30 30 watt Class A combo amp. The amp uses 5 12AX7s and 4 El84s. I re-tubed with JJs across the board and it sounds GREAT.

This amp is Cathode Biased and has no bias adjustment. The manual specifically says drop in a matched QUAD of EL84s and go....but...being the curious individual I am, I started taking a few measurements and determined that my EL84s are idling at around 15.75 WATTS. Everything I've read says not to exceed 12 watts. The tubes sound fine, plates aren't glowing red, etc.

I measured a plate voltage of 363V, and I'm dropping 13V across a 130 ohm Cathode resistor for each Pair which leaves me with 350V across the tubes. 13V across 130 ohms is 100 mA cathode current for a pair, i.e. 50 mA per tube. I subtracted out the screen current (about 5 mA) to leave me with a plate current of 45 mA, and a voltage of 350V,i.e. about 15.75 watts.

So... if the amp sounds fine and the tubes aren't glowing, should I leave it alone and just plan on replacing tubes a bit more often ?

What am I missing

[David Root]

Just the "Class A" in the name of the amp. This amp is designed to run in class A probably up to about 22W or so, when it will move into high AB1. The 130 ohm cathode resistor on each pair determines that.

Usually Class A bias is recommended at 100% of Pa rated. You can exceed this a bit but the further you go down that road the shorter your tube life will be. To be exact, since you no doubt have a cathode bypass cap on each 130 ohm resistor you are running class A2.

What's important is do you like the tone? Seems like you do, so... EL84s are not that expensive (NOS excepted). So I think your diagnosis is good.

If you want something a little tougher in an EL84 tube, there is a Russian NOS (Sovietski era) tube that fits that bill, I can't recall its number, could be 6NP14 but Lord Valve sells it and it can take 500V plate, supertuff, higher Pa, and it also has a lot more clean headroom. If lowdown mucho breakup is your bag you might not like them, on the other hand you can push the s**t out of these and they will take it all day and all nite. Won't cost you an arm and a leg to find out either.

[guitardude57]

I would try 7189 tubes. Heavy duty EL 84 pin out that likes 400 volt plus range. They should last forever in your config.

[jjman]

EL84s get no respect with regard to idle. I increased the cathode resistance in my 2xel84 amp.

Note that doing this will move the phony Class A towards honest Class AB. Can you handle the truth?

[Andy Le Blanc]

Cathode bias doesn't mean class A, post a conduction angle figure and then
make a valid statement, (can you handle the truth?).

2x cos-1(-Iq/Ip rms) thats zero signal current divided by max signal current.

If you can handle it

it gives you a negative .xxxx figure, the closer to -1 it is, the closer to the
strict definition of class A. Even class A amps often don't meet the figure.
It tells you where between A and B the operating point really is.


I've seen 50+ year old amps cathode biased hot running fine on original tubes.
Its the safest way to go. Do check your plate dissipation figure.
If you want to choose a different operating point, feel free.

[krash]

As they said, that's not atypical for cathode biased EL84s. That is a pretty high B+ but I am running my prototype amp right at those same numbers with 125 ohm shared cathode R per pair of EL84s and it works just fine, been running for years with this setup and has not eaten a single tube yet. I ran it on the Peavey branded Sovteks for a long time and also with everything from JJs to formerly-NOS Valvos and they all work great.

If you do anything you might consider evaluating the screen resistor. If you want to change something, I suggest a 2.2K or 2.7K x 3W screen R for enhanced tube life with modern tubes, especially considering that high B+.

Some Vox quad EL84 amps ran upwards of 400V plates and I have seen some with 50 ohm shared cathode R for four tubes. That's hot.

BTW ... my own subjective opinion, having built a number of 4xEL84 amps (Revelation 72s), is that running one 68 ohm bypassed with >47uF unlocks the magic, better than per-pair Rk/Ck. I have no technical explanation of why, but every amp I have built that way has just had a better "feel" and more complex and organic tone than the ones with 2x 130 ohm Rk and 22uF Ck.

[M Fowler]

Krash,

Your running 68R/47uf per pair rather than 130R/22uf? Is that what your saying, how hot is that running those EL84's?

[Andy Le Blanc]

You measuring V to the cathode or ground?

[krash]

no that's 68R for a quad of EL84s, all cathodes together rather than 130 per pair like the amp you are describing.

Voltages I was talking about are B+ voltages, of course measured to ground. Cathodes tend to run around 11V.