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Tube mono-block amp with SIX 6L6 outut tubes



 
 
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  #11  
Old December 29th 18, 02:20 AM posted to sci.electronics.design,rec.audio.tubes
bitrex
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Posts: 5
Default Tube mono-block amp with SIX 6L6 outut tubes

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

>
> 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)

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  #12  
Old December 29th 18, 07:27 PM posted to sci.electronics.design,rec.audio.tubes
Tauno Voipio
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Posts: 6
Default Tube mono-block amp with SIX 6L6 outut tubes

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.

--

-TV

  #13  
Old December 30th 18, 06:51 PM posted to sci.electronics.design,rec.audio.tubes
Les Cargill[_4_]
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Posts: 1,367
Default Tube mono-block amp with SIX 6L6 outut tubes

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.
>
>


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
  #14  
Old December 31st 18, 08:14 PM posted to sci.electronics.design,rec.audio.tubes
Big Bad Bob
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Posts: 348
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"
  #15  
Old December 31st 18, 08:25 PM posted to sci.electronics.design,rec.audio.tubes
Big Bad Bob
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Posts: 348
Default Tube mono-block amp with SIX 6L6 outut tubes

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.


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"
  #16  
Old January 1st 19, 05:32 AM posted to sci.electronics.design,rec.audio.tubes
Tim Williams[_3_]
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Posts: 4
Default Tube mono-block amp with SIX 6L6 outut tubes

"Big Bad Bob" > wrote in message
...
> 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/

  #17  
Old January 1st 19, 05:39 AM posted to sci.electronics.design,rec.audio.tubes
Tim Williams[_3_]
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Posts: 4
Default Tube mono-block amp with SIX 6L6 outut tubes

"Big Bad Bob" > wrote in message
...
> 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/

  #18  
Old January 1st 19, 07:32 AM posted to sci.electronics.design,rec.audio.tubes
bitrex
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Posts: 5
Default Tube mono-block amp with SIX 6L6 outut tubes

On 12/31/2018 11:32 PM, Tim Williams wrote:
> "Big Bad Bob" > wrote in message
> ...
>> 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!
  #19  
Old January 1st 19, 01:21 PM posted to sci.electronics.design,rec.audio.tubes
[email protected]
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Posts: 9
Default Tube mono-block amp with SIX 6L6 outut tubes

On Mon, 31 Dec 2018 11:14:33 -0800, Big Bad Bob
> wrote:

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


<clip>


>>> 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.

  #20  
Old January 1st 19, 01:34 PM posted to sci.electronics.design,rec.audio.tubes
[email protected]
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Posts: 9
Default Tube mono-block amp with SIX 6L6 outut tubes

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

>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 :-)

 




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