Help on SRPP line amp!

[jazzpeter65]

HI;

I'm going to build my first tube line amp. I'm considering a SRPP-circuit because of high gain and low outputimpedance.

Found this schematic at http://www.supertube.de/amps/amps.htm

Anyone that knows this circuit?

Should I go for 6SN7GT or 6N3P tubes?

Best,

Peter

[jazzpeter65]

This is how the PCB looks like. Notice that all the resistors looks like 470K.

In the schematic they are wiped out. 470K? Can it be true?

[jazzpeter65]

This is another circuit that I'm considering. It uses the Russion equivalen to 5057 called 6N3P?

Any comments?

[ashok]

I think you can get some SRPP resistor values on some circuits I have seen on the web.
The other thing you can do is to simplify the circuit by using a 1K resistor in the cathode of the lower triode. Use a 1K resistor between the plate of the lower tube and the upper triode's cathode. The upper triode grid goes directly to the plate of the lower triode.
All the other resistors and capacitor shown between the upper and lower triodes are thrown away. Keep the input and output components. That will work. I like the 6SN7. Sounds very nice.
I have the 6N3P but have never used it as I am not sure of its characteristics.
Cheers.

Edit: 1K resistor between pins 6 and 2.
1 K resistor between pin 3 and ground. ( all 1/2 watt )
Connect pin 4 and 2 directly.
Note : if you have any hum due to the heater, ground one heater terminal through a 1uF 250V capacitor. I found this to be better than a floating heater.

[jazzpeter65]

This is another one that I'm considering:

[jazzpeter65]

...or this one:

What should I shoose?

[ashok]

The last 6SN7 circuit looks fine.
The 1.5K or 1K I mentioned earlier only determines the operating point ( current ) through the tube. It will determine how much you can load the tube before clipping and possibly a different distortion figure. But in practice I think it might not make much of an audible difference.
Remember to ground the heater. Don't do this directly ( use the 1uF/250V cap) because the heater to cathode rating I think is about 100 volts and the upper cathode will be at 1/2 the supply voltage.
Cheers.

Check the sound with no capacitors in the bottom cathode resistor. It might surprise you !

[jazzpeter65]

HI Ashok,

1.How many db do you think I can get out of the latest circuit? I think I need about 22-25db to drive my solid state power amp.

2. What is the input impedance of this circuit? Shouldn't there be a resistor from input grid to ground?

3. And output impedance? Is it the 470K at the output that sets this?

4. The DC-coupling cap at the output: Should this be decoupled by another cap of lesser value?

Hope you can help!


Best,

Peter

[jazzpeter65]

...I forgot to ask about the volumen pot at the input. Normally you tend to reduce this vaulue as much as possible - starting at 10K. I allready have a good ALPS pot of 50K.

Is this an issue?

- Peter

[Brett]

Originally posted by jazzpeter65
I'm considering a SRPP-circuit because of high gain and low outputimpedance.

Why do you want high gain (about 20 in this cct) in a linestage? You don't need it and you'll just end up attenuating it away in the volume control.

Found this schematic at http://www.supertube.de/amps/amps.htm

Anyone that knows this circuit?

It's not really an SRPP, but a mu-follower or self-biassed totem pole. It's still better than your average SRPP.