Helios + Pultec tube EQ rack prototype.

I have just finished building a prototype 3U rack containing a modified tube Pultec EQ (like an EQP1-A with an added overlapping mid dip) and a tube version of the Helios Type 69EQ. Here are a few pics.












The two vertical slots at the front are where I will; later fit an LED VU.

Cheers

Ian

impressive work !
x2 EQs ??

edit: i thought the helios had a mic pre...

Awesome  8)

Is 'Hleios' in the topic title a typo or irony? Sorry 

;D ;D ;D

isophase said:

edit: i thought the helios had a mic pre...

Click to expand...

It did. This is just the EQ section.

Cheers

Ian

Holger said:

Is 'Hleios' in the topic title a typo or irony? Sorry  

Click to expand...

Unfortunately not irony. You would have thought after 60 years my brain would have slowed down to the speed of my fingers.

Cheers

Ian

Hmm...mid cut, how?  Gain makeup stage, with a cut pcb...what is it? Danbury mains transformer, stock item for HT? Different brand transformer for 6.3 V. Lots of knobs on the Pultec side..Q, boost and?
Nice project, tell us more
J

looks good man. So it's one pultec and one hleios in the same box? 

mitsos said:

looks good man. So it's one pultec and one hleios in the same box?   

Click to expand...

That's right.

Cheers

ian

Joechris said:

Hmm...mid cut, how?  Gain makeup stage, with a cut pcb...what is it? Danbury mains transformer, stock item for HT? Different brand transformer for 6.3 V. Lots of knobs on the Pultec side..Q, boost and?
Nice project, tell us more

Click to expand...

The Danbury mains transformer also has a 6.3VAC  heater winding. The other transformer is for the (not yet implemented) 12V supply for the LED VU meters.

The Pultec EQ starts with the EQP1-A but modified to work with a 10K bridging input transformer rather than the 600 ohm one of the original. The low end frequencies have been extended up to include the lower ones of mid dip section of the MEQ5. Similarly the high end frequencies have been extended downwards to include higher ones of the MEQ5 mid dip section. Then I have added what is essentially an MEQ5 mid dip section, the idea being to extend the Pultec simultaneous peak/dip capability across the entire audio spectrum(see pic)



The Helios is much like the original type 69 but with extension similar to the arctic version i.e. and extra mid peak/trough at 16Khz and a low boost at 30Hz. So far as I know this is the first tube Type 69 (see pic)



Cheers

Ian

Great work, love it!

I have always dreamed of adding a mid cut to my pultecs, did you slap this in series with the standard EQP1 filter circuit before the interstage transformer?
how much make up gain do you need for the final amp stage?

martthie_08 said:

Great work, love it!

I have always dreamed of adding a mid cut to my pultecs, did you slap this in series with the standard EQP1 filter circuit before the interstage transformer?
how much make up gain do you need for the final amp stage?

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Yes and no.My EQP1 circuit is non standard in that it operates with a 10K:10K input bridging transformer instead of the Pultec 600:600 one but other than that the basic topology of the EQP1 part is standard. The mid cut follows the EQP1 part and then to the gain make up. A total of just under 26dB of gain make up is required.

Cheers

Ian

Thanks. Ok, so you have made the the hi cut pot larger than the stock 1k, and recalculated all other values upwards so it will not load the source more than 10K?
J

Nice work Ian - looks great!

Yes and no.My EQP1 circuit is non standard in that it operates with a 10K:10K input bridging transformer instead of the Pultec 600:600 one but other than that the basic topology of the EQP1 part is standard. The mid cut follows the EQP1 part and then to the gain make up. A total of just under 26dB of gain make up is required.

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That's very interesting - I didn't know that could be done with that high a source impedance and still maintain the same relative boost amounts as the original EQP1a.  Do you plan on sharing the schematic for this one?

lassoharp said:

That's very interesting - I didn't know that could be done with that high a source impedance and still maintain the same relative boost amounts as the original EQP1a.  Do you plan on sharing the schematic for this one?

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It still requires a relatively low source impedance because that same impedance, reflected into the secondary of the input transformer, is still a factor on determining the Q of the Hi boost circuit, but my circuit does not load the source anywhere as heavily as the original Pultec. As I am sure you know there are plenty of tube circuits with a low output impedance that will happily drive 10K but struggle with 1K.

Yes, I am happy to share the circuit. I'll scan it and post it in a separate post.

Cheers

Ian

Circuit of modified Pultec EQ as promised:



Cheers

Ian

Thanks Ian!    Look forward to building up a version on the bench. 

Would the output be happy going into say, a 500K grid resistor of a tube make up stage?   

lassoharp said:

Thanks Ian!    Look forward to building up a version on the bench. 

Would the output be happy going into say, a 500K grid resistor of a tube make up stage?     

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It currently goes into 1Meg but 500K should be OK. By the way, the inductors are by Carnhill. L1 is the VTB9042 and L2 is the VTB9050. You will notice the boost and cut values are not quite as high as in the original EQP1 but that is one of the small compromises necessary to manage the total insertion loss. The input transformer is a Sowter 3575.

Cheers

Ian

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?