EMI BRED806 preamp

[rotation]

Hi guys,

i'm thinking about building this preamp for some time, but can't find any information about output transformer and quality of this preamp. Input is probably nothing special, i don't even need multiple primaries as on schematic.
But output is new to me, looks like Ampex 351, but single ended and cap coupled, see Ampex:
http://www.cinemag.biz/application_notes/AN-106.pdf
Any thoughts about possible output replacement? Would this Cinemag CM-9660 work? There are some voltages shown on schematic, it looks like output is step down, so Cinemag might be just fine.
Btw, i contacted technical engineer from Abbey Road. He didn't know anything about this preamp but finds it interesting and would also like to know more about it. There is a long thread about it at PSW, a lot of talking and no real information about the sound or transformers.

Miha

 

[emrr]

can you point out the discussion elsewhere?  I can't find anything.

There's no current in that output; any single-ended type with a center tapped primary will work.  No gapping required. Just have to find something with a tertiary winding for feedback. 

 

[mad.ax]

I guess the thread at PSW is this one:
http://recforums.prosoundweb.com/index.php/mv/msg/23253/0/0/0/

Quite a bit of discussion about restoring the faceplate or not, and who got it from who, yada yada... Fascinating from an historical POV, but a little frustrating on the technical level...
The original poster didn't fix his two units after 2 years...

Axel

 

[rotation]

Here it is:
http://recforums.prosoundweb.com/index.php/t/23253/18892/
I contacted several people who discussed there and no one knew more details or at least how it sounds. Discussed preamp was returned to the owner in non-working condition.

One thing; did you notice that screen grid of EF86 is connected to anode? Why would they want to use precious pentode triode connected?
Another strange thing (to me) is very small cap from screen grid to ground on ECF80. I normally see this cap much larger.

It is pretty interesting circuit and looks pretty well designed. Or just over complicated, i will build it and see.

Miha

 

[Kingston]

One thing; did you notice that screen grid of EF86 is connected to anode? Why would they want to use precious pentode triode connected?

That's a very common preamp input stage configuration apparently. Used by Gates in some of their broadcast consoles with both 5879 and EF86 as well. Gain is some 20-25dB for that block. Guess it's just a cheap way to use a pentode when there's no need for another dual triode.

oh, what is that DM70 block doing by the way? I don't quite understand its function.

 

[tv]

There's a description of "triode-strapped pentode" sound:

http://boozhoundlabs.com/howto/

...but a pentode can be made to act like a triode by connecting one of the grids to the plate. This is called triode mode, or triode-strapping. The sound of a triode strapped pentode is characterized as being a bit more "forward" than that of a true triode, which seems to mean a more pronounced midrange.

 

[Kingston]

Here are some more probable reasons manufacturers strapped pentodes as triodes, from NYD's one bottle thread, the information applies to this EMI design and EF86 also (except they aren't so cheap these days):

> I always wanted to build a pentode pre too

Just FYI: NYDave's plan has you buy a Pentode and then convert it to a Triode. A waste of two grids!

However, triode is better here: Pentodes have noise issues, and we don't want the tremendous gain that a pentode is capable of. He picked that particular 5879 tube because it was manufactured for audio applications, including low-noise when triode strapped. There are a half-ton of other triodes you could use instead, but you may have to buy several extras and select for low noise. With 5879, you are unlikely to run into a hisser, and it isn't expensive. (It has also been featured in some notoriously famous audio gear, but so what?)

PRR, those are exactly the reasons why I used a triode-wired 5879. Good call! The fact that it only draws 150mA of heater was a big plus, too, since I'm planning to build mine using a power transformer that isn't going to have much extra heater current to spare in the first place.

Certainly hints why Gates may have used these in consoles: low noise with decent gain, and relatively small heater current needs. For similar reasons it might be a "slap-on" last minute addition to this EMI design. Sound was probably not a concern in those days; they all aimed for linearity and reasonably low cost.

 

[mad.ax]

Rotation wrote :
“I contacted several people who discussed there and no one knew more details or at least how it sounds.”

