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Tube mono-block amp with SIX 6L6 outut tubes

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Tim Williams
Guest

Tue Jan 01, 2019 5:45 am   



"Big Bad Bob" <BigBadBob-at-mrp3-dot-com_at_testing.local> wrote in message
news:R6mdnYGk2c0E97fBnZ2dnUU7-LPNnZ2d_at_earthlink.com...
Quote:
sounds like a gimmick. Tube plate/cathode currents are just way too small
to properly drive a speaker without an impedence matching transformer.


So what, nothing comes out?...

They were definitely below the maximum power point (at clipping), if that's
more accurately what you meant.

6AS7/6080, 6S33S and the other regulator tubes have peak cathode current in
the ballpark of an ampere, so a modest number of tubes offers a modest power
output, say 20 or 40W, preferably into a higher load like 16 ohms. The
efficiency is poor, with more heat dissipated in the heaters alone, than
delivered to the output.

If you put dozens in parallel, the efficiency keeps going up as you get
closer to matching, but now your whole system consumes multiple kilowatts...

There was also the Philips "SEPP" with a pair of EL86? driving a 100s-ohms
voice coil at good efficiency, but those speakers are so rare that this is
practically a unique case.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

Tim Williams
Guest

Tue Jan 01, 2019 5:45 am   



"Big Bad Bob" <BigBadBob-at-mrp3-dot-com_at_testing.local> wrote in message
news:bsGdneTGabiI8LfBnZ2dnUU7-aHNnZ2d_at_earthlink.com...
Quote:
I think it works ok without "all that". fuses where needed of course, to
avoid things catching on fire. But tubes are amazingly strong at
absorbing conditions that transistors would melt under. Sure the plates
turn pink [like when half of a push-pull transformer melts away] but if
you don't push them to the edge of physics they generally take it ok and
survive getting a new output transformer.


A lot of sweep tube datasheets contain the rating, "short term overload
duration: 220W for 60s" or something like that. Transistors can only dream
of such abuse, gone in milliseconds!

But the purpose of that rating must be understood. These were TV tubes, and
the sweep tube in particular often took a beating as other tubes heated up
and other signals stabilized: horizontal oscillator and sync, and the damper
diode especially -- its high cathode voltage isolation takes a long time to
warm up.

There was definitely no money in adding a protection circuit! Burn a tube,
pop it out, take it down to the corner drugstore and buy a new one for a
buck or a few.

Nowadays, with both tubes and transformers being rather pricey, let alone
the repairman -- the balance changes, and especially with how little
hardware is involved in adding a protection circuit (if one does not mind
that it contains silicon), it's well worth it.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

bitrex
Guest

Tue Jan 01, 2019 7:45 am   



On 12/31/2018 11:32 PM, Tim Williams wrote:
Quote:
"Big Bad Bob" <BigBadBob-at-mrp3-dot-com_at_testing.local> wrote in message
news:bsGdneTGabiI8LfBnZ2dnUU7-aHNnZ2d_at_earthlink.com...
I think it works ok without "all that".† fuses where needed of course,
to avoid things catching on fire.† But tubes are amazingly strong at
absorbing conditions that transistors would melt under.† Sure the plates
turn pink [like when half of a push-pull transformer melts away] but
if you don't push them to the edge of physics they generally take it
ok and
survive getting a new output transformer.

A lot of sweep tube datasheets contain the rating, "short term overload
duration: 220W for 60s" or something like that.† Transistors can only
dream of such abuse, gone in milliseconds!

But the purpose of that rating must be understood.† These were TV tubes,
and the sweep tube in particular often took a beating as other tubes
heated up and other signals stabilized: horizontal oscillator and sync,
and the damper diode especially -- its high cathode voltage isolation
takes a long time to warm up.

There was definitely no money in adding a protection circuit!† Burn a
tube, pop it out, take it down to the corner drugstore and buy a new one
for a buck or a few.

Nowadays, with both tubes and transformers being rather pricey, let
alone the repairman -- the balance changes, and especially with how
little hardware is involved in adding a protection circuit (if one does
not mind that it contains silicon), it's well worth it.

Tim


Yep, the OP is talking about monoblock amp with 6, 8, ten power tubes.
The power supply iron and output transformer will be large, possibly
custom, and not cheap.

$5-10 worth of microcontroller or ICs and relays to at the least monitor
tube cathode currents and grid voltages and cut the HT if things start
going tits-up compared to a melted half of an output transformer that
might cost $3-500. Big BUMMER!


