Best way to reduce gain in this circuit?

[kodabmx]

The input is capacitor coupled (not shown).

I'm thinking NFB, but I'm used to driving NFB into a cathode, and this circuit inverts... What about using the unused grid or the cathode of the same triode?

Any idea are appreciated. And yes, I know 2 cents for a 2 resistor divider is also a solution...

 

[deafbykhorns]

Take out a gain stage?

 

[kodabmx]

There is only one, without it it would be a buffer... The gain is about 10 now. 5-8 would be better.

 

[JonSnell Electronic]

Introduce negative feedback between the anode of the 6N6P pin 6 and the cathode of the 6N14P pin 8.Try 4k7 resistor in series with a 470n capacitor.

 

[kodabmx]

Thank you, Jon. 470n == 0.047u I think? 0.1u good enough? Also, the cap and resistor values are dependant on each other? I couldn't use a 10k pot instead to "dial-a-gain" could I?

 

[Tjj226]

I wouldn't use NFB on a preamp. It just seems like an unnecessary complication when all you really need to do is tweak the circuit to accept an input tube with a lower amplification factor. 5687s for instance are cheap and have an amplification factor of 15 compared to 22 on the 6N14P.

 

[kodabmx]

Different pinout though... And they weren't cheap the last time I bought them (which is why I switched to 6N6P)

I'd rather use a 2 resistor divider on the input that rewire the sockets.

 

[rayma]

For this circuit the gain is mu/2, and the tubes tend to cancel their distortion.
Any change of topology (other than an input or output attenutator) would degrade
the operation of the circuit.

 

[RajkoM]

I second for input attenuator (L or T type)

 

[Ketje]

Before you make a nice oscillator with Jon's proposal , try it this way.
Mona

 

[chede]

I use a choice of 6N6P, 5687, E182CC, E80CC, ... in my Aikido as the first stage to vary gain.

No rewiring, I use adapters I bought off the big auction site (used to build my own adapters, but there are nice ones available).

Look for example for "E182CC to 6H30 Vacuum tube adapter socket". The seller I bought from also did some custom adapters for me at my request.

Need only to change cathode resistors for the first valve to re-set the bias curent to the desired value. I have socketed the cathode resistors ...

I can also easily re-configure the Aikido as buffer / ACF if I need no gain by just taking out the first valve and inserting a small MKP capacitor into the socket to route the input signal directly to the buffer stage. In your circuit in post #1, that would be inserting a cap between pin 7 and pin 6 of the socket for the first valve.


Regards, Claas

 

[rayma]

I can also easily re-configure the Aikido as buffer / ACF if I need no gain by just taking out the first valve and inserting a small MKP capacitor into the socket to route the input signal directly to the buffer stage.

That would remove the bias DC voltage from the upper grid, which is
normally provided by the input stage, and the buffer would not work.

 

[kodabmx]

Before you make a nice oscillator with Jon's proposal , try it this way.
Mona

I thought the FB would be positive (too good to be true?).

Really though? Simply returning the 1M to output instead of ground? I'll gladly try it...

 

[kodabmx]

That would remove the bias DC voltage from the upper grid, which is
normally provided by the input stage, and the buffer would not work.

In this case, you simply strap the grid to ground and B+ each through 1M resistors.

 

[TonyTecson]

use a lower mu triode for the gain stage, 12au7, 6gu7, or even 6080....

 

[kodabmx]

12AU7 would result in virtually the same gain though... 6080 is silly because it has crap linearity, and it's heater requirements... 6GU7 is slightly better (never heard of it before, thank you.) but still almost as out of touch as using a step down transformer. Voltage divider seemingly for a low cost solution. The rest are past the point of diminishing returns IMO.

 

[Tjj226]

Different pinout though... And they weren't cheap the last time I bought them (which is why I switched to 6N6P)

I'd rather use a 2 resistor divider on the input that rewire the sockets.

I don't know what prices are like in canada, but here in the US I can find them for ~20 bucks on ebay.

But in any case, I am sure there has to be some tube out there that you can pretty much plop in and cut the gain down without rewiring the sockets.

 

[TonyTecson]

12AU7 would result in virtually the same gain though... 6080 is silly because it has crap linearity, and it's heater requirements... 6GU7 is slightly better (never heard of it before, thank you.) but still almost as out of touch as using a step down transformer. Voltage divider seemingly for a low cost solution. The rest are past the point of diminishing returns IMO.

silly 6080? tell that to Peter Millette,

you can check this out.....Low-mu preamp

 

[chede]

In this case, you simply strap the grid to ground and B+ each through 1M resistors.

Right, sorry ... didn't look at the schematic in post #1 closely enough to see that this was not there yet. In the Aikido, John Broskie had put it in ...


Regards, Claas

 

[Lampie519]

Put the volume control between the gain stage and the buffer (Floating DC). It does not reduce the gain but it does not ruin your buffer stage.