Info about Collins and other old cool vintage tube preamps?

[emrr]

….and 1st version SA-70 had no series NFB cap at all….

 

[MidnightArrakis]

Hi Nate,
Here is a rough updated/cobbled together schematic that shows the layout I use. I want to preface I'm somewhat of a novice, so others may have better perspectives or tweaks to what I'm sharing.

The pot is a 250k linear pot. The bass control is on a 6 position make before break switch. I haven't implemented the high end/treble control as of yet. I found the bass and negative feedback control gave me what I was looking for. View attachment 125030

1) Is it advised to place the VU-meter directly across the output? There isn't any need for a buffer/driver to both drive and calibrate the VU-meter with to the signal?

2) Which EDCOR transformers would you recommend for both T1 and T2?

3) Would all of the "Bass Control" capacitors also be 400VDC types?

4) A] What is the actual voltage on Pin-3 of V2? B] Why does V2 say " X2 "? Are there 2 of them? If so.....where?

>> A couple of weeks ago I received a package containing 50-pieces of 47uF @ 450VDC electrolytic capacitors!!! Now this circuit could give me a reason to use them in something!!! YAY!!!

/

 

[heatwalk]

I always have one position on the capacitor switch with no contact, so that the negative feedback is completely switched off. I like it, depending on the source.

If I remember correct the pair that I built I found that with a 250k pot at full resistance it was almost the same as taking the loop out.

HF control you can look at one of the other schems that exists to get a sense of. Instead of a 10k there is a 56k paralleled by a cap. Switching that around alters HF response. It doesnt give boost though, only hf cut.

One other thing I added was HPF in form of switchable coupling cap (C3)

 

[beezer4]

1) Is it advised to place the VU-meter directly across the output? There isn't any need for a buffer/driver to both drive and calibrate the VU-meter with to the signal?

2) Which EDCOR transformers would you recommend for both T1 and T2?

3) Would all of the "Bass Control" capacitors also be 400VDC types?

4) A] What is the actual voltage on Pin-3 of V2? B] Why does V2 say " X2 "? Are there 2 of them? If so.....where?

>> A couple of weeks ago I received a package containing 50-pieces of 47uF @ 450VDC electrolytic capacitors!!! Now this circuit could give me a reason to use them in something!!! YAY!!!

/

1.) I don't always use a VU meter. If I do, I put it on a switch and have typically leveraged vintage style VU meters with an internal diode bridge. Adding a VU meter across the output without a buffer will introduce additional harmonic distortion, which can be good or bad depending on preference.
2.) I've only ever used Edcor for the output position: WSM Series 15k - 600
3.) Yes and I typically use these capacitors: Mallory, 400V, 150s, Axial Lead
4.) A] I don't have the actually voltage measurements sorry -- B] There are not two of them, just one. Not sure why is has the X2 there.

Best of luck on the build!

 

[MidnightArrakis]

1.) I don't always use a VU meter. If I do, I put it on a switch and have typically leveraged vintage style VU meters with an internal diode bridge. Adding a VU meter across the output without a buffer will introduce additional harmonic distortion, which can be good or bad depending on preference.
2.) I've only ever used Edcor for the output position: WSM Series 15k - 600
3.) Yes and I typically use these capacitors: Mallory, 400V, 150s, Axial Lead
4.) A] I don't have the actually voltage measurements sorry -- B] There are not two of them, just one. Not sure why is has the X2 there.

Best of luck on the build!

>> THANKS!!! for the EDCOR "WSM" info. I personally like this style of the "same thing, only different" transformer:

https://edcorusa.com/products/mxl-series-1-2w-balanced-or-unbalanced-line-matching-transformers

And.....I like this style of EDCOR transformer for the input (200-Ohm : 50K):

https://edcorusa.com/products/umm-s...1&_fid=b7700780b&_ss=c&variant=42317234143481

[Adding a VU meter across the output without a buffer will introduce additional harmonic distortion] -- So, now I guess I need to find a good VU-meter buffer/driver circuit, huh???

[Yes and I typically use these capacitors: Mallory, 400V, 150s, Axial Lead] -- The capacitors that I bought are in a small -- radial -- package and were only $0.40 each.

[I don't have the actually voltage measurements sorry] -- Since I'm not a circuit-designer type of person, certain things easily confuse me when I'm looking at a schematic. Looking at "R7", it shows that there is 240VDC on one side of the resistor, but on the other side it is showing -- 2.84V RMS -- and connecting to V2/Pin-3, so that makes me wonder what the voltage rating of the "Bass Control" capacitors should be.

[There are not two of them, just one. Not sure why is has the X2 there] -- One of life's many mysteries, huh???

>> So.....were looking at: 1 - 6C5, 1 - 6J7 and 1 - 6X5.....right???