Guest

Tue Jan 01, 2019 1:45 pm   



On Mon, 31 Dec 2018 11:14:33 -0800, Big Bad Bob
<BigBadBob-at-mrp3-dot-com_at_testing.local> wrote:

Quote:
On 12/29/18 10:27, Tauno Voipio wrote:
On 29.12.18 01:32, Chris wrote:


<clip>


Quote:
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.


Search for 6082 "50 ohm" and you find quite a lot hits with single
pair driving 50 ohms.

With tree pairs, the load could be 16 ohms.


Guest

Tue Jan 01, 2019 1:45 pm   



On Mon, 31 Dec 2018 22:39:02 -0600, "Tim Williams"
<tiwill_at_seventransistorlabs.com> wrote:

Quote:
There was also the Philips "SEPP" with a pair of EL86? driving a 100s-ohms
voice coil at good efficiency, but those speakers are so rare that this is
practically a unique case.


Usually closer to 1000 ohms.

These speakers were sold for a few years, but later on, they went on
sale. Adding an 8:1000 ohm transformer and you could cheaply build a
good quality speaker box for an ordinary 8 ohm tube or transistor
amplifier Smile

Kevin Aylward
Guest

Tue Jan 01, 2019 4:45 pm   



"Big Bad Bob" wrote in message
news:bsGdneTGabiI8LfBnZ2dnUU7-aHNnZ2d_at_earthlink.com...

On 12/28/18 09:16, bitrex wrote:

Quote:
All amps of that scale should have active protection circuitry/monitoring
of currents, biases, and temperatures via microprocessor to avoid
potentially catastrophic faults.

Even briefly overvolt or overcurrent on a typical transistor, and you'll be
replacing it VERY soon. I made the mistake of designing a circuit that
operated close to the maximum Vceo (these 60V transistors should be able to
handle 45-50V right?), and the transistors never lasted long under load
(replaced 3 times, and 3 blown fuses that were supposed to protect them).
Replaced with transistors that had twice the Vceo and no problem.


Well... overcurrent on a mosfet output device is usually pretty safe for
quite a while. Second breakdown is the problem with bipolar. Their rating at
high voltage is much lower than their power ratings would imply.

I designed mosfet amp in the early 80s. I tested it by putting in a full
level signal with an output s/c. The only protection was a zener across the
gates to limit the current to the device ratings. I left it cycling with its
90 Deg heatsink thermal cutout for 3 days. No problems.

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html

Tauno Voipio
Guest

Tue Jan 01, 2019 8:45 pm   



On 1.1.19 06:39, Tim Williams wrote:
Quote:
"Big Bad Bob" <BigBadBob-at-mrp3-dot-com_at_testing.local> wrote in message
news:R6mdnYGk2c0E97fBnZ2dnUU7-LPNnZ2d_at_earthlink.com...
sounds like a gimmick.† Tube plate/cathode currents are just way too
small to properly drive a speaker without an impedence matching
transformer.


So what, nothing comes out?...

They were definitely below the maximum power point (at clipping), if
that's more accurately what you meant.

6AS7/6080, 6S33S and the other regulator tubes have peak cathode current
in the ballpark of an ampere, so a modest number of tubes offers a
modest power output, say 20 or 40W, preferably into a higher load like
16 ohms.† The efficiency is poor, with more heat dissipated in the
heaters alone, than delivered to the output.

If you put dozens in parallel, the efficiency keeps going up as you get
closer to matching, but now your whole system consumes multiple
kilowatts...

There was also the Philips "SEPP" with a pair of EL86? driving a
100s-ohms voice coil at good efficiency, but those speakers are so rare
that this is practically a unique case.

Tim


The final triodes (6080) are special high-current tubes. The amplifier
designer rated the thing at 25 W into a 16 ohm voice-coil speaker.

--

-TV

bitrex
Guest

Wed Jan 02, 2019 12:45 am   



On 01/01/2019 10:27 AM, Kevin Aylward wrote:
Quote:
"Big Bad Bob"  wrote in message
news:bsGdneTGabiI8LfBnZ2dnUU7-aHNnZ2d_at_earthlink.com...

On 12/28/18 09:16, bitrex wrote:

All amps of that scale should have active protection
circuitry/monitoring of currents, biases, and temperatures via
microprocessor to avoid potentially catastrophic faults.

Even briefly overvolt or overcurrent on a typical transistor, and
you'll be replacing it VERY soon.  I made the mistake of designing a
circuit that operated close to the maximum Vceo (these 60V transistors
should be able to handle 45-50V right?), and the transistors never
lasted long under load (replaced 3 times, and 3 blown fuses that were
supposed to protect them). Replaced with transistors that had twice
the Vceo and no problem.

