Redd.47 output control

Hi fellow diy's,

I am building a pair of Redd.47 clones. I have searched hi and low and am wondering; is there any reason that I can't replace the grid resistor on V2 of a Redd.47 with a 1 meg log pot?  Inserting a volume control at this point seems to be common with other preamps, the MILA and UA610 as examples. I understand that this will change the feedback gain control switch levels, and not allow me to drive the output stage hard, but if I want to lower the output and keep it clean without an attenuator after the output transformer, would this be a good idea?
If not, the Chandler Redd47 seems to use a pot (or stepped attenuator, depending on order) that is tied near the output transformer (looking at the available internal pics). Any comments on how this is implemented?
Cheers!

Hi
I do something like you tell(1Mlog pot replace 1M resistors from 1k to grid on v2), but this solution, bring us more distortion on lower gain(don't remember how many...), if you want have always the same THD. Bridget T Attenuator is better,...Chandler, and Drip do this.

Put 1K log pot on output,  each leg to pot has ~300ohm resistor. Just tried how quiet preamp is this way, can't measure or hear any difference, actually it is a bit quieter because of better layout, sounds the same.
Don't put pot at R10 or IT secondary, it will not work well, at R10 because it would be in feedback loop.  Many did it this way without NFB.
With this preamp it is good to have -20dB input attenuator ala Jensen and output control like 1k pot or another attenuator that works.
I attached it, use A and wire secondary of OT to ~300ohms resistors before input of the pot (upper and lower leg).  Take output from mid pin to out xlr pin 2, from lower leg to pin 3. Works almost anywhere and doesn't mess with the sound, specifications, etc. It is cheap too. Good luck!

Robinsonics said:

Hi fellow diy's,

I am building a pair of Redd.47 clones. I have searched hi and low and am wondering; is there any reason that I can't replace the grid resistor on V2 of a Redd.47 with a 1 meg log pot?  Inserting a volume control at this point seems to be common with other preamps, the MILA and UA610 as examples. I understand that this will change the feedback gain control switch levels, and not allow me to drive the output stage hard, but if I want to lower the output and keep it clean without an attenuator after the output transformer, would this be a good idea?

Click to expand...

Preamps like the 610 have two separate gain stages with a gain control in between them. The REDD47 has two tubes with overall negative feedback creating a single gain stage. If you put a volume control between them all you do is alter the open loop gain and the negative feedback will try to compensate and keep the gain constant. If you want to lower the output and keep it clean then you should use a pad at the input.

If not, the Chandler Redd47 seems to use a pot (or stepped attenuator, depending on order) that is tied near the output transformer (looking at the available internal pics). Any comments on how this is implemented?
Cheers!

Click to expand...

Most likely an attenuator after the output transformer.

Cheers

Ian

Thanks Everyone,

I suspected the pot was too easy. 
I have been trying to calculate the extra feedback resistors for the 3 extra gain positions that the Chandler has. Is this even useful? And is it easily calculated?

Cheers,

This is link to 600R Atten... i just check and redrawn values from https://groupdiy.com/index.php?topic=58750.0

Awesome!
Thankyou.

Robinsonics said:

Thanks Everyone,

I suspected the pot was too easy. 
I have been trying to calculate the extra feedback resistors for the 3 extra gain positions that the Chandler has. Is this even useful? And is it easily calculated?

Cheers,

Click to expand...

No and no. The problem is maintaining stability as the gain is changed. As feedback increases, stability worsens. That is why the original only has a limited range of gain settings. For lower gains you need more feedback which often leads to instability. The only way to combat this is to reduce the open loop gain at low gain settings to regain stability. I don't know if Chandler do this.

Cheers

Ian

Robinsonics said:

I have been trying to calculate the extra feedback resistors for the 3 extra gain positions that the Chandler has. Is this even useful? And is it easily calculated?

Click to expand...

You can try to change resistors in the feedback path and see what you will get. I did it recently with no problems with an instability.

Robinsonics said:

Thanks Everyone,

I suspected the pot was too easy. 
I have been trying to calculate the extra feedback resistors for the 3 extra gain positions that the Chandler has. Is this even useful? And is it easily calculated?

Cheers,

Click to expand...

Aren't easy and well working solutions best? You could use Bourns T pad attenuator if 1k pot doesn't work for some reason, it does for many others like API. Another solution would be stepped input attenuator, although with noise penality.
Adding more NFB positions will change the sound between them even more. I noticed original lowest gain position doesn't sound as good as other two, so it is like Ian says although i didn't measure why it happens. This changing sound with gain positions is not something i like very much. NFB between two stages like here doesn't allow pot between them, it can be very helpful for modern production.
Another question are (frequency compensation?) those caps, guess they would have to be calculated for more position if stability even allows them.