ECL82 (6BM8) PP Stereo

[Family_Dog]

Hi Guys,

I've been browsing the archives for some time now looking for a suitable schematic to build using 4 of the above valves, two per channel. I'm limited to only 4 valves in total due to the size of the (recovered) chassis that I intend using. Although I have found a multitude of ECL86 schematics, I would rather use ECL82 valves due to easier (and cheaper!) availibity.

After about a week of searching the archives (and getting side-tracked every time due to the interesting articles) I am no closer to finding a circuit, other than the "Stoetkit" Junior Mk2. Would appreciate some suggestions here.

-Eric

[PRR]

> schematic to build using 4 of the above valves, two per channel. I'm limited to only 4 valves in total

How many ways are there to use two ECL82s? Oh, I can think of several silly ways: use the pentodes as volt-amps, the high-Mu triodes as a bad power amp. But seriously, only a couple/three ways. Given your space, a V- supply for fixed-bias or LTP driver doesn't make sense. So it really comes down to some minor variation on this:



The "about 270K" resistor should be trimmed so the first plate sits at 1/4 to 1/3rd of the supply for that stage (so the cathodyne is well biased). The output stage cathode resistor is about 200 ohms, and may work better with a bypass resistor. I've set NFB with 2K7 and 27K fairly arbitrarily, change the 27K to taste.

[moamps]

http://www.bonavolta.ch/hobby/en/audio/ecl82.htm

[Sjef]

Try this one, sounds pretty damn good for a small amp, better than the Stoetkit (yes, I have compared them) Coupling caps can be as small as 22nF. I used a pretty heavy power supply with diode rectification --> 550uF --> 10H --> 550uF. the 30uF for the driver stage was a paper in oil cap I had lying around, coupling caps where Wima FKP's cheaply bought at a dumpstore.

[Eli Duttman]

Please notice that both PRR and Sjef suggested a common cathode gain block driving a "concertina" phase splitter. It's most definitely a workable solution.

A possible tweak to the concept is to regulate the the screen grid B+ of the "finals" with a Zener diode stack. Regulating screen grid B+ in pentodes and beam tetrodes improves linearity. Loop NFB is still needed to lower the O/P impedance for damping factor considerations.

[Tubes4e4]

Hi,

an alternative to the dc-coupled gain/concertina stage would be a CCSed diff amp. Elegant way to inject FB at the grid of the 2nd diff amp triode section, too.

Tom

[dstockwell]

Sorry, these are SE.

[Family_Dog]

Guys,

Thanks. These circuits all look good. They are also simple enough to allow some experimenting with as well.

I am probably going to use OPTs designed for an EL84, not sure if they are made for 6.6k or 8k Primary impedances, they do have screen tappings for UL operation. I don't thinlk the impedance (mis)match would be a train-smash in this application.

Now I have to determine whether the mains transformer I have is up to the application by placing suitable loads on the filament & HT windings!

-Eric

[Morgan L]

dstockwell,

Looks like a neat little amp. What worries me is the pentode-connected
output without any form of feedback. Most speakers will misbehave
due to the high output impedance (may yield weird frequency response
with sloppy bass). Also, the input sensitivity will be very high,
which may be a problem in some cases (any noise issues?).
I've also built an SE PCL82, and like it a lot. It's switchable between
pentode and triode connected output (useful to match different music),
but more importantly, it uses plate-to-grid feedback, which really works
well. The input triode is run far hotter (in fact, I use a plate CCS to
"pull" the triode into a higher-current, more linear spot) and there is
a resistor from pentode's plate to triode's plate (in AC terms the same
as pentode's control grid). A triode "current pump" (V/I converter)
feeding the pentode "I/V converter" (not quite). That's all. Pretty
easy to try.
I've since learnt, from the excellent PRR, that a smaller plate-to-B+
resistor instead of a CCS will work almost as well. A rough calc. gives
about 15k triode plate resistor, 390 ohm cathode resistor. I think I
used 180k plate-to-plate resistor. No triode cathode bypass, and
remove or lessen the 22k B+ dropping resistor to maximize available
voltage to the triode. (This will demand big current swings from the
triode, in fact, just barely enough. A plate CCS at 2.7 mA replacing
the 15k plate resistor would improve the margin.)
Not a lot of feedback, but just enough to get a manageable Zout
and to clear up the sound just a bit without going overboard.
One disadvantage: power supply rejection gets worse, so good
filtering is required, but you seem to have that.
Just a suggestion, but IME well worth trying as an alternative to
global NFB, which can be tricky to get right without an o-scope and
the knowledge to use it (I'm talking about myself!). Also, this will
work with any output transformer, so is more universally applicable.

Morgan

[Shoog]

The reason you are finding more circuits for the ECL86 over the ECL82, is because on paper at least, the ECL86 is a better valve.

Wont stop me using the ECL82 in my own headphone amp though.
Anyone think of a use for a PCL83 ??

Shoog