[Engineer[_2_]]

Hi, Vacuumlanders
I have just parted-out a huge tube-based PA amplifier, over 60 lbs
weight... alleged to be 250 watts output. It had four 807's in
parallel push-pull, transformer driven by a push-pull 2 x 6F6 driver
stage. It was a homebrew and terribly constructed - dry joints, wires
all over the place, electrician's tape on joints, etc, ghastly... so
rather than refurbish it I may just re-build it. It had no less than
9 Hammond transformers in it (power(2), output(1), filament(1),
coupling(2), chokes(3).
But the large output transformer is a conumdrum. It's Hammond type
1682 from their PA line in catalog #65 (not "Hi-Fi", rather for PA
with the extra line output.) The P-P impedance is 1900 ohms (it tests
to this.) It's also rated at 120 watts (not 250) so perhaps that's
all the original amplifier produced... if it ever worked, that is!
Anyway, 1900 ohms P-P seems a bit low to me, even for 4 O/P tubes.
It means 3800 ohms P-P for two 807's in ordinary push-pull. I've not
drawn any load lines yet, but does this seem reasonable? Remember, it
was a homebrew: did the original "builder" (butcher?) pick the right
OPT? If the 1682 OPT would work with four 807's, then I may re-build
this thing to my own design... but I'm not sure for what use! (I
already have two 200 watt s/s subwoofers.) If not, then all the
"iron" (except a 10 H choke that I want to keep) will be for sale at
the radio club (http://www.ovra.ca), or a local hamfest (I certainly
won't ship it!)
Thanks for all replies.
Cheers,
Roger

[Phil Allison[_3_]]

"Engineer"


But the large output transformer is a conumdrum. It's Hammond type
1682 from their PA line in catalog #65 (not "Hi-Fi", rather for PA
with the extra line output.) The P-P impedance is 1900 ohms (it tests
to this.) It's also rated at 120 watts (not 250) so perhaps that's
all the original amplifier produced... if it ever worked, that is!
Anyway, 1900 ohms P-P seems a bit low to me, even for 4 O/P tubes.
It means 3800 ohms P-P for two 807's in ordinary push-pull. I've not
drawn any load lines yet, but does this seem reasonable?

** For maximum possible power output it is about right.

I take it the unloaded B+ is about 450 to 500 volts for plate and screens ?

A pair of 807s or 6L6GCs can do 60 watts like that.

For a PA amp, bias current would be set low near to Class B conditions and
the load presented by a 70V or 100 V line speaker array is all over the
place anyhow.

BTW: An output tranny can always handle more than its nominal power rating
provided you tolerate higher percentage losses and/or restricted output at
the lowest frequencies.



.... Phil

[Patrick Turner]

On Nov 19, 9:08*am, Engineer wrote:
Hi, Vacuumlanders
I have just parted-out a huge tube-based PA amplifier, over 60 lbs
weight... alleged to be 250 watts output. It had four 807's in
parallel push-pull, transformer driven by a push-pull 2 x 6F6 driver
stage. *It was a homebrew and terribly constructed - dry joints, wires
all over the place, electrician's tape on joints, etc, ghastly... so
rather than refurbish it I may just re-build it. *It had no less than
9 Hammond transformers in it (power(2), output(1), filament(1),
coupling(2), chokes(3).
But the large output transformer is a conumdrum. *It's Hammond type
1682 from their PA line in catalog #65 (not "Hi-Fi", rather for PA
with the extra line output.) *The P-P impedance is 1900 ohms (it tests
to this.) *It's also rated at 120 watts (not 250) so perhaps that's
all the original amplifier produced... if it ever worked, that is!
Anyway, 1900 ohms P-P seems a bit low to me, even for 4 O/P *tubes.
It means 3800 ohms P-P for two 807's in ordinary push-pull. *I've not
drawn any load lines yet, but does this seem reasonable? *Remember, it
was a homebrew: did the original "builder" (butcher?) pick the right
OPT? *If the 1682 OPT would work with four 807's, then I may re-build
this thing to my own design... but I'm not sure for what use! (I
already have two 200 watt s/s subwoofers.) *If not, then all the
"iron" (except a 10 H choke that I want to keep) will be for sale at
the radio club (http://www.ovra.ca), or a local hamfest (I certainly
won't ship it!)
Thanks for all replies.
Cheers,
Roger


With B+ of 600V and Eg2 = 300V and operating in class AB2, a pair of
807 are good for 80W, so a quad could give 160W.
But the Hammond OPT 1k9 to 4,8,16 ohms sounds low, so do the load line
analysis to work out the right load.

