Altec 1556a clone - Hiss and Hum!

[Atarijetman]

I've just assembled an altec 1556a clone but im having dreadful problems with Hum and a rather loud hissing....

Ive tried implementing D/C filaments but to no real improvement...

Im using an EDCOR 600/15K input tranny which I realise is unshielded. I have a crappy mic with an on/off switch, and i notice when I turn the mic ON the hum problem almost doubles. Does this suggest the imput transformer isnt suitable?

There might be a few issues at stake here, but if anyone can suggest something I'm all ears!

 

[joaquins]

Take the input transformer and use a resistance instead, no input signal, I think 10k or 15k is a proper value, depending on the transformer used, for yours secondary is 15k with 600Ω mic, so for standard 150Ω mic 3k9 would be a good reference for EIN, which is quite low for that preamp, 1:4 tx is more suitable for a 312 mic pre for example, for a tube preamp you should look closer to 1:10 or maybe a little more sometimes. In any case this is a easy and good way of knowing if the noise is coming from the transformer or not, hiss should be the same, hum eliminated. To lower hiss, if it's not coming from any other part of the circuit, as bad caps or resistors, depending on what you are calling hiss, but with a higher ratio transformer should be lower. If you are going to keep this transformer or any other unshielded you can shield them with cooper or aluminum sheet as a first approach, with iron plumbing as a better way or with mumetal sheet which could be get on ebay for example as the ultimate option, is quite expensive but is the correct material for the task, iron is quite good since a 1.2mm square iron tube will make some decent magnetic shielding which won't be done by cooper or aluminum, try it out, sure you have some scraps near to try it out building a nasty box to insert the transformer in, if it works better you can go for a better one and paint it down.

JS

 

[ruffrecords]

How does the hum vary with the gain control setting?

Cheers

ian

 

[Bill Wilson]

Better go with a decent quality mike input transformer that iswell shielded. The input impedance should be 200 ohms to 600 ohms. The secondary impedance should be in the neighborhood of 40-50k ohms for a tube input stage. The gain of the mike preamp should be reduced by changing the cathode resistor to 2.2k ohms or 2.7k, as Altec always designed the first stage of a tube mike sre with too much gain. This introduced unwantedhiss & other tube noise.

 

[lassoharp]

Ditto what Bill said.  You will likely get nothing but grief trying to use that transformer for a mic input.

If too much gain try 12au7 or 12ay7 and scaling down the plate and bias resistors.