Well... overcurrent on a mosfet output device is usually pretty safe for
quite a while. Second breakdown is the problem with bipolar. Their
rating at high voltage is much lower than their power ratings would imply.


Ya, BJT amps tend to have oversized output devices as compared to what
their rated maximum "RMS power" output would imply. 75 watt-rated
devices in 25 watt amps. The problem is risk of second breakdown when
working into reaactive loads, not de-rating BJTs appropriately when
they're handing significant powers into reactive loads common newbie
mistake

Quote:
I designed  mosfet amp in the early 80s. I tested it by putting in a
full level signal with an output s/c. The only protection was a zener
across the gates to limit the current to the device ratings. I left it
cycling with its 90 Deg heatsink thermal cutout for 3 days. No problems.

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html


Tim Williams
Guest

Wed Jan 02, 2019 7:45 am   



"Kevin Aylward" <kevinRemovAT_at_kevinaylward.co.uk> wrote in message
news:Pdedne3rNa5uG7bBnZ2dnUU7-aHNnZ2d_at_giganews.com...
Quote:
Well... overcurrent on a mosfet output device is usually pretty safe for
quite a while. Second breakdown is the problem with bipolar. Their rating
at high voltage is much lower than their power ratings would imply.


Used to be true -- modern MOSFETs are more than current-dense enough to
exhibit 2nd breakdown. I shouldn't actually say modern, because apparently
SuperJunction process has... PTC source connections or something? I haven't
seen one without a square SOA yet I don't think. So by now, it's actually
previous generation that you have to watch out for. I forget if lower
voltage (SJ goes away under ~400V I think it was?) processes are still
prone.

Tim

--
Seven Transistor Labs, LLC
Electrical Engineering Consultation and Design
Website: https://www.seventransistorlabs.com/

Kevin Aylward
Guest

Thu Jan 03, 2019 10:45 pm   



Quote:
"Tim Williams" wrote in message news:q0hkvr$7dm$1_at_dont-email.me...

"Kevin Aylward" <kevinRemovAT_at_kevinaylward.co.uk> wrote in message
news:Pdedne3rNa5uG7bBnZ2dnUU7-aHNnZ2d_at_giganews.com...
Well... overcurrent on a mosfet output device is usually pretty safe for
quite a while. Second breakdown is the problem with bipolar. Their rating
at high voltage is much lower than their power ratings would imply.

Used to be true -- modern MOSFETs are more than current-dense enough to
exhibit 2nd breakdown. I shouldn't actually say modern, because apparently
SuperJunction process has... PTC source connections or something? I
haven't seen one without a square SOA yet I don't think. So by now, it's
actually previous generation that you have to watch out for. I forget if
lower voltage (SJ goes away under ~400V I think it was?) processes are
still prone.


Exicon are the audio mosfets of choice today, apparently. They are laterals.

http://www.exicon.info/

They have the usual power limited SOA.

I don't know who actually makes them, but my guess is someone like XFAB.

Standard fab vendors will make any asic for any fabless company, even if the
asic is just the one big transistor!

-- Kevin Aylward
http://www.anasoft.co.uk - SuperSpice
http://www.kevinaylward.co.uk/ee/index.html


Guest

Fri Jan 04, 2019 12:45 am   



On Fri, 28 Dec 2018 14:19:50 +0200, Tauno Voipio
<tauno.voipio_at_notused.fi.invalid> wrote:

Quote:
On 28.12.18 10:45, upsidedown_at_downunder.com wrote:
On Thu, 27 Dec 2018 21:11:35 -0600, tubeguy_at_myshop.com wrote:

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.

Tubes may age at different rates, so the characteristics might be
quite different after a long time. It might not make sense to replace
a single (failed) tube and you may have to replace the whole set of
tubes with new tubes preferably from the same manufacturing batch.
This can be quite expensive :-)


In the 1960's, I made a guitar amplifier with six EL500's. The tubes
are not characterized for linear use, and they were not matched in
any way. Each tube had an own cathode resistor, and there was no
evidence of any overload from unbalanced operation.

The tubes had a tendency of parasitic oscillation, so I added a
ferrite bead on the control grids and an inductor-resistor parallel
combination on each plate (for constructions, see nearest ham handbook).


Back in the late 60s early 70s, I had several (self refurbished)
mono-block power amps that used four 6L6 output tubes. I did not even
know about matching those tubes. I just put in any tube marked as a 6L6.
I recall having both the glass (GC) types mixed with the black metal
ones. I always had good sound and lots of power. I do recall that
replacing the metal cased ones with 6L6GC did increase my power though.
But until I could afford new tubes, I used what I had. And even with the
new tubes, I never matched them.

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