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    Building, troubleshooting and testing of these amplifiers should only be
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    the safety precautions around high voltages.

Frank’s 6SN7 preamp/linestage

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THD+N

THD+N

Member
Joined 2007
Since the cathode resistor (first stage) in Frank's design is un-bypassed, there is going to be some degeneration of the signal voltage. I am not near my equipment right now (nor have a messed around with the circuit in awhile) but I would say the voltage gain of the circuit would be about 10, so if 100mV was the input, you could expect to get 1V out.
 
D

DualTriode

Member
Joined 2009
Hello,
I just plugged the first stage of franks preamplifier into the simulator. As is without a cathode bypass capacitor the gain looks to be in the range of 9. With the cathode bypassed the gain is in the range of 17, right on the requested target.
A fun trick you do not see in words often is using a couple of resistors in series that add up to the original cathode resistor value and connect the cathode bypass capacitor at the divider point between the resistors. Using this technique you can adjust the cathode resistor feedback and stage gain to any point between full capacitor bypass and none. If you have the bench tools you can measure gain, 2nd and 3rd harmonics. Tune them to where you want.
DT
All just for fun!
 
T

tubenut33

Member
Joined 2011
Hey , long time...your sugestion was well received.Now I have stumbled upon a neat program freeware;It`s called ``visual analyser``.com.See for yourself,and there are many others.With this program I get 2 trace occ.distortion analyser ,function generator and much more.It irrates me when I contemplated the purchase of rta programs...no need
 
R

rhythmsandy

Member
Joined 2012
can anyone post the schematic of the RPA preamp? no links on the net are working since the thread is about a decade old.

Secondly my observations towards using just choke for filtering improved the sound quality considerably. I'm using a solid-state rectifier but want to move to tube 5AR4 and then move further up to get better sq.

I have few doubts can anyone clarify on them please.

I have used common toroidal and r core transformer for the tube preamplifier and found many issues when using solid-state bridge blocks etc.

So do we need to use independent transformers for B+ and Heaters?

If so which type of transformers to be used for B+ and which type for Heaters?
 
rob211264

rob211264

Member
Joined 2015
GordonW said:
Funny. I was looking for a simple preamp for a small integrated amp I was building- I wanted to use a 6CG7 (6SN7 exact equivalent in miniature bottle).

Just played around in TubeCad... and damn if I didn't come up with just about the exact same values as Frank's preamp. Didn't even see this post until I had built two units with the design I came up with.

On my preamp, I wound up decreasing the gain a bit (using a 22K 2w resistor instead of a 47K 2w) due to the large amount of gain in the amp section... but otherwise, it's the same values. I can say- this design (at least those gain stage values) works VERY well!

Regards,
Gordon.
Click to expand...
I'm curious of your design got any final details on your layout using the 6CG7's.
 
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