Poor Man's Pultec EQP1-A Build Support Thread

[ruffrecords]

Thanks Ian I was hoping to wire my pair similar to Daveseqp-1ahookup.pdf on your dropbox documents. I went for the stereo pair of mk2 boards with x2 poor mans make up and psu board. I thought I only needed the 1 transformer to power it all. 

Stuffing the boards is giving me time to figure this part out so not rushing into it

You can use one transformer but it needs to have two separate secondary windings; one for the heaters and one for the HT. Check out this post for recommended types:

http://groupdiy.com/index.php?topic=44213.msg766586#msg766586

Cheers

Ian

 

[unklepablo]

Ok last try at being cheap! What about this one? http://www.ampmaker.com/store/240V-6.3V-toroid-power-transformer.html

If not this im gonna have bite the bullet and go for the antec.

Cheers

Paul

 

[ruffrecords]

Ok last try at being cheap! What about this one? http://www.ampmaker.com/store/240V-6.3V-toroid-power-transformer.html

If not this im gonna have bite the bullet and go for the antec.

Cheers

Paul

That is absolutely perfect! I have a funny feeling we came across that one sometime earlier in the thread. I think it might be a good idea to create a list of suitable mains transformers and add it to the DIY section of my web site.

Cheers

Ian

 

[p0ulp]

For european , I think this one is also ok

http://www.ebay.de/itm/Ringkern-Roehren-Trafo-17-VA-240V-6-3V-Transformer-Roehre-Tube-Preamp-/291368326969?

sorry for the ebay link , if it's not ok I ll remove it, anyway I buy one and let you know if I can find info about the manufacturer and how does it looks like.

Sorry for UK product I m sure they are great quality but shipping is way more expensive to France :/, no need to say about US Antek ...

 

[Humner]

hi ian,

this might be a silly question. I'm trying to understand the PAD for the EQ bypass.

As per the note, I used a 10K resistor + 1k trimmer for the pad and when I engage the pad I can only get the output to 2db less than the EQ, and if I turn the 1k trim the other way, I can get rid of the entire signal and it drops back to the noise floor.

I'm now a little bit confused as to why I can reduce the signal to nothing. It doesn't seem to behave has expected.

 

[ruffrecords]

hi ian,

this might be a silly question. I'm trying to understand the PAD for the EQ bypass.

As per the note, I used a 10K resistor + 1k trimmer for the pad and when I engage the pad I can only get the output to 2db less than the EQ, and if I turn the 1k trim the other way, I can get rid of the entire signal and it drops back to the noise floor.

I'm now a little bit confused as to why I can reduce the signal to nothing. It doesn't seem to behave has expected.

Looks like you have missed out the 470R resistor  between the bottom of the trim and 0V.

Cheers

Ian

 

[Humner]

you are right, I completely missed it, I just put it in place and its now sitting in proper range.

thank you!

 

[Rocinante]

Ooh lest I forget.  MY D-Pultec PM-EQP-2A.
Damn lovely.


THANKS IAN!!!!!!!

 

[ruffrecords]

Ooh lest I forget.  MY D-Pultec PM-EQP-2A.
Damn lovely.


THANKS IAN!!!!!!!

Cool! Would you like to email the pics to Pierre so he can add them to his gallery of my projects?

Cheers

ian

 

[ruffrecords]

While I remember,  I recently posted a note to my white market thread to announce the retirement of the HT350 PCB. It was originally intended for providing HT for a complete mixer so it was always a bit of an overkill for a couple of PMTGMU stages. I have now replaced it with the HT250 board. It uses smaller/cheaper 220uF caps, a low cost DIL recrtifier and the PCB is nearly half the size of the HT350. The new HT250 PCB cost just £4.

Cheers

Ian

 

[Rocinante]

Ian,
I'd be happy to email some pics to you.  I also have finished the Classic Twin tube preamp (which the ht250 would've been perfect for as well) just need to take a couple of pics.
It'll also get its own thread.
Another great project sir. Thanks again.

 

[johnheath]

Ian...

First of all I must say that you have a very interesting project here

Even though evereything is perfectly presented I still have a few questions:

Have you experienced any problems by having the output transformer inside the chassis together with the power transformer? If so how did you solve it? =)

The rotary switches are the shorting or not? And since I am a sucker for PTP and want to give it a try and I wonder if there are some rotary switches that you recommend specifically for this EQ?

And finally... The make up stage... Is your choice of tube critical or will anything close to it do?


Best regards

/John

 

[ruffrecords]

Ian...

First of all I must say that you have a very interesting project here

Even though evereything is perfectly presented I still have a few questions:

Have you experienced any problems by having the output transformer inside the chassis together with the power transformer? If so how did you solve it? =)

Yes, but it depends on the transformer. The only ones I have had any slight hum problem with are the Carnhill ones. Sowter and Edcor have been fine. The solution is to keep them well apart and adjust their relative orientation for minimum hum. Having said this, the hum level level I have experienced has been down at -85dBu so I expect most people will jot have noticed it.

