Engineer wrote:
On Feb 18, 9:20 am, Patrick Turner wrote:
Patrick many thanks for the thoughtful reply. Some questions
below...
(snip)
You can set up a PP pair of class AB1 6080 or aka 6AS7 with
RL about 1k, a-a and Ea = 200V and if the Pda of the two tubes = 40W,
then you get about 10Watts of clean power, or about 100Vrms a-a into the
1k,
and gain at about 1.8, so you need about 30Vrms at the grid which is
none more than for 6550 in triode.
Do you mean two 6080 in mono P-P. i.e. two paralleled triodes per
side, total 4 triodes each 1/2 a 6080, or just one 6080 in P-P (two
1/2 6080's)? I'm thinking about the latter, i.e. one mono PP amp with
one 6080 (duplicated for stereo if it works.)
3k is to high, and the Vswing will be limited by the Ea, which cannot be
much above 200V because the tube
just doesn't like it, it tends to run away with current if the Ea is too
high
no matter what the applied grid bias voltage is.
Pity, as I have the 3K option with the two 125E OPT's in the spares
box.
Before leaping into a project, I always work out the load lines for
myself.
Other wise I am guessing what might work, or might not,
and what I have said here about 6080 is little more than guesswork
because I have not done the graphical analysis to get a projected
outcome
which could be within 10% of being accurate.
There is a lot you could learn about such analysis from the old books,
or at
the many good websites around, perhaps even including my own,
http://www.turneraudio.com.au
But I can say a 3k anode to anode load will NOT hurt a pair of 6080
if you connect such a load.
Perhaps you'll wonder why there is so little power, but
one needs to consider basic triode class A1 efficiency which won't
ever be more than about 30%, if RL is made a lot larger than Ra,
which means you must have a high as possible Ea, and as low as possible
Ia which
still gives about 80% of the rated data maximum Pda for the triode,
and so if you have 40watts of Pda from two tubes with two triodes within
each, 10W each,
then 12 watts is the class A outer limit of power. But its very NICE
power,
because it will have low thd/imd, and the Rout of the amp measured at
the sec will be very low.
12 watts into 3k means you must have 190Vrms anode to anode, which is
95Vrms of opposite phased signal
at each anode,
and a mains transformer which can take 240V across its centre tapped
primary
would be OK, although saturation would be at about 40Hz at 1.2Tesla.
48 watts into 3k is 380Vrms a-a, and the OPT would saturate at
80Hz if it was a mains tranny, so its NO GOOD.
But if you reduced the a-a RL from 3k to 1k, and still have your 190Vrms
a-a but using 6 x 6080, then you'd get 36 watts, and saturation would
still be at 40Hz, assuming it was the same OPT but with a lower sec load
to reduce the
primary load, and with the same turns per volt.
Because you'd have a much higher and useful power ceiling with 6 tubes,
you can afford to back the bias off to allow class AB1,
and more relaxed tube temperatures and longer life, and there would be
9 watts available at only 1/2 the output voltage level, so the
saturation F would fall to 20Hz.
Every way you look at it, more tubes = a better amp.
The peak Ia available is VERY high for a tube, and ppl have used a quad
of 6080 for
a basic OTL amp, and Atmasphere makes OTL with lots of 6AS7 as a
commercial example.
Not planning for OTL, not enough tubes, anyway.
Its also possible to series connect the 6AS7 and use a cap coupled OPT
which is a line
voltage speaker tranny, Altronics of Western Aust make a toroidal one
which I thought would work
with the series PP.
Might look into it...
But a good use is the circlotron circuit with a low Z load on the
primary.
Two 200V floating supplies are needed.
Starting to look contrived... using the 6080 is not worth that
effort!
Nah, the circlotron only takes two B+ supplies, not just one for a
channel,
and its not all that difficult at all.
There are benefits in that the OPT does not have to be centre tapped and
the
"anode to anode" load is 1/4 of conventional a-a load used in class A
amps.
The conventional layout is all one needs though, and the circlowhizzy
IS slightly more difficult for the diyer who must spend more on PTs,
diodes and supply caps.
Some toroidal mains transformers are OK to use because with say 240V : 2
x 35V windings
you have 240 : 35 with both secs paralleled, giving TR = 6.86, ZR = 47:1
= 376 ohms to 8 ohms,
and about right for a suitable tube circuit if the Ra-a is about 90
ohms.
1/2 a 6080 Ra = 280 ohms, and a whole one has 140 ohms,
a pair have 280 ohms a-a, and a six pack have Ra-a = 93 ohms, just what
you want
and you could get about 50 watts AB1 easily. They'l also go AB2,
but maybe not much is gained except more thd, which isn't wanted.
