A Audio and hi-fi forum. AudioBanter

Go Back   Home » AudioBanter forum » rec.audio » Vacuum Tubes
Site Map Home Register Authors List Search Today's Posts Mark Forums Read Web Partners

Tube mono-block amp with SIX 6L6 outut tubes



 
 
Thread Tools Display Modes
  #1  
Old December 28th 18, 04:11 AM posted to sci.electronics.design,rec.audio.tubes
[email protected]
external usenet poster
 
Posts: 15
Default Tube mono-block amp with SIX 6L6 outut tubes

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.


Ads
  #2  
Old December 28th 18, 09:40 AM posted to sci.electronics.design,rec.audio.tubes
Tim Williams[_3_]
external usenet poster
 
Posts: 4
Default 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 ****ty --
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/

> wrote in message
...
>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.
>
>


  #5  
Old December 28th 18, 02:11 PM posted to sci.electronics.design,rec.audio.tubes
[email protected]
external usenet poster
 
Posts: 9
Default Tube mono-block amp with SIX 6L6 outut tubes

On Fri, 28 Dec 2018 14:19:50 +0200, Tauno Voipio
> wrote:

>On 28.12.18 10:45, wrote:
>> On Thu, 27 Dec 2018 21:11:35 -0600,
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.

>
>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).


  #6  
Old December 28th 18, 02:49 PM posted to rec.audio.tubes
Peter Wieck[_2_]
external usenet poster
 
Posts: 104
Default Tube mono-block amp with SIX 6L6 outut tubes

RCA Theater amps back in the day used between two and six 6L6 tubes depending on the required function. They were all monoblocks, sometimes from a common power-supply. Towards the end of their heyday, when Stereo sound-tracks were becoming more common, the center channel (the speaker directly behind the screen) would run from the optical sound-track, and the left and right from the magnetic sound track.

Peter Wieck
Melrose Park, PA
  #9  
Old December 29th 18, 12:32 AM posted to sci.electronics.design,rec.audio.tubes
Chris[_13_]
external usenet poster
 
Posts: 1
Default Tube mono-block amp with SIX 6L6 outut tubes

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...
  #10  
Old December 29th 18, 02:18 AM posted to sci.electronics.design,rec.audio.tubes
bitrex
external usenet poster
 
Posts: 5
Default Tube mono-block amp with SIX 6L6 outut tubes

On 12/28/2018 06:32 PM, 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...


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.
 




Thread Tools
Display Modes

Posting Rules
You may not post new threads
You may not post replies
You may not post attachments
You may not edit your posts

vB code is On
Smilies are On
[IMG] code is On
HTML code is Off

Similar Threads
Thread Thread Starter Forum Replies Last Post
FA: Mono Tube Amp w/ 807 tubes, premium UTC LS-6L3 OPT Stephen F. Marsh Marketplace 0 September 29th 05 02:29 PM
FA: Pair of 2A3 Tube Mono Block "Project" Amplifiers Jon Yaeger Marketplace 2 January 9th 05 07:30 AM
FA: Pair of 2A3 Tube Mono Block "Project" Amplifiers Jon Yaeger Vacuum Tubes 2 January 9th 05 07:30 AM
problem with old mac mono block [email protected] Vacuum Tubes 5 October 10th 03 05:08 PM
Jl 500/1 vs. Alpine 500 watt mono block John Car Audio 1 September 27th 03 10:48 PM


All times are GMT +1. The time now is 06:13 PM.


Powered by vBulletin® Version 3.6.4
Copyright ©2000 - 2019, Jelsoft Enterprises Ltd.
Copyright İ2004-2019 AudioBanter.
The comments are property of their posters.