The rotary switches are the shorting or not? And since I am a sucker for PTP and want to give it a try and I wonder if there are some rotary switches that you recommend specifically for this EQ?

The switched are non shorting. The PMEQP1A PCB and the MK2 version are both designed to used low cost Lorlin rotary switches. The 3 band Pultec PCB is designed to use the (more expensive)  Grayhill 71 series switches.

And finally... The make up stage... Is your choice of tube critical or will anything close to it do?


Best regards

/John

The original PMTGMU stage was designed to use a 6CG7 tube mainly because it was a straight copy of one stage of one of my early mic pre designs. However, it does have limited drive capability which means it can only feed a 10K bridging load (not normally a problem). Later mic pre designs centred on the 6922 (E88CC) tube. This has a much lower plate resistance and hence a better drive capability. It produces more distortion than a 6CG7 but if run at a higher current it is nearly as distortion free as the 6CG7 and the higher current also improves its drive capability still further. More recently I have developed a third version using a 12AU7. The 12AU7 has nearly the drive capability of the 6922 but more distortion. Although not ideal, the 12AU7 version does at least provide the opportunity to operate from 12V heaters (which was why it was developed).

All the above is described in more detail on my web site. Under the DIY tab:

http://www.customtubeconsoles.com/diy

select the PMEQP1A folder and then the PMTGMU folder.



Did you have any particular tube in mind?

Cheers

Ian

 

[johnheath]

Hi Ian

Well, I can't really say that I had a specific tube in mind even though I have had good practice and experience with 12AU7 as driver tube in SRPP configuration. I am open to hear what people say about other tubes.

The 12V heater possibility is nice if you want it but have you had any problems with hum from the 6,3vac for heaters in this project... I guess it is a matter of proper layout with a touch o luck when designing the layout?


Best regards

/John

 

[ruffrecords]

Hi Ian

Well, I can't really say that I had a specific tube in mind even though I have had good practice and experience with 12AU7 as driver tube in SRPP configuration. I am open to hear what people say about other tubes.

The 12AU7 is well known to have relatively high intrinsic distortion. An SRPP topology generally produces more distortion than a mu follower so a 12AU7 SRPP is definitely not a recipe for low distortion. That said, this is often a sound that people like. If you are happy with it then go for it.

The 12V heater possibility is nice if you want it but have you had any problems with hum from the 6,3vac for heaters in this project... I guess it is a matter of proper layout with a touch o luck when designing the layout?


Best regards

/John

Not had any problems with ac heaters on this design at 6V or 12V. With mu followers and SRPP you do need to use elevated heaters. This means the cathode of the lower triode is reverse biased relative to the heater so heater hum does not get coupled to the cathode. If you use NOS tubes with poor heater cathode insulation the elevated heaters will cause a lot of hum in the output - it is an excellent detector of dodgy NOS tubes.

The original EZTubeMixer designs used dc heaters because of my concerns about possible heater hum especially in sensitive mic pres. However, in the last few months I have  conducted a wide range of tests on them using ac heaters. I fully expected hum to be a problem and anticipated having to make considerable modifications to get them working with ac heaters.  However, I am pleased to report that they all work without modification  without any sign of heater hum.

Cheers

Ian

 

[johnheath]

Yes, I know about the heater elevation... been troubling with that before. But I thought that the 12AU7 was a good choice for SRPP but if you say it is not I might consider another tube for that  topology?

I also have been thinking about using ac for heaters since it is a low cost and low labour alternative to dc fixes... and as you say it is a concern about hum caused from this ac. I have ordered some cahssis to start testing myself and I am glad to hear that you don't have any trouble with it so far.

I use mostly sowter transformers and they are great I think (there might be better ones I don't know).

So... from your experience what would be a GREAT tube for SRPP final stage of a mic pre?

Best regards

/John

 

[ruffrecords]

I use mostly sowter transformers and they are great I think (there might be better ones I don't know).

There are a number of transformer manufacturers in Sowters leaugue. Cinemag, Carnhill, , Jensen and Llundahl come to mind.

So... from your experience what would be a GREAT tube for SRPP final stage of a mic pre?

The output stage of my Eurochannel mic pre uses a 6922 configured as an SRPP principally for its output drive capability.  On its own this stage produces mostly second harmonic distortion which many people find pleasant. In the Eurochannel design its distortion is reduced by about 20dB of negative feedback.

Cheers

IAn
Best regards

/John
[/quote]

 

[ilfungo]

Hi
I'm working in a DOA make up stage.
If I use the schematics attached there are component  values to be changed?
Thanks!

 

[ruffrecords]

Hi
I'm working in a DOA make up stage.
If I use the schematics attached there are component  values to be changed?
Thanks!

This question was asked some time ago. I have added my recommended 2520 configuration in the PMTGMU folder of my web site.

https://drive.google.com/file/d/0B_n67A1hN3qtNzBHSGN5VkIxSFE/view?usp=sharing

Cheers

Ian

 

[ilfungo]

Sorry and Thanks Ian!!!!
Just one question
only 3 resistor? no caps?
Thanks!!!!