"low watt" 6L6 and EL34

[C Moore]

The thread that Katopan has going makes me wonder.
I sometimes see dual EL34 and 6L6 amps advertised as 30 watts.
I am pretty sure the amps of question are fixed bias, class AB circuits.
Considering the tubes are 25 and 30 watts respectively, how is the lower power achieved.?
Is it simply that the OT of choice will not stay clean after 30 watts.?
Do they bias the tubes on the cold side.?
Thank You

[LeftyStrat]

It's mostly the amount of voltage on the tubes. Lower voltage = lower power.

If you look at the application data, say for an EL34:

http://tdsl.duncanamps.com/show.php?des=EL34

The Va column is the plate voltage, and the Pout column is the output power. If you put 315 volts on the plates in AB1 P/P, a pair of EL34's will produce around 35 watts.

But 775 volts on the plates, and that pair is capable of 100 watts.

[katopan]

And OT primary impedance, which can be used to set the load line well below the knee of the plate curves and down in an area of less output power delivered.

First thing I'm going to do when I build my Express (other than play it for a bit) is measure the delivered output peak voltage on the verge of clipping and see if it corresponds to the power I've calculated for EL34s at 400V B+ and a 6K6 load.

[C Moore]

Thanks for the replies.
Pardon my ignorance, I imagine it can be trying.....
but what is the difference between having a lower than "normal" plate voltage for said tubes, and reducing the bias voltage.?
If the tube is dissipating 15 watts, is there an advantage of plate voltage over plate current.?
Thank You

[Andy Le Blanc]

the power supply limits the available current under peak conditions

a power test is what generates the figure, under peak conditions...

opt for a PS tranny that gives you the figure you want under those test conditions

and then you too can quote the same figures

[katopan]

In theory you can have a lower voltage with lower OT impedance and higher voltage with higher OT impedance amps both delivering the same output power. But the tone will be different and it all depends on what sound you want out of said power valves.

[Andy Le Blanc]

sarcasm aside, what you typically see in tube data is the increase of the gird to grid AF signal through the operating classes as it relates to plate voltage.

what may be 5w class A may increase to 35w AB2 for the same amp, alot depends upon definition.

you can use any valve you want as long as it can handle the voltage and/or current that the design is asking it to handle.

I've seen many lower power designs use a larger plate load and a smaller PT
they just cant develop the watts because the PT is designed to safely supply only so many milli-amps.

it becomes a design restriction, what can I do with only 120ma and a 8.5k load

and that 120ma PT will only get you 80ma full load before it starts to heat up

so say you have 120ma 660v/ct tranny, peak V with a full wave works out to around 460v, the average v works out to around 300.
so at best you say 18w dissipation per tube .04 x 460, and 12w .o4 x 300, the reality should be between the two, what tube type do you use?

cranking little 6v6 amp, about right for 6l6, 6l6-g, 6l6-gb, and cold for 6l6GC and 6ca7, so if you go with 6l6GC/6ca7 the specs for the amp will look very low powered,
and depending upon the plate load the power test may look even lower

[Structo]

So if the power transformer is not capable of keeping up with the demand of the power amp, won't it get really hot?

I mean, how do you determine the right load for the transformer?

[C Moore]

Thanks Guys -
Andy put it into (for me) perspective.
It pisses me off now.....
I had a Carol-Ann OD2r that was rated at 30 watts with 2x6L6.
I should have taken the opportunity to investigate further while I still owned. It was a real nice amp. The best amp I ever had from the perspective of having a very nice Clean Channel, as well a a very good Dirty Channel.
But the electronics had a lot of that "Black Goop" covering the board, and I kind of gave up poking around in the chassis because of that.
Thanks Again

[Andy Le Blanc]

the load is your bias, thats what the bias is for, a tube amp is speaker attached to a power supply, the tube are there to modulate the power supply.

that static bias figure is really you setting the idle on the PT...

Its a wonderful thing to know what the current rating of a PT is.
warm is one thing, hot running potting is bad JU JU.

I have a rig that I set up for KT88, it uses 650ct/200ma PT
Thats a comfortable fender voltage 450ish, but the kt88 can be run up to 40w each if I tried to max them I'd have bias around 175ma static
the PT would do it, but pushing the amp past class A with input could be very bad, but if I bias to 100ma the amp will run forever and I can pound the crap out of it with my guitar and all on 11

[Firestorm]

I think what everybody said here is pretty much spot on. Output tube plate ratings are measured in terms of heat, not audio power (though those are related). But if you want to measure the power produced at the speaker (as opposed to the tube plates and ot) you may find speaker efficiency can have a significant audible impact.