Gates M5215 questions

Help Support GroupDIY:

Deepdark

Well-known member
Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
Hi

I found this schematic and I think I'll give it a try for fun one day. I found the complete document about the mixer, which this preamp is part of. So, first thing first, the transformers.

Is it thinkable to try a 1:1 ratio, maybe about 15K/15K, maybe like teh CMLI-15/15 or a 150/15K and output, 10K/600 or 150, somethinh like that. Sound good?

Secondo, if I want to put a gain pot, is puting a pot at R5 instead of a fixed resistor sounds like a good idea? So the wiper goes to the grid of V2.

 

Attachments

  • gates 5215.jpg
    gates 5215.jpg
    112.7 KB · Views: 175

emrr

Well-known member
Joined
Apr 12, 2006
Messages
7,893
Location
NC, USA
You can't use a pot there because it's inside the feedback loop. 

I have a number of the original output transformers if anyone wants 'the real deal'.  I think it's 15K:600-ish. 

Gain will be a little lower without a 150:50K input. 

Gain will be higher with a 1:1 output. 
 

Deepdark

Well-known member
Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
emrr said:
You can't use a pot there because it's inside the feedback loop. 

I have a number of the original output transformers if anyone wants 'the real deal'.  I think it's 15K:600-ish. 

Gain will be a little lower without a 150:50K input. 

Gain will be higher with a 1:1 output.

Thanks. Where would be the best place to insert a pot? Make R2 variable? or putting a pot between the sec. input transformer and the grid of V1 (something that match the impedance??)
 

Deepdark

Well-known member
Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
emrr said:
R1 variable is what can be done.

ok, so I would tie the wiper and teh bottom leg togheter, as a variable resistor or I would be better off to tie the wiper to C4 and bottom pin to ground?
 

Deepdark

Well-known member
Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
I just found a revised schematic here: http://www.americanradiohistory.com/Archive-Catalogs/Gates-Harris/Gates-Studioette-Console-1962.pdf

Some component has changed: R1 has a bypass capacitor, and is now 68K, R8 become 12K instead of 6k2, R5 is now 120k  and they changed the 5879 tubes for EF86. Maybe the changes are to accomodate that tube.

Curious to hear the difference between both tube in that circuit
 

rmburrow

Well-known member
Joined
Oct 8, 2013
Messages
183
The Gates M5215 series mic preamps were used in various Gates consoles for radio stations.  The 5879 version and the EF86 version are both nice sounding preamps.  You need to select the tubes and find the quietest ones for use on the input.

As for adding a level control to the preamp, this is NOT recommended.  The Gates preamps were built for fixed gain for use in their radio consoles.  Typically, a dynamic broadcast mic was plugged into the console and went straight into the preamp, no attenuation.  The output of the preamp worked into a constant impedance variable ladder attenuator  on the console which essentially was part of the console's mix buss (using variable ladder attenuators of the Daven  or Shallcross variety) for the various inputs.

My suggestion for the Gates M5215...do not modify it for a level control.  Find a 600/600 "tee" or "H" variable attenuator of the Daven or Shallcross variety and connect it (correctly!) across the output of the preamp, and feed the output of the attenuator into the 600 ohm line of your board or other device.

If you are using high output condenser microphones and get excessive distortion, pad down the input using either a variable 150/250 or 250/250 "tee" or "H" attenuator, either fixed or variable.  I find that 12 or 18 db works fine.  I build my fixed "H" style attenuators in Switchcraft S3FM "barrel" enclosures so these can be plugged between the XL mic plug and the preamp as needed.  The resistor values for the various attenuation and working impedances can be calculated.  Use 1/4 watt film resistors for the attenuators.

Last but not least, Gates used a regulated power supply to power the consoles.  It put out approximately 300 volts B plus and 6.3 volts ac for the heaters.  Feed the M5215 preamp clean B plus and operate the tube heaters with DC for best performance.  You don't need the Gates supply...it is way overrated for powering just mic preamps...a suitable small low current supply can be built to power these preamps...
 

