How many watts from a pair of EL84's?

for the DIY ST35, the Dynakit and every other PP EL84

Postby EWBrown » Tue Mar 24, 2009 5:20 pm

Interesting..... This is definitely taking these tubes to the next level of their performance capabilities. Definitely an "out of the box" approach
to amp design, something beyond my experiences.

I guess I'm much too used to the usual 365-380V B+, 35 mA cathode biasing and UL mode for these tubes.

I've gunned them up to around 25W, pentode mode, 400V B+ and fixed bias, and 6600 ohm OPTs, but lesser EL84s (like EIs and most J/J EL84s need not apply for the job) ;)

The Russian EL84M / 6P14P-EVs are pretty tough, I've pushed them well beyond the normal PD ratings, up to 45 mA cathode curents, but at lower (360-400V), B+ levels than the RM-10's 700V

FWIW, has anyone tried 6P14P-ERs? I've heard good things about these, but haven't yet tried any.

I also wonder.... Since 6GK6s are basically 6BQ5s with a different pinout, how well would (or not) these stand up to 700V B+ and 350V screens... Then there are the lower cost 10GK6, 16GK6 and 29GK6s.

Roger has also done some interesting designs with our favorites, 6EM7 and 13EM7:

/ed B
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Postby Geek » Tue Mar 24, 2009 9:45 pm

Roger A Modjeski wrote:Note the 350V screen, 15mA/tube bias and 13,000 ohm load.

Any flyback diodes across that to control peaks and increase efficiency?
(like on 100W guitar amps)

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screens,diodes and guitar amps

Postby Roger A Modjeski » Wed Mar 25, 2009 9:22 am

The RM-10 is very easy on tubes. I have tried EL-84s from Russia, China, USA and Eastern Europe. They are all very happy in the RM-10 because of the low screen voltage (lower than the ST-35). Horizontal output tubes are typically rated for 5,000 volts on the plate, even the ones without the plate cap like the 6HB5. If a tube has a good vacuum it does not mind very high plate voltages as long as dissipation is observed. But the screen is an entirely different issue. One should also look at the curves of screen current vs. plate voltage. When the plate dips down toward ground at full output swing the screen current goes up dramatically. The higher the screen voltage the higher the dissipation and this often results in a glowing or melted screen wire. That's one of the reasons for the little holes in the plate, so we can look at our screen grids in operation. Anything above 350 V screen is getting into dangerous territory.

Simply put, the standard EL-84 circuits generally use the same screen and plate voltage which the data books hault at 300 V in my data books. That puts the lid on power for UL and typical Pentode amps where the screen voltage equals the plate voltage. The other important point is that cathode bias always limits output power in that the cathode current cannot increase as needed for peaks. So these cathode biased circuits are not happy when the impedance dips unlike the RM-10 where there is significant reserve current. See the table below.

As I was musing over this yesterday I also considered the voltage swing limitations of the typical EL-84 amps. Because the power supply is always drawn down by the hign idle current, the amp has no ability to produce high peaks at low average power. Piano music can have a peak to average ratio of 20 dB. That's 100 to 1 in power. Take your ST-35 amplifier power divide it by 100 and that's your non-clipping max average power. Thats .17 watts per channel average. Even if you take a peak to average ration of 10 db, thats 10 to 1 so your 17.5 watt amp can run 1.75 watts and produce 90 db average (as read on an SPL meter) on a 87 spl/watt speaker.

A little history on power-rating abuse. If you look at old Lafayette, Allied Radio, Radio Shack catalogs you will find something called Peak to Peak power which is 4 times the Music Power. The ratio of Music Power to RMS cannot not be estimated, it must be measured and here is how you do it. The actual process is to take the amplifier (usually transistor when this was at it's most aggregious) and use a variac or external power supply to hold the B+ to the idle value. Measure the RMS and that is a valid amount of power when playing without clipping. In a poorly designed amp you do it fast so as not to over dissipate the output devices.

Here's the Allied 399 solid-state Stereo Receiver from the 1968 catalog. "Power output 244 watts peak, 122 watts total IHF, 43 watts per channel continuous sinewave at 4 ohms". Note the use of "total" for the peak but "per channel" for the RMS.

For their last act, the power raters did something that made no sense at all. They took the peak to peak voltage, which cannot ever be across the load and squared that. Double the voltage squared gives 4 times the power. That's how 20 watt amps with 25 watts Music Power got up beyond 100 watts. They called this peak to peak power which is something that has no real world meaning.

I use a 2 cycle tone burst at 1 KHz from a generator I built. You can likely generate one off your computer with the right software. It's doesn't have to look perfect. There may be a CD available or make one. Check the accuracy of volts per division on your scope. Measure the peak voltage when the burst clips. You might use a peak-hold meter but I like to see what the wave looks like. Now take that voltage, divide by 1.4 to get the RMS equivalent, square it and divide by your load. Here are the results from the RM-10 measured this morning. Note the impedance where the max power occurs is around 5-6.4 ohms, the typical power of a 8 ohm speaker. The following load impedances were measured on the 8 ohm tap.

Load Music Pwr
12.5---32 watts

Note that the 4 ohm power on the 8 ohm tap is still above the 35 watt rating. Most amplifiers drop to 60-80% when loaded at half the tap impedance. This is a real problem as we know speakers dip, especially electrostats, which the RM-10 drives really well. I didn't measure this when I had my Stereo 35 but I bet it's going to be pretty low. Anyone out there care to do it?
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Postby Roger A Modjeski » Wed Mar 25, 2009 11:00 am

Geek wrote:
Roger A Modjeski wrote:Note the 350V screen, 15mA/tube bias and 13,000 ohm load.

Any flyback diodes across that to control peaks and increase efficiency?
(like on 100W guitar amps)


It is my belief that the diodes across the primary are to keep things under control under full clipping conditions that guitar amplifiers are subjected to on a regular basis. I don't see how they would increase efficiency, nor do I see any reason to have them in my amplifiers.
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Postby dhuebert » Wed Mar 25, 2009 3:11 pm

I had a set of transformers left over from a former Diytube ST-35 so I built a guitar amp with the power and output transformers. With a carefully matched quartet of EL84s in fixed bias and ultralinear with ~375V B+ it makes 30 watts RMS across a 4 ohm resistive load at 1KHz.

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