I have been intriqued for some time now with the idea of building an
output transformerless tube audio power amplifier. I have seen
schematics of two different versions of this, one in an old 1962 radio
book that I have and another on the web which was published in the
February 1990 issue of a magazine which I've never heard of call
"Glass Audio". One design uses 1682 triodes for the output and the
other uses 6AS7's. The 1682 is just a 26 volt filament version of the
6AS7 and so the heaters can be wired in series eliminating a filament
transformer. In fact this particular design uses no transformer for B+
either with AC neutral being connected to one side of the chassis. As
long as you polarize the plug everything should be ok the article goes
on to say, however I don't know how crazy about that scheme I am
though. Each design boasts 25 and 35 watts respectivly into 16 ohms
with .50 percent total distortion at full rated output. Frequency
response is not limited by the transformer and and so extends from
10HZ - 250KHZ on both. The circuit for the 35 watt design is very
interesting and can be seen at:
http://www.bonavolta.ch/hobby/en/audio/6as7_2.htm#top
If I'm going to the trouble to build something though, I would really
like to wind up with a unit that can do at least 70 watts or so. By
the looks of these designs, perhaps just adding more triodes in the
output and beefing up the power supply would do it but I was wondering
if anyone else had any further information on this type of project, or
had looked into this?
Thanks, Lenny Stein, Barlen Electronics.

Lenny wrote:
I have been intriqued for some time now with the idea of building an
output transformerless tube audio power amplifier. I have seen
schematics of two different versions of this, one in an old 1962 radio
book that I have and another on the web which was published in the
February 1990 issue of a magazine which I've never heard of call
"Glass Audio". One design uses 1682 triodes for the output and the
other uses 6AS7's. The 1682 is just a 26 volt filament version of the
6AS7 and so the heaters can be wired in series eliminating a filament
transformer. In fact this particular design uses no transformer for B+
either with AC neutral being connected to one side of the chassis. As
long as you polarize the plug everything should be ok the article goes
on to say, however I don't know how crazy about that scheme I am
though. Each design boasts 25 and 35 watts respectivly into 16 ohms
with .50 percent total distortion at full rated output. Frequency
response is not limited by the transformer and and so extends from
10HZ - 250KHZ on both. The circuit for the 35 watt design is very
interesting and can be seen at:
http://www.bonavolta.ch/hobby/en/audio/6as7_2.htm#top
If I'm going to the trouble to build something though, I would really
like to wind up with a unit that can do at least 70 watts or so. By
the looks of these designs, perhaps just adding more triodes in the
output and beefing up the power supply would do it but I was wondering
if anyone else had any further information on this type of project, or
had looked into this?
Thanks, Lenny Stein, Barlen Electronics.

Doubling the power to 70 watts doubles the number of parts, the amount
of heat generated, and indirectly the expectable longevity of the
amplifier. And it only gives you three more decibels of headroom. Is it
really worth it?

You'd probably be better of getting a set of speakers that was 3 dB more
efficient. ;-)

Regards,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

"Fred Nachbaur" wrote in message
news:waQic.12102$NG2.3455@edtnps84...
Doubling the power to 70 watts doubles the number of parts, the amount
of heat generated, and indirectly the expectable longevity of the
amplifier. And it only gives you three more decibels of headroom. Is it
really worth it?

You'd probably be better of getting a set of speakers that was 3 dB
more efficient. ;-)

Psst, hey Fred, heat and parts only go up 3dB as well. BG

Tim

--
"I have misplaced my pants." - Homer Simpson | Electronics,
- - - - - - - - - - - - - - - - - - - - - - --+ Metalcasting
and Games: http://webpages.charter.net/dawill/tmoranwms

Lenny wrote:

