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Big Bad Bob Big Bad Bob is offline
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Default Tube mono-block amp with SIX 6L6 outut tubes

On 12/29/18 10:27, Tauno Voipio wrote:
On 29.12.18 01:32, Chris wrote:
On 12/28/18 03:11, wrote:
I found a schematic of a stereo amplifier containing TWELVE 6L6 audio
output tubes. But aside from sharing the power supply, it's two
mono-blocks with each having SIX 6L6 output tubes. Unfortunately, I was
not able to get any real detailed information on this.

I compared it to several guitar amp schematics which use FOUR 6L6 tubes
in push-pull parallel, and this is very similar. It just has 3 tubes on
each side of the phase, rather than two. The plates are all wired
together and cathode / grid components are duplicates to each tube.

They did use as bias adjust potentiometer on each tube, which I have not
seen on the guitar amps.

Obviously the purpose is to increase wattage output.

I'm not intending on building this, but it's interesting and makes me
wonder if someone could use EIGHT 6L6 tubes, or TEN?
(Of course adding more tubes mean bigger audio output transformers and
heftier power supplies.

Another question that comes to mind is the impedience of the primary on
the audio output transformer. Assuming I could find a transformer that
would handle the wattage, would the impedence be the same as those used
with FOUR 6L6 tubes, or would that change since there are two more tube
plates connected in series.

Anyhow, looking at this schematic makes me think that any PPP amp with
FOUR tubes could have more output tubes added, as long as power supply
current is available and a suitable audio output transformer is
obtainable.




If you want to see an interesting audio amp, look up the Editors and
Engineers Radio Handbook from the mid 60's. There's a design that
which uses 6 or 8 6080 double triodes to build a direct coupled
amplifier, no output transformer and dual power rails, positive and
negative. Never built built it here, but a very original design. Could
probably update that to use power mosfets...


I found it in my copy, 16th edition, 1963.

The amplifier is not direct coupled, there are several stages
with coupling capacitors. The final is series-connected with
3 series pairs of 6082 triodes in parallel and direct feed to
a 16 ohm speaker. The power supplies are + and - 140 V.


sounds like a gimmick. Tube plate/cathode currents are just way too
small to properly drive a speaker without an impedence matching transformer.

Had they thought of it "back then" they could've used toroidal
transformers with PWM push-pull amplifiers modulated up to 70 or 80
percent as a class 'G' amplifier. By the time that was invented, it was
all transistors, though. [and no benfit to using tubes].

I suspect that the push-pull configuration's advantages [and
disadvantages] disappear when you use a transformerless configuration,
and new ones take their place.

If you want high power, you should consider using KT88's and a matching
transformer from Hammond. A single pair should get you at least 100W
RMS. I saw a stereo amplifier built using these at a state fair once,
in the 70's, basically a clone of the GE tube manual's reference design.
It was on an oversized chassis, though. I would've preferred it as 2
"monoblock" amplifiers that could fit together on a shelf...

even the high heater current requirements of a KT88 would be smaller
than 6 or 8 6L6's [and is physically smaller]. But I suppose it looks
'cooler' to have all of those bottles grouped together.


--
(aka 'Bombastic Bob' in case you wondered)

'Feeling with my fingers, and thinking with my brain' - me

'your story is so touching, but it sounds just like a lie'
"Straighten up and fly right"