A 150Watt mains toroidal tranny could be fine, and is worth a try.
Such mains trannies are dead useless for other large octal tubes, 6550
etc, because the
AB load for a pair is typically 5k for 50 watts AB1 into 4 ohms, and
7k for 35 watts into 8 ohms, with most of the power class A.
"Universal" mains toroidal trannies with two 120V primary windings are
needed, and the secondary voltages of say 35V are a guide only, and
could vary
down to say 18V + 18V for a transistor amp PS.
240V : 18V gives 1.42k : 8 ohms.
The 240V across the primary gives a Bmax of about 1.2Tesla at 50Hz.
For good low bass you want 1.2T at say 20Hz, so the voltage across the
primary
should be no more than 240V / ( 50/20 ) = 100V only.
240Vrms into 1.42k = 40 watts only.
For a guitar amp the mains toroidal is usable with 6080 but for hi-fi
the available
turns per volt are simply not high enough, no matter what the VA of the
tranny is,
because it will saturate at too high an F, especially when used
with the high Z of bass speakers below 100Hz.
This is starting to look interesting... I just happen to have several
salvaged mains transformers (from my company's dumpster - we used to
build our own linear P/S's in the 1980's and have just chucked out the
old parts.)
But what sort of mains trannies are they?
If they are toroidal types, perhaps you are in luck since
they will have a good HF response maybe.
But if they are E&I bobbin type trannies, perhaps you may find the HF
response
quite dismal, maybe you get 5kHz, and this is if they have
concentric windings. If the windings are side by side with a plastic
divider
between each side for the P and S group of windings, HF
performance will be even worse, and nothing can fix this.
They are PRI: 240, 220, 208, 133, 120, 0 VAC
This gives what looks to be a 240V winding with CT. But probably, the
240V winding has two wire sizes, for 0-133, its thick wire, then maybe
from
133 its much thinner wire because I in with 240V P is 1/2 the I in with
120V,
and this means you have assymetrical P 1/2 windings. It won't matter
much and will still work,
but other serious assymetries may exist such as very different and large
amounts of
leakage inductance and shunt capacitance.
If you are building a guitar amp, or modulator for an amateur radio
station, the HF roll off just above 3khz
that you are likely to find may not matter one bit.
to SEC: 27.6
VAC CT, i.e. 13.8-0-13.8 VAC. Thhese have a CT 240 VAC PRI to 13.8
VAC SEC (using 1/2 of it.) For 8 an ohm speakers I make that about
2420 ohms plate-to-plate. Low enough for one 6080?
Theoretically its OK for a pair of tubes, ie two tubes with the two
triodes within each
paralleled. Even ONE x 6080 would work, but power would be very low
unless you begin to work well into class AB.
transformers are rated 63 VA, also 50-60 Hz. They have quite large
iron cores but I don't know the design Bmax is.
As long as the mains tranny VA is about twice the intended maximum audio
power output is,
there is no worry about power handling; the limitations are saturation
at LF and HF response.
Your 20 Hz calculation (above) shows 100 VAC max across the "240" VAC
primary for 1.2 Tesla (assumed), giving 5.75 VAC across the the 8 ohm
speaker for only 4.13 watts. Not at all good!
Well nearly all commercially designed mains trannies are designed to
have a magnetic field strength
maximum of between 0.8Tesla for hi-fi amp PTs, to 1.3T in hot running
industrial PTs.
The less turns per volt, the hotter the core runs and the greater the
distortions,
and the nearer you are to complete saturation. In good trannies with
GOSS cores,
and many don't have such a good grade of iron, the turns used produce
1.2T
and 3% THD, but if you tried to run the 240V at 25Hz instead of 50Hz,
the core would seriously overheat and distortion might be 60%,
and a disater is immiment. But you could run the 240V tranny at 120Vrms
at 25Hz
OK, and the Bmax is 1.2T.
With an amp, it will rarely need to have the ability to make a full 240V
at any F
and so having the 20Hz saturation at 100Vrms, 1.2T, is OK for non
critical audio.
For real hi-fi, all such proposed use of mains trannies is complete BS,
because we'd want to get full power down to 13Hz perhaps, so
this means very many more turns per volt wound on the core; the whole
design
is far more complex and must provide also for the interleaving needed
for a decent HF.
Hammond quality is where one starts when doing diy, but for serious
hi-fi one has to
go to Plitron, Lundahll, Sowter, or some other respected brand, or else
gird the loins
and build a lathe, and learn all about it and wind your own OPT.
The latter option is where i went, and its all explained at my website.
The Hammond option isn't expensive in the US, where freight costs are
confined
but here in Oz the freight is a major component.
