Bias info

[luthiernc]

I'm building a Hoffman AC30 preamp and running it through a Peavey Classic 50 power amp that I want cathode biased. I have a 50 ohm 10 watt resistor and 220 uf cap attached to the cathodes. B+ at idle is 425vdc and under load is 390 vdc. I'm reading 12vdc across the cathode res/cap. Seems a little low to me. Any big ideas out there to bias this?

[sluckey]

The classic 50 is fixed bias. Before you convert to cathode bias, measure the negative voltage on the grid of the EL84s (pin 2). Schematic shows -27v but it's not clear that is the actual bias voltage.

Now convert to cathode bias and choose a resistor that will give you a positive voltage that's the same as the previous fixed bias grid voltage.

[PRR]

> 50 ohm ..... 390 vdc. I'm reading 12vdc across the cathode res/cap. Seems a little low to me.

How bright are those plates glowing? Hot enough to roast weenies? (Figures 24 Watts each of four 12W tubes.)

>Any big ideas out there to bias this?

When you Fix-Bias, you can and always do jack the B+ WAY up. You can because with fix-bias you can avoid melting at idle; and you do because why else would you risk fix-bias?

To "go back" to self-bias you probably have to go way down on B+. Is it worth the mental strain plus loss of loudness?

Then what Sluckey said.

But also note that the EL84 is a 330V tube, and working it at 360V-400V will accelerate troubles. Yes I know many new EL84 won't go BOOM right away at 331V, but still this is the cheapest in-production power tube on the planet and it does not owe you any extra.

[pdf64]

Think of what you’re wanting as being a square peg - round hole scenario. For cathode bias with a HT up above 400V, a pair of beefy octals, eg EL34 or 6L6GC, seem much more suitable than 4 x EL84.

[Bieworm]

"But also note that the EL84 is a 330V tube, and working it at 360V-400V will accelerate troubles. Yes I know many new EL84 won't go BOOM right away at 331V, but still this is the cheapest in-production power tube on the planet and it does not owe you any extra."
[/quote]

I run my EL84 in my 18 watts at 355V. I like their behaviour at those voltages. The cathode resistor is 180R.
But!!! The tubes are not actual EL84.. they are NOS russian 6P14P anf should handle those voltages just fine. The amps are being played like that for the past 6 months..no issues so far. Nor redplating either...

[HotBluePlates]

... 355V. I like their behaviour at those voltages. The cathode resistor is 180R.
But!!! The tubes are not actual EL84.. they are NOS russian 6P14P anf should handle those voltages just fine. ...

6π14π data sheet

The Russian data shows the screens should not be over 300v.  Do you happen to know if your screens are that low (as they're often near-plate-voltage in guitar amps).

"But also note that the EL84 is a 330V tube, and working it at 360V-400V will accelerate troubles. Yes I know many new EL84 won't go BOOM right away at 331V, but still this is the cheapest in-production power tube on the planet and it does not owe you any extra."

I run my EL84 in my 18 watts at 355V. ... they are NOS russian 6P14P anf should handle those voltages just fine. ...

Note "Ua" with Note 2 on the data sheet: Plate Voltage shouldn't exceed 300v if plate dissipation is ≥8 watts.  Yeah, the folks touting 6P14P tubes breeze right past that...

Sure, tubes don't die the moment you exceed their ratings; there is some uncertain amount of margin built into the ratings.  You've got 6 months on your 6P14P's, and I have ~15 years on a pair of 6P3S tubes running near-but-below ratings.  Who do you think might have to swap tubes first?

[luthiernc]

Ok gang, I went back to fixed bias with a grounded cathode and a 1 ohm resistor on each tube before ground. Now I'm reading B+ at 410 and -17v on grids and 28 ma across the one ohm resistors. I guess I'm asking what the ma reading should be at that voltage. Is 28 ma near enough or do I need to alter bias circuit? It's putting out a clean 50 watts into 8 ohm load. The plates never were red hot. No wienie roast for sure now!

[shooter]

i'm at 11W/tube

[HotBluePlates]

... B+ at 410 and -17v on grids and 28 ma ... Is 28 ma near enough or do I need to alter bias circuit? It's putting out a clean 50 watts into 8 ohm load. ...

If you have 28mA per tube, you're good (especially since the amp seems to be outputting full-power).  If you're not getting red plating when driven for full-power, you're good.

Many tube types are run past their rated voltages in guitar amps.  The challenge is we never know how close to the edge the tubes are.

[AmberB]

I'm a bit confused...you can get 50 watts from a quad of EL84s?
First time I've heard of that...

[PRR]

..you can get 50 watts from a quad of EL84s?...

A pair of 7189 at 400V will do 24W at low distortion. 7189 is a high-rated (400V) model of EL84 family. Many modern EL84 are not too fragile at 400V. G-amps can rate higher THD than Hi-Fi. So yes, it is not impossible.

[HotBluePlates]

...you can get 50 watts from a quad of EL84s?
First time I've heard of that...

Yes, if they're biased cool into Class AB.

EL84 is a "12w class tube."  In other words, the rated plate dissipation is 12w, just like (some ratings for) the 6V6.  Jim Kelley built 4x 6V6 amps making 60w.

The way this usually gets done is the plate voltage is made large, and the idle current is made small with a large bias voltage.  The low idle current helps keep the output tubes from overheating over the signal cycle when the output tubes are pushed to high a.c. plate current.

The large a.c. plate current times the large a.c. voltage dropped across the OT primary impedance (a.c. plate volts = a.c.plate current x OT primary impedance) results in high a.c. power output.

The difference between the EL84 and 6V6 is that 6V6s had a higher rating for plate voltage.  Which was then exceeded in many guitar amps, as manufacturers figured out the tube could handle more than the data sheet said.  That's how the Kelley amps did large power output from 6V6s.

But the EL84 is rated for 300v on the plate; as we saw from the 6P14P data sheet, it also gets conservative about claimed plate voltage rating depending on how hot the tube is operated. 

You may be used to looking at Vox amps as an example for EL84 amps.  Except the plate voltage in those is only a bit over 300v, and the bias around -12.5v.  These amps are "cathode-biased hot Class AB" (to use my terminology), kind of like a tweed Deluxe where the amp only gets "a little bit beyond Class A."

But the Classic 50 has a plate voltage of ~400vdc, and a bias of more like -27vdc (on the 2nd page).  These tubes run cooler at idle, and are much further into Class AB than the Vox amps, more like the Jim Kelley example.

As long as the tubes don't overheat near max power output, and don't suffer any negative effects from voltage-stress (like arcing between pins), then it's a valid way to run the tubes.