Measuring the primary of an OT

[pbman1953]

I'm talking to Hammond about a 4/8 OT for my Super Twin. They asked me for the Primary impedance.
How do I get that with a multimeter?


I read to take the meter and connect to the OT while it's disconnected from the amp. First to test the Brown and Blue , then brown to B+, then Blue to B+. I got figures but he's figuring that the OT should be 2000 ohms




The test -


Brn /Blue- 40 ohms
Brn/Red- 22.6
Blue/Red- 21.1


4 ohm load


Am I doing this right?


Thanks

[tubeswell]

I'm talking to Hammond about a 4/8 OT for my Super Twin. They asked me for the Primary impedance.
How do I get that with a multimeter?

I read to take the meter and connect to the OT while it's disconnected from the amp. First to test the Brown and Blue , then brown to B+, then Blue to B+. I got figures but he's figuring that the OT should be 2000 ohms

The test -

Brn /Blue- 40 ohms
Brn/Red- 22.6
Blue/Red- 21.1

4 ohm load

Am I doing this right?

Thanks

No.
Impedance is 'resistance-to-A/C', which you can't ohm-out with a R-meter. You need to feed one of the OT windings with some VAC and measure the VAC on that winding and the VAC on the other winding to get the Pri:Sec VAC ratio. The Pri:Sec Impedance ratio will be the square of the Pri:Sec VAC ratio. And then the Impedance your output tube plates will see on the OT primary will be the product of your speaker impedance (hooked up to the secondary) x the Pri:Sec Impedance Ratio.


So say you hooked up 5VAC to the OT secondary winding and measured 158VAC across the Primary winding, that's 158:5 (or a VAC ratio of 36.6:1). 36.6:1 x 36.6:1 = 1000:1 impedance ratio. 1000:1 x 8R speaker = 8k Impedance on the Primary.

[sluckey]

he's figuring that the OT should be 2000 ohms

That's right.

[pbman1953]

he's figuring that the OT should be 2000 ohms

That's right.

Ok, I'll tell them 2000 ohms

edit... fixed quote

[PRR]

...They asked me for the Primary impedance. ...How do I get that with a multimeter?...

Thinking hurts. I would find as many Super Twin replacement OTs as possible, and poll their specs.

At the risk of thinking: Fender soon settled on 4K/pair for all/most 5881/6L6GC amps. So two pair? 2K. Like the man said. 

[J Fletcher]

 Super Twin , 3 pairs of 6L6's . Primary impedance should be less than 2000 ohms . No ?

[sluckey]

good point

[pbman1953]

I ended up talking to a Hammond Tech Sales rep. A 40 year vet at the company. He feels that it's 2000 ohm. They have a unit that is a 1750 WR rated at 135 watts for 6- 6L6's. He explained that even though it's published at 135 watts, they always over build and are conservative. He will ask engineering what the limits are of the OT and get back to me either to advise or if they will take that unit design and re-engineer one for me.

https://www.hammfg.com/files/parts/pdf/1750WR.pdf



Another huge tip he gave was the name of one of his distributors that had THE best pricing in the county.


Hawk Electronics in Elk Grove, ILL

 

[PRR]

Super Twin , 3 pairs of 6L6's . Primary impedance should be less than 2000 ohms . No ?

Is this the 180 Watt from 1978? With a 6C10??
https://www.thetubestore.com/lib/thetubestore/schematics/Fender/Fender-Super-Twin-Reverb-180W-Schematic.pdf
Do you actually need or want 180 Watts from a 45 year old chassis?
Yes: then load in about 1,333r.
No, you like the looks and tone but don't need to fill SoFi Stadium?
Then use quad-6L6 iron @2k and be happy.

I was not ashamed to cut-back a burnt Ampeg VT-40 from 590V to 400V and 60 TOO-LOUD Watts to 20 studio Watts. It's been a long time since I tried or wanted to maximize power. (Today I'd rather have a sweet idle, but my Honda is doing the rupp-rulp-ruff again.)

[pbman1953]

I use this as a bass amp, so yes the 185 watts is nice to have
6-6l6


7025's
12at7

[AmberB]

I use this as a bass amp, so yes the 185 watts is nice to have
6-6l6


7025's
12at7

I understand your desire for power.  I played bass through a Fender Bassman 100 from the early 1970s, I couldn't keep up with the volume of the lead guitarist playing through a 50 watt Marshall and one 4-12 cab...

[pbman1953]

Super Twin , 3 pairs of 6L6's . Primary impedance should be less than 2000 ohms . No ?

Is this the 180 Watt from 1978? With a 6C10?? no  , the first version, Super Twin, w/o reverb
This one-
https://el34world.com/charts/Schematics/files/Fender/Fender_super_twin_180w_schem.pdf

Do you actually need or want 180 Watts from a 45 year old chassis?  It's either the 180 watts or use a Class D with 800w
Yes: then load in about 1,333r. Not sure what this is
No, you like the looks and tone but don't need to fill SoFi Stadium? Definity like the looks and  tone, but no need for SoFi
Then use quad-6L6 iron @2k and be happy.  6L6's, yes I'd be happy

I was not ashamed to cut-back a burnt Ampeg VT-40 from 590V to 400V and 60 TOO-LOUD Watts to 20 studio Watts. It's been a long time since I tried or wanted to maximize power. (Today I'd rather have a sweet idle, but my Honda is doing the rupp-rulp-ruff again.)

I used it last night , for the first time , at a gig, it did really well with my 8 ohm cab. The PT at the end of the night was warm , but the OT was a bit hot. I'd need a second cab to run the amp at the rated impedance, 4 Ohm. Probably wouldn't have to work a hard

{edit: dangling quote -PRR}