/

 

[MidnightArrakis]

Because I tend to design things a bit "differently" than some other members on this forum, here are the power transformers I have selected because I like to make things relatively "small" and in a 1U rack-mount chassis:

This one (Model #DSZ5-12) is for the heaters/filaments (6.3VAC @ 2.0A):

https://powervoltgroup.com/ProductDetails.aspx?ProductID=147&InputID=125

And, this one (Model #FZ230-50) is for the B+/Plates (230VDC CT @ 50mA):

https://powervoltgroup.com/ProductDetails.aspx?ProductID=137&InputID=67

So.....two power-transformers from this company and two signal-transformers from EDCOR should do it!!! How well do you think that these transformers would work within your circuit? And, by the way.....this power-transformer company is located in Addison, IL should you be so interested!!!

/

 

[Matt Syson]

Putting a usual VU meter with 3K6 resistor across a 600 ohm output circuit (600 ohms source impedance) adds about 0.3% distortion if I remember correctly so you need a buffer circuit unless you want the distortion.

 

[MidnightArrakis]

Putting a usual VU meter with 3K6 resistor across a 600 ohm output circuit (600 ohms source impedance) adds about 0.3% distortion if I remember correctly so you need a buffer circuit unless you want the distortion.

>> I'll go for a VU-meter buffer/driver circuit that also has a calibration trimpot. That way.....I can inject an input reference signal and then adjust the meter for a "0 VU" reading to that reference. Have any suggestions???

/

 

[CalavoBob]

Here's a meter buffer circuit that MCI used on their JH-24 tape deck meter bridge. I made a bunch of these to build a meter bridge for a mixer I have. The Collins tube circuitry won't have the +/- 18V supply that this uses though, so you'd need to add a small dual supply for these. To get more adjustment from the pot I replaced the 270 ohm resistor on the left end of the pot with 100 ohms. The board location drawing on the left shows holes where the meter screws pop through the circuit board to hold it on. You don't need the record and input status lamps shown at the bottom of the page of course, but you might consider running some voltage divider resistors to the meter housings for some lights in them. So as to not load my bus down so much, I doubled the two 2k ohm input resistors to 4k ohms. A good way to get rid of some of those uA741s that have been piling up in your spare parts drawer!

 

[MidnightArrakis]

Here's a meter buffer circuit that MCI used on their JH-24 tape deck meter bridge. I made a bunch of these to build a meter bridge for a mixer I have. The Collins tube circuitry won't have the +/- 18V supply that this uses though, so you'd need to add a small dual supply for these. To get more adjustment from the pot I replaced the 270 ohm resistor on the left end of the pot with 100 ohms. The board location drawing on the left shows holes where the meter screws pop through the circuit board to hold it on. You don't need the record and input status lamps shown at the bottom of the page of course, but you might consider running some voltage divider resistors to the meter housings for some lights in them. So as to not load my bus down so much, I doubled the two 2k ohm input resistors to 4k ohms. A good way to get rid of some of those uA741s that have been piling up in your spare parts drawer!

[Here's a meter buffer circuit that MCI used on their JH-24 tape deck meter bridge] -- THANKS!!! I appreciate it very much!!!

[you'd need to add a small dual supply for these] -- I'm not worrying about that. First.....I can buy NE5532's for less money than what an LM741 costs to start off with and, Second.....I can easily build a +/- 15VDC power-supply for around 6-bucks by using two of these little AC/DC converters and some caps, diodes and inductors:




[running some voltage divider resistors to the meter housings for some lights in them] -- Which I could do either by tapping off the +15VDC rail from the above AC/DC converter or by using yet another one of these small converters just to power the VU-meter lights.


[I made a bunch of these to build a meter bridge for a mixer I have] -- As I had mentioned in my Post #26, "I tend to design things a bit -- differently -- than some other members on this forum". Here's a project I had built several years ago when I was just beginning my DIY journey. I performed the mechanical design of the 4U rack-chassis itself as well as several of the PCB's, most of the cable assemblies and all of the chassis assembly shown in this photo:



>> I am hoping that "Beezer" will comment on the transformers I have selected.

/

 

[MidnightArrakis]

Here's my take on the heater and B+ transformers:

>> 115/230 Primary -- 6.3VAC @ 3.2A Secondary


>> 115/230 Primary -- 230VAC CT @ 50mA Secondary


Any comments? Anyone??? Bueller? Bueller?? Bueller???

/

 

[dawsonaudio]

Thanks for the reply. I was away for a bit so just got back to the board here.

I'll give the feedback control a go as you explained. And thanks for the visual description. Looks like it will add some nice tone control. I'm just finishing up a few of these gates pre's with some spare components I have around I'll be sure to post some photos when I'm finished.

Thanks again,
Nate

 

[dawsonaudio]

The negative feedback adjust is pretty cool! It gets pretty beefy.... Great feature!

For the low end adjustment, I had a 4 position switch and put some random caps in... they are .022/.01/.0047/.0018. Seems the low end drops a little like a shelf with each different value. The larger the cap, the less low end. With the .0018 providing the greatest low end. Is this correct?

Thanks,
Nate

 

[beezer4]

Here's my take on the heater and B+ transformers:

>> 115/230 Primary -- 6.3VAC @ 3.2A Secondary
View attachment 125392

>> 115/230 Primary -- 230VAC CT @ 50mA Secondary
View attachment 125390

Any comments? Anyone??? Bueller? Bueller?? Bueller???