If the speakers are 8 ohms, then connection of them to the 4 ohm tap
gives RLa-a = 3k8, or 7k6 per pair of tubes which might better suit
the Ea of 600V.

But forget 250W. You need 6 x 6550 to get this ***reliably***.
Best would be TT21, which are like KT88 with top caps like the 807, a
pair can give 140W with Ea = 800V, class AB2, so a quad could give
280W in theory,

4 x 6550 might just get you 200W with Ea at 600V.

[Patrick Turner]

On Nov 19, 10:24*am, "Phil Allison" wrote:
"Engineer"

But the large output transformer is a conumdrum. *It's Hammond type
1682 from their PA line in catalog #65 (not "Hi-Fi", rather for PA
with the extra line output.) *The P-P impedance is 1900 ohms (it tests
to this.) *It's also rated at 120 watts (not 250) so perhaps that's
all the original amplifier produced... if it ever worked, that is!
Anyway, 1900 ohms P-P seems a bit low to me, even for 4 O/P *tubes.
It means 3800 ohms P-P for two 807's in ordinary push-pull. *I've not
drawn any load lines yet, but does this seem reasonable?

** For maximum possible power output it is about right.

I take it the unloaded B+ is about 450 to 500 volts for plate and screens ?

A pair of 807s or 6L6GCs can do 60 watts like that.

For a PA amp, bias current would be set low near to Class B conditions and
the load presented by a 70V or 100 V line speaker array is all over the
place anyhow.

BTW: *An output tranny can always handle more than its nominal power rating
provided you tolerate higher percentage losses and/or restricted output at
the lowest frequencies.

Indeed, for most of the time you are right. 807 as I said in another
post are good for 80W a pair with Ea at 600V but fixed screen voltage
at 300V, or else the screens cook. Probably 6L6GC would also be OK,
but also with screen supply low at 300V. but you risk arcing from
anode pin 3 to pin 2 earthy heater pin.

One might always use a 120W rated tranny to give 250W if the primary
Idc current density is low, ie, 3A/sq.mm max. A Hammond tranny rated
for 120W for RLa-a = 1k9 means that 477Vrms may be applied across the
OPT primary. to get 250W, Va-a will be 689Vrms, and the losses have
not increased at all if the load is the same. But If the core
saturation is 32Hz, at 477V it will move up to 50Hz with 689Va-a.
My guess is that 1k9 is too low a load to consider for higest possible
PO from 4 x 807 class AB2, and 3k8 would be better, and then if one
could get 250W from the OPT, you'd have Va-a = 775 Vrms, and Fsat =
65Hz. For PA, perhaps this is OK. It certainly used to be in 1955 when
many high powered PA amps with multiple output tubes, or even just a
pair of large size transmit tubes such as 211 or 845 etc. Fsat was
allowed to be 75Hz, which meant small cores with few turns could be
used. The winding losses with a 8 ohm speaker connected to the 4 ohm
tap of an OPT with 4,8, 16 taps reduces considerably, not by half,
because the loss % is always highest at the 4 ohm tap because only 1/2
the sec turns are used.

Winding losses always increase a lot when connecting an 8 ohm speaker
to a tap output labelled 16 ohms. The RLa-a load of say 1k9 would
reduce to 950 ohms and SMOKE soon comes from a PA amp.

If the OP wants 250W from tubes, he should consider the large Ampeg
amp with 6 x 6550.
Or he could use 8 x 807, 6L6GC, EL34, but only after he learns a lot
more about amp design.

Patrick Turner.