Its better to spend on a Hammond for say US $100 than put a badly
matched mains tranny
worth $50 in they amp, IMHO.
If we accept a roll off at modest 40 Hz we could allow 240/(50/40) =
192 VAC across the primary to get 11 volts across the 8 ohm speaker
for just over 15 watts. A lot better, not at all bad for one O/P
tube.
So, I'll draw a few lines on the 6080 plate characteristics and see
what gives. BTW, the tube rating goes to 250 VDC so might as well push
it there.
Don't push your luck with 6080 too far by going to the rated maxVdc Ea
for the tube.
You may not have to and maybe you'd still get the wanted anode V swing
and power.
I would use cathode bias to keep it a bit safer, then,
allowing for over 100 VDC of bias the B+ would be close to 350 VDC.
In your opinion, is this worth a try?
Cheers,
Roger
All teaching exercizes are worth a try, smoke generation in the work
shed
is always a questionable goal :-)
Without any load on the amp and with just the primary winding,
at 1khz the primary load is very high at about 1,000 x 6.28 x inductance
at 1kHz.
Now the primary L at 1khz isn't the same as it is at its maximum at
about 20Hz,
depending on the irn type, and the applied voltage, and often the 1khz L
is 1/20 of the value at 20Hz, but nevertheless the 1kHz load with no
secondary load is
well above the intended R load in ohms, and the fact it is purely
inductive won't matter
because the Ra-a of the tubes will be a far lower ohm value than the ZL,
the reactance in ohms of the
LP at 1kHz.
But at 1kHz is where you should first get the amp to work,
and use a sine wave. Use square waves only when you have things going
right with a sine wave
If you have cathode bias, use separate RC networks for each cathode.
Start loading the amp with 64 ohms and work downwards to 2 ohms
in steps of 64, 32, 24, 16, 12, 8, 6, 5, 4, 3, 2.
Record the output voltage at clipping into each load, and draw a graph
of the power vs load curve. It will be an arch.
Record where the cathode Ek rises more than 10%; this is where the class
A operation
ceases, and class AB is deemed to have begun. With 2 ohms,
the Ek may double.
The peak in the arch should be at loads under 4 ohms and at about 3
ohms.
If you are happy with the power levels into 4 ohms without more than 30%
of Ek rise,
then the amp is loaded about right, and will still provide useful power
into all loads between 3 and 16 ohms, and most speakers lie within this
range.
Some may say it doesn't matter about the Ek rise with class AB, and AB2
op would be fine,
but then thd/imd leaps to large numbers.
Extracting huge audio power from a minimum of glassware involves higher
tolerance for
poor technical excellence in performance. Perhaps its OK, even desirable
if you have a guitar amp
where the more distortion the merrier, but not for hi-fi.
The 6080 can produce huge peak currents ( for a tube ), for short period
signal swings, hence their ability in OTL amps. The same current ability
exists in transformer coupled
output stages but is rarely used, unless the load is reduced to 1 ohm,
but then the OPT behaves like a resistance
of low value because the winding resistance is like a fixed value of R
in series with the input to the primary
if considered a perfect tranny with no winding resistance.
So with a 1 ohm load, most of the power goes to heat the winding wire
rather than give much output power.
Don't prolong tests with 1 ohm. Such tests of 20 seconds won't hurt at
clipping,
but Pda will be perhaps over the rating and tubes can turn red and DIE
BY MELTDOWN
if left running into too low a load for too long; and you don't need to
have a high level
of output volume to cause the meltdown, and its how many amps are killed
by their owners; they neglect
and/or don't notice when one channel has a shorted speaker lead, or
partially shorted speaker.
More critical choices for loading can be done using varied arrangements
of secondary windings
on the OPT but if you have a mains tranny to begin with you probably
won't have the luxury
of such an option that would be correct.
two 13.8V windings in parallel might match give 2 ohms, but in series
they match 8 ohms,
and this lack the flexibilty of also having a 4 ohm match, so all
speakers
between 1 ohm and 4 ohms use the 2 ohm outlet, and all speakers 4 ohms
and above use the 8 ohm outlet.
vey few speakers need to be connected to 2 ohms, because their average Z
hovers arong 5 ohms these days.
The slow drift to low Z speakers over the last 50 years is due to the
emergence of SS amps
which can cope quite OK with one ohm loads if there are enough output
transistors
and you don't need a high rail voltage for huge powers.
Patrick Turner.
Have fun, but I prefer the 6550, KT88, KT90, EL34, 6AC7, 2A3, 300B etc.
Patrick Turner.- Hide quoted text -
- Show quoted text -