rainton

Hey guys,

I'm just about to do a Pultec EQP1-A  all p2p and as close to the original as possible.
Even though I already found most of the original switches, knobs, input tranny etc...
...I realized how much of an unobtanium the Triad HS29 and the Peerless S217D are nowadays.

I know that Sowter makes some trannys for use in a Pultec but I discovered that their HS29 replacement has a totally different impedance compared to the original.
5K/20k to 80K/20K  in the original

2.5K to 10K in the Sowter replacement??

Doesn't the difference in impedance make a difference in sound

And then I figured, why not rather use a Triad HS27 which has the same impedance as the HS29, but a ratio of 1:1.72 compared to 1:2 of the original?

Or - even easier available the Triad HS23? It has an impedance of 15K to 110K (but still closer than the Sowter replacement) and a ratio of 1 : 2.7 instead of 1:2

What would I need to change in the circuit if I wanted to use the HS23 instead of the HS29?
Having a ratio of 1 : 2.7 the signal would hit the amp circuit at a slightly higher level and I would need to compensate for that, right?
Any suggestions?
What about the difference in impedance?

Thanks for any advice!

Martin


Pultec interstage transformer
« Reply #1 on: May 19, 2015, 02:30:38 AM »
Any suggestions?
UTC A-16
UTC A-18 (slightly higher ratio)
Hammond 832
Any of the above would function fine.
Create an artificial center tap if necessary, adjust network across the interstage transformer secondary for optimum response.
« Last Edit: May 19, 2015, 02:33:40 AM by gridcurrent »

CJ

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #2 on: May 19, 2015, 03:07:33 AM »
that Sowter innerstage sounds just as good if not better than the Triad,

that MEQ-5 Pultec eq sounds very good also and it has no innerstage xfmr at all,

so that  innerstage  is not a critical component to the sound,

now that output transformer, with the nickel core, jus sayin...
If I can't fix it, I can fix it so nobody else can!
Frank's Tube Page: www.mif.pg.gda.pl/homepages/frank/vs.html
Guitar Amps: http://bmamps.com/Tech_sch.html

rainton

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #3 on: May 19, 2015, 05:36:01 AM »
Great information - thank you so much!!

I checked the Hammond 832 and it seems almost a perfect match, but the freq. response shows  only 50Hz to 15khz
compared to 20-20khz of the HS29

Isn't that a problem given the filter circuit can boost/cut frequencies from 20Hz to 20khz?

By adjusting the network you mean the values of the resistor & cap across the secondaries, right?
But what would the optimum response be? flat? What's the response of the original unit in bypass?

What issues would I be running into if I used a tranny with a larger ratio (e.g. 1: 2.7 like the HS23)?


Anyway good to know that the interstage is not really critical to the sound :)

But yes the output...well, I know it was brought up here in the past already, but all the threads kind of lead to nowhere.

What could I use here as an replacement to remain as close as possible to the 217d? I know the 217d has a tertiary winding and that's maybe what makes it difficult to find an appropriate replacement (except the Sowter), right?

I was even starting a thread recently calling out to you CJ and the rest of the transformer guys if there's really no way for you to make a replica, or at least one that is very close to the original, but it remained unheard...

http://groupdiy.com/index.php?topic=59395.msg754449#msg754449

Again big thanks for your input!


Pultec interstage
« Reply #4 on: May 19, 2015, 04:02:29 PM »
I checked the Hammond 832 and it seems almost a perfect match, but the freq. response shows  only 50Hz to 15khz
compared to 20-20khz of the HS29
Isn't that a problem given the filter circuit can boost/cut frequencies from 20Hz to 20khz?
Hammond transformers are conservatively rated.  I suspect but cannot confirm that the frequency response is power.
Quote
By adjusting the network you mean the values of the resistor & cap across the secondaries, right?
Yes.
Quote
But what would the optimum response be? flat?
Flat; unless one wishes to start a discussion of the "Gibb's Effect".
Quote
What's the response of the original unit in bypass?
measured from 600 ohms and into 600 ohms.
EQP-1a sn 6745
20 Hz:  -0.34 dB
1000 Hz:  0 dB
20 kHz:  +0.5 dB