I have been intriqued for some time now with the idea of building an
output transformerless tube audio power amplifier. I have seen
schematics of two different versions of this, one in an old 1962 radio
book that I have and another on the web which was published in the
February 1990 issue of a magazine which I've never heard of call
"Glass Audio". One design uses 1682 triodes for the output and the
other uses 6AS7's. The 1682 is just a 26 volt filament version of the
6AS7 and so the heaters can be wired in series eliminating a filament
transformer. In fact this particular design uses no transformer for B+
either with AC neutral being connected to one side of the chassis. As
long as you polarize the plug everything should be ok the article goes
on to say, however I don't know how crazy about that scheme I am
though. Each design boasts 25 and 35 watts respectivly into 16 ohms
with .50 percent total distortion at full rated output. Frequency
response is not limited by the transformer and and so extends from
10HZ - 250KHZ on both. The circuit for the 35 watt design is very
interesting and can be seen at:
http://www.bonavolta.ch/hobby/en/audio/6as7_2.htm#top
If I'm going to the trouble to build something though, I would really
like to wind up with a unit that can do at least 70 watts or so. By
the looks of these designs, perhaps just adding more triodes in the
output and beefing up the power supply would do it but I was wondering
if anyone else had any further information on this type of project, or
had looked into this?
Thanks, Lenny Stein, Barlen Electronics.

Clive Locke had his OTL design published in Glass Audio
in 1992, volume 4, No 4.
It was capable of around 60 watts into 8 ohms and used 8 x 6AS7G tubes.
He got 50w into 4 ohms, and 65w into 16 ohms.
There would be more power available into higher ohm loads, which is the
reverse
of transformer coupled amps.
The more tubes one uses, the higher the output power, and the better the
load match to each
tube used.
Consider the use of 8 x 6AS7G in class A.
A single 1/2 of a 6AS7 has Ra = 280 ohms, so for 16 half triode sections,

Ra is 280/16 = 17.5 ohms.

The load should be about 35 ohms at least for low thd.

The totem pole output stage without any loop FB
is partially in follower mode for the top tubes,
so the Ro is below 17.5 ohms, perhaps 10 ohms approx, in theory.

Clive's article claims his amp using 8 x 6AS7G
operates with +/- 100 volt rails, and with fixed bias
of approx -100v to each triode, and with 200mA total Ia,
so each tube disspates 5 watts, well down from the rating
of around 20 watts.
With 40 watts of total dissipation in the 8 tubes, Clive says there
is 10 watts of class A into an 8 ohm load, and I would say
only 4 watts of class A into a 4 ohm load.

50 watts into 4 ohms is 3.5 amps rms = 5 peak amps, so if there are
8 x 1/2 triode sections, each one has to make a peak 0.625 amps.
If the anode supply voltage was 100v to each triode, there would be a drop
in tube voltage
of only 20 peak volts since the 4 ohm load voltage change = the anode to
cathode voltage change,
and the tube has a peak dissipation of 80v x 0.625 amps = 50 watts,
and when converting this to average power dissipation,
it is I guess about 16 watts per triode section, or 32 watts per tube,
which is a bit high, considering the heater power is another 16 watts per
tube.
I would therefore suggest that testing continuously with a sine wave
with 4 ohms be done very briefly.

But music is easier on the tubes than a sine wave, so although the tube do
have to
work harder than a tranny coupled amp, they might last.
An 8 ohm load reduces the sine wave max dissipation to lower levels, but
its still very high.
And not all the tubes might equally share current flow.

With only 4 x 6AS7G, the situation with real world 4 ohm loads is
dramatically
worse.

And you want 70 watts, so 10 x 6AS7 would be about right.

The heater power is about 16 watts per tube, so 160 watts all up,
and the idle Ea x Ia condition needs 50 watts, so the amp
requires 210 watts of idle power to make a maximum of
70 watts.
Clive used a pair of cascaded longtail pairs as a driver amp using 6DJ8,
with two more 6DJ8 acting as cathode follower buffers to drive the output
tubes into grid current. These 4 tubes would consume about less than 10
watts.


The use of a well rated 400VA power tranny is needed,
or a couple of 300VA trannies, one for each rail.

There are some toroidal transistor amp power trannies with
two 35v secondaries, which can be seriesed for
70v, to produce a +100v supply.

Clive also quotes his amp has 0.05% thd at 50 watts into 8 ohms,
which is a good figure. There must be a quite large amount of NFB used to
achieve this,
probably 40 dB, which is possible due to no OPT.

I happen to think the circlotron type of circuit is better than
the seriesed totem pole config, since each output tube is in a
symetrically balanced
twin PP follower array.
But two floating 100v supplies are needed per channel,
so that's a total of 4 x 160 watt toroidals.

