Tube preamp topologies and music

If you want to start with tube amps my recommendation is a nicely restored Dynaco ST70. A proven design (250,000 sold!) with adequate power (35wpc) for most speakers. If more power is desired adding a second one and paralleling the outputs on both will yield 70wpc. The ST70 has stood the test of time. Prices are reasonable, well under $1K. Output tubes are readily available (EL34's). I have two - one with a triode input board. The second one is a custom-build from a friend with the stock output transformers and a much larger power transformer.

Push-pull can have superior signal-to-noise (SNR) ratio and less harmonic distortion due to the self-canceling effect of a balanced circuit. Also there is no DC current flowing in the output transformer of a push-pull circuit. 

A push-pull 211 can have double the power in watts of a PP KT88/6550 amp. With correspondingly more expensive parts/output transformer. Doubling the watts only gives a 3db increase in volume level. Not much!

AudioXpress magazine had a good article on building a single-end 211 amp some years ago. The authors cautioned that such a project was not for the inexperienced! I suggest you track down this article! I no longer have my copy. Another valuable source of information on tube amp design are the issues of Sound Practices - recommended! Lastly, find issues of Glass Audio - more information on design and construction of tube amps.

 All single-end tube amps are best used when power levels don’t exceed 25% of maximum wattage. Otherwise distortion climbs rapidly. Plus signal-to-noise (SNR) ratios are poor due to lack of common-mode noise rejection available in push-pull designs.

The radio-transmitter tubes - 211, 805, 845 - require high plate voltages on the order of 1K + volts DC. Building output transformers able to handle a kilo volt without insulation breakdown is difficult. Plus having such a high voltage circuit in a chassis makes it a potential execution device!

In my tube amp collection I have several single-end amps: a 45, a 2A3, a 6BG6 and a 6L6GC. The last is the Golden Tube SE40, which uses three 6L6GC beam tetrodes per channel to yield 20 watts. Much more usable than the 45 (1.75 watts), 2A3 (3.5 watts) and the 6BG6 tetrode (7 watts). 

I do not recommend single-end (one output tube per channel) amps because they typically have low power and high distortion/noise with high output impedances. The last means that the amp will track the impedance curve of the speaker, meaning it will yield a non-flat response, acting like a tone control. Something to be avoided!

A nice thing about the ST70 is the availability of various tube options for the input stage using different dual triodes to replace the stock pentode/triode. Mine uses three dual triode 6922's along with higher-quality capacitors. Plus you can try different brands of EL34's for different sonic "flavors".

An 8 watt single-end 300B amp would be strained to drive a Klipschorn to adequate levels for hard rock/metal because past 2 watts distortion skyrockets to 5 - 10%. Add the inability to provide enough current for the typical 10 - 12 inch woofer and you have an amp with very limited usability.

There's a Jadis Defy 7 for sale now on eBay for $2636 + $105 shipping from The Music Room in Colorado. Six EL34:s per channel for 100 watts. It looks real nice with black transformer covers and gold chassis!