rainton's True To The Original PULTEC EQP1-A chassis kit discussion thread

[hugo]

Well, the original units used carbon comps in all positions except for the 470R 2W I think.
I did the same with my unit.
I haven't tried to build it with metal film to compare it.

I did an experiment like that many years ago with 1176s though - I built one with metal film in the signal path and one with carbon comps, ran the same signal through both units, recorded it and had several people listen to both files in a blind test.
And even though the majority preferred the carbon comp one for a little bit warmer sound I think they sounded very close.
But it might very well be the "out of tolerance" factor

In regards to using tropical fish caps - I don't know if there's different kinds of those caps. The ones I found are Polyester film caps which I wouldn't use in a passive EQ circuit. I prefer polystyrene or polypropylene.
There's a very interesting article by Chris Preston @ vintagewindings.com about filter caps for the Pultec. I posted a link to it in my White Market Thread.

But if you have the time I would always suggest to not close and pot the filter units before you tried various different kinds of caps for each Frequency position.
I did so with both - my EQP-1A and my MEQ-5 - always measured the peak frequency and listened to how it sounds and chose the one I liked best.
I spent a lot of time doing that and it was very interesting!
Different kind of caps sometimes sounded VERY different - even though they measured  almost the same when measuring  them for capacitance with a DMM.
From "oh that's rather harsh" to goosebumps  ;D
And I tried all kinds of caps: PIO, polyester film, polypropylene, polystyrene, metallized paper, etc...
Most of them vintage ones or NOS - I guess I thought if i want it to sound like a vintage unit it makes more sense to use components from that era (as I did throughout my units)

Anyway it's really worth trying it for yourself, because only then you will know for sure
I ended up using mostly vintage Good-All polystyrene caps pulled from a Hammond organ (most of them were still very close to spec) for the filter unit, because I really liked their sound, but you might find others to sound much better to your ears. That's the beauty about DIYing one of these gems - you can create YOUR Pultec and select caps for YOUR sound

Martin

Hi Martin,
I looked inside some real pultecs and saw the 470R carbon composition.. So i am very curious about this 470R 2W metal.. I went with cc resistors all the way. One other thing.. The BOM needs a 20pf capacitor ( i took it in ceramic, but maybe a bad choice, would like to have your advise..and the Layout needs an update with a wire between HS-29 pin5 and 12AX7 pin 2

Regards,
Hugo

 

[rainton]

Hi Martin,
I looked inside some real pultecs and saw the 470R carbon composition.. So i am very curious about this 470R 2W metal.. I went with cc resistors all the way. One other thing.. The BOM needs a 20pf capacitor ( i took it in ceramic, but maybe a bad choice, would like to have your advise..and the Layout needs an update with a wire between HS-29 pin5 and 12AX7 pin 2

Regards,
Hugo

Hey Hugo - thanks for the feedback - I just updated the layout to rev. 1.30 - make sure you use that one:
http://www.mediafire.com/file/by1pyjuymzs3ujd/Pultec_EQP-1A__layout_rev1_30.pdf

The 20pf is a ceramic cap in the original units as well - but mostly an ERIE branded tubular ceramic cap - very hard to find...

 

[hugo]

2 units matched completed.. I am waiting for rainton's next project. Maybe a good tube preamp ? ^^

 

[rainton]

2 units matched completed.. I am waiting for rainton's next project. Maybe a good tube preamp ? ^^

man I'm sorry that connections was there in all earlier revision - I must have erased it by mistake when I made the changes to V1.30...

going to fix it
Martin

 

[rainton]

UPDATED: latest revision of the layout rev 1.31:

http://www.mediafire.com/file/2b8py0igkenkh7l/Pultec_EQP-1A__layout_rev1_31.pdf

There was even another connection that got lost on the way - the in output wasn't connected to chassis ground anymore in rev 1.30

I hope it's all good now - otherwise please report back here

And make sure to use rev. 1.31 ONLY from now on
Thanks Hugo
Martin

 

[hugo]

Stereo !

 

[rainton]

Stereo !

I LIKE!!  8)

 

[salomonander]

i was reading that paper by chris again... the section where he disserts the original filter unit is puzzling to me. the way he did his re-drawing of the original filter caps makes no sense at all considering the "known" pultec caps.... or am i not getting something here?
the values do not even seem to be in the balllpark of any pultec schematic i have seen so far....

 

[mike]

Just placed an order for 2 EQP-1A cases!

A couple of questions.

What did you guys use for the on/off rotary and the original style bypass switch?

And which ceramic switches did you use for the freq selectors? There's a lot of options but I only know about the more expensive Elma ones, I have no idea what cheaper ones may be good and which ones are garbage. I assume non-shorting is the option I want for this project?

 

[kosi]

You get the original on off switch from Tedss, it's an ARROW-H&H

http://tedss.com/Catalog/Browse?searchString=ARROW-H%26H&inCategory=ROTARY%20SWITCH%20%3E%20FULL%20SIZE

there are also other switches, that work

Bypass switches are Switchcraft Telever 16xxxx series  DPDT

Ceramic switches are russian and ucrainian ebay ones, most of them are good, but I also had 2 bad ones.
They are sometimes very strong, so you have to bow the metal. that holds the position, back.
If they work, they work very good, the feeling is solid good. Like a heavy cardor. Grayhill work too. Much "finer" feeling.

1 Switch 2x7 High Frequency
ceramic rotary switch (2x11 reduced to 7 pos.)
1 Switch 2x4 Low Frequency
ceramic rotary switch (2x5 reduced to 4 pos.)
1 Switch 1x3 Atten Sel
ceramic rotary switch (3x3pos.)

If you like, come over to our facebook group:
https://www.facebook.com/groups/1615927638648181/

 

[mike]

Thanks for the links! I just made a tedss order before i saw this, haha
Still have more stuff i need to order from them though so i'll throw this on my list

that FB link goes to a page unavailable message

looks like maybe centralab were the original switches. I found a few but they are expensive.

 

[mike]

I got the PEC pots and the don-audio knobs - is it normal for the knob marking to go a few mm (like 4-5mm maybe) past 0 and 10? i.e. the pot rotation is wider than the panel markings.

I got the RV4NAYSD series pots but it looks like standard 24mm alpha pots would do the same thing.

 

[rainton]

I got the PEC pots and the don-audio knobs - is it normal for the knob marking to go a few mm (like 4-5mm maybe) past 0 and 10? i.e. the pot rotation is wider than the panel markings.

I got the RV4NAYSD series pots but it looks like standard 24mm alpha pots would do the same thing.

Yes it is -  was exactly the same with vintage EQP-1As

 

[mike]

Perfect, thank you! 

 

[Flowjow]

Hi all,

I'm really thinking of building a pair of those replicas. Many thanks to Martin and all the contributors for sharing such a hard, time consuming, accurate, well documented, clever and beautiful work!

Ideally, I would really like to have such beasts in 2U racks, so they would look like EQP-1A3. Really impressed by the finish and the conformity to the original of these chassis (allowing an original layout) I was wondering if it would be easy to use your MEQ-5 chassis to build EQP-1A3s (EQP-1A in 2U racks).

Martin, do you think it's an easy job? Do you see any more obstacles than a few drills to add the interstage transformer? Will the pre-assembled filterbox from vintage windings fit in place of the one provided with your chassis? Would you sell 2 chassis without the MEQ-5 faceplates?

Thanks again!

 

[rainton]

Martin, do you think it's an easy job? Do you see any more obstacles than a few drills to add the interstage transformer?

Thanks again!

Yes when rack space is at a premium it can make sense to go for the EQP-1A3 layout, even though I feel the original EQP-1A layout is more iconic

Anyway, I think it can work to build an EQP-1A into an MEQ-5 chassis.
If you check the pics of my prototype in my WM thread you can see there's two can caps to the right of the filter unit:

https://groupdiy.com/index.php?topic=63423.0

As far as I remember only the cutout for the upper can cap made it to the production run. So you could either use the space below the upper can cap to drill a hole for the interstage transformer, or you could omit the upper can cap and use that cutout for the interstage.
The original EQP-1A had two can caps to the left of the filter unit with 2x40uf ea. and on the right side there was a 2x50uf can cap.
If you chose to use the cutout on the right hand side for the transformer, you could use one 4x40uf plus one 2x50uf on the left

Will the pre-assembled filterbox from vintage windings fit in place of the one provided with your chassis?

No, it won't, since the dimensions of the vintage-windings filter box are not made to original specs.
Also Chris with Vintagewindings put some of the resistors that are supposed to be soldered straight to the rotary switches inside the box too, which was meant to make it easier, but is actually more confusing if you want to follow the original wiring layout.
Apart from that another forum member found out that the vintagewindings inductor tends to ring in very high frequencies and many other builders measured the same. Unfortunately Chris has responded to neither of us regarding that issue.
Forum member AusTex64 found out that the inductors sold by audiomaintenance.com are very close to the original ones and don't show that ringing - and they're also much cheaper.

Therefore - at this point I would suggest to get the inductors from audiomaintenance, the filter caps of your choice and put it all into the filter box as per the graphic layout also available in my WM thread. This also gives you the option to tune everything to your liking

Would you sell 2 chassis without the MEQ-5 faceplates?

No, unfortunately I can't do that, since all the kits are already packed up at don-audio's warehouse who takes care of shipping & handling for me.

But I'm 200% sure that if you offer the faceplates here on the forum they'll be gone in no time.
I had some spare panels left over from the production run I offered here and they were all gone within a day

hope that helps
Martin

 

[Flowjow]

I feel the original EQP-1A layout is more iconic

Well, I might be a little bit of an iconoclast! Space is definitely an issue for me. I'd like to take them with me in different studios.

The original EQP-1A had two can caps to the left of the filter unit with 2x40uf ea. and on the right side there was a 2x50uf can cap.
If you chose to use the cutout on the right hand side for the transformer, you could use one 4x40uf plus one 2x50uf on the left

That's really smart! I'll probably go  for that solution and group the 4x40uf into one can cap.

No, unfortunately I can't do that, since all the kits are already packed up at don-audio's warehouse who takes care of shipping & handling for me. But I'm 200% sure that if you offer the faceplates here on the forum they'll be gone in no time.

That still sounds cool to me. I'll PM you for order as soon as I get my BOM sourced and my budget done.

Thanks a lot Martin!

 

[jefepeters]

Are there any photos or documentation on now the filter units should be put together? I've never seen an original and the only images I've found are here:

http://www.studio-techservices.com/Trouble_Sooting_Tips/Trouble_Shooting_Gudies/Pultec_Filter_fix.html

Why were the originals encased in wax? And how are all of you doing this build putting them together?

Thanks,
Justin

 

[kosi]

e.g.

 

[hugo]

Are there any photos or documentation on now the filter units should be put together? I've never seen an original and the only images I've found are here:

http://www.studio-techservices.com/Trouble_Sooting_Tips/Trouble_Shooting_Gudies/Pultec_Filter_fix.html

Why were the originals encased in wax? And how are all of you doing this build putting them together?

Thanks,
Justin

Put foam inside, just as a clean package for shipping..it allows you to maintain without all those wax problems, and to ensure your caps are protected..Wax is not a right option IMO,  you are never sure that caps are not "killed" by heatness...

Hugo