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Pre Amp
http://diyaudioprojects.com/Forum/viewtopic.php?f=6&t=6697		 Page 1 of 1
Author:  simo [ 12 Jan 2020, 00:07 ]
Post subject:  Pre Amp
I am building my second "DIY SE KT88 / 6L6 / EL34 / 6CA7 Tube Amplifier" http://diyaudioprojects.com/Tubes/KT88/

The first version of this amp I built, I used a separate tube pre amp with a 12AX7 tube and it worked fine.

However, on this unit I want to build the pre-amp into the same unit. The max plate voltage on the 12ax7 is 300 volts and this amp has a B+ of around 400 volts. I am trying to avoid running two transformers and power supplies in the one unit. The other option is to use a pre amp with a higher plate voltage.

Any suggestions on this would be helpful.

John
Author:  Suncalc [ 12 Jan 2020, 10:54 ]
Post subject:  Re: Pre Amp
simo wrote:
The max plate voltage on the 12ax7 is 300 volts and this amp has a B+ of around 400 volts.
Maximum plate voltage and maximum B+ are two separate entities. The maximum working plate voltage (plate-to-cathode- differential) of a 12AX7 is 330vdc. The maximum zero current plate voltage (i.e. B+) is 450vdc. You'll be fine with a 400v B+.
Author:  simo [ 12 Jan 2020, 16:12 ]
Post subject:  Re: Pre Amp
Thanks for your help. I was confused on the plate voltage and B+ but that explains it.

Thank you
Author:  simo [ 15 Jan 2020, 01:48 ]
Post subject:  Re: Pre Amp
I also want to add the facility for the preamp to accept a phono (pickup head) as well as other inputs. The schematic I am using has a 470k Ohm resistor on the grid input to ground and a 10k resistor in line to the grid.

http://diyaudioprojects.com/Tubes/Unive ... amplifier/

I want to be able to switch the following input devices - Tuner, Ipad, Phono, CD player. Any advice on the most efficient way to achieve this would be great.

Regards, John
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