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300B SE Amp Design: Some Questions

Posted by Xenithon (A) on August 6, 2008 at 01:11:28

Hi all. I am having an amplifier built by a local builder. Following discussions about my preferred music, equipment, requirements, etc. we decided on a Single Ended 300B amp. I am not too technically skilled or experienced and there are a number of questions/options I have in regard to the design. I’ve sent these through to the builder, but would really appreciate any input and suggestions you may have.

Ideally, I’d like to maintain the 300B magic (three dimensionality, tone, texture, musicality) but without too great a sacrifice of bass weight and control (I want to avoid it being too “tubey”, euphonic, woolly, syrupy etc.)

1. Tube complement

I have looked at various designs on the Internet - as well as high-end amplifiers available on the market - for the various options of tubes to use with the 300B. I have come across a wide variety but have not come across info about the differences or preferred ones to use. The options include:

- 300B, 5687, 12AU7 (e.g., Ayon Audio)
- 300B, 5687, 12AV7 (e.g., Silbatone)
- 300B, 6SN7, 6DJ8 (e.g., Prestige Audio)
- 300B, 6SN7, 6J5 (e.g., DIY design)
- 300B, 6SN7, 6SL7 (e.g., Cayin Audio)
- 300B, 6SN7, 12AT7 (e.g., Tonewin)
- 300B, 12BH7A, 12AU7A (e.g., Air Tight)

The builder also proposed this Aikido design: http://www.tubecad.com/2004/blog0011.htm

2. Chokes

From my limited understanding, chokes can add to speed and clarity, and reduced hum/noise. But, they add size, weight and cost to an amp. Do you guys suggested using chokes?

3. Headphone jack

One of the criteria/requirements is that the amp will have a headphone jack in addition to speaker taps. I have read that this can sometimes be a problem – something to do the high noise floor created by the output impedance of 200 ohms or higher at the jack (and headphones expecting close to 0 ohms)?

I have also read that people employ series resistors between the tube output stage and the headphone jack. Do you think this is the way to go? Do you think it is required? The headphones I will use with it are around 70 ohms, so not very low (like many Grados at 32 ohm) nor very high (like many studio headphones at 300-600 ohm).

Thanks in advance for any feedback!
X

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