Help on tube preamp

[3nity]

Hello,.blessed new year to all.
I'm helping a friend on this tube mixer it's a Philips EL6405..he wants to convert to mic preamp s however I wonder whats the part labeled as 1, 2 and 3 are!?
And another question what you think about the preamp design?

 

[dmp]

sockets for input transformers?

 

[3nity]

That's what I think but there's no info about input transformers. I haven't seen the uni yetm

 

[user 130681]

They are sockets for matching transformers or a selector module. Very common in the past, when balanced line amplifiers were very expensive, (they required two pre amp stages to null the signal). Didn't have op amps back in those days.
Socket 3 is for an instrument; electric guitar etc.
Socket 4 has no pre amp, line level input.

A balanced signal from a twisted pair feed connected to the input of the low impedance transformer, pins 1 & 2 and the secondary winding supplied voltage between the ground wire, pin 3.
From memory, this is just after compressed carbon granule microphones and the early days of moving coil microphones.

 

[rock soderstrom]

Here in this photo you can see the IPTs socket in noval tube sockets. Note the description on the power transformer for optional versions of the IPT you can plug in.


Edit: It looks to me like you can configure the power mixer for high impedance or professional low impedance microphones with an optional transformer. The parts shown in the photo are probably the connectors that replace the transformer for use with high impedance microphones.

 

[ruffrecords]

Circuit design is a little odd. The 47K in series with the grid of the EF86 will worsen the noise considerably.

Cheers

Ian

 

[3nity]

Thanks guys for all your answers.
Will definitely take a look at those transformers.

Ian I was having the same thoughts on those resistors will indeed have to mod it in order to have a nice preamp.

 

[user 130681]

The reason there is a 47k is purely as an antiparasitic device. With a 10M Ohm grid leak, I doubt it will increase any sort of noise floor, on the contrary.

 

[ruffrecords]

The reason there is a 47k is purely as an antiparasitic device. With a 10M Ohm grid leak, I doubt it will increase any sort of noise floor, on the contrary.

I understand why it is there but unfortunately, unlike the 10M grid leak, the 47K is in series with the input signal and will therefore add significantly to the noise. The EF86 is strapped as a triode which will be quieter than if it were used as a pentode so it seems a shame to throw away that advantage with a huge value grid stopper.

Cheers

Ian