For sale is a hand built integrated 6EM7 DC coupled tube amp based on the Gary Kaufman DC coupled circuit as outlined here: http://www.the-planet.org/6EM7.html. It has the battery biased signal section and is obviously direct coupled. This is a cathode biased circuit also called self biased which is nice since no rebiasing is necessary as with the fixed biased designs.I took some liberties with some modest changes based on what I considered improvements from the experience garnered from building a good variety of tube amps. For example, the elevated voltage reference to keep the filament to cathode voltage within the allowable limit was done off the power supply vs at the cathode. I've found it makes for a quiter amp and less modulation distortion at the output. The operating point was tapered down a little since the Kaufman circuit runs the output section near maximum plate dissipation. That was not a bad strategy a decade ago when 6EM7 tubes could be had NOS for $1 or $2 but since they've gone up, dropping the dissipation by 10% or so can increase their lifespan orders of magnitude.
The output tubes are Edcor GXSE series with considerably better bass than the Hammond transformers he used. The power supply has a choke in it, but overall has increased filtering making power supply ripple very low. The power supply was modeled in Duncan amps PSUD II and designed not only for low ripple but to be well behaved during audio transients etc... As a result, it is a very quiet amp for use with high efficiency speakers. The volume is controlled with a NOS potentiometer made by CTS in the USA. It's a carbon trace pot which I like and a linear pot that has been modified to provide a log taper similar to that as outlined on the Elliott sound productions website.
The amp is tube rectified with a 5R4 rectifier (or you could use a 5U4GB as shown in the picture as well). For those not familiar with the 6EM7 tube it is a dual triode with a signal section similar to a 6SL7 and a triode output section with a mu of 5 and is similar to a low powered 2A3 tube except it is indirectly heated. This is a true all tube single ended triode tube (SET) and only puts out about 2 watts. But they are a wonderful 2 watts with zero negative feedback, but you will need high efficiency speakers. They are currently running a pair of EV Interface C series II speakers and will do so with all the volume I would ever want, but in general you'll need speakers of 93dbs into 8 ohms or greater. I'll be selling the EV speakers in the upcoming months since I'm putting together a horn speaker system (the reason I'm selling this amp is to fund that project).
I normally put a bottom on the amps, but left this one open for better ventilation. Although direct coupled amps do not have a capacitor coupling the signal and output sections the cost is that the cathode resistors are large and they waste a lot of power which is converted to a lot of heat. I placed those resistors as close to the aluminum plinth as possible (directly against it) to act as a heatsink to draw the heat outside the amp. So with this amp, ventilation is important and the open bottom helps with that process. Finally, the amp has a thermistor which helps with the current inrush on the filaments increasing tube life. Although compared to 300B tubes etc... the 6EM7 is inexpensive, I like to get as much life as I can out of any tube as a matter of philosophy not just economics, especially when there's not any sonic penalty associated with that extra life. More music for longer makes for a happy tubeophile!
The amp comes with the pair of tubes in it and a brand new (in boxes) pair of Philco 6EM7 tubes as an additional pair.
The amp weighs about 20 lbs and will ship from 85087. Buyer pays shipping and we'll split Paypal fees.
Shipping will be via USPS only.
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