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Guest

Fri Dec 28, 2018 4:45 am   



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.

Tim Williams
Guest

Fri Dec 28, 2018 9:45 am   



Well, yeah, it just scales. More in parallel, more (total) bias and load
current, lower load impedance.

By the time you get to 8 or 10 or more, you should probably be asking
yourself if a one of those fancy DSP pedals plus a class D amp is cheaper
(not to mention more efficient, as we start running into the limitations of
a 120V 15A circuit around this level..), or at least another tube type with
better economy of scale (i.e. a transmitter tube of some sort), give or take
whether we're talking production here (6L6s are still around, if shitty --
the cheap ones that is) or one-offs from NOS.

Heh, funny, come to think of it, that matched tubes are very common, and
independent grid bias is reasonably common, but independent grid drive level
is not at all common. Really all that matters is balance at the OPT, for
magnetic reasons. The more tubes you wire in parallel, the less critical
their matching is (assuming independent variables, and assuming any
individual does not exceed its plate dissipation rating).

Tim

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

<tubeguy_at_myshop.com> wrote in message
news:435b2ehccbuss8l6utp1tmic2pqh6i0tjm_at_4ax.com...
Quote:
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.




Guest

Fri Dec 28, 2018 10:45 am   



On Thu, 27 Dec 2018 21:11:35 -0600, tubeguy_at_myshop.com wrote:

Quote:
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 Smile

Tauno Voipio
Guest

Fri Dec 28, 2018 1:45 pm   



On 28.12.18 10:45, upsidedown_at_downunder.com wrote:
Quote:
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 Smile


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).

--

-TV


Guest

Fri Dec 28, 2018 2:45 pm   



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.


Using individual cathode resistors to make the whole grid bias helps a
lot to equalize the current in each tube.

In general one tries to avoid cathode resistor bias in power stages,
since quite a lot of the output voltage swing is lost in the cathode
resistor(s) and use fixed low power negative grid bias supply instead,
but this may require separate bias adjustment for each tube.

I am not familiar with 6L6, but the similar EL34 is used in pairs for
30 W for HiFi, using 4 or 6 for "100 W" guitar amplifier with 450 V
anode voltage. One bass qui tar amplifier is rated at 100 W using only
two EL34 tubes, but runs at 800 V, but only Telefunken "Special
quality" EL34s seemed to survive more than a few gigs.

Quote:

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).


Phil Hobbs
Guest

Fri Dec 28, 2018 4:45 pm   



On 12/28/18 3:45 AM, upsidedown_at_downunder.com wrote:
Quote:
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 :-)



Unless the tubes are arranged truly symmetrically, e.g. in a circle with
vertical air flow, they won't all run at the same envelope temperature,
which means they won't have the same cathode temperature either.

Cheers

Phil Hobbs

--
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC / Hobbs ElectroOptics
Optics, Electro-optics, Photonics, Analog Electronics
Briarcliff Manor NY 10510

http://electrooptical.net
http://hobbs-eo.com

bitrex
Guest

Fri Dec 28, 2018 6:45 pm   



On 12/28/2018 03:45 AM, upsidedown_at_downunder.com wrote:
Quote:
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 :-)



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

Not putting that into an amp with the kind of non-inconsiderable expense
one will sink into building it on the grounds of "purism" or whatever is
mad, the kind of irresponsible behavior that the less well-heeled of the
world get called out on. Work hard. Save money. Don't burn up 2 grand of
parts/risk your life cuz one thinks silicon is too new-fangled.

Chris
Guest

Sat Dec 29, 2018 12:45 am   



On 12/28/18 03:11, tubeguy_at_myshop.com wrote:
Quote:
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...

bitrex
Guest

Sat Dec 29, 2018 2:45 am   



On 12/28/2018 06:32 PM, Chris wrote:
Quote:
On 12/28/18 03:11, tubeguy_at_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...


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.

bitrex
Guest

Sat Dec 29, 2018 2:45 am   



On 12/28/2018 08:18 PM, bitrex wrote:

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...

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.


These issues can be addressed but it requires more work than I'm
guessing they put in in the mid 60s (can't immediately find the article
in question online)

Tauno Voipio
Guest

Sat Dec 29, 2018 7:45 pm   



On 29.12.18 01:32, Chris wrote:
Quote:
On 12/28/18 03:11, tubeguy_at_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.

--

-TV

Les Cargill
Guest

Sun Dec 30, 2018 7:45 pm   



tubeguy_at_myshop.com wrote:
Quote:
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.



The largest production tube MI amp I am aware of is the
Mesa Boogie 400+. It uses ( I believe ) 12 power tubes,
essentially 6 push-pull pairs ).

SFAIK, it exists only to compete with the Ampeg SVT.


I wouldn't want to lift it.

--
Les Cargill

Big Bad Bob
Guest

Mon Dec 31, 2018 8:45 pm   



On 12/29/18 10:27, Tauno Voipio wrote:
Quote:
On 29.12.18 01:32, Chris wrote:
On 12/28/18 03:11, tubeguy_at_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.


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"

Big Bad Bob
Guest

Mon Dec 31, 2018 8:45 pm   



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.


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. Or if a single tube fails in
multi-pair configuration, the others will take up most of the slack
without too much bad behavior. You'll probably hear the bad quality
sound at high volumes, but without extended operation "that way" the
other tubes should survive.

the more you have paralleled, the less impact a single tube failure will
have on the other tubes.

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.


--
(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"


Guest

Tue Jan 01, 2019 12:45 am   



On Monday, 31 December 2018 19:14:42 UTC, Big Bad Bob wrote:
Quote:
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.


No-one tell him about electrostatics, piezo arrays or high impedance moving coils.


NT

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