Designing a 2XEL84 PP amp with a smallish PT (190-0-190 65mA)

[Hardtailed]

I built a PCB Firefly last year, which uses as 12AT7 output stage in push-pull.
It's a fun little toy, but still too loud to crank in my living room. I bring it along as a spare amp but I wish it was a bit louder.

So I have this crazy idea of building a 2XEL84 PP amp with a EF86 input stage + a 12AX7 PI reusing as many parts as possible.

The PT is a Hammond 269EX and the OT is a 125C (8W).

I've calculated the heaters current draw at 2.02A and the PT is rated for 2.5A, so no problem there.

The problem is the 65mA max current draw on the HT line.
I know that Hammond are notoriously underrated, so I'm guessing I can push my luck up to maybe 75-80mA, but I'm trying to stay as safe as possible.

I've checked http://tdsl.duncanamps.com/show.php?des=EL84

With the 190-0-190 rectified and filtered, I should be getting around 250-260V. From what I can see here, in Pentode mode, the screen current brings it over the top, but it can word in Triode mode with the OT configured with 10K primaries (125 serie allows many impedance options).

Problem is... I love pentodes!!! That's why I'm also using a pentode input stage (EF86).
And in triode mode with 250V and a 130ohm share cathode resistor, they're giving 3.4W. I'd like at least double that.

How about if I use larger screen resistors?
Or if I bring B+ lower a bit
Or use a higher impedance?

I'm sure there is a way of getting about 7-8W out of a pair of EL84 in pentode mode while still having some current left for the preamp (should be about 6-7mA if I understand this correctly).

Any help/suggestion would be appreciated!

Thanks

[Robert M. Martinelli]

Hi,
I wouldn't worry that much, as EL84s @ 250 VDC + B in P-P AB1 draw about 30 mA with a cathode resistor around 270 Ohms ( or 135 Ohms per tube if you're going to use separate cathode resistors ). Add some 3-4 mAmps for the screens, and bear in mind that your preamp is not going to need 7-8 mAmps, but ( just an "educated guess" ) more likely some 1-2 mAmps, plus about 1 mAmp for the PI if you're using a LTPI ( each triode in the PI will draw about 0,5 mAmp ).

You'll probably end up with about 10 W output power, and the overall +B current draw should be around 70 mAmps with the amp fully cranked. Also, bear in mind that a cathode biased PP stage is self-regulating to some degree, because as the current across the cathode resistor increases, so does the cathode voltage, so less voltage will tend to drop across the tubes ( cathode bias "sag" ). All in all I'd say there's nothing to worry about.

Hope this helps

Best regards

Bob

[Hardtailed]

Is it not the other way around? 270 split or 135 shared?

As for the preamp, I'm reading 3mA + 0.6 for the screens on the EF86, and 1.2mA (I'm guessing per side) on the 12AX7. Hence the 7mA figure.

I understand that by increasing both the primary OT impedance and the cathode resistor, I can bring the current draw (and thus power) down a little while keeping a similar bias point.
I'm not familiar with load lines, I usually just copy existing design.

Like, if I increase the primary to say 12K, and the cathode resistor to about 200ohm instead of 130. What happens then?

I'm also assuming that not bypassing the cathode resistor with a cap will keep power down (or does it just affect gain and headroom stays the same?)

[Bruce / Mission Amps]

Is it not the other way around? 270 split or 135 shared?

... Yes, he merely stated in backwards.

[Hardtailed]

... Yes, he merely stated in backwards.

Hehe, thanks!
For a second I was wondering if my electric skills were depleting!

Oh well... I think I'll just build a real "18w" style amp with new trannies.

Still want to do something more interesting with my Firefly.
I might try swapping an ECC99 for the power amp, with a lower primary impedance, it's supposed to be able to push over 4W.
Then add a long tail PI and perhaps the EF86 as the input stage (or paralleled triodes).

I've read that the ECC99 is very similar to a pair of triode-strapped EL84, so why go the trouble! I can throw it in there and just swap the cathode resistor to a lower value!

[Robert M. Martinelli]

... Yes, he merely stated in backwards.

......OOoooooops!

You're right, I thought it right and wrote it backwards....

Hope this doesn't mean I need brain surgery....

Sorry!

Bob