Helios + Pultec tube EQ rack prototype.

[ruffrecords]

I thought I recognized a couple of those L tap values as Carnhills.  The max cut & boost values look plenty ample.  The MEQ 5 clone I just did is only designed for 10db of boost/cut and it covers most common situations, so yours is a step up from that and only a db or 2 here and there away from the EQP1A.


Is this the design that's going into your tube mixer?

The reason I built the rack is that I have two prospective clients, one of whom wants a tube mixer with the modified Pultec EQ, and the other who wants it with a tube based Helios Type 69 EQ. So, thinking I could kill two birds with one stone, I decided to build a rack mounted demo unit of both so they could play with them and I can also use it to demo EQ possibilities to other prospective clients. The rack unit goes in the post to client number one this week.

Of the two EQs, and having played with them quite a bit myself, I think I prefer the simpler Helios. The HF is great for adding clarity. The mid peak/trough is good for emphasis and the extra frequency at 16KHhz for air. The bass cut, though simple, is not shelving so it just goes on cutting forever and the bass boost being inductive based has a nice ring to it. I am not surprised the Helios EQ is considered very musical.

Cheers

Ian

 

[skal1]

looks very nice ian, can you tell us about what tube circuit you  are use to make up the gain loss , are you using the EQP1A out stage ?

skal1

 

[ruffrecords]

looks very nice ian, can you tell us about what tube circuit you  are use to make up the gain loss , are you using the EQP1A out stage ?

skal1

The gain make up stage is a 6CG7 based mu follower running at about 5mA which provides 26dB of gain. Output is via a Sowter type 5069 1:1 transformer. This combination will provide +22dBu into 10K at less than 0.1% harmonic distortion (almost entirely 2nd harmonic). It is not intended to drive a 600 ohm load.

Cheers

Ian

 

[tommypiper]

Ian, thanks for sharing.  What are the green PCBs you are using for PS and output amp?

 

[strangeandbouncy]

HI Ian,



  that looks wondrful. I have a few bits of Helios, and had been wondering about a tube makeup stage, as i have a partial channel, with all the eq intact. I am absolutely fascinated about your extra 16kHz air band. Would it be asking too much to  ask how your did it . . . .



      Kindest regards,


      ANdyP

 

[hobiesound]

the 16khz is probably a +/- 470pF capacitor in series with the inductor of the mid-band. At least that's what i'm using for my prototype. I'm using a carnhill vtb type for my mid-band can't remember the model name.

greetings,

Thomas

 

[ruffrecords]

Ian, thanks for sharing.  What are the green PCBs you are using for PS and output amp?

They are two PCBs I designed myself. The one with tubes on is one I designed a couple of years ago when I was planning on building tube mic pres in die cast boxes. The PCB is laid out for three identical mu followers with space for a Sowter mic transformer. It is designed to fit into the PCB mounting lsots of a die cast box. I just cut off two mu followers worth! here is a pic of the whole PCB:

The other one is a CRCRCRC board I designed as a general purpose HT board. It has the rectifiers, snubber cap, filter RCs plus a pot divider to give a 75V heater elevation voltage for the mu followers.

Cheers

Ian

 

[ruffrecords]

the 16khz is probably a +/- 470pF capacitor in series with the inductor of the mid-band. At least that's what i'm using for my prototype. I'm using a carnhill vtb type for my mid-band can't remember the model name.

Yes that's exactly it. I am using the Carnhill VTB9042 inductor.

Cheers

Ian

 

[Biasrocks]

Cool build Ian, I'd love to hear some sound clips if you get a chance.

Any chance you could share a schematic of your Helios EQ implementation, I'd like to build a couple of these guys and it would really help.

Mark

 

[ruffrecords]

Cool build Ian, I'd love to hear some sound clips if you get a chance.

The unit is on loan to a London recording studio right now and will be used on sessions over the next few weeks. I'll ask them if they can provide some sound clips after they have evaluated it but it will not be for a few weeks.

Any chance you could share a schematic of your Helios EQ implementation, I'd like to build a couple of these guys and it would really help.

The basic circuit is available all over the www. The HF cut/boost and LF cut is pretty much identical to these. (see pic).

The bits I had to design anew were the mid peak/trough and the bass boost using currently available inductors.

For the mid peak/trough I used a Carnhill VTB9042 inductor as follows:

f               L               C
700            1H              56nF
1K             1H              27nF
1K4            1H              12nF
2K             1H              6.2nF
2K8           0.6H             5.6nF
3K5           0.6H             3.3nF
4K5           0.3H             3.9nF
6K            0.2H             3.3nF
16K           0.1H             1nF

For the bass boost is used a Carnhill VTB 9043 as follows:

f               L               C
30             10H           2200nF
80             10H            390nF
120             7H            220nF
240             7H             56nF
400             7H             22nF

Cheers

Ian

 

[strangeandbouncy]

Hi Ian,





      Thanks for that info. Glad that my schemo helped - thats my writing on it. Yay . . . I have 2 working, and  in bits of that module.



    Kindest regards,


      ANdyP

 

[ruffrecords]

Thanks for that info. Glad that my schemo helped - thats my writing on it. Yay . . . I have 2 working, and  in bits of that module.

Hi Andy,

I did not know that was your schemo! I got it off the net, must be at least a couple of years, ago along with several other similar one but yours seems to me to be the most complete. If I have violated any copyright of yours then of course I'll gladly remove it.

Cheers

Ian

 

[TimS]

I'm curious: how effective did you find the Q control on the MEQ mid dip to be? I read in the ioaudio MEQ500 thread that the bandwidth change didn't have a noticeable effect and I was just wondering if you experienced the same results or not.

Thanks!

 

[desol]

It would be nice to build a couple channels of the Helios eq!

 

[G-Sun]

Awesome work Ian!
Thanks for sharing!

 

[ruffrecords]

It would be nice to build a couple channels of the Helios eq!

Well, now you can. This same Helios EQ is now available as a PCB as part of my EZTubeMixer project. Check out the HELIOS69 PCB on my white page:

http://www.groupdiy.com/index.php?topic=48986.0

Cheers

Ian

 

[ruffrecords]

I'm curious: how effective did you find the Q control on the MEQ mid dip to be? I read in the ioaudio MEQ500 thread that the bandwidth change didn't have a noticeable effect and I was just wondering if you experienced the same results or not.

Thanks!

I was never very happy with this version of the Pultec EQ and the guy testing it in a London studio definitely preferred the Helios.

This was my first attempt at my own version of a Pultec EQ and it was with some difficulty that I managed to combine parts of the EQP1A and the MEQ5. One of the compromises resulted in the loss of the famous Pultec bump at the LF end and the tacked on mid cut was never very satisfactory.

I then did the poor man's EQP1A which I am pleased to say was a bit of a runaway success, especially as it retained the Pultec bump. You can imagine that I was a little reluctant when someone asked if I could add a mid boost/cut to this EQ - I was concerned it might compromise the whole EQ. Anyway, to cut a long story short, after a lot of experimentation I came up with a simplified mid cut/boost which simultaneously varies Q and boost/cut which I am also pleased to say has been very well received.

So to answer your original question, no I did not find the Q control of the MEQ dip to be very effective but the new combined Q/boost/cut control seems to work pretty well.

Cheers

Ian