... * Phil

[Phil Allison[_3_]]

"Patrick Turner"


Indeed, for most of the time you are right. 807 as I said in another
post are good for 80W a pair with Ea at 600V but fixed screen voltage
at 300V, or else the screens cook.

** But NOT with 3800 ohms plate to plate impedance as is REQUIRED by the
OP's question.




.... Phil

[Engineer[_2_]]

On Nov 18, 8:51*pm, "Phil Allison" wrote:
"Patrick Turner"

Indeed, for most of the time you are right. 807 as I said in another
post are good for 80W a pair with Ea at 600V but fixed screen voltage
at 300V, or else the screens cook.

** But *NOT with 3800 ohms plate to plate impedance as is REQUIRED by the
OP's question.

... *Phil

Thanks, Patrick and Phil, useful information as always. This thing
will not get rebuilt as I don't need the resulting 150 watt, or so,
"boat anchor". But I can still admire all that iron! I've never seen
such a large PT, a Hammond type 717. It's 510-0-510 VAC at around 1/2
an amp.
My next tube audio project will likely be a second Williamson near-
clone ( two pentode connected 6L6's) to go with the first one I built
a few months ago (to get stereo) - with VLF shelf stabilization, of
course. Can't use the 1682 OPT, though. That would have been fun just
for the physical size... it's huge!
Cheers,
Roger

[Phil Allison[_3_]]

"Engineer"


Thanks, Patrick and Phil, useful information as always. This thing
will not get rebuilt as I don't need the resulting 150 watt, or so,
"boat anchor". But I can still admire all that iron! I've never seen
such a large PT, a Hammond type 717. It's 510-0-510 VAC at around 1/2
an amp.

** Used with a choke input filter give about 400VDC at 650mA.



.... Phil

[John L Stewart]

Originally Posted by Engineer[_2_]

Hi, Vacuumlanders
I have just parted-out a huge tube-based PA amplifier, over 60 lbs
weight... alleged to be 250 watts output. It had four 807's in
parallel push-pull, transformer driven by a push-pull 2 x 6F6 driver
stage. It was a homebrew and terribly constructed - dry joints, wires
all over the place, electrician's tape on joints, etc, ghastly... so
rather than refurbish it I may just re-build it. It had no less than
9 Hammond transformers in it (power(2), output(1), filament(1),
coupling(2), chokes(3).
But the large output transformer is a conumdrum. It's Hammond type
1682 from their PA line in catalog #65 (not "Hi-Fi", rather for PA
with the extra line output.) The P-P impedance is 1900 ohms (it tests
to this.) It's also rated at 120 watts (not 250) so perhaps that's
all the original amplifier produced... if it ever worked, that is!
Anyway, 1900 ohms P-P seems a bit low to me, even for 4 O/P tubes.
It means 3800 ohms P-P for two 807's in ordinary push-pull. I've not
drawn any load lines yet, but does this seem reasonable? Remember, it
was a homebrew: did the original "builder" (butcher?) pick the right
OPT? If the 1682 OPT would work with four 807's, then I may re-build
this thing to my own design... but I'm not sure for what use! (I
already have two 200 watt s/s subwoofers.) If not, then all the
"iron" (except a 10 H choke that I want to keep) will be for sale at
the radio club (http://www.ovra.ca), or a local hamfest (I certainly
won't ship it!)
Thanks for all replies.
Cheers,
Roger

Hi Roger- What driver transformer does it use?

PS probably choke input as was common back in the day. 250 Watts unlikely, half that is possible in AB2. The low p-p OPT needed while pushing up into the pentode knee of the plate current to get full output while the G1's are driven +ve.

What is driving the 6F6's?

All sounds like fun but as you have said kind of large & cumbersome!

Cheers, John

[Engineer]

On Nov 19, 9:53*am, John L Stewart John.L.Stewart.
wrote:
Engineer[_2_ Wrote:

(snip)

Hi Roger- What driver transformer does it use?

