Redd 47 Type Build Troubleshooting

[Dr Tony Balls]

Hi all.....first time poster, long time lurker/knowledge sponge. I'm an accomplished builder in the guitar electronics world with over 60 tube amp/preamp circuits built and too many pedals to mention. That is all to say that i'm not a complete dummy/newbie to building but this one is a step outside of my normal comfort zone into the world of recording gear, specifically a mic preamp. I was asked by a friend if I had ever attempted a tube mic preamp and I decided to give it a shot by starting with the EMI REDD 47 circuit. As i'm sure you all know, there are many versions of the schematic floating around and I consulted many, but this was a good one that I used:

My thought was to try and simplify things a bit both technically and economically, at least for a test build. So for input and output transformers instead of opting for picier versions I tried the Hammond 1140-MN-A (1:10) and the Edcor WSM 15K/600 (5:1), respectively. I added an input pad, and I also simplified the power supply a bit, taking nods from the MILA-1 circuit, with a more simple full-wave rectified design that provides ~290VDC. My filament supply is AC, coming right of the secondary taps on my PT. Here's my resulting schematic with voltages:

For the layout & construction I went with tagboard with almost all componentry mounted to it, including tubes and input transformer. My layout and pics of the build are here:

Anyway, here's where the problems start. Its passing signal and the controls seem to affect the signal as they should but the amount of amplification is very low. I'm just doing rough tests with a dynamic mic and a small guitar amp to verify that its working before testing it in studio but the signal level is much lower than the mic level alone without the preamp. Also the EF86 position is *wildly* microphonic. I've tried it with two tubes of different manufacturer and the problem persists. It even happens without an input signal connected. I've built plenty of EF86 amps before and know that they can have microphonic issues but never have I encountered something like this. It will self-oscillate and grow into a feedback-like sound in higher gain settings.

So after unloading all of that info....can anyone shed any light on where my problems might lie? I've gone over the schematics and my layout may times and everything looks correct, but maybe i'm placing sensitive components in the wrong place without realizing it. Or maybe my whole construction technique is problematic. ANY HELP IS APPRECIATED!

 

[rock soderstrom]

Nice build!

Your problem might have something to do with the NFB, I would test disabling the whole "over all" NFB branch.

Measure your total gain with and without NFB.

Are your input and output transformers wired correctly?

Change the position of the output transformer as it has no mu metal shield, there is a risk of inductive coupling.

The position of the input XLR is suboptimal, again risk of capacitative coupling and self oszillation. Keep input and output away from each other. Is there a DI input?

Bypass all pads and other upgrades, focus on the basic amp for now.

 

[Rob Flinn]

Did you check your build against the test voltages on the very first diagram you posted ???

If you look at your tag board layout pic you have something wrong there. Your gain switch connects to the ground side of the fine pot & not the other side which is where it should be according to the circuit diagram. i.e you have the output driving into ground through the gain switch resistor network. That would probably reduce the gain a bit.

 

[Dr Tony Balls]

Nice build!

Your problem might have something to do with the NFB, I would test disabling the whole "over all" NFB branch.

Measure your total gain with and without NFB.

Are your input and output transformers wired correctly?

Change the position of the output transformer as it has no mu metal shield, there is a risk of inductive coupling.

The position of the input XLR is suboptimal, again risk of capacitative coupling and self oszillation. Keep input and output away from each other. Is there a DI input?

Bypass all pads and other upgrades, focus on the basic amp for now.

Well thanks for you help. I got as far as checking my transformer wiring and realized my error which was of course the dumbest thing imaginable. My XLR connections were wired incorrectly. I fixed those and things are working more or less as they should. My at-home test setup (SM57 and a guitar amp) is of course very lacking but it's amplifying signal appropriately. At the higher gain/less padded settings its feeding back but without a proper test setup I suspect that's normal. I did notice that with no microphone connected and the pad at minimum (i.e. bypassed) i'm getting a 60Hz hum that i'm not getting with the pad engaged. Not sure what that's about but i'll do some thinking on it.

To answer questions that were asked:

Input Transformer (https://www.hammfg.com/files/parts/pdf/1140-MN-A.pdf)
Orange primary :: Input pin 2 (+)
Brown primary :: Input pin 3 (-)
Green secondary :: out to grid of V1
Green/Yellow secondary :: ground
Gray core connection :: ground
Violet can/shied connection :: ground

Output Transformer (WSM Series - 1/2W balanced or unbalanced line matching transformers.)
Primary pin 1 :: output from C8
Primary pin 2 (ct) :: no connect
Primary pin 4 :: ground
Secondary pin 5 :: output pin 2 (+)
Secondary pin 6 (ct) :: no connect
Secondary pin 8 :: output pin 3 (-)

Voltages (against a reference schematic)
EF86 pin 6 :: 73V (theirs) :: 67V (mine)
EF86 pin 3 :: 1.63V (theirs) :: 1.7V (mine)
E88CC pins 1,6 :: 138V (theirs) :: 131V (mine)
E88CC pins 3,8 :: 3.7V (theirs) :: 3.8V (mine)

Fine pot connection on layout: This was drawn incorrectly but on my build it was correct to schematic. It should also maybe be noted that on the layout im using pin 12 of the gain switch and pin 6 of the pad for connections, but these are dead pins that i'm just using as solder points. Here's an updated layout for completeness:

 

[My3gger]

I built too many of these with p2p layouts following schematic/amp current flow, only had mechanical problem in one. That means 1k grid stopper resistors directly on socket pins (helps against oscillations), V1 components are around V1, not V2 or elsewhere, lytics away from hot E88CC, mic input tx secondary connected to the load and tube by very short connections (minimizes hum, doesn't change frequency response), etc. I prefer boars with turrets because they allow placement of the components right where needed.
Older threads have lots of basic information about layouts for small signal devices, old books are also helpful, back then i could not find videos with explanations. Guitar stuff makes me paranoid because connections tend to be very crossed, sometimes components exposed to high temperatures, too far away from related tube and other layout problems.

 

[rock soderstrom]

Well thanks for you help. I got as far as checking my transformer wiring and realized my error which was of course the dumbest thing imaginable. My XLR connections were wired incorrectly. I fixed those and things are working more or less as they should.

You are not the first to make stupid mistakes here in the forum, guaranteed. It happens to everyone who actually does something and doesn't just talk about it, so it's a good thing!

At the higher gain/less padded settings its feeding back but without a proper test setup I suspect that's normal.

Not really, I think that can be optimised. One should always keep an eye on the relationships between the conductors. Therefore, keep the inputs and outputs of the amplifier stages at a distance.

just my 2€ cents