[Engineer[_2_]]

Hi, Vacuumlanders.
Here are the 6080 results I got while experimenting with a SE OPT
(from a 50C5 o/p tube stereo receiver - have two, planned on stereo
from one 6080) - see original post from a few days ago.
I presented all this at the Ontario Vintage Radio Association (OVRA)
meeting on October 18, 2009 with some pics (none here, of course!)

Circuit:
1/2 a 6080, Rk = 1500 ohms WW (fixed for all tests)
Ck = 100 uF (200 VDC working)
Vintage Heathkit variable HV supply
Small SE OPT 2500 to 8 ohms (17.7 to 1); tests done at 6, 8 and 10
ohms load
Driver: (kluged!) a 10 watt s/s amplifier driving a small OPT
"reversed" so as to get up to 50 Vrms drive to the 6080 grid (loaded
for 8 ohms at s/s amp. o/p.) This tube needs lot of drive and I did
not want to get into driver stage design in case this project went
nowhere!
Pre-set power resistor up to 10 ohms across OPT secondary.
Audio sig. gen. to the s/s amplifier input.
All voltages measured by a Fluke 111.
Tektronix 561B across load to check waveform.

Results:
Most tests at 250 Hz.
I(p) set to 40, 50, 60 65, 66 mA by Heathkit regulated variable
voltage P/S
B+ a residual number from any given I(p)

Test 1: RL = 8 ohms
I(p) = 60 mA
V(p-k) = 290(B+) - 90(Vbias) - 12.7(OPT DCR drop) = 187.3 VDC
Power dissipation (plate) = 183 X 0.06 = 11 watts
AC volts across OPT primary = 85.5 VAC RMS (measured)
OPT sec. voltage = 3.8VAC RMS (clipping)
Power output = 1.8 watts

Gain tests (at I(p) = 50 mA and B+ 250 VDC):
Grid 1 drive voltage = 6.28, 8.39, 16.30, 40.33 VAC RMS, all at 250
Hz.
Primary OPT voltage = 9.76, 13.03, 25.25, 62.70 VAC RMS
Average Gain = 1.55 Looks low vs. theoretical gain, viz:
u = 2
RL = 2500 ohms with 8 ohm speaker load
Rp = 280 ohms (at V(p-k) = 125 VDC and I(p) = 125 mA)
Thus, gain = (u x RL)/(Rp x RL) = 1.79
Notes: Different operating points.

Test 2: RL = 8 ohms
I(p) = 66 mA
Max. rating, limit of variable P/S:
Plate power = 13.4 watts
V(p-k) = 315(B+) - 99(Vbias) - 12.8(OPT DCR drop) = 203.2 VDC
Max. o/p voltage = 4.1 VAC (clipping)
Power out = 2.1 watts
Power dissipation (plate) = 203.2 X 0.066 = 12.19 watts
Max "design rating" 13 watts (RCA tube book.)

Test 3: RL = 8 ohms
I(p) = 50 mA
Plate power = 8.2 watts
V(p-k) = 250(B+) - 75(Vbias) - 10.7(OPT DCR drop) = 163.3 VDC
Max. o/p voltage = 3.9 VAC (clipping)
Power out = 1.9 watts

Test 4: RL = 8 ohms
I(p) = 60 mA
Plate power = 11.3 watts
V(p-k) = 290(B+) - 90(Vbias) - 12.7(OPT DCR drop) = 187.3 VDC
Max. o/p voltage = 4.1 VAC (clipping)
Power out = 2.1 watts

Test 5: RL = 8 ohms
I(p) = 65 mA
V(p-k) = 320(B+) - 97.5(Vbias) - 14(OPT DCR drop) = 208.5 VDC
Plate power = 13.6 watts
Max. o/p voltage = 3.4 VAC (clipping)
Power out = 1.45 watts