EQP-1a sn 6138
20 Hz:  -0.45 dB
1000 Hz:  0 dB
20 kHz:  +0.5 dB

EQP-1a3 sn 8078
20 Hz:  -0.6 dB
1000 Hz:  0 dB
20 kHz:  +0.2 dB

EQP-1a3 sn 7509
20 Hz:  -0.55 dB
1000 Hz:  0 dB
20 kHz:  +0.55 dB

Quote
What issues would I be running into if I used a tranny with a larger ratio (e.g. 1: 2.7 like the HS23)?
In my opinion, greater sonic signature.
Quote
Anyway good to know that the interstage is not really critical to the sound :)
Compare your amplifier with the recovery amplifier such as the one found in the EQH or MEQ which needs no interstage transformer.  In my opinion, those simpler circuits have a more pleasing character despite being a bit down on maximum output compared to the EQP design.
Quote
What could I use here as an replacement to remain as close as possible to the 217d? I know the 217d has a tertiary winding and that's maybe what makes it difficult to find an appropriate replacement (except the Sowter), right?
I've used  the Sowter replacements and consider them superb.

rainton

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #5 on: May 20, 2015, 12:11:47 PM »
Wow!
Thanks gridcurrent!! I didn't expect such detailed information - absolutely awesome! :)

You wrote something about "creating an artificial center tap" - how can I do that if the secondary of the  transformer only consists of 1 coil as the UTC A16 or the Triad HS23 for example?


Hammond transformers are conservatively rated.  I suspect but cannot confirm that the frequency response is power.


So you think the Hammond could be used without running the risk of narrowing down the freq. response?
Because I checked and the Hammond appeared on ebay recently for next to nothing, so it could be really tempting.

On the other hand if I get the chance to grab one of the Triad HS-series that I can make fit,  for about the same price as the Sowter replacement, I would probably go the Triad route.

I got you about the Sowters though - I guess I will buy the OT from them ;)

One last question, since you seem to have a lot of knowledge on the Pultec:
I was considering to buy a hammond PT for the pultec, but I realized the Chicago or Triad PTs can still be had for relatively little money - though it's a little harder to find the exact replacement.

But i.e. I saw the Chicago PSC-66 on eBay - but it has 300V secondaries, so 600V CT instead of 270V/540V CT of the PCC55
Could I still use it if I adjusted the resistor after the 6x4 rectifier tube?

I also saw many people using replacements with 500V CT,  But also they would have to adjust the voltage after the tube, right?
And even some Triads with 500V CT can be had for very little money! Much less than the Hammond is over here!

I'm well aware that those Triads and Chicago PTs have 115V primaries, but I still have some pretty highend voltage converters sitting around unused here that I bought for some stuff I brought along from the States in the past.
So I figured if I could get a PT with the original looks for a couple of bucks I would rather get that one instead of spending like a 100 bucks for the hammond...

Again thank you so much for all your input!
You really help me lern a lot here!!
Martin

My3gger

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #6 on: May 20, 2015, 10:24:32 PM »
600V or 540V with CT is very little difference for a tube circuit like this one, reuse those nice transformers if possible. You might want to adjust that 470r resistor, looking at another thread original trafo seems to have more like 500V:
http://groupdiy.com/index.php?topic=29304.0
Don't forget to check if replacement transformer has enough current and correct heater tap. You could use pilot light similar to original and put led diode inside. It takes some time to get it right, but it is worth it because led uses so low U and I, i do this in my projects with great success.

rainton

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #7 on: May 21, 2015, 05:07:25 PM »
Thanks My3gger!

I did a little more research and found that the Triad-R7A PT used by the later EQP1-As indeed had 600V CT!  ;)
The Chicago PCC-55 offered 540V CT.

And the Chicago PSC-60 that I found also has 600V CT, but it's also a little bigger than the PCC55...
...anyway I'll find a solution ;)

But still it would be good to know about creating the "artificial" center tap on the interstage (e.g. UTC A16 or Triad HS-23) and if the Hammond 832 could be used as interstage without running the risk of narrowing down the freq. response?

Gridcurrent? Anyone?

I really appreciate your help!