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,278
Location
Marcelland
Deepdark said:
Secondo, if I want to put a gain pot, is puting a pot at R5 instead of a fixed resistor sounds like a good idea? So the wiper goes to the grid of V2.
This circuit is not terribly different than a V72, which many have modded for variable gain, by replacing the NFB resistor with a pot + resistor in series. You need to experiment because the frequency response varies with gain. This variation was deemed unacceptable by the German broadcast, but many studio users are happy with this mod.
 

Deepdark

Well-known member
Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
abbey road d enfer said:
Deepdark said:
Secondo, if I want to put a gain pot, is puting a pot at R5 instead of a fixed resistor sounds like a good idea? So the wiper goes to the grid of V2.
This circuit is not terribly different than a V72, which many have modded for variable gain, by replacing the NFB resistor with a pot + resistor in series. You need to experiment because the frequency response varies with gain. This variation was deemed unacceptable by the German broadcast, but many studio users are happy with this mod.

Thanks for the reply.

First I wanted to make a kind of variable gain selector with R5, which is Inside the NFB loop. But I fear that reducing the NFB will increase distortion and hum. By making R1 variable, it will act a little like the gain selector on the REDD47 I suppose. Or, as rmburrow suggest, simply put variable pads and live with it. I don't know yet what I'll do with it. Maybe I'll give the NFB loop a try.
 

Deepdark

Well-known member
Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
I just looked at teh RCA BA2 and the way they do with R4 is a little what I wanted first to do. Maybe that's what you were talking about, John?
 

Attachments

  • jqj2gz.jpg
    jqj2gz.jpg
    96.1 KB · Views: 84

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,278
Location
Marcelland
Deepdark said:
the combination of R11/C8 is probably a freq. booster to compensate for the freq. response variating with gain ??
Partly yes, but also in order to compensate an overall decreasing HF response, due to transformer losses and Miller effect.
 

Deepdark

Well-known member
Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
abbey road d enfer said:
Deepdark said:
the combination of R11/C8 is probably a freq. booster to compensate for the freq. response variating with gain ??
Partly yes, but also in order to compensate an overall decreasing HF response, due to transformer losses and Miller effect.

I guess something like this would somewhat work? The value of the // cap would be determined in real life test to find the better value, right?
 

Attachments

  • gates m5215 mod.pdf
    539.3 KB · Views: 24

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,278
Location
Marcelland
Deepdark said:
I guess something like this would somewhat work? The value of the // cap would be determined in real life test to find the better value, right?
That wouldn't work, because the whole circuit is under control of the overall NFB loop (C4, R1, R2). The only way to implement gain control is either changing the NFB loop transmittance (modding R1 - within limits) or changing outside the NFB loop at the input side (attenuator before T1, pot after T1 or altering the ratio of T1 - all these with consequences on S/N), or at the output side (pot before T2 - clumsy - attenuator after T2, or altering the ratio of T2 - all these with consequences on headroom).
As I wrote before, if I was in your shoes, I'd experiment with putting a 100k pot (connected as a rheostat) in series with a 10k resistor in place of R1. The variations in frequency response could probably be ignored (or easily dealt with); you may find that for higher values of gain, THD may deteriorate, as well as HF response, and that for lower values, there may be stability issues (oscillation) and/or loss of headroom.
 

Deepdark

Well-known member
Joined
May 19, 2013
Messages
1,321
Location
Quebec, Canada
Thanks Abbey. That make sense. I just redrawn it. I would like to know why a 100k pot, if R1 was originally 56K? and the serie resistor, is it to prevent the pot to completly short the signal if fully close, or to always have a load?

I guess that if I would like to put a Pot after T1, I would wired it like, for exemple, an LA2A. So, the secondary High is strap to the pot pin 3, wipper strap to the grid pin 1 and pot pin 1 to ground, right? And 100K would be a good guess?
 