Test 6: RL = 10 ohms plate load = 3125 ohms
I(p) = 50 mA
V(p-k) = 250(B+) - 75(Vbias) - 10.7(OPT DCR drop) = 163.3 VDC
Plate power = 8.2 watts
Max. o/p voltage = 4.0 VAC (clipping)
Power out = 1.6 watts

Test 7: RL = 10 ohms plate load = 3125 ohms
I(p) = 60 mA
V(p-k) = 290(B+) - 90(Vbias) - 12.7(OPT DCR drop) = 187.3 VDC
Plate power = 11.3 watts
Max. o/p voltage = 4.0 VAC (clipping)
Power out = 1.6 watts

Test 8: RL = 10 ohms plate load = 3125 ohms
I(p) = 65 mA
V(p-k) = 320(B+) - 97.5(Vbias) - 14(OPT DCR drop) = 208.5 VDC
Plate power = 13.6 watts
Max. o/p voltage = 3.1 VAC (clipping)
Power out = 1.0 watts

Test 9: RL = 6 ohms plate load = 1875 ohms
I(p) = 50 mA
V(p-k) = 250(B+) - 75(Vbias) - 10.7(OPT DCR drop) = 163.3 VDC
Plate power = 8.2 watts
Max. o/p voltage = 3.1 VAC (clipping)
Power out = 1.6 watts

Test 10: RL = 6 ohms plate load = 1875 ohms
I(p) = 60 mA
V(p-k) = 290(B+) - 90(Vbias) - 12.7(OPT DCR drop) = 187.3 VDC
Plate power = 11.3 watts
Max. o/p voltage = 2.8 VAC (clipping)
Power out = 1.3 watts

Test 11: RL = 6 ohms plate load = 1875 ohms
I(p) = 65 mA
V(p-k) = 320(B+) - 97.5(Vbias) - 14(OPT DCR drop) = 208.5 VDC
Plate power = 13.6 watts
Max. o/p voltage = 2.6 VAC (clipping, distorted)
Power out = 1.13 watts

Test 12: RL = 6 ohms plate load = 1875 ohms
I(p) = 40 mA
V(p-k) = 205(B+) - 60(Vbias) - 8.5(OPT DCR drop) = 136.5 VDC
Plate power = 5.5 watts
Max. o/p voltage = 2.9 VAC (clipping)
Power out = 1.4 watts

Test 13: RL = 8 ohms plate load = 2500 ohms
I(p) = 40 mA
V(p-k) = 205(B+) - 60(Vbias) - 8.5(OPT DCR drop) = 136.5 VDC
Plate power = 5.5 watts
Max. o/p voltage = 3.2 VAC (clipping)
Power out = 1.3 watts

Conclusions
6080 not a very useful audio tube for SE stereo amplifier - pity,
would have been fun to use for low level listening... and heating the
house!
Might benefit from a higher B+
Would benefit from a better OPT (see below.)
SE Power o/p low, between 1 and 2 watts (as tested)
Plate current high at around 50 mA, needing a large core OPT with air
gap.
Waveforms below 100 Hz quite horrible over 1.5 watts.
Small radio OPT was likely inadequate - need much more iron. The
closest type I've seen is $45 from a US supplier (someone sent me the
link in the earlier thread - thanks) but "shipped, customed and
taxed" (and X/C rated) to get it here (near Toronto, Canada) is just
not on for me!

Started to experiment with a 6080 in P-P configuration with a Hammond
125E P-P OPT to hand (will try several plate-to-plate impedances.)
Grid test drive from a small, "reversed" P-P OPT, 40:1 ratio. Rk -
2.5 Kohms, for now.
More later...
Cheers,
Roger

[Patrick Turner]

On Oct 21, 9:48*am, Engineer wrote:
Hi, Vacuumlanders.
Here are the 6080 results I got while experimenting with a SE OPT
(from a 50C5 o/p tube stereo receiver - have two, planned on stereo
from one 6080) - see original post from a few days ago.
I presented all this at the Ontario Vintage Radio Association (OVRA)
meeting on October 18, 2009 with some pics (none here, of course!)