************** The 807 grids are driven by a Hammond Type 937
interstage transformer.
From Hammond catalog #65:
Primary: 2500/10000 ohms, CT
Secondary: 1100/4400 ohms, CT
Max. primary unbalance: 3 mA
DC per leg: 40 mA
Max level: 35 dBM
For P-P 6F6, 6V6, 2A3, 6AQ5, to P-P 805 (and I presume also 807.)
Max. primary DC 50 mA (but must balance)
(The original "boat anchor" PA chassis used two 6F6's)


PS probably choke input as was common back in the day.

************** No, it had a CLC B+ filter from a 510-0-510 PT, so
loaded B+ would be quite high, e.g. 600VDC or more. The choke was
huge, too.
Type 10-300X Choke
10 H @ 300 mA
63 ohms
Max 800 V winding to core/chassis

250 Watts unlikely, half that is possible in AB2.

************* Agreed. 250 was just the speculation of the chap I got
if from.

The low p-p OPT needed while
pushing up into the pentode knee of the plate current to get full output
while the G1's are driven +ve.

************ Noted. What might Patrick and Phil think of that? :-)

What is driving the 6F6's?

************ They are driven by a Hammond type 426.
Line to P-P grids
Primary: 500 CT
Secondary: 20,000/80,000 ohms, CT
1:12.6 ratio
Max primary DC: 100 mA
80 - 7,000 Hz.
There was a separate pre-amp with a line output to the 426, but it was
such a dog's breakfast of dry-jointed bird's nest that I declined to
take it! Had I rebuilt it I would have used a normal pre-amp with an
AUX-level input signal.

All sounds like fun but as you have said kind of large & cumbersome!

************* Yes, indeed! I have a Fisher CA273, a 100 WPC s/s
stereo amplifier that uses two 100 watt STK modules... much smaller!
Unfortunately, it has an intermittent linear VC tapped for loudness
(nasty design, the linear format I mean) that does not respond to
Deoxit D5, so I'm looking for a replacement part. Markings: Noble
74190 15AM 100K(ohms) W6. The search is on!


Cheers, John

--
John L Stewart

Hi, John,
My answers in your text (above) after **************
Cheers,
Roger

[Phil Allison[_3_]]

"Engineer"


PS probably choke input as was common back in the day.

************** No, it had a CLC B+ filter from a 510-0-510 PT, so
loaded B+ would be quite high, e.g. 600VDC or more. The choke was
huge, too.
Type 10-300X Choke
10 H @ 300 mA
63 ohms
Max 800 V winding to core/chassis

250 Watts unlikely, half that is possible in AB2.

************* Agreed. 250 was just the speculation of the chap I got
if from.

** Not so far off what is likely.

See RCA data on the 807:

http://www.triodeel.com/807_p4.gif

In your case, the DC supply under load is about 600 to 650 volts.

The screens would be fed from a voltage divider and electro cap to ground to
give some peak current ability with speech.

The Hammond OT would be run with double the rated load impedance so the
primary Z is also double or 3800 ohms.

The conditions in the column beginning with 600V then apply to 4 tubes.

So 160 watts or so.


..... Phil

[John L Stewart]

Originally Posted by Engineer[_2_]

Hi, Vacuumlanders
I have just parted-out a huge tube-based PA amplifier, over 60 lbs
weight... alleged to be 250 watts output. It had four 807's in
parallel push-pull, transformer driven by a push-pull 2 x 6F6 driver
stage. It was a homebrew and terribly constructed - dry joints, wires
all over the place, electrician's tape on joints, etc, ghastly... so
rather than refurbish it I may just re-build it. It had no less than
9 Hammond transformers in it (power(2), output(1), filament(1),
coupling(2), chokes(3).
But the large output transformer is a conumdrum. It's Hammond type
1682 from their PA line in catalog #65 (not "Hi-Fi", rather for PA
with the extra line output.) The P-P impedance is 1900 ohms (it tests
to this.) It's also rated at 120 watts (not 250) so perhaps that's
all the original amplifier produced... if it ever worked, that is!
Anyway, 1900 ohms P-P seems a bit low to me, even for 4 O/P tubes.
It means 3800 ohms P-P for two 807's in ordinary push-pull. I've not
drawn any load lines yet, but does this seem reasonable? Remember, it
was a homebrew: did the original "builder" (butcher?) pick the right
OPT? If the 1682 OPT would work with four 807's, then I may re-build
this thing to my own design... but I'm not sure for what use! (I
already have two 200 watt s/s subwoofers.) If not, then all the
"iron" (except a 10 H choke that I want to keep) will be for sale at
the radio club (http://www.ovra.ca), or a local hamfest (I certainly
won't ship it!)
Thanks for all replies.
Cheers,
Roger

