how to lower gain from this schematic

[milen007]

hi Passtube

thanks for the reply.

how bout voltage divider of 10K dan 3K to ground. which attenuate 10db. while giving Zin=13K (which might not be problem as i am using 2.2uF output caps from monica) Zout of 2.3K. is this good enough to feed the input impedance of 27K of my LDR volume pot?

my preamp has gain of 6. which is 16db, am i right? hence i still have 6db gain which i think is enough

suggestion?

tia
erwin

[KenC]

Hi erwin,

You really really want to reduce the gain?

Wanna make it into a buffer amp?

Are you brave enough to re-configure your amp?

From SRPP to a boot strapped Cathode Follower with tube active load...

Here goes,

I'll just maintain the operating points of the tubes, that will save me some calculations chore.
I won't even have to know the Q point of the tube
and should anything go wrong, nothing should burn

Top triode mod,

• Connect a 1M 1/4W resistor from top triode grid to the anode of the bottom triode ( or in another word, replace the wire between the top triode grid to bottom triode anode with a 1M 1/4w resistor)
  Connect a 0.01uF/600V film cap as input cap to the grid.
  Connect a 1M 1/4W grid resistor on the other end (input end) of the film cap to ground.
  Connect the volume pot to the input end of the cap.

#many ppl omit this input grid resistor to save 10 cent and use the volume pot to do two jobs, to me this kills the sensitivity. but beware, with the improved sensitivity comes all sort of noise which were blurred prviously... such as filament hum, poor layout noise... etc etc... more improvement shoud come. If no such noise, then call yourself lucky.

Bottom triode,
Connect a 1M 1/4 grid resistor from the grid to the ground end of the cathode resistor of the bottom triode.

Others should remain unchange.

Too lazy to draw the circuit.

you should get gain of mu/(1+mu)=0.93

Zin? Very high higher then you can dream of,
Zo? should be at least lower then 800ohm.

All velues are not critical, except for the capacitor voltage rating, just make sure it is higher then your B+ will do. So don't beat yourself if you only have 1.2M or 860k resistors, or 0.1uf.

Won't know how it will sound but surely nothing should burn

Have a go.

KenC


## Ops its a mu-follower not SRPP

[milen007]

hi kenc

thanks alot for tips that i can experiment.

i attached my sketches see if i got your instruction right. is the B+ is 138Vdc?

thanks alot.

one more thing, whats the input caps for? as this is feed by monica. does it still serve the purpose?

erwin

[KenC]

Almost, Except for the imput.

Connect the grid side of the input capacitor to th top triode grid.

[KenC]

Remember signal is in AC form. Caps pass only AC and block DC.
There are 2 reasons for this input cap.
1) the DC voltage at the top triode is high way above 0V, hence preventing each stage to mess up each others' operating condition.
2) prevent DC current flowing in the 2 stage, hence protection to each other from burning each other.

Cheers

Ken

[milen007]

hi kenc

this way?
ops forget to put down the value for the input ground resistor of 1M.

thanks for the inputs

erwin

[KenC]

that 1m which you mention is suppose to be the volume pot.

[milen007]

hi kenc
K got it. Thanks alot

Just confirmation that I see few basic mu follower shcematic and the input stage is at the bottom triode.

Thanks in advance

Erwin

[KenC]

Hi erwin,
We are converting it to a boot strapped Cathode follower with active load.

The top triode is the cathode follower with the 1M resistor boot strapped to the cathode resistor to set the bias. Signal is taken from the cathode. and capacitor couple with the next stage.

The bottom triode is the active load configured as a CCS, it is cathode bias to set the current and receive no signal.


Mu-follower is actually the other way round, the top tube is a sort of active load configured but the bottom tube is used as common cathode amplifier.

hope that clears the air.

Cheers

Kenc

[KenC]

If you really can't live with cathode follower in any form, there is another mod which you can do. The Totem Pole, Disadvantage is the Zo will be much higher, similar to any common cathode amplifier, Advantage is that this will half the gain and improve on linearity you may loose the warm tube sound and go toward neutral sound characteristic.

Very very simple.

Take the output off the anode pin (Pin 1,7) of the bottom triode.
Keep everything else

That it.

Cheers

KenC

[milen007]

hii kenc

thanks alot for the tips

will update here when i got the time to do it

again, thanks for the taking the time on this

erwin

[KenC]

Erwin,

Any progress?

[sss333]

How about increasing the cathode resistor e.g from 75ohm to 150R or 220R?

[KenC]

SSS333,
Sure have effect; Don't know how much will affect. I don't have the spec sheet with me & no intention to build such circuit. You can try and tell us.

Cheers

Ken

[sss333]

for SRPP, as i know changing the plate resistor can really affect the gain. higher resistor value higher the gain.

for cathode resistor higher the value , lower the gain...

the srpp i use and build for friend is using 5687 tube, cathode resistor 1.5k. palte voltage ~250-280v