The choice of power trannies is critical, because many
toroidals bought cheaply hum quite badly, even though they run fairly
cool.

Patrick Turner.

Tim Williams wrote:

"Fred Nachbaur" wrote in message
news:waQic.12102$NG2.3455@edtnps84...

Doubling the power to 70 watts doubles the number of parts, the amount
of heat generated, and indirectly the expectable longevity of the
amplifier. And it only gives you three more decibels of headroom. Is it
really worth it?

You'd probably be better of getting a set of speakers that was 3 dB
more efficient. ;-)


Psst, hey Fred, heat and parts only go up 3dB as well. BG

Tim

No way. Hey.. wait.. you're right! :-p

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

"Lenny" wrote in message

In fact this particular design uses no transformer for B+
either with AC neutral being connected to one side of the chassis. As
long as you polarize the plug everything should be ok the article goes
on to say,



** Don't even dream of doing such a **crazy thing** - it is totally
ILLEGAL and LETHAL.

Neither of the AC supply wires should EVER be connected to the chassis of
an amplifier or any metal parts that can be touched. The neutral wire must
be treated the SAME as the active wire since it is so common for the two to
become reversed PLUS if there is simply *no connection* to the neutral
wire of the appliance the chassis ( plus anything attached) can be at full
active voltage and hence capable of delivering a lethal shock.






............ Phil

Try these links for some pretty exotic OTL's

http://www2.gol.com/users/tube/otl.html

http://www.ne.jp/asahi/evo/amp/el509/intro.htm

http://www.ne.jp/asahi/evo/amp/6c33c/intro.htm

Carroll


"Fred Nachbaur" wrote in message
news:QTTic.9908$en3.7965@edtnps89...


Tim Williams wrote:

"Fred Nachbaur" wrote in message
news:waQic.12102$NG2.3455@edtnps84...

Doubling the power to 70 watts doubles the number of parts, the amount
of heat generated, and indirectly the expectable longevity of the
amplifier. And it only gives you three more decibels of headroom. Is it
really worth it?

You'd probably be better of getting a set of speakers that was 3 dB
more efficient. ;-)


Psst, hey Fred, heat and parts only go up 3dB as well. BG

Tim

No way. Hey.. wait.. you're right! :-p

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

In article ,
says...

I have been intriqued for some time now with the idea of building an
output transformerless tube audio power amplifier...

Check out:

http://www.transcendentsound.com/audioreality.htm

In article JY0jc.21929$0u6.3827423@attbi_s03,
says...

In article ,
says...

I have been intriqued for some time now with the idea of building an
output transformerless tube audio power amplifier...

Check out:

http://www.transcendentsound.com/audioreality.htm

and

http://mywebpages.comcast.net/cbrodersen/T8OTL.html

Carroll Conklin wrote:

Try these links for some pretty exotic OTL's

http://www2.gol.com/users/tube/otl.html

http://www.ne.jp/asahi/evo/amp/el509/intro.htm

http://www.ne.jp/asahi/evo/amp/6c33c/intro.htm

Carroll

Interesting stuff. The lower two sites show SS input circuitry.

When using say 8 x 6AS7 for an OTL connected to 4 ohms,
the class B load seen by each 1/2 triode section would be
8 x 4 = 32 ohms.

The 6AS7 G has a U =2, and Ra = 280 ohms,
so the gain into 32 ohms will be 2 x 32 / ( 280 + 32) = 0.205,
so to get 14vrms of output into 32ohms, ( 6.1 watts ) then 14 / 0.205 vrms is
needed
at the input, ie, 68 vrms, ie, nearly 100 peak volts,
which is about the maximum able to be applied to the grid,
with respect to the cathode voltage.
Consider the circlotron.
Suppose the anode supply voltage was +100v
To get 14 v at the load of 4 ohms, which is 50 watts,
the voltage at the two sets of commoned cathodes
is oppositely phased 7 volt signals, (ie, 10v peak.)
The signal voltage swing between catode and anode is 14v,
and the grid input voltage is 100v plus the 7v at the cathode,
so 107v is needed at each grid input.

So the driver amp should be one capable of some high voltage at low distortion.

The circlotron has a dual floating supply for each channel, which is a pita.