While at U of T Physics in the early 60's I built something similar. It used a set of four 6DQ5's in PPP, 980 volts B+ and as I recall driving into a Hammond 1678 OPT. It easily pushed 120 watts of audio into 16R while running AB1.

The PS was a Hammond 270 Series running into a SS FWB, so that as well as the 980 volts, half that was available to run the rest. A series regulator got the volts down to something acceptable to the 6DQ5 screens.

Was used in an experinment on electroluminescent materials, it was the high voltage AC off the plates that was the objective. If I should come upon the schema I will post it for the curious.

Cheers, John

[Engineer]

On Nov 21, 10:23*am, John L Stewart John.L.Stewart.
wrote:
'Engineer[_2_ Wrote:









;942884']Hi, Vacuumlanders
I have just parted-out a huge tube-based PA amplifier, over 60 lbs
weight... alleged to be 250 watts output. It had four 807's in
parallel push-pull, transformer driven by a push-pull 2 x 6F6 driver
stage. *It was a homebrew and terribly constructed - dry joints, wires
all over the place, electrician's tape on joints, etc, ghastly... so
rather than refurbish it I may just re-build it. *It had no less than
9 Hammond transformers in it (power(2), output(1), filament(1),
coupling(2), chokes(3).
But the large output transformer is a conumdrum. *It's Hammond type
1682 from their PA line in catalog #65 (not "Hi-Fi", rather for PA
with the extra line output.) *The P-P impedance is 1900 ohms (it tests
to this.) *It's also rated at 120 watts (not 250) so perhaps that's
all the original amplifier produced... if it ever worked, that is!
Anyway, 1900 ohms P-P seems a bit low to me, even for 4 O/P *tubes.
It means 3800 ohms P-P for two 807's in ordinary push-pull. *I've not
drawn any load lines yet, but does this seem reasonable? *Remember, it
was a homebrew: did the original "builder" (butcher?) pick the right
OPT? *If the 1682 OPT would work with four 807's, then I may re-build
this thing to my own design... but I'm not sure for what use! (I
already have two 200 watt s/s subwoofers.) *If not, then all the
"iron" (except a 10 H choke that I want to keep) will be for sale at
the radio club (http://www.ovra.ca), or a local hamfest (I certainly
won't ship it!)
Thanks for all replies.
Cheers,
Roger

While at U of T Physics in the early 60's I built something similar. It
used a set of four 6DQ5's in PPP, 980 volts B+ and as I recall driving
into a Hammond 1678 OPT. It easily pushed 120 watts of audio into 16R
while running AB1.

The PS was a Hammond 270 Series running into a SS FWB, so that as well
as the 980 volts, half that was available to run the rest. A series
regulator got the volts down to something acceptable to the 6DQ5
screens.

Was used in an experiment on electroluminescent materials, it was the
high voltage AC off the plates that was the objective. If I should come
upon the schema I will post it for the curious.

Cheers, John

--
John L Stewart

John,
Well, it looks like a rebuild is in the air... just for fun! I've
cleared the chassis of all bits (what a chore!) and sent to my old
company to be sandblasted. A Winter project when I've fixed a 1935
Majestic type 59 radio! I'll be testing the parallel 807's with drive
from a 10 watt amplifier via a small P-P output transformer in
reverse... also an external power supply. If OK, I'll build the
rest. Project name: "Scratch 100".
BTW, your R&S 'scope was donated to the OVRA with attribution...
auctioned for coffee money... the chaps send their thanks.
Cheers,
Roger