Theres a link on page 4 of the PSW thread, did you also tried to contact him?
http://www.pomaudiodesign.com/page3/page3.html
POM (stands for Pierre-Olivier Margerand) didn't participate in the discussion, but obviously he worked on a rack of 8 X 806 pres, and even upgraded them. He must know a bit about those for sure...


@ Kingston :
The DM70 is a sub-mini tuning indicator, or 'magic eye'. Here it is used as a level meter.
You can see how it works here:
http://pagesperso-orange.fr/michel.terrier/radiocol/detail2003/tubes-magics/dm70-anim.gif

Axel

 

[emrr]

Looking at in more closely, you could just leave that level indicator off entirely, and not worry about the additional primary tap. 

RCA, Gates, Langevin, Altec, Western Electric; all used pentodes strapped for triode operation. 

 

[Kingston]

Looking at in more closely, you could just leave that level indicator off entirely, and not worry about the additional primary tap.  

and with one less plate eating away that barebones B+, noise decreases. Certainly with the stock CRCRCRC setup it would make an improvement. Then again if DM70's can be had for cheap the magic eye would be a nice bling bling touch.

 

[xmvlk]

Input is probably nothing special

I do not agree with You - input is very special. Input tranny is loaded by Miller resistor, what
is noise-optimal.
This is common and proven topology with triodes from late 1930-ies. Maybe it works badly with
pentode {too much great Ri, nonstable} and it is reason to use triode connection. { it conserves transconductancy gm and lowers Ri and noise- modern high gm pentode is quitely common tube.}

 

[rotation]

Rotation wrote :
“I contacted several people who discussed there and no one knew more details or at least how it sounds.”

Theres a link on page 4 of the PSW thread, did you also tried to contact him?
http://www.pomaudiodesign.com/page3/page3.html
POM (stands for Pierre-Olivier Margerand) didn't participate in the discussion, but obviously he worked on a rack of 8 X 806 pres, and even upgraded them. He must know a bit about those for sure...

Hey, i didn't realize he was the guy who racked those preamps. I will contact him and post my findings if there are any!

@Kingston: can you name those Gates mixers and their preamp models? I would like to compare schematics.

It is nice that i got so much information from you guys, it will help with my little research. Btw, my professor/mentor was pretty surprised by the level of  knowledge at this place when i showed to him some discussion. We are for sure making a favor to future generations.

Miha

 

[emrr]

95% of the gates preamps use triode strapped pentodes, anything they made from 1954-1961 or so uses 5879's, then they switched to 6267's in the same designs.  All of the consoles (Yard, etc), and the various remote amps. 

I agree there's nothing special about the input.  It looks like the standard variety used in that era, anything from 25K to 60K secondary impedance I would expect. 

 

[rotation]

Yeah, i checked some schematics and they mostly have pentodes wired as triodes.

Some time ago you told me that some difference in transformer ratios doesn't really matter in practice (for example Sowter often doesn't have exact ratios in their recreations); i tested a few combinations and it is truth. There are some small differences in gain and that's it. Sound is similar when i use the same type but different ratios..

 

[Kingston]

Some time ago you told me that some difference in transformer ratios doesn't really matter in practice (for example Sowter often doesn't have exact ratios in their recreations); i tested a few combinations and it is truth. There are some small differences in gain and that's it. Sound is similar when i use the same type but different ratios..

Well you do need a reasonable amount of step-up to effectively use up the headroom of these type of amps. No point wasting any. But yes indeed, anything from like 1:2 (not really optimal) to 1:10 (sweet spot) works for mic input. Moving above that and bandwidth starts to suffer. That's why there are little or none modern manufacturers who do 1:20 (like 150/80k) step up for example. Winding one is difficult apparently. And amp headroom vs. gain needs more optimisation for those as well.

 

[emrr]

The transformer set from a Collins 356A, or Langevin 5116 (not 5116-B), or 116A/B would work here.  But, they might sound too 'american'.