Frank's Pultec Support

[mitsos]

hey briomusic, ever fix your filter? I thought I saw a schemo here for this, but maybe it was someone else (I think a guy named Steffen had one also?)... not sure anyway.

I am going to etch some PCBs, but I'm confused about the versions. I have files marked "V1" and "V1.4(untested)" ... There seem to be differences, it's a bit hard to see without a schematic.... What PCB are people using and is it working OK? 

thanks!

 

[briomusic]

hi mitsos,
yes i did get frank's filterboards to work with the gyraf tube amp stage.
it needs quite a few modifications but they can all be done with offboard wiring and without cutting any traces. the following is my corrected wiring scheme. I have no idea how the filterboard interfaces with frank's op amp stage, but with the tube amp it works like this and not as described in the original documentation - at least for me.

cheers briomusic

 

[mitsos]

wow, that's quite a few mods. Thanks for posting that.  Good job getting that sorted.  So you have V1.4..  Did you modify the circuit to match some other pultec schemo floating around? 
I'm thinking to just etch V1 at this point...

Anyone have v1.4 working stock?

 

[creal]

wow very good topic for pulec builder.

I have order the compement for the EQP & the MEQ5.
So i looking for more informations aubout the MEQ5, if you have any info to share

Thx Cyril

 

[briomusic]

@mitsos: yes. basically if you look at my post before, it's now like the gyraf schematic/topology but without the bypass option and with the additional 'enhanced' frequencies. if you're going to self etch, at least change the layout so the groundpanes are connected at the top. maybe even find a way to incorporate the two additional resistors, it's not nice having them hanging in the wiring. that being said, the thing sounds absolutely great with plain old wima caps and ioaudio's custom inductor.

@creal: sorry, haven't done the meq5, so I can't comment on that one.

 

[mitsos]

Cool, man, thanks for the info!  Good job getting it done.. enjoy!

 

[mitsos]

Hey Briomusic and everyone..

FWIW, the V1 PCB (Frank's original enhanced one, not the V1.4 which is the one that was fabbed/sold) has those resistors in place.  I traced out the schematic and it looks a bit different from the gyraf schematic, but not much that would make a diffference (I think)... it looks kind of like Steffen's pultec schematic.

It would lead me to believe that those mods might be necessary for all the different make-up amps?

Anyway, I am (still) waiting for my etch file transparency from the printers, had a holiday this week and no one wants to work...  :

 

[creal]

Hi all,

Did you think's we can replace the NE5532 make-up amps by a 2520 DOA make-up amps?

 

[mitsos]

Of course, that's what the original solid state pultecs were. Check the EQ meta, there are pics of a solidstate one by DrPat with original parts.

HS56 -> filter ->1uf cap -> 2520 ->HS56

 

[creal]

Thx mitsos, i found the post
http://www.groupdiy.com/index.php?topic=26322

and another by khstudio
http://www.groupdiy.com/index.php?topic=26790.120

That's exacly what i like for my meq5
But i can't find the SS Pultec circuit (Draw by PAT).

i like to use the APP2050 or APP2050E to replace the 2520. What did you think about this DOA?
http://www.groupdiy.com/index.php?topic=33576.0

 

[mitsos]

Creal, I don't personally know either of those opamps. Maybe ask PPA?

I finally got my board etched, populated and hooked up. I used a 600:600 trafo on input and 150:600 on output. With it on and everything set to 0 (flat) I get a flat freq curve from 20-20K. Excellent. But then things get weird. The attens seem to work fine, but the boosts are weird. I haven't done extensive testing, but for example the low boost works OK at a slight boost (maybe up to 2dB according to fuzzmeasure), but if you go to the middle or max boost it freaks out.. see pdf attached, the graphs are slight boost(up to 2dB or so), medium boost (after passing 2dB or so), and max boost, all at 20Hz.    

The high boost works somewhat, but at the cusp of the boost it turns down? Can this have to do with my handwound toroid inductor?

I used frank's V1 board (which has the above-named resistors on it which the V1.4 apparently does not).  I traced out the schemo and it looks similar to the gyraf just rearranged a bit, kind of like Steffen's. I can try to clean it up and scan it if that helps.

Any idea what to look for here?

thanks!

