Tube mono-block amp with SIX 6L6 outut tubes

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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.



Re: Tube mono-block amp with SIX 6L6 outut tubes
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
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Re: Tube mono-block amp with SIX 6L6 outut tubes
On Thu, 27 Dec 2018 21:11:35 -0600, snipped-for-privacy@myshop.com wrote:

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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 :-)



Re: Tube mono-block amp with SIX 6L6 outut tubes
On 28.12.18 10:45, snipped-for-privacy@downunder.com wrote:
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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



Re: Tube mono-block amp with SIX 6L6 outut tubes
On Fri, 28 Dec 2018 14:19:50 +0200, Tauno Voipio

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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.  

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Re: Tube mono-block amp with SIX 6L6 outut tubes
On Fri, 28 Dec 2018 14:19:50 +0200, Tauno Voipio

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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.


Re: Tube mono-block amp with SIX 6L6 outut tubes
On 12/28/18 3:45 AM, snipped-for-privacy@downunder.com wrote:
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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
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Re: Tube mono-block amp with SIX 6L6 outut tubes
On 12/28/2018 03:45 AM, snipped-for-privacy@downunder.com wrote:
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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.

Re: Tube mono-block amp with SIX 6L6 outut tubes
On 12/28/18 09:16, bitrex wrote:

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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

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Re: Tube mono-block amp with SIX 6L6 outut tubes
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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
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Re: Tube mono-block amp with SIX 6L6 outut tubes
On 12/31/2018 11:32 PM, Tim Williams wrote:



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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!

Re: Tube mono-block amp with SIX 6L6 outut tubes
"Big Bad Bob"  wrote in message  

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

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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


Re: Tube mono-block amp with SIX 6L6 outut tubes
On 01/01/2019 10:27 AM, Kevin Aylward wrote:

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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


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Re: Tube mono-block amp with SIX 6L6 outut tubes
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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
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Re: Tube mono-block amp with SIX 6L6 outut tubes

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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


Re: Tube mono-block amp with SIX 6L6 outut tubes
On 12/28/18 03:11, snipped-for-privacy@myshop.com wrote:
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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...

Re: Tube mono-block amp with SIX 6L6 outut tubes
On 12/28/2018 06:32 PM, Chris wrote:
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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.

Re: Tube mono-block amp with SIX 6L6 outut tubes
On 12/28/2018 08:18 PM, bitrex wrote:

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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)


Re: Tube mono-block amp with SIX 6L6 outut tubes
On 29.12.18 01:32, Chris wrote:
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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


Re: Tube mono-block amp with SIX 6L6 outut tubes
On 12/29/18 10:27, Tauno Voipio wrote:
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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

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