Attachments

  • GATES M5215 - POT.pdf
    537.4 KB · Views: 24

abbey road d enfer

Well-known member
Moderator
Joined
Jan 22, 2008
Messages
13,278
Location
Marcelland
Deepdark said:
Thanks Abbey. That make sense. I just redrawn it. I would like to know why a 100k pot, if R1 was originally 56K?
That would allow for about 6dB more gain.
and the serie resistor, is it to prevent the pot to completly short the signal if fully close, or to always have a load?
For stability and headroom reasons, the resistance cannot be zero.
I guess that if I would like to put a Pot after T1, I would wired it like, for exemple, an LA2A. So, the secondary High is strap to the pot pin 3, wipper strap to the grid pin 1 and pot pin 1 to ground, right? And 100K would be a good guess?
Anything between 100k and 1Meg should work fine.
 

thomasdf

Well-known member
Joined
Aug 10, 2011
Messages
484
Location
Paris, France
Hi all

I will soon have 6 of these preamps and I would like to rack them in a proper clean case.

How much does one module draw ? Any pointers for a PSU design ? I was thinking straightforward diode rectified PSU (ruffrecords 350HT?) , and DC heaters.

Thanks in advance,

Thomas
 

MidnightArrakis

Well-known member
Joined
Sep 25, 2015
Messages
116
Hi all

I will soon have 6 of these preamps and I would like to rack them in a proper clean case.

How much does one module draw ? Any pointers for a PSU design ? I was thinking straightforward diode rectified PSU (ruffrecords 350HT?) , and DC heaters.

Thanks in advance,

Thomas
[I would like to rack them in a proper clean case] -- In my hopes that I am not banned from this forum for making you this offer, but I am an "electronics" Mechanical Designer and I have personally designed all manner of rack-chassis for broadcast equipment, professional audio gear, professional video electronics and even telecommunications and medical electronics equipment. Depending upon the exact size of the chassis that you will need, I can either custom-design a rack-chassis for you or, I know of vendors that supply off-the-shelf rack-chassis and all that is needed is to provide them a drawing showing where all of your hole cutouts are located. In fact, I know of one rack-chassis vendor that supplies 3D CAD-models of their chassis of which I can import into my CAD-design program, and then all I have to do is to send back to them the modified 3D CAD-model detailing what you need!!!

So, if you wish to have a "proper clean case" for your 6 preamps, I can help you out.....but, only if you wish.
> I AM NOT SELLING YOU ANYTHING!!! < I am merely offering you my rack-chassis mechanical design expertise to help you along with your project. You can take a look at some of my rack-chassis mechanical designs within the attached PDF file.

/
 

Attachments

  • JBW-Designed - RACK STUFF -- Assemblies-Chassis-Enclosures-Panels-Systems.pdf
    6.2 MB · Views: 8

MidnightArrakis

Well-known member
Joined
Sep 25, 2015
Messages
116
Hi all

I will soon have 6 of these preamps and I would like to rack them in a proper clean case.

How much does one module draw ? Any pointers for a PSU design ? I was thinking straightforward diode rectified PSU (ruffrecords 350HT?) , and DC heaters.

Thanks in advance,

Thomas
HELLO, again!!! I just now noticed that you are located in France. The vendor that I mentioned in my original response that provides the 3D CAD-models of their rack-chassis has a sales office in France, as I believe that they are a German-based company. Here's their address:

24-28 avenue Graham Bell
Espace Vinci - Le Renoir 19A
77600 Bussy Saint Georges

I am guessing, should you decide upon using one of their rack-chassis for your project, that I could download the appropriate rack-chassis 3D CAD-model for your project and modify it as necessary. Then, when I would upload my modified 3D CAD-model back to them for fabrication, that they would ship you your finished rack-chassis from their Germany facility. Again, I'm just guessing that is how the scenario would work out. But, I do know that you would end up with one (or, more!!!) really nifty-looking and professionally made rack-chassis!!!

Just my 2-cents worth!!! (0.016818742 Euros)

/
 

Latest posts

Top