One way to keep away from the totem pole seriesed tubes is to
have a simple single supply of say +110v, and feed the anodes of each half
of the PP circuit with a choke with a CT, just like the primary
of an OPT.
Or you could have two gapped separate chokes, IE, parafeed,
and simply couple the load anode to anode, and with the CT choke,
the load is connected directly across the choke.

The trouble is the load would then all be at the +110v potential,
so the output 6ASG are better used as CF, although there is little Ro reduction

due to the follower config.

The CT choke could be a toroidal mains transformer which a 100 watt rated type
with two 50 volt sec windings, with the mains primary left unconnected.

The fedback if any could be taken from each output cathode, and applied
via a balanced network to an input LTP.
The driver amp needs to be two cascaded LTPs,
say a 12AT7 for the first input LTP, then the second LTP is a pair of 6CG7,
and then a buffer of another pair of 6CG7 for CF drive to the output tubes to
be able
to cope with the AB2 conditions.
So 4 x 6CG7 are needed in the input stage, although 6SN7 might look better
alongside the 6AS7G.
The output stage requires a 168 vrms grid to grid drive voltage, and the
2nd LTP driver would need 10.5vrms grid to grid, and the input needs
0.32 vrms, and after 12 dB of fedback is applied, the input voltage is 1.31
vrms g-g,
so a concertina phase inverter is still required ahead of all that for an
unbalanced input.
So we need 4 drive stages including the buffers, and all this just to avoid
winding or buying an
output tranny.
A typical tranny driven class AB1 amp using 2 x 6550 to make the same power
as 8 x 6AS7 needs only 1 x 6CG7 SET input stage, another one for a driver in an
LTP,
and we only have to make 25 vrms to drive the output tubes.
The transformer transforms the high Ra of the output tubes down to managable
levels,
and the feedback applied produces similar Ro and thd as the OTL,
except we have used less stages to do it, and only 2 output tubes instead of 8.

Is the avoidance of the OPT really worth trying to do?
I don't think the absense of the OPT makes any sonic difference in itself.
OK, one throws out the OPT, but then your tubes have
terribly sub optimal load matching, and its 1 step forward, and 3 backward,
and then one needs more FB than the tranny amp to compete.
The heater power at least is a lot higher to pay for.
Great for winter, lousy for summer.

The other thing which can be done with 6AS7G hooked up to a
toroidal choke with CT is to have symetrical taps on the choke
which then allow the choke to be an auto transformer, and
if the one winding has a tap in the centre of each half winding,
we get a 2:1 voltage reduction, which is a 4:1 impedance transformation
so the 8 x 6AS7G would see a 16 ohm load when a 4 ohm load was connected.
The bandwidth of such a toroidal auto transformer can be from 20 Hz to 100 kHz,

at full power, without risk of LF saturation.
An 8 ohm load would appear to the tubes as 32 ohms, and the amount of initial
class A would rise
considerably, and sound better, and need less FB.
While in class A each triode half section sees the same load, which would be
128 ohms,
which is a much nicer load for the tubes to drive in PP than without an OPT.


Patrick Turner.




"Fred Nachbaur" wrote in message
news:QTTic.9908$en3.7965@edtnps89...


Tim Williams wrote:

"Fred Nachbaur" wrote in message
news:waQic.12102$NG2.3455@edtnps84...

Doubling the power to 70 watts doubles the number of parts, the amount
of heat generated, and indirectly the expectable longevity of the
amplifier. And it only gives you three more decibels of headroom. Is it
really worth it?

You'd probably be better of getting a set of speakers that was 3 dB
more efficient. ;-)


Psst, hey Fred, heat and parts only go up 3dB as well. BG

Tim

No way. Hey.. wait.. you're right! :-p

Cheers,
Fred
--
+--------------------------------------------+
| Music: http://www3.telus.net/dogstarmusic/ |
| Projects: http://dogstar.dantimax.dk |
+--------------------------------------------+

I have been intriqued for some time now with the idea of building an
output transformerless tube audio power amplifier.
If you want to work on gear like this, consider buying a good isolation
transformer for $100 or so. Take a look at:
http://terryking.us/equiphome.htm#TE...CH%20ISOLATION
where I'm selling a couple of extras I have...

--
Regards, Terry King ...In The Woods In Vermont

Capturing Live Music in Sound and Images
http://www.terryking.us