On 12/28/18 03:11, tubeguy@myshop.com 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...
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...
the two big issues with OTL tube amps is the intrinsic impedance
mismatch of a tube cathode vs a tube plate in a totem pole arrangement looking into the load, and related lack of any intrinsic power supply
noise rejection in the output stage the way a push-pull
transformer-coupled output stage has when both halves have similar
output impedances working into the same reflected load.
Global negative feedback can't do anything about the second and there
isn't usually enough open-loop gain available to do a good job of
correcting for distortion caused by the first across the audio band.
often leading to a kinda poor-performing amp.
On 12/28/18 03:11, tubeguy@myshop.com 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 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.
On 29.12.18 01:32, Chris wrote:
On 12/28/18 03:11, tubeguy@myshop.com 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.
All amps of that scale should have active protection
circuitry/monitoring of currents, biases, and temperatures via
microprocessor to avoid potentially catastrophic faults.
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.
sounds like a gimmick. Tube plate/cathode currents are just way too small
to properly drive a speaker without an impedence matching transformer.
"Big Bad Bob" <BigBadBob-at-mrp3-dot-com@testing.local> wrote in message news:bsGdneTGabiI8LfBnZ2dnUU7-aHNnZ2d@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
On 12/29/18 10:27, Tauno Voipio wrote:
On 29.12.18 01:32, Chris wrote:
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.
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.
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.
"Big Bad Bob" <BigBadBob-at-mrp3-dot-com@testing.local> wrote in message news:R6mdnYGk2c0E97fBnZ2dnUU7-LPNnZ2d@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
"Big Bad Bob"Â wrote in message news:bsGdneTGabiI8LfBnZ2dnUU7-aHNnZ2d@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.
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
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.
"Tim Williams" wrote in message news:q0hkvr$7dm$1@dont-email.me...
"Kevin Aylward" <kevinRemovAT@kevinaylward.co.uk> wrote in message >news:Pdedne3rNa5uG7bBnZ2dnUU7-aHNnZ2d@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.
On 28.12.18 10:45, upsidedown@downunder.com wrote:
On Thu, 27 Dec 2018 21:11:35 -0600, tubeguy@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).
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