610B preamp build with EQ

[Spencerleehorton]

Hi All,

Wanting now to see if I could build either a channel or a pair of 610B tube preamps with EQ.
I have a 610A schematic with the low and Hi cut and boost but not the 610B EQ section which I’m assuming would add into the circuit in a similar fashion?
Could anyone shed any light on the fabled 610B EQ section?

 

[Spencerleehorton]

Should be able to knock up both input and output transformers.
What’s the best way to do the -15db pad?
Phase switch is no problem and could have that after the output transformers.
EQ selection of 70hz, 100hz and 200hz with +/- 9db on 3 way switch
Along with 4.5k, 7k, 10k with +/- 9db
Don’t know if the original unit had the +/- 12db output ? But I can put a output pot as master.

Is there any similar schematics with the same style EQ section?

 

[Spencerleehorton]

Is the EQ section inductor based?

 

[C.B. - Boudio]

I'm probably wrong, but the 610-B seems to exist only as a plugin?

 

[Spencerleehorton]

Here it is as the preamp part in the modern version.
Not sure if the original had all these bits?

 

[Spencerleehorton]

Here you see inside the 610 solo unit.

 

[C.B. - Boudio]

Maybe you could try a normal shelving feedback EQ, but the different frequencies will require (too?) much switching it seems.

15dB pad is, of course, the easiest after the OT, or in-between stages, but overloading of the first stages could be a logical issue then.

 

[Spencerleehorton]

Maybe one of these?

 

[C.B. - Boudio]

Probably not, because the 610A was an active EQ, so I would suspect the B is too. A Pultec is passive, so it needs makeup gain afterwards.

 

[ruffrecords]

I wonder if this is some kind of variant of the 508A plug in equaliser that was designed to work with the 1008A mic pre and possibly also with the 100D?

Do you know if the boost/cut controls on the 610B are rotary switches or pots

Cheers

Ian

 

[Spencerleehorton]

I wonder if this is some kind of variant of the 508A plug in equaliser that was designed to work with the 1008A mic pre and possibly also with the 100D?

Do you know if the boost/cut controls on the 610B are rotary switches or pots

Cheers

Ian

Yes the boost cut is rotary pot set at centre for 9 boost and 9 cut

 

[Spencerleehorton]

View attachment 148064
Here it is as the preamp part in the modern version.
Not sure if the original had all these bits?

Ian here is a picture

 

[Spencerleehorton]

some discussion on it:
https://groupdiy.com/threads/1108-508eq.51627/

the 508A looks like it does 3db gain per switch to +9 -6 rather than the 610b which does 1.5db per click and +9/-9.
but i suppose it could be worked out, also the 508A doesnt seem to have the same frequencies?

 

[Spencerleehorton]

here is the 1008 and you can see the EQ inserts in pins 1 and 3.

 

[ruffrecords]

Yes, I have seen the 1008 schematic bit not the 508 one. It is as I suspected, it is much easier to do different things for cut and boost if you use switches because you can have 2 gangs (as in the low frequency section). But it is much harder to do this with just a couple of pots. Getting boost OR cut on a pot is no problem but getting both on one pot is the hard part.

Cheers

Ian

 

[Spencerleehorton]

Yes, I have seen the 1008 schematic bit not the 508 one. It is as I suspected, it is much easier to do different things for cut and boost if you use switches because you can have 2 gangs (as in the low frequency section). But it is much harder to do this with just a couple of pots. Getting boost OR cut on a pot is no problem but getting both on one pot is the hard part.

Cheers

Ian

sorry, just came off night shift, its switched pots, so very probably multi gang.

 

[ruffrecords]

sorry, just came off night shift, its switched pots, so very probably multi gang.

Ah, that makes it somewhat easier. It is probably similar to the 508 but with switched frequencies and a wider boost cut range.

Cheers

Ian