Preamp for reel-to-reel

[studeb]

You mean BK 875B? A hand-held one? It could be just not good enough to measure the heads. I am using the Racal-Dana 9343M and that is barely OK for that purpose. The best meter I know of to measure heads (and I had a chance to use it) is the HP4192A as it allows to measure coils at a very low AC voltage (down to 10mV) , avoiding the saturation, and at any frequency from 5Hz to 13MHz, both avoiding and finding resonances.

That, at least with a good meter, should not be a problem, I do regularly measure heads together with connecting cables and it usually gives me only a small (within few percent) error.

Cheers

Alex

Hi Alex,

Yes, BK not B&K my mistake. I can never get those two sorted out.
It may well not be up to the task, but its all i have. I will mess around with them again tonight, try a fresh battery. I am trying to find the service manual, i could at least check the freq and see if that is off.

 

[studeb]

That meter is out, i measure 990Hz at the terminals.
I swapped the leads to another meter, and the resistance dropped from 382 to 218 ohms.
A more believable number.
I thought i had the service manual for it, but i cannot find it.
So this experiment will have to wait a while longer, but we have Alex's numbers to use.

 

[stellavox]

Most playback heads that I have measured are 100 to 200 mH for feeding SS and 300 to 600mh for feeding tubes. Ohmic readings are "in the ballpark" of the mH readings. Anything lower than 80mH or so is probably a record head although some manufacturers used very low impedance heads with stepup transformers.

If you have a SS preamp with a MM phono input that has RIAA ACTIVE feedback, try "disabling" one side of that network and substitute a simple series RC network for NAB/IEC. Start with 22 to 27nF as the cap and a 5K pot to tweak the highs. You may need an extra R of around 200K across the cap to dampen the LF headbumps. Keeping the 47K MM input resistor should be OK.

Cheers

Charles