REDD47 need more gain...

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simonsez

Well-known member
Joined
Nov 14, 2008
Messages
679
Location
Jakarta, ID
What the best way to increase gain in this unit?

1. lower ratio output transformer?
2. Adding resistor at feedback network?
3. increase input transformer ratio? (1:10)
4. increase plate resistor of EF86?


Thanks,


Simon
 
Probably reduce the ratio of the output transformer. Provided you feed it into a bridging load it should be OK.

Cheers

Ian
 
ruffrecords said:
Probably reduce the ratio of the output transformer. Provided you feed it into a bridging load it should be OK.

Cheers

Ian

I change the OT 1:7 to 1:5, it's gave me about +3.5db  output gain, now i need about 5 dB more gain.  Increase plate resistor to 220k or increase feedback resistor?
 
You could also increase the screen voltage some to provide more gain, depending on how much you are getting out of the pentode. Might be able to achieve an additional 3dB or so.

And yes, increasing the plate resistance will add gain. Maybe 150K to 200K from the 100K stock.

Of course, changing the operating point of the plate and screen changes the nature of the pentode sound.
So keep that in mind if you change these.

There is a mod floating about in one of the many redd47 threads, for the feedback atten network which adds some gain. I think that is your 'adding resistor at the feedback network'.

I guess noise will begin to be an issue at a certain extra gain level?


 
simonsez said:
ruffrecords said:
Probably reduce the ratio of the output transformer. Provided you feed it into a bridging load it should be OK.

Cheers

Ian

I change the OT 1:7 to 1:5, it's gave me about +3.5db  output gain, now i need about 5 dB more gain.  Increase plate resistor to 220k or increase feedback resistor?

Making any changes to the gain of either of the tubes will not affect the overall gain as they are surrounded by a negative feedback loop and it is this which determines the gain. So if you increase the EF86 gain all you do is possibly reduce distortion and possibly increase the risk of instability. Neiither is what you are after.

So, the second best method is to increase the negative feedback resistor. This will lead to an increase in distortion by exactly the amount by which you increase the gain.

Cheers

Ian
 
OK - I stand corrected :) 

Maybe because I use my ones at max gain, where negative feedback attenuation is at a minimum, I found that varying the plate/screen resistances/voltages increased the overall gain.

I'll need to look at this more closely on my current builds.

Thanks for corrected info!
 
alexc said:
OK - I stand corrected :) 

Maybe because I use my ones at max gain, where negative feedback attenuation is at a minimum, I found that varying the plate/screen resistances/voltages increased the overall gain.

I'll need to look at this more closely on my current builds.

Thanks for corrected info!

If you operate at max gain, the amount of NFB is small. The text book equations for gain do not apply because they assume infinite open loop gain. So in your case, if you increase the open loop gain by tweaking the ef86 cct you might we see a measurable gain increase. So although you were not completely right you were not completely wrong either!

Cheers

Ian
 
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