[Bob H.]

Hi all

The past discussion of 6080 pp circuits brought up an interesting
point to me.

Many lower impedance transformers, such as those wound for 7591's can
be used for lower impedance by putting 8 ohm speakers on 16 or 32 ohm
taps.
However, Patrick mentioned the winding resistance, which can rob
output power, and around 500 ohms was normal for vintage
transformers. The loss is pretty significant at 500 ohms.

I measured an output transformer on my bench wound for 7591's, and got
220 ohms. I haven't checked the reflected impedance, but suspect it's
probably around 4.5 k or so.

Maybe we could check whatever pp transformers we have lying around for
winding resistance, and thereby establish a list of suspects good for
low rp tubes. Of course, they would have to have at least a 16 ohm
tap.

The output I measured was from a Heathkit HF-181, using a pair of
7591's. 220 ohms plate to plate, 4,8,16 ohm taps.

I have some others I can measure, when I get a chance.

Bob H.

[Engineer]

On Feb 19, 8:23 am, "Bob H." wrote:
Hi all

The past discussion of 6080 pp circuits brought up an interesting
point to me.

Many lower impedance transformers, such as those wound for 7591's can
be used for lower impedance by putting 8 ohm speakers on 16 or 32 ohm
taps.
However, Patrick mentioned the winding resistance, which can rob
output power, and around 500 ohms was normal for vintage
transformers. The loss is pretty significant at 500 ohms.

I measured an output transformer on my bench wound for 7591's, and got
220 ohms. I haven't checked the reflected impedance, but suspect it's
probably around 4.5 k or so.

Maybe we could check whatever pp transformers we have lying around for
winding resistance, and thereby establish a list of suspects good for
low rp tubes. Of course, they would have to have at least a 16 ohm
tap.

The output I measured was from a Heathkit HF-181, using a pair of
7591's. 220 ohms plate to plate, 4,8,16 ohm taps.

I have some others I can measure, when I get a chance.

Bob H.

Here are the resistance specifications of the industrial power
transformer (speculated for 6080 OPT use) that is the subject of the
other post... very low indeed!
PRIMARY
0-240 VAC 10 ohms (+0%, -20%)
0-120 VAC 3.5 ohms (+0%, -20%) (part of the above, of course)
SECONDARY
27.6 VAC CT 0.2 ohms, +/- 10%
Also 63 VA, as I said earlier .
Just for comparison, here are the DC plate-to-plate resistances of 3
OPT's I happen to have lying around, all Hammond brand (measured on an
old AVO Model 8):
A new 1650F: 200 ohms
A vintage 468: 480 ohms
A new 125E: 150 ohms.
I didn't measure any secondaries.
Clearly, the mains transformer is a very different animal.
Cheers,
Roger

[Patrick Turner]

Engineer wrote:

On Feb 19, 8:23 am, "Bob H." wrote:
Hi all

The past discussion of 6080 pp circuits brought up an interesting
point to me.

Many lower impedance transformers, such as those wound for 7591's can
be used for lower impedance by putting 8 ohm speakers on 16 or 32 ohm
taps.
However, Patrick mentioned the winding resistance, which can rob
output power, and around 500 ohms was normal for vintage
transformers. The loss is pretty significant at 500 ohms.

I measured an output transformer on my bench wound for 7591's, and got
220 ohms. I haven't checked the reflected impedance, but suspect it's
probably around 4.5 k or so.

Maybe we could check whatever pp transformers we have lying around for
winding resistance, and thereby establish a list of suspects good for
low rp tubes. Of course, they would have to have at least a 16 ohm
tap.

The output I measured was from a Heathkit HF-181, using a pair of
7591's. 220 ohms plate to plate, 4,8,16 ohm taps.

I have some others I can measure, when I get a chance.

Bob H.

Here are the resistance specifications of the industrial power
transformer (speculated for 6080 OPT use) that is the subject of the
other post... very low indeed!
PRIMARY
0-240 VAC 10 ohms (+0%, -20%)
0-120 VAC 3.5 ohms (+0%, -20%) (part of the above, of course)
SECONDARY
27.6 VAC CT 0.2 ohms, +/- 10%
Also 63 VA, as I said earlier .
Just for comparison, here are the DC plate-to-plate resistances of 3
OPT's I happen to have lying around, all Hammond brand (measured on an
old AVO Model 8):
A new 1650F: 200 ohms
A vintage 468: 480 ohms
A new 125E: 150 ohms.
I didn't measure any secondaries.
Clearly, the mains transformer is a very different animal.
Cheers,
Roger

The higher dcr of the P windings for tube amps indicate
the designer's intent:- to not waste copper,
to have maybe 4 times the turns per volt for the OPT compared to the
mains tranny,
To accomodate a barely high enough VA or power rating, and to make a
good profit.

A mains tranny for a Quad-II power amp will have quite high winding
resistances.
Not all mains trannies have low winding R, especially those designed
60 years ago when much copper was lying on the seabed after WW2.

Patrick Turner.