Multiple line amps in single enclosure

[Goodizzy]

Good evening,

I love this website.  I've been lurking longer than I care to admit and have learned a lot from the many useful posts. 

I am now embarking on creating a very small valve mixer based off the REDD.47.  I am hoping to mount this in a spare 19" audio rack I have laying around.  I intend to mount te actual line amps themselves in a repurposed military surplus signal generator rack unit.  The chassis is already drilled to accomodate 11 octal-sized valves and I would like to adapt this to housing 2-4 of these line amps. 

I have an older 4-channel valve Vortexion mixer as well that was designed for the broadcast industry.  Underneath the chassis there are very small, (approx 1.5" high by x 3.5" long) dividing walls which separate the valve sockets.  I'm unsure what they are made of, but no doubt something non-ferrous.

My question is- should I incorporate these, or something similar,  into the line amps' chassis?  Besides proper line dressing, is there anything else that should be done to shield the REDD.47's sensititive EF86 input stage?  I know the originals were houses in cassettes but they seem to have been sandwiched together fairly closely without apparent crosstalk or interference. 

I'd appreciate any insight or suggestions.  Thanks!

 

[emrr]

It all depends on layout.  Plenty of broadcast consoles with multiple preamps built on the same sheet of metal with no shielding between channels.  Of course shielding won't hurt anything either. 

 

[My3gger]

Claims about sensitivity most likely come from guitar combos like Vox where EF86 can suffer from microphonics. I haven't found this problem in preamps like Redd47 where the same tube is used, even in the long run they don't need much sorting or replacement so many people are afraid of.
Placement of transformers seems to be more important, try rotating power transformer until you find lowest interference. It is also a good practice to separate input tx from output, following schematic (input tx, EF86, things in between, output tube, output tx) mostly gave me nice clean layout and really quiet preamps. I find it essential to really understand grounding in anything audio.

 

[abbey road d enfer]

I have an older 4-channel valve Vortexion mixer as well that was designed for the broadcast industry.  Underneath the chassis there are very small, (approx 1.5" high by x 3.5" long) dividing walls which separate the valve sockets.  I'm unsure what they are made of, but no doubt something non-ferrous.

Looks to me like it's been designed by RF engineers. In RF design, that kind of shielding was very common. There should be no need for such in an audio mixer. It can' do no harm, though.

My question is- should I incorporate these, or something similar,  into the line amps' chassis?  Besides proper line dressing, is there anything else that should be done to shield the REDD.47's sensititive EF86 input stage?  I know the originals were houses in cassettes but they seem to have been sandwiched together fairly closely without apparent crosstalk or interference. 

Cassettes ARE the shield! Indeed, you need some sort of separation between sections, in order to avoid capacitive coupling, but in fact the most sensitive part is the EF86 itself and the grid circuit. The EF86 is a well-shielded tube, so you have to care only of the grid circuit; if there is any length to it, you must use shielded cable.
There is also magnetic coupling, that appears between transformers. You should mount the input xfmrs far enough from the output xfmrs, AND find the proper relative orientation. Typically, flux lines should cross at 90° angle.

 

[Goodizzy]

Thank you all for the fantastic information. 

 

[ruffrecords]

I seem to remember in the original REDD47 the EF86 was mounted in a shock mount socket - little more than a couple of grommets.

Cheers

Ian