Circuit:
1/2 a 6080, Rk = 1500 ohms WW (fixed for all tests)
Ck = 100 uF (200 VDC working)
Vintage Heathkit variable HV supply
Small SE OPT 2500 to 8 ohms (17.7 to 1); tests done at 6, 8 and 10
ohms load
Driver: (kluged!) a 10 watt s/s amplifier driving a small OPT
"reversed" so as to get up to 50 Vrms drive to the 6080 grid (loaded
for 8 ohms at s/s amp. o/p.) This tube needs lot of drive and I did
not want to get into driver stage design in case this project went
nowhere!
Pre-set power resistor up to 10 ohms across OPT secondary.
Audio sig. gen. to the s/s amplifier input.
All voltages measured by a Fluke 111.
Tektronix 561B across load to check waveform.

Results:
Most tests at 250 Hz.
I(p) set to 40, 50, 60 65, 66 mA by Heathkit regulated variable
voltage P/S
B+ a residual number from any given I(p)

Test 1: RL = 8 ohms
I(p) = 60 mA
V(p-k) = 290(B+) - 90(Vbias) - 12.7(OPT DCR drop) = 187.3 VDC
Power dissipation (plate) = 183 X 0.06 = 11 watts
AC volts across OPT primary = 85.5 VAC RMS (measured)
OPT sec. voltage = 3.8VAC RMS (clipping)
Power output = 1.8 watts

Gain tests (at I(p) = 50 mA and B+ 250 VDC):
Grid 1 drive voltage = 6.28, 8.39, 16.30, 40.33 VAC RMS, all at 250
Hz.
Primary OPT voltage = 9.76, 13.03, 25.25, 62.70 VAC RMS
Average Gain = 1.55 *Looks low vs. theoretical gain, viz:
u = 2
RL = 2500 ohms with 8 ohm speaker load
Rp = 280 ohms (at V(p-k) = 125 VDC and I(p) = 125 mA)
Thus, gain = (u x RL)/(Rp x RL) = 1.79
Notes: Different operating points.

Test 2: RL = 8 ohms
I(p) = 66 mA
Max. rating, limit of variable P/S:
Plate power = 13.4 watts
V(p-k) = 315(B+) - 99(Vbias) - 12.8(OPT DCR drop) = 203.2 VDC
Max. o/p voltage = 4.1 VAC (clipping)
Power out = 2.1 watts
Power dissipation (plate) = 203.2 X 0.066 = 12.19 watts
Max "design rating" 13 watts (RCA tube book.)

Test 3: RL = 8 ohms
I(p) = 50 mA
Plate power = 8.2 watts
V(p-k) = 250(B+) - 75(Vbias) - 10.7(OPT DCR drop) = 163.3 VDC
Max. o/p voltage = 3.9 VAC (clipping)
Power out = 1.9 watts

Test 4: RL = 8 ohms
I(p) = 60 mA
Plate power = 11.3 watts
V(p-k) = 290(B+) - 90(Vbias) - 12.7(OPT DCR drop) = 187.3 VDC
Max. o/p voltage = 4.1 VAC (clipping)
Power out = 2.1 watts

Test 5: RL = 8 ohms
I(p) = 65 mA
V(p-k) = 320(B+) - 97.5(Vbias) - 14(OPT DCR drop) = 208.5 VDC
Plate power = 13.6 watts
Max. o/p voltage = 3.4 VAC (clipping)
Power out = 1.45 watts

Test 6: RL = 10 ohms plate load = 3125 ohms
I(p) = 50 mA
V(p-k) = 250(B+) - 75(Vbias) - 10.7(OPT DCR drop) = 163.3 VDC
Plate power = 8.2 watts
Max. o/p voltage = 4.0 VAC (clipping)
Power out = 1.6 watts

