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Old 26th March 2011, 01:45 PM   #1
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Default Unexpectedly good EL84 amp

I had been endlessly trying to get a scrapheap amp back working and sounding somewhat good, but just could not get her working. See the thread over here.




Anyway, in the end I just gave up and in an effort to salvage something, anything, I hit upon this schematic, courtesy of tubecad:

Click the image to open in full size.


The thing is, this one sounds absolutely fantastic. Nothing special on any of the parts, all are stuff I had lying around, although I think my transformers are sowters. Anyway, after sorting a few issues with hum etc (thanks again trobbins), this amp is sounding just great. Its easily on a par with the F5s that I've built, and having compared with other amps I've heard, it stands up very well. Soundstage is wide and deep, treble is sparkly and clear with no harshness, bass is tight - although I have heard it in bigger quantities. It really is well worth a mention here as a nice amp to build, lowish to medium gain, and about 8W so good enough for horns.

Its simple enough to build with a low enough parts count. From the tubecad site:

Quote:
EL84/6BQ5 Push-Pull Power Amplifier
Few tube amplifiers sound as sweet as two triode-connected EL84s in push-pull. These amazing little power tubes are quite efficient, requiring only 15Vpp to bring them to full output. Thus, with just a single input/phase-splitter tube, such as a 6N1P or 12AT7, we build a fine push-pull amplifier.The 6N1P is a fine little tube and about 1Vpk of input signal will bring this amplifier to full output (about 8W) and no feedback loop is employed. The input stage and split-load phase splitter define a CCDA and present an equal amount of B+ noise to the output tubes' grids, which will help reduce the noise at the output. (By the way, the 0.47F coupling capacitors could be replaced with something smaller, say 0.1F.) I would use old Dynaco ST35 (also SCA-35) output transformers (about 8k P-to-P), which are stellar performers.

Fran
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Old 26th March 2011, 02:45 PM   #2
el156 is offline el156  Portugal
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The EL84 are great tubes! They sound exelent!
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Old 26th March 2011, 02:52 PM   #3
tinitus is offline tinitus  Europe
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nice to have another working curcuit
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Old 26th March 2011, 04:50 PM   #4
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The EL84 is a great tube - difficult to mess up.

However I think in this circuits case it is the differential output stage which is bringing most of the benefits. Everything I have built with differential outputs has had excellent soundstage and imaging, and crisp delivery.

Good work Fran.
A shoot out some time in the summer seems in order

Shoog
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Old 26th March 2011, 05:00 PM   #5
kevinkr is offline kevinkr  United States
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Seems like a couple of minor tweaks could improve it still further. The operating point of the phase inverter is not really optimum IMHO. I'd aim for something like 100V on the cathode of the split load inverter by adjusting the value of the lower 1M to something like 1.5M and I would probably decrease the load resistors to 20K or less in order to get the plate current up.

A red led on the cathode of the first section of the 6N1P would give you a little more gain, and possibly somewhat better linearity as well, but might make the design more sensitive to variations in transconductance from tube to tube.

Looks like a good, simple and tweakable design..
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Last edited by kevinkr; 26th March 2011 at 05:02 PM.
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Old 26th March 2011, 05:00 PM   #6
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... and a BBQ maybe?


It sounds lovely, really does.


EDIt: Sorry Kevin, simulpost! Might look at those numbers. I also wondered about using a CCS but want to listen for a while and get a handle on it.

Last edited by woodturner-fran; 26th March 2011 at 05:05 PM.
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Old 26th March 2011, 05:05 PM   #7
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When my studies are over for the year - at the end of May - I would love to come over. No meat on that BBQ for me though.
Currently trying to get my hands on some old Gaertz field coil drivers for another OB build. Lots of new ideas.

Sorry looking at it again its not differential - just a nice simple circuit.

Shoog

Last edited by Shoog; 26th March 2011 at 05:12 PM.
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Old 26th March 2011, 05:11 PM   #8
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You know, I'd love to have a go at some open baffles. You bring the drivers, I'll cut the ply....


I'll keep the veggies separate!
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Old 26th March 2011, 05:36 PM   #9
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I would think power O/P is less than 8 WPC. Look at page 3 of the GE 6BQ5 data sheet, where some PP AB1 triode mode conditions are given.

The 6Н1П (6n1p) is an excellent driver, thanks in part to its large cathodes.

There's lots of sonic mojo in "12" W. multi-grid power O/P types, in pentode, UL, and triode modes. Poindexter's "Musical Machine" uses a very similar O/P stage constructed around 6V6 "finals".
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Old 26th March 2011, 05:51 PM   #10
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Eli - haven't tested the power output myself (don't even think I have the gear to do it), just quoting broskie from his post.

Sounds plenty loud with the horns - and all my sources put out 1v rather than the customary 2. Using a dcb1 buffer up front.

Fran
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