# Confirmation of Primary Impedance?

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#### krzanik

I recently salvaged a power transformer and two single ended output transformers from an old reel to reel. I have been using guidance on the web to determine the windings ratio and subsequent primary impedance of the output transformer. I wanted to post my methodology for review and comment:

-I put 125 volts (measured on the meeter) from the wall socket into the output transformer and measured 3.1 volts out.

-I then divided 125 by 3.1 to determine the turns ratio and got 40:1

-I then squared the turns ratio (40 time 40) to get an impedance ratio of 1600:1

-So...if I hook this transformer up to my 8 Ohm speaker the impedance that will be reflected to the primary will be the impedance ratio (1600) times the load (8 ohms) to get a primary of 12.8 K? And if I used 4 ohm speakers the primary would be 6.4K?

If someone could comment on wether I did this correctly I would be grateful. Thanks.

Krzanik

Yes, your calculations appear to be correct. However, if you do them without rounding in the intermediate stages, you'll find that the primary impedance is around 13K for an 8W load (not that the difference is particularly significant).

#### planet10

Paid Member
That is correct... i'd guess EL84/6BQ5 or ECL82/6BM8 and 4 ohm speakers.... what tape deck?

dave

#### krzanik

Thanks for the replies

I appreciate the help. I was kind of hoping I did something wrong. Dave- you are right. the reel to reel (Ampex early 60's) was a 6BM8 with EZ81 rectifier and 12ATX driver stage. You know your tubes.

I have 8Ohm speakers (fostex FE207E's) and the amp I was going to build uses 5K primary output transformers. Oh well I guess I'll have to spend some money. Any opinions on those \$16 SE output transformers (4K-10K/8watts) at Antique Electronics Supply http://www.tubesandmore.com/ ? The other choice is the \$13 5K/5watts at triode electronics http://store.yahoo.com/triodeel/tfchxfwi48oh.html . If you had your pick, which would you pick?

thanks,

#### alejo

this transformers OPT is not for Hi-Fi (need 20hz-20khz). They are guitar output transformer (arround 100hz. to 15khz).

Yours Fostex 207E´s is very good driver, need good Hifi OPT.

See around The "UBT-2 from One Electron" in tubesandmore arround \$89.95 or HAMMOND P-T 1628SE \$97.52

if you dont take the money see general purpose Hammond P-T125SE series; not HiFi but very good irom.

good build.

#### planet10

Paid Member
Re: Thanks for the replies

krzanik said:
I appreciate the help. I was kind of hoping I did something wrong. Dave- you are right. the reel to reel (Ampex early 60's) was a 6BM8 with EZ81 rectifier and 12ATX driver stage. You know your tubes.

I have 8Ohm speakers (fostex FE207E's) and the amp I was going to build uses 5K primary output transformers.

Not all is lost... what amp were you going to build? The 6BM8 is limited to about 3.5W so your OPT may also be... You could build an amp with EL95s and have close to the same power & since these like really high impedance, your OPT may be fine....

BTW the 12AT7 would be the mic pre-amp tube, each 6BM8 has its own driver triode inside.

dave

#### krzanik

Not all is lost

Planet 10, I was going to build the "super triode connection" 6BM8, the schematic for which can be found at the link below:

This amp caught my eye a while ago and when I came across the 6BM8's in the reel to reel I thought I'd try it. The 13K Primary output transformers I got from the reel to reel won't work with this circuit though. I looked at the EL95 and it's pretty inexpensive (cheaper than buying new output transformers) so I guess I'll start looking for schematics. 2 or 3 watts output is fine. If you have suggestions on a good circuit please let me know. Thanks.

#### planet10

Paid Member
Re: Not all is lost

krzanik said:
I was going to build the "super triode connection" 6BM8, the schematic for which can be found at the link below: