transformer impedance and DCR?..

I found a couple of Marshall reverb driver transformers in my drawer today. and knowing the are suposed to drive a 250ohm reverb from a 12A?7, I figured they could be great for a tubepre output, or driving my 250 ohm Beyer headphones.
sending a signal trough them reveals a turnratio of 5:1...or 15000:600..I could hang this on a 12au7 and have an output  ;D
..but what's pusseling me is the DCR..if this is made to drive 250 ohm, shouldnt the DCR be something like 25ohm or less?..what I get with my meter is 300ohm DCR on the secondary...is this just a poor transformer, or is it intentionaly made this way not to push bass into the reberbtank or something. ..or what does it mean?...could they be ok for audiowork anyway?
they look like this by the way..

any thoughts or ideas?..
thank's
johan

Yeah, you're all the way on the other side of the nominal Z; not even under it at all.  Not so sure about that.  Sure you've got it right, and sure they're good? 

There are RCA and Langevin 600 ohm outputs that are as high as 400 ohms DCR.    I'm still not clear as to exactly why, and what sort of choices were made. 

Reverb tank is customarily driven with HIGH impedance. Pentode, or strained triode, or with series resistance, or current feedback.

So using too-thin wire, especially on the secondary, is not bad for reverb, and saves many pennies.

Put it on a 12AU7, load the secondary in 600 ohms, measure output and frequency response. I suspect it has poor bass, but the price and availability are much better than most other DC-rated 5:1 plate to load transformers.

True, suspecting poor bass is wise in the first place given the type.  Does it handle DC on the pri (i'm not looking it up)?  If it doesn't the $10-12 Edcors are quite unbeatable in a competitive sense, and full range.  Sometimes free costs a lot. 

emrr said:

True, suspecting poor bass is wise in the first place given the type.  Does it handle DC on the pri ...  Sometimes free costs a lot. 

Click to expand...

I guess I'll have to take PRR's advise and hook it up and try..
it should be able to take the DC on the primary, in the amp it comes from, it sits with one end to the plate and the other straigt up to HT (450volt)...yeah, sometimes free cost a lot, but I'd like to see if they can be used for anything else than what they were for..if I find they could be usefull, I'll report back...

thank's to both of you..

j

ok, so I did just that...built a simple 12au7 circuit. powered with 170volt and sent a signal through it all..hung a 600ohm resistor and a VU on the output.
now, my tonegenerator leaves a lot to wish for. it has a set of 5 preselected frequences(40,400,1000,10k and 15k) so the test isn't really accurate and might not tell all...but anyway.
with a 1kHz I adjusted the level so the VU read "0"
at both 400Hz an 10kHz it stayed at "0", at 15kHz it was up to +1 and 40Hz was down to about -4...so flat it is not, but better than I hoped and perhaps with a little bit of feedback or some other tweaking..

also, when measuring with signal through circuit, it seems I must have messed up with the ratio. at 1Khz and measuring the signal across primary/secondary I now get 9:1, so sorry about that..(perhaps I did my previous measuring at 40Hz?...) so perhaps it would be better with a 12AT7 or even an 12AX7?...

anyway..thank's for your thought and input..we'll see what I can/may use them for, or if it's back in the drawer again...

j 

you could correct for bottom droop with NFB, at the expense of some gain.    You probably know this, but be sure to confirm signal generator output levels at each freq too, many will vary in level. 

> my tonegenerator leaves a lot to wish for. it has a set of 5 preselected requences(40,400,1000,10k and 15k)

Transformer is NOT likely to show a narrow peak-dip in bass or midrange.

Working on a guitar amp, I did most tests at 400Hz, then switched to 80Hz and 8KHz to see the band-edge trend. Only when it "basically worked" did I bother with a full sweep.

> 1kHz I adjusted the level so the VU read "0"
> 400Hz an 10kHz it stayed at "0"
> 15kHz it was up to +1
> 40Hz was down to about -4

That's astonishingly flat for a reverb transformer. (Doug is right: check your sig-gen against your meter; either one may have errors this large.)

> perhaps it would be better with a 12AT7 or even an 12AX7?...

Bass response will be better with a -lower- impedance tube. The original may have used 12AT7 and fallen an octave or two short of what you get with 12AU7.

> primary/secondary I now get 9:1

Yeah, that is a non-optimum load for 12AU7. Low distortion with low output. Still you can work 300V and 6mA, get well over a half Watt or +27dBm... well, the 300 ohm DCR cuts that down, but far over +20dBm which is plenty for an SE line-amp.

This does not replace "professional broadcast studio" iron. It IS a cheap table-radio transformer wound to different impedance, and apparently wound cheaply (the 300 DCR suggests they use the same gauge for secondary as primary).

The droop at 40Hz will come up with a low-ressistance (high current) triode, or with NFB from plate to an earlier point. Many but not all old mike-amps had the NFB. Today this is a problem because we like to put a gain-pot between stages to control our too-high levels.

The "+1dB at 15KHz" needs more study. If you only have to be flat to 15KHz, it is easy to find a way to droop 15KHz elsewhere. However if that +1dB @ 15KHz becomes a +10dB narrow resonance at 33KHz, every cymbal hit throws subsequent amplifiers into a tizzzy. A R-C network like 300r+5nFd may tame it, or may just move it lower. Here is where you probably do need a full sweep or even noise and an FFT, to find best compromise values.