Test 7: RL = 10 ohms plate load = 3125 ohms
I(p) = 60 mA
V(p-k) = 290(B+) - 90(Vbias) - 12.7(OPT DCR drop) = 187.3 VDC
Plate power = 11.3 watts
Max. o/p voltage = 4.0 VAC (clipping)
Power out = 1.6 watts

Test 8: RL = 10 ohms plate load = 3125 ohms
I(p) = 65 mA
V(p-k) = 320(B+) - 97.5(Vbias) - 14(OPT DCR drop) = 208.5 VDC
Plate power = 13.6 watts
Max. o/p voltage = 3.1 VAC (clipping)
Power out = 1.0 watts

Test 9: RL = 6 ohms plate load = 1875 ohms
I(p) = 50 mA
V(p-k) = 250(B+) - 75(Vbias) - 10.7(OPT DCR drop) = 163.3 VDC
Plate power = 8.2 watts
Max. o/p voltage = 3.1 VAC (clipping)
Power out = 1.6 watts

Test 10: RL = 6 ohms plate load = 1875 ohms
I(p) = 60 mA
V(p-k) = 290(B+) - 90(Vbias) - 12.7(OPT DCR drop) = 187.3 VDC
Plate power = 11.3 watts
Max. o/p voltage = 2.8 VAC (clipping)
Power out = 1.3 watts

Test 11: RL = 6 ohms plate load = 1875 ohms
I(p) = 65 mA
V(p-k) = 320(B+) - 97.5(Vbias) - 14(OPT DCR drop) = 208.5 VDC
Plate power = 13.6 watts
Max. o/p voltage = 2.6 VAC (clipping, distorted)
Power out = 1.13 watts

Test 12: RL = 6 ohms plate load = 1875 ohms
I(p) = 40 mA
V(p-k) = 205(B+) - 60(Vbias) - 8.5(OPT DCR drop) = 136.5 VDC
Plate power = 5.5 watts
Max. o/p voltage = 2.9 VAC (clipping)
Power out = 1.4 watts

Test 13: RL = 8 ohms plate load = 2500 ohms
I(p) = 40 mA
V(p-k) = 205(B+) - 60(Vbias) - 8.5(OPT DCR drop) = 136.5 VDC
Plate power = 5.5 watts
Max. o/p voltage = 3.2 VAC (clipping)
Power out = 1.3 watts

Conclusions
6080 not a very useful audio tube for SE stereo amplifier - pity,
would have been fun to use for low level listening... and heating the
house!
Might benefit from a higher B+
Would benefit from a better OPT (see below.)
SE Power o/p low, between 1 and 2 watts (as tested)
Plate current high at around 50 mA, needing a large core OPT with air
gap.
Waveforms below 100 Hz quite horrible over 1.5 watts.
Small radio OPT was likely inadequate - need much more iron. *The
closest type I've seen is $45 from a US supplier (someone sent me the
link in the earlier thread - thanks) but "shipped, customed and
taxed" (and X/C rated) to get it here (near Toronto, Canada) is just
not on for me!

Started to experiment with a 6080 in P-P configuration with a Hammond
125E P-P OPT to hand (will try several plate-to-plate impedances.)
Grid test drive from a small, "reversed" P-P OPT, 40:1 ratio. *Rk -
2.5 Kohms, for now.
More later...
Cheers,
Roger

Gee Roger, lots of tests led you to a conclusion which could be
challenged.

The 6AS7 is just another triode, and you should be able to get 30%
class A anode efficiency, ie, if you have 20W of total anode power
input to the two triodes in one bottle, then you should get 6.6 watts
of audio power in class A and even more in class AB1, AB2.

The B+ does not want to be too high because the grid Vdc bias becomes
too high and can't hold the Ia down if Ea is too high; its not like an
EL34 or 6550.

Have you done a load line analysis?

At least though, the 6AS7 is a fine tube for headphones where the
anode efficiency does not matter if the RLa-a is a much higher value
than used for powering a speaker.

Patrick Turner.

[Bret L]

If you're looking for 'cheap' you might try line transformers. The
problem there, besides commodity line transformers not being the best
(what do you expect for 'cheap?) is DC balance. I use a current mirror
under the tubes for that and it works rather well.

What you'd be looking for is a multi-tap (to set 'power') primary. The
lowest power tap is your highest impedance (one plate) with ground
being the other plate. That's your plate to plate impedance. Then you
need a 4x power (center) tap for B+

I don't know what line transformers you have available locally in
Canada but, for example, the TIC SP70T 20W 70V line transformer has
2.5, 5, 10, 15, and 20 Watt taps (at least they say so, I've not used
this one). The 2.5 Watt tap comes to 2k PP with the 10 Watt tap
center.


If fidelity is the goal there is no substitute for a proper output
transformer. If $45 apiece is too high then you're playing in the
wrong park here and need to go home and quit wasting everyone's time.

There is a reason the 6080 and 6AS7 are cheap and have been forever.
They are not a cost effective path to results because of their
excessively low mu. That isn't to say it can't be done just an
expensive one.

Using a driver xfmr means not only the cost but the impossibility of
putting feedback across the stage. Is NFB absolutely necessary? No,
but it sure helps. An amplifier without it could still be OK for a lot
of applications but damping factor will suffer. You also need to use
cathode bias with this tube which is not too desireable either from an
efficiency standpoint.

That said, with GOOD iron such an amp could drive a K-horn pretty
well with one output tube, and in the long run be economic if you were
okay with the high transformer expense.I'd actually figure $100 to
$200 for the output and the driver-each. But a lifetime of NOS output
tubes would be less than fifty bucks.

[Engineer[_2_]]

On Oct 20, 6:48*pm, Engineer wrote:
Hi, Vacuumlanders.
Here are the 6080 results I got while experimenting with a SE OPT
(from a 50C5 o/p tube stereo receiver - have two, planned on stereo
from one 6080) - see original post from a few days ago.
I presented all this at the Ontario Vintage Radio Association (OVRA)
meeting on October 18, 2009 with some pics (none here, of course!)

(snip)


Started to experiment with a 6080 in P-P configuration with a Hammond
125E P-P OPT to hand (will try several plate-to-plate impedances.)
Grid test drive from a small, "reversed" P-P OPT, 40:1 ratio.

Too small, overloaded even as a driver, scrapped - see below.

*Rk = 2.5 Kohms, for now.
More later...
Cheers,
Roger

Thanks for all replies. I generally agree with the technical
comments.
Re. costs: yes, this tube needs good SE iron, so I'd have to bend my
rules a bit, perhaps chalk it up to entertainment. The original idea
was cheap "flea power" from virtually free tubes just for fun and to
see if I could do it. Since I have several P-P amplifiers with 6L6's,
6V6's, etc, and good iron I'm not tube-audio deprived! Also, the P/S
for any 6080 project would be virtually free as I have a couple of
spare ones on the shelf (both 25 Hz iron, too - looks great!)

I tried P-P using the Hammond 125E OPT. Best continuous output
(nearly clipping) was a nominal 8 watts (250 Hz) at 3000 ohms plate to
plate, with 2.5 Kohm in each 6080 cathode, B+ = 375 VDC (but 125 VDC
"lost" as bias at 50 mA per tube), still using the kludged driver
(another 125E backwards from a s/s amplifier drive source.) RC
coupled driver design would be a challenge as I don't want to use
transformer coupling; anyway, don't have any and I want to use global
NFB!
Since the 125E OPT's are reserved for a future "proper" stereo pentode
amplifier I may just drop the 6080 notion. There's always the
headphone amplifier, of course...
Cheers,
Roger