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Old 23rd August 2012, 11:04 AM   #11
palmas is offline palmas  Portugal
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Originally Posted by palmas View Post
With that voltage you can use the 6SN7 in a griunded cathode configuration, with both triods in parallel (Rplate=10k to 12k and Rcathode=1K and 8uF).
You'll get about 23dB of gain (14x), but you'll have only about 60kOhm of input inpedance.
I suggest you use one triode only, making the triode 1 for the left channel and the 2 for the right one, so you can trade tubes between channels and have a pair of "new" triods.
For that you can use the same values for resistors and cap and get about 20dB gain (12x), but have an input impedance of more than 160kOhm.

Let me know if you need some recalculations or new specification.
Sorry, I straied a bit....
Using the 2 in parallel will reduce gain marginally to what you have now (maybe 1dB less).
You can use one in ground cathode using the same base you have (cathode resistor 1k5 and 100uF), just need a 10k plate resistor, and gain will drop to 21dB (11x).
Would that be OK? I would replace the 100uF (electrolitic) witha 8 -10uF film foil cap. Better sound and longer life for the tube.

Last edited by palmas; 23rd August 2012 at 11:09 AM.
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Old 23rd August 2012, 11:56 AM   #12
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Quote:
Originally Posted by palmas View Post
With that voltage you can use the 6SN7 in a griunded cathode configuration, with both triods in parallel (Rplate=10k to 12k and Rcathode=1K and 8uF).
You'll get about 23dB of gain (14x), but you'll have only about 60kOhm of input inpedance.
I suggest you use one triode only, making the triode 1 for the left channel and the 2 for the right one, so you can trade tubes between channels and have a pair of "new" triods.
For that you can use the same values for resistors and cap and get about 20dB gain (12x), but have an input impedance of more than 160kOhm.

Let me know if you need some recalculations or new specification.
Many thanks for your advised with all the valuable data provided...

Taking the consideration of the "one triode" option, what will be the value of the Rplate do you recommended? around 64K ohm? is that right?

Another direction I'm thinking about may be just to put a 100K ohm Alps vol. connected between the input jack and the 1st triode (keep everything else unchange)... not sure if it will lose some of the details that I remembered I have this experience on another project before. How do you think?

Last edited by Angelpat; 23rd August 2012 at 11:58 AM. Reason: typo
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Old 23rd August 2012, 01:06 PM   #13
palmas is offline palmas  Portugal
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NOP, plate resistor should be 10k to 15kOhm. Use good quality and high power to keep it from heating too much.
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Old 23rd August 2012, 02:56 PM   #14
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So may I know what is the input impedance of my current SRPP connect of the input stage? Thanks in advance !
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Old 23rd August 2012, 04:13 PM   #15
palmas is offline palmas  Portugal
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So may I know what is the input impedance of my current SRPP connect of the input stage? Thanks in advance !
from simulation, you have at present:
Gain, 23,8dB (15,6x)
Input impedance: 128k
Output impedance: 1,46k
Plate current: 4,2mA

With one tridoe in grounded cathode you'll get:
Gain, 20,7dB (10,8x)
Input impedance: 155k
Output impedance: 4,17k
Plate current: 7,1mA


These are only results from simulation, and thay can differ from triode to triode, but usually pretty accurate.
Hope this helps
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Old 23rd August 2012, 05:13 PM   #16
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Originally Posted by palmas View Post
You can use one in ground cathode using the same base you have (cathode resistor 1k5 and 100uF), just need a 10k plate resistor, and gain will drop to 21dB (11x).
Would that be OK? I would replace the 100uF (electrolitic) witha 8 -10uF film foil cap. Better sound and longer life for the tube.
How about if I not used the bypass capacitor (the 8 uF cap that you recommended) The gain will be lower down further. But just not sure what is the down side. Any idea?
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Old 23rd August 2012, 05:27 PM   #17
palmas is offline palmas  Portugal
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Yes, it would lower the gain. I tryed it some times, but never liked the sound of it
But please tell me again why you want to reduce gain?
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Old 23rd August 2012, 05:32 PM   #18
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The gain of my preamp is so high so it came up with hiss noise from the tweeters.

To back to the power amp. I found the response is much faster that I removed the cathode bypass capacitor.
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Old 23rd August 2012, 05:39 PM   #19
palmas is offline palmas  Portugal
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Originally Posted by Angelpat View Post
The gain of my preamp is so high so it came up with hiss noise from the tweeters.

To back to the power amp. I found the response is much faster that I removed the cathode bypass capacitor.
If you remove the cap, gain goes to 19dB (9x).
Bu the big difference is that with cap the max signal on the input within linear zone is 2V and without cap is almost 9V.
Do you have a correct idea of what the gain should be for the correct use of your pre?
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Old 23rd August 2012, 06:12 PM   #20
palmas is offline palmas  Portugal
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Best solution:
remove both srpp (I'm not a big fan), use the 6SN7 ground cathode single triode with 400V plate voltage, 22kOhm plate resistor and the 1,5k cathode resistor bypassed with a film cap of 8 to 10uF.
This has much lower gain (14x) than the product of both srpp, can take high signal at input and can provide all swing needed to drive the 300B, and will sound much better than the origínal project.

If you can make it simpler why make it worst?
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