/

Here is the power transformer I typically use: https://www.hammfg.com/part/269GX
Your B+ transformer should be okay as long as the 230VAC is not getting split down to 115 with the center tap. Power transformer specs can be confusing for me from time to time.

 

[beezer4]

The negative feedback adjust is pretty cool! It gets pretty beefy.... Great feature!

For the low end adjustment, I had a 4 position switch and put some random caps in... they are .022/.01/.0047/.0018. Seems the low end drops a little like a shelf with each different value. The larger the cap, the less low end. With the .0018 providing the greatest low end. Is this correct?

Thanks,
Nate

Hey Nate,
That is correct. The smaller the cap value the more bass This is the set I typically use and yours is not far off:

• .001 uf
• .0022 uf
• .0056 uf
• .008 uf
• .01 uf

 

[beezer4]

>> THANKS!!! for the EDCOR "WSM" info. I personally like this style of the "same thing, only different" transformer:

https://edcorusa.com/products/mxl-series-1-2w-balanced-or-unbalanced-line-matching-transformers

And.....I like this style of EDCOR transformer for the input (200-Ohm : 50K):

https://edcorusa.com/products/umm-s...1&_fid=b7700780b&_ss=c&variant=42317234143481

[Adding a VU meter across the output without a buffer will introduce additional harmonic distortion] -- So, now I guess I need to find a good VU-meter buffer/driver circuit, huh???

[Yes and I typically use these capacitors: Mallory, 400V, 150s, Axial Lead] -- The capacitors that I bought are in a small -- radial -- package and were only $0.40 each.

[I don't have the actually voltage measurements sorry] -- Since I'm not a circuit-designer type of person, certain things easily confuse me when I'm looking at a schematic. Looking at "R7", it shows that there is 240VDC on one side of the resistor, but on the other side it is showing -- 2.84V RMS -- and connecting to V2/Pin-3, so that makes me wonder what the voltage rating of the "Bass Control" capacitors should be.

[There are not two of them, just one. Not sure why is has the X2 there] -- One of life's many mysteries, huh???

>> So.....were looking at: 1 - 6C5, 1 - 6J7 and 1 - 6X5.....right???

/

One call out on the Edcor step-up input transformer. It doesn't have any shielding, so it may introduce noise into the preamp and placement will be important. Typically, this is why I leverage more traditional new or vintage step-up input transformers that have additional shielding, which can be more pricey. I'll be interested to hear how it works for ya.

 

[MidnightArrakis]

It doesn't have any shielding, so it may introduce noise into the preamp and placement will be important. I'll be interested to hear how it works for ya.

Going along within my "designing things a bit differently" philosophy.....I am not opposed to enclosing both of my PCB-mount power transformers within -- shielded enclosures -- as shown below. And, actually.....this is something that many others on this forum should also be doing.....anyway!!!

While this style of -- shielded enclosure -- is primarily used on PCB's to shield sensitive "RF" circuitry (I have used these on PCB's I have designed for "covert intelligence-gathering and surveillance" projects for Homeland Security and military-related projects), these devices can be used with anything that needs to be shielded. In fact, these shields can even be "tuned" to shield against a particular band of frequencies. So.....there's that!!!

[Your B+ transformer should be okay as long as the 230VAC is not getting split down to 115 with the center tap] -- You are correct!!! In fact, the B+ transformer -- SHOULD -- be called out as a "460V CT" transformer!!!

[Power transformer specs can be confusing for me from time to time] -- You're not alone in that department!!!



/

 

[ruffrecords]

MidnightArrakis makes a good point - when it comes to tube mic pres, the physical implementation is at least as important as the circuit design. Tubes are high impedance devices quite capable of picking up the minutest of interference. They should therefore be housed in a fully shielded enclosure. Even if the are not completely shielded themselves, they should be fully shielded when fitted into the overall enclosure.

Cheers

Ian

 

[Ilya]

When it comes to placing a power tx inside the same enclosure, these are rules that worked for me:
1. Distance is your friend. The further away from the most low-signal and sensitive part (this is often an input tx) your power tx is, the better.
2. Properly designed toroid power tx is your friend. The power field doesn't radiate as much with this type of transformers (IF they don't overload!)
3. Turning a power tx around and/or on its side can make a huge difference in the induced 50Hz hum and harmonics.

I tried shielding and it didn't help much. Besides, for shielding low frequency it's suggested to use special alloys that are very expensive. So for me, proper tx design and placement worked much better than shielding.

 

[dawsonaudio]

Here's a link of a photo of four channels of the gates sa-70's I built. The channels are powered by a 250 volt and 5 volt linear supply from International Power. I've got another three going with my last three altec 4722 mic input transformers. I have a bunch of audio bits and pieces that I inherited from a family member who was in the audio biz so I'm just trying to use them all up.

I'm actually looking for a wsm 15k/600 to try out in one of the new channels. I've currently got a channel with utc a-25, a-24 and two with cinemag cm-27101. If anybody has an extra/surplus one they could sell me, I'd appreciate it. Looks like the lead time is a month or more and want to get these other three finished.

Thanks,
Nate