Pultec interstage
« Reply #8 on: May 21, 2015, 05:23:57 PM »
But still it would be good to know about creating the "artificial" center tap on the interstage (e.g. UTC A16 or Triad HS-23) and if the Hammond 832 could be used as interstage without running the risk of narrowing down the freq. response?
While not pertaining specifically to a Pultec, here is a quick look of frequency response of a  compressor in which an A-16 was implemented in the front end, wired 1:2.  The A-16  secondary was terminated for "optimum" frequency response,
@ +4 dBu input:
20 Hz:   -0.5 dB
100 Hz:  -0.2 dB
1000 Hz:  0 dB
10 kHz:  -0.1 dB
20 kHz:  -0.6 dB
30 kHz:  -2.2 dB
« Last Edit: May 21, 2015, 05:30:32 PM by gridcurrent »

rainton

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #9 on: May 21, 2015, 05:36:30 PM »
But still it would be good to know about creating the "artificial" center tap on the interstage (e.g. UTC A16 or Triad HS-23) and if the Hammond 832 could be used as interstage without running the risk of narrowing down the freq. response?
While not pertaining specifically to a Pultec, here is a quick look of frequency response of a A-16 implemented in the front end of a special compressor, wired 1:2.  The secondary was terminated for "optimum" frequency response,
@ +4 dBu input:
20 Hz:   -0.5 dB
100 Hz:  -0.2 dB
1000 Hz:  0 dB
10 kHz:  -0.1 dB
20 kHz:  -0.6 dB
30 kHz:  -2.2 dB

Looks great, but "terminated" again means adjusted values of the cap and resistor between the secondaries and not really a center tap to ground as in the EQP1-a schematics, because the A16 secondaries only consist of one coil, right?
Any issues if not having a grounded center tap there?



rainton

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #10 on: May 23, 2015, 05:05:22 AM »
anyone?

shabtek

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #11 on: May 23, 2015, 07:58:01 PM »
The grids need a path to ground to bias: either center tap or resistors to ground
"really fine players do not use stomp boxes or master volume, they match the amp to the room and turn it up to 11.  Stevie Ray, BB King, Albert King, Duane Allman, Dicky Betts, Louis Armstrong"
   -CJ

rainton

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #12 on: May 23, 2015, 08:04:17 PM »
The grids need a path to ground to bias: either center tap or resistors to ground

ok got it!
You mean a matched pair of 100Ohm resistors, right?
Like when using resistors instead of a center tap for the 6.3V heater supply?
I just read about it in another thread and then searched the web about it... :)

Thanks!!

vari-mu

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #13 on: May 26, 2015, 08:26:49 AM »
Hi rainton!
You can try UTC O-16 under this scheme :



I have done many times transformers for my PULTECs :




rainton

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #14 on: May 26, 2015, 12:10:12 PM »
Hi rainton!
You can try UTC O-16 under this scheme :




Wow thanks vari-mu!
Why exactly did you choose 22kohm as reference to ground?

Your transformers look interesting!
Did you follow the original blueprints on the S217d?
More information on your repro would be awesome! Did you get the chance to compare them to the original?

vari-mu

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #15 on: May 28, 2015, 06:46:22 AM »
Why exactly did you choose 22kohm as reference to ground?
My thoughts: we need a primary impedance of 10k,
Ktr = 1: 2, then 10k / 40k,   40k / 2 = 20k .
But if the transformer UTC A-16 has a single coil, it may not work well,
will be distortion at medium-high frequencies.
If the two coils inside the 90% is a good result.

 
Quote
Your transformers look interesting!
Did you follow the original blueprints on the S217d?
  Carefully read the CJ`s posts can be made  transformers  for PULTEC
    For example:
  TRIAD HS-56 :
http://groupdiy.com/index.php?topic=58093.msg739559#msg739559

  PEERLESS S-217D :
http://groupdiy.com/index.php?topic=58106.0
http://groupdiy.com/index.php?topic=51046.0

 TRIAD HS-29 :
http://groupdiy.com/index.php?topic=57092.0

  Thank you very much CJ !

core13

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #16 on: February 16, 2018, 10:31:05 PM »
I tried 22K for center tap on the A-16, the noise disappear but it seem to create conflict with the zobel network.

I measured with fuzz-measure and the high end is decreasing.

I removed the zobel and it's better but it's still decreasing the high.

there is an other value I can try for the center tap resistor?

thanks



bancho

Re: Triad HS23 or 27 instead of a HS29 as Pultec interstage?
« Reply #17 on: March 11, 2019, 07:54:20 PM »
I also have the A-16 intended for interstage.  Could it be done?