 

[mitsos]

here is the graph for max boost @ 1KHz. Just like the low boost, the curve looks fine until about 2dB boost, after that it turns into the one in the PDF.

 

[mitsos]

OK, I swapped the inductor for a pot core audio one and got the same results on the low boost. I can't check all the high boost frequencies since the inductor is missing the 312mH tap, but it still gives me that inverted peak at the 128mH (highest mH tap) that the other inductor gave me... SO... maybe this rules out the inductor (or they both suck  :  )....

Anyone have an idea? I will post a traced-out schemo of the filter PCB soon, it doesn't look too much different except maybe the low cut part... Anyway, will get that up in a bit.

thanks!

 

[mitsos]

OK, I  THINK I was overdriving the input of the interface.... I don't know what signal level comes out of Fuzzmeasure, there is no control/indicator that I could find, but I think with the boosting going on the returning signal have been over-driving the input of the interface.  After I put a pad on the input of the interface, I get a normal-looking graph for both boosts, no downward dip.

So it could be that everything is working, we'll see with a couple more days of tests, but I'm a bit relieved.

cheers! Where are the rest of the enhanced pultec-ers? 

 

[helterbelter]

Hi,
Well, I'm working on mine. For some reason only the mid band is working correctly....  Maybe your mod will make a change, I'll try tomorrow. I already have built a PTP gyraf type one with a discreet Dannertype solid state amp, and that one works fine so I haven't got a clue why my Frank's isn't working. All soldering is done right.

And by the way, I'm not sure what amp I'll use. Maybe an SSL LM394 type amp, maybe Siemens V274's, or maybe a pair of reliqiuas without the input trannies.

 

[creal]

And by the way, I'm not sure what amp I'll use. Maybe an SSL LM394 type amp, maybe Siemens V274's, or maybe a pair of reliqiuas without the input trannies.

hi all

i think to use his make up for my MEQ5.
http://i164.photobucket.com/albums/u14/Garbagelarge/API_325.jpg
It's maybe the most close to the original pultec.
I don't know if it needs mods.

 

[benlindell]

OK, I  THINK I was overdriving the input of the interface.... I don't know what signal level comes out of Fuzzmeasure, there is no control/indicator that I could find, but I think with the boosting going on the returning signal have been over-driving the input of the interface.  After I put a pad on the input of the interface, I get a normal-looking graph for both boosts, no downward dip.

So it could be that everything is working, we'll see with a couple more days of tests, but I'm a bit relieved.

cheers! Where are the rest of the enhanced pultec-ers? 

Totally what those graphs show, took me forever to figure that out too, thought all my EQs were busted, till I turned down my output. What interface are you using? I believe you can turn down the amplitude in the sweep settings as well.

 

[benlindell]

I'm comparing the V1 and V1.5 Wiring and Components Diagrams and there a ton of differences:

the extra two 1K or 10K resistors near the filter output
the the 0mh connection goes to a different leg of the HF boost
the HF bandwidth connects the opposite leg to the High freq Selector pad A.

Is the V1.4 board and V1.5 Wiring guide working for anyone using pots?

I feel like these changes would make the filter circuit way way different between the 2 versions.

 

[benlindell]

I got my first channel fired up tonight with my stepped switches.

I'm getting some boost both Lows and High, but I have a whole host of problems. When I turn them all the way down I get no signal, which looks almost like what's supposed to happen the signal has a straight path to ground. Did I screw something up by using stepped switches instead of pots? A whole bunch of other problems like the bass boost frequency not moving, both boosts adding more gain to the entire signal than just their frequency, the bypass is off by +6db.... Gonna take this one step at a time.

[EDIT: this was supposed to go before the one where I pointed out the differences between the two versions. I think I'm getting the same problem briomusic was getting, has anyone else had to make those mods to get the V1.4 boards to work?]

 

[benlindell]

Brio,

Brilliant work. Following your modifications made mine work perfectly!!

Thanks for sharing that diagram.

hi mitsos,
yes i did get frank's filterboards to work with the gyraf tube amp stage.
it needs quite a few modifications but they can all be done with offboard wiring and without cutting any traces. the following is my corrected wiring scheme. I have no idea how the filterboard interfaces with frank's op amp stage, but with the tube amp it works like this and not as described in the original documentation